Fluid Ear Pressure
Low-Temperature Mounting Wax
Velocity or displacement measurement
Load cell on True Force
Standing Jump Measurement
Method 213B Shock test
Flight Control, Pitch & Roll
Pressure transducer for brake chamber
Impact Testing
Vacuum Gage Environment
High temperature application
Capacitive vs PR accels
Out-of-Plane Vibrations
Tangential Acceleration
Site Phase Measurement Difference
Cable Attachment
Remote Charge Convertor
Wireless Radio Transmission
Motorcycle Helmet Impact
Polysilicon Pressure Sensor
Non-linearity Interpretation
Accelerometer Mounting
Smart Sensors
Vocal Measurement
MEMS Sensors
Cable Connections
Electrical Output Units
Signal Conditioner
Glue Solvent
Bi-polar or Unipolar Excitation
Output Voltage
In-Line Load Cells
Subwoofer Driver Acceleration Measurement
Piezite & Quartz characteristics
Shock Limitation
Wind Measurement
Acceleration/Deceleration
Product Compatibility
Calibration Certificates
Heart Sound Measurement
Accelerometer Accuracy
Automated Equipment and LabView
Cableless Sensors
Minimum Voltage
Radiation Sensitivity of Accelerometers

Fluid Ear Pressure
We are looking for a means to measure dynamic fluid pressure in the human inner ear (frequency range 0...6000Hz, required threshold around 0.1 Pa rms, 2mm max diameter). Any suggestions? I was looking into your models 8514 and 8507C. In addition, could you please provide sensitivities in mV/Pa and noise spectra in V?/Hz?

Moderator's Response:
The 8507C-2 does seem like the most appropriate candidate for your test. It will provide you a linear (5%)frequency response well beyond the 6 kHz you need (out to about 15 kHz), but it is a tad wider than you want it to be. The 8507C, being cylindrically shaped, has a nominal diameter of 2.60 mm. You're looking for a diameter of 2 mm. The broadband noise floor of the 8507C-2 is typically about 5 microvolts-rms. This equates to about 0.000033 psi-rms, or 0.23 Pa-rms. The broadband noise floor mentioned is typical. It is possible, although rare, for the noise floor to be 10 times higher--2.3 Pa-rms. The 8507C-2's nominal sensitivity of 150 mV/psi is equivalent to 21.76 mV/kPa, or 0.0217 mV/Pa.

Low-Temperature Mounting Wax
Can you recommend on an accelerometer mounting wax to be used in a low temperature environment (zero and bellow degrees celsius) or another easy removable mean for a curved surface?

Moderator's Response:
Although we're not actually sure what will happen with beeswax and petro wax at temperatures below freezing, we're confident that both will lose a considerable portion of their adhesion properties. The cyanoacrylate adhesive that we recommend for temporary mounting, however, is rated for use down to -65 F (-54 C). It is Loctite 430 and can be ordered from Endevco in a 3-gram tube as part number EHX399. The solvent that works best with Loctite 430 is Loctite X-NMS. A 59-milliliter bottle can be ordered from Endevco as part number EHX398.

Velocity or displacement measurement
Can an accelerometer be used to measure velocity or displacement?

Moderator's Response:
It is possible to use accelerometers to measure velocity and displacement. In fact, it is quite common. To obtain velocity information from an accelerometer requires single-integration of an accelerometer's signal. Displacement requires double-integration. Integration of an acceleration signal can be accomplished in one of two ways--in the analog domain or the digital domain. Acceleration signals that correspond to measuring vibratory or dynamic phenomena are usually but, by no means only, integrated in the analog domain at the signal conditioner. Accelerometer outputs that are non-vibratory or static/ quasi-static in nature (acceleration of an automobile or flight path of a rocket) are typically integrated in the digital domain, downstream of the signal conditioner. Piezoelectric and ISOTRON® accelerometers are typically used to measure dynamic acceleration and, therefore, dynamic velocity and displacement. They should not be used for measurement of static or quasi-static accelerations, velocities, or displacements. At frequencies approaching 0 Hz, piezoelectric and ISOTRON accelerometers cannot, with the accuracy required for integration, represent the accelerations an object is subject to. When this slight inaccuracy is integrated in order to determine velocity and displacement, it becomes quite large. As a result, the velocity and displacement data are grossly inaccurate. If one wants to integrate a static or quasi-static acceleration signal, one should use either a piezoresistive or variable-capacitance accelerometer. These technologies measure accelerations and, therefore, velocities and displacements accurately at frequencies approaching 0 Hz.

Load cell on True Force
We are performing a test on a pullout switch which requires us to measure a force of up to 750 lbs. at pullout velocities as high as 25 ft/s, basically an impact force. We’ve found through tests and mathematical modeling that inertial forces are dominating the measured force. Since we can’t measure the force directly at the application, we are using a solid rod between the pullout and the transducer (acting as a reaction mass). We do not believe that simply using a dynamic load cell will alleviate this dominance. Could you recommend a load cell and methods (calibration or design) to reduce or eliminate this superposition and measure the “true” force.

Moderator's Response:
My understanding of your situation is as follows: You have no interest in measuring the inertial force of this pull-out switch. You only want to measure the contact force necessary to engage/disengage the switch. If this is incorrect, feel free to give the Applications Engineering Department a call at 877-ENDEVCO, or send an e-mail to + applications@endevco.com to clarify, correct, and/or elaborate. To rid of the inertial component in your measurement, I suggest you: Use the current load cell you're using, but make sure you subject its output to a suitable high-pass filter. The inertial-force component of your measurement should be of a low-frequency nature. If you can high-pass filter your load cell's output at 2, 5, or 10 Hz, you'll probably effectively eliminate the signal due to inertial forces. The decision of which filter corner to use is best left up to you. If you can, execute a test and capture your load cell's output. Subject the captured data to a Fourier Transform so you can see what frequency components are present. If, as you say, the inertial components are dominant, you'll probably notice most of the output at lower frequencies. For starters, just make an educated guess as to which frequency components are inertial. Filter these out and see if the data you're left with is something you can more readily accept. If you need a DC amplifier that has a programmable filter, you should have a look at our 4430A. It can be found on our Web site.

Standing Jump Measurement
I have never used any kind of accelerometer before so I was wondering if you could suggest one for me to use in this class project. I am building a device that determines how high someone jumped from a stand still. It will be worn around the waist so you can not cheat by lifting your feet higher. The device will measure the acceleration of the person jumping and then calculate their displacement. I have the coding taken care of but I don't have a clue about the accelerometer. If you could point me in the right direction, it would be greatly appreciated. The microcontroller I'm using has a 12bit AD converter built in.

Moderator's Response:
You'll need to use a very sensitive accelerometer with a frequency response that is linear down to 0 Hz. You might consider using any of the following Endevco variable-capacitance accelerometers: 7290A-2, 7596-2, or 7593A. Some comments I'd like to add about your application: - you'll need to do your best to ensure that the accelerometer's axis of sensitivity is aligned vertically. A person's body will change its orientation during a vertical jump. Since your accelerometer will be coupled to a subject's body, it too will change its orientation. When it does, the accelerometer will only be measuring a component of the vertical acceleration (as I see it, this won't really matter once a person's feet have left the ground--see the following comments). Perhaps it will be easiest if you affix your accelerometer to a helmet, have your subjects wear it, and have them look straight ahead while they jump. - The equation that describes your subjects' vertical motion is "s = (0.5gt)(t) + vt" where s = vertical displacement g = acceleration of Earth's gravity = 9.8 m/s/s t = time subject is in flight v = initial vertical velocity -I would use the accelerometer to obtain two bits of information: "t" and "v." You'll be able to determine time-of-flight from your acceleration-vs-time curve. Whilst in flight, as shown in the equation above, the only acceleration your subjects will be exposed to is gravity. The acceleration-vs-time curve should, therefore, look very much like a straight line, as Earth's gravity is constant (this assumes the accelerometer's axis of sensitivity is perfectly aligned with the vertical). You'll need to integrate the initial part of the acceleration curve to determine what the initial velocity (v) is. With this information, you can define the two variables necessary to solve the equation above. You'll be able to determine the vertical displacement (s) at any time (t) during the flight. - The accelerometer will have an output before you begin any test. Because the accelerometer can measure down to 0 Hz, it will be measuring Earth's gravity (1 g). If you don't zero this output before you begin your test, your integration to determine your initial vertical velocity will be grossly inaccurate. - The equation above applies only to projectile motion near the Earth's surface. While a person's feet are on the ground, he/she is not subjected to gravitational acceleration alone. The contact force supplied by the ground is associated with an acceleration equal and opposite to that of gravity. In other words, whilst standing still, a person is not accelerating vertically at all.

Method 213B Shock test
I have a requirement to perform a MIL STD 202F, Method 213B Shock test. A requirement of 213B is 2.2.3, Transducer Calibration. Can this accelerometer be calibrated from the 2 Hz to 5 kHz range (ASA STD S2.2-1959)? Is this an appropriate sensor for this application, or can you suggest a different model?

Moderator's Response:
The 7201-50 can be calibrated between 2 Hz and 5 kHz. The standard calibration that comes with the purchase of the 7201-50 is from 20 Hz up to 50 kHz. If you'd like to extend the calibration down to 1 Hz, we can do so without a problem. Just ask for Calibration Service 130L. To determine whether or not the 7201-50 is appropriate for your test, you'll need to consider at least 3 things: 1) how many picocoulombs will saturate your charge amplifier; 2) how much amplitude non-linearity can you tolerate; 3) is the frequency response of the 7201-50 adequate? Charge amplifiers and charge converters are what are typically interfaced with charge-output piezoelectric accelerometers like the 7201-50. They have limits as to the amount of charge they can process. Endevco's model 133, for example, can only tolerate 30,000 pC. This will equate to about 600 g's with the 7201-50. Many shock tests far exceed 600 g's. The amplitude-linearity specification (really should be thought of as amplitude NON-linearity) for the 7201-50 is 1 percent per 250 g's, up to 2000 g's. What this means is that the sensitivity of the 7201-50 will be exaggerated by approximately 1% per every 250 g's. If the sensitivity of a 7201-50 at 1 g is 50 pC, for example, at 200 g's it will be 50.5. At 2000 g's the sensitivity will be about 54 pC/g. The frequency response of the 7201-50 is linear out to about 6 kHz. If you think your shock test might involve frequencies above 6 kHz (some shock tests do, some don't), the 7201-50 might not be the best bet. If your test will involve a broad pulse-width (several milliseconds) half-sine transient, then your test will likely not involve frequencies in excess of 6 kHz. If the transient is better described in terms of microseconds, then it is likely you will see frequencies above 6 kHz.

Flight Control, Pitch & Roll
I'm part of a project group working on flight control. We have 2 accelerometers mounted in a radio controlled plane to measure pitch and roll angles. However, as soon as we start up the engine (a larger than usual 2-stroke), our readings become unusable. Not only is there high frequency noise (which I assume we can filter out) but also a large change in the average value. For example, with the plane sitting at an angle of 35deg, starting up the engine would produce a reading of around -20deg, and this gets worse as the revs are increased. Has anyone encountered a similar problem before, and is there any way to rectify it?

Moderator's Response:
Filtration is the best way I know of dealing with your problem. The acceleration measurements necessary to determine roll and pitch angles are components of gravity. Gravity is, of course, a static, or 0-Hz, acceleration. The components you will be measuring will not necessarily be static. I cannot, however, imagine them being any more than 1- or 2-Hz phenomenona. You should, therefore, utilize a low-pass filter circuit to pass everything below 2 Hz--maybe 5 or 10 Hz to be safe. The sharper you can make the filter roll-off, the better. This should effectively rid of the motor-induced vibration information that your accelerometers are measuring in addition to the gravitational components of interest. One issue that troubles me is flight-path acceleration. I assume that the accelerometer that you are using to measure pitch angle has its axis of sensitivity aligned with the main axis and, therefore, flight path of the plane. If this is the case, this accelerometer will be measuring the plane's linear acceleration in addition to the gravitational component that defines the pitch angle. This flight-path acceleration will be, like the pitch-angle information, a low-frequency event. The filtration will, therefore, have little or no effect on it. As a result, this flight-path acceleration will "cloud" the pitch-angle information, making it difficult for you to make an accurate pitch-angle measurement. The way around this type of a problem is a six-degree-of-freedom sensor. This type of a sensor defines every parameter of motion for any object and is commonly used in inertial navigation. A six-degree-of-freedom sensor can measure the three axes of linear motion that model our reality and the rotation of an object about these axes.

Pressure transducer for brake chamber
I am working on the brake chamber of a commercial vehicle. I am a graduate student. Its towards my research. I need to know what pressure transducer I should buy, what are specifications I am looking at and what is the cost i am looking at.

Moderator's Response:
You should have a look at our 8530B series of piezoresistive pressure transducers. One particular iteration of the 8530B, the 8530B-2KM37, was designed specifically for automotive brake-line pressure measurements in Anti-Lock-Braking systems. It is temperature-compensated between 0 and 200 F, but is capable of being used between -65 and 250 F. The 8530B-2KM37 also features a removable cable to simplify installation. If you won't be measuring up to 2,000 p.s.i. with your system, the 8530B is also available in 200-, 500-, and 1000-p.s.i. ranges. "M37" denotes the detachable-cable option described previously and it is available on most of the rest of our pressure transducers, including the 8530B.

Impact Testing
We are building an impact testing rig for motorcycle helmets and need to know which type of sensor and associated measuring device would best suit this application.

Moderator's Response:
Most helmet-impact testing standards adhere to a more general test specification published by the Society of Automotive Engineers (SAE) known as SAE J211. The key tenet of this specification, for the purposes of this discussion, is that a linear frequency response (between +0.5 and –4 dB) is required between 0 and 1650 Hz. SAE J211 is a fairly elaborate test-recommendation practice, of course, but the frequency-response requirement is what will help you select the most appropriate accelerometer. Another parameter you’ll need to consider is the expected acceleration level. Many test specifications consider a helmet to have failed if if the accelerometer in the headform measures more than 300-400 g’s. Luckily for you, most accelerometers that meet the linear-frequency-response requirement of 0-1650 Hz can also measure at least 500 g’s. Some of the accelerometers that might be suitable for your testing, therefore, include: the 2262CA-1000, 2262CA-2000, 7231C-750, 7264B-500, 7264B-2000, 7264C-500, 7264C-2000, 7265AM3, and the 7267A. In my own personal experience, I’ve encountered labs that have used 2262CA-2000’s and 7231C-750’s for helmet-impact testing.

Vacuum Gage Environment
Hi I found Endevco accelerometer Model 7754-1000 Suitable for my needs. But it should be installed in a vacuum cage. 1e-7 Torr. Please let me know if it is designed for this purpose.

Moderator's Response:
The 7754-1000 accelerometer has been used in vacuum environments many, many, many times with success. It is hermetically sealed courtesy of glass-to-metal seals and various welds. As such, its case can easily resist the stresses caused by the pressure inside the case and the absence of pressure outside. The 7754's case would have no problem withstanding a much greater pressure differential. If you're concerned about outgassing, you'll want to remove the blue silicone-rubber strain relief at the 10-32 end of the supplied cable assembly. The 3061 cable assembly will not outgas much, but if you want as little outgassing as possible, it's helpful to bake the assembly at 250-275 F for 3-4 hours to release any organics trapped in the cable's jacket.

High temperature application
We are working on a process where a sheet of paper and plastic material is "crimped" between two rolls heated (max 200°C) and pressed against each other at a surface speed of 180 m/min . We want now to measure the interference (i.e. the roll deformation on the contact line) during the process. If this is not possible, a statical measure would anyway be useful. I have contacted a contactless displacement sensors (eddy-current based) supplier but the temperature made the measure impossible.

Moderator's Response:
Putting accelerometers on your rollers' bearing houses might do the trick. If there is a deformation that passes through the rollers, the accelerometers will measure the deformation as an acceleration pulse. This acceleration data can be integrated to determine velocity and displacement. If you integrate to displacement, you will be able to determine the height of the wrinkle, or imperfection, in your sheet. This method assumes that the rollers are spring-loaded against one another. 200 C is no problem for a piezoelectric accelerometer. In fact, Endevco offers piezoelectric accelerometers that can be used at temperatures as high as 760 C. I don't know how big your rollers and, therefore, bearing housings are, but I'll offer a suggestion nonetheless (the mass of the accelerometer should be, at most, 1/10th the mass of the bearing housings). The 7703A-100 accelerometer and 6634B signal conditioner are probably a good choice for your application. You'll need an accelerometer, like the 7703A-100, with a fairly high sensitivity (100 pC/g). The 6634B will allow you to integrate your accelerometer's output to determine displacement. You can also integrate your accelerometer's output in the digital domain with the appropriate software. Reputable data-acquisition manufacturers frequently offer such software as an accessory to their hardware.

Capacitive vs PR accels
I want to measure linear acceleration(~10g, DC to 20Hz) on a rail vehicle. From looking at your range of accelerometers either a capacitive accelerometer(7290A) or piezoresistive accelerometer(2262A) would suit. What are the advantages and disadvantages of capacitive and piezoresistive accelerometers? (ie. shock survival, accuracy, drift, overrange recovery time)

Moderator's Response:
For your application, the variable-capacitance (VC) accelerometer has it all over the piezoresistive (PR). VC accelerometers are generally available with much higher sensitivities than PR accelerometers are available with. The VC accelerometer best suited for your test, the 7290A-10, has a nominal sensitivity of 200 mV/g. The most closely competitive PR unit, the 7265A-HS, only has a sensitivity of 25 mV/g. Because your test will likely expose a chosen accelerometer to fractions of a "g" at some point, you'll want to go with a higher-sensitivity unit in order to achieve a more desirable signal-to-noise ratio. VC and PR accelerometers do share two similarities: 1) they both require simple signal conditioning (DC power supply and differential output) and 2) they both can measure accelerations down to 0 Hz (i.e., gravity). The main difference between the two, as stated above, is sensitivity. VC accelerometers are generally much more sensitive than PR units. Endevco's VC accelerometers also generally have much higher shock limits. To continue the example above, the 7290A-10 (VC) has a shock limit of 5,000 g's and the 7265A-HS (PR) has a shock limit of 2,000 g's. The next higher-g-range variable-capacitance model, the 7290A-30, has a shock limit of 10,000 g's. It should be noted that high-shock survivability is a characteristic unique to Endevco's variable-capacitance accelerometers. There are others VC units out there that offer equivalent sensitivities, but they are only capable of surviving mechanical shocks 30%-60% the level of g's that Endevco's units can. In the end, perhaps it's best to think of VC and PR units in terms of which is most appropriate for a given application. VC accelerometers are ideally suited to measure low-frequency, low-level accelerations (acceleration of an automobile; wind-induced sway on a skyscraper). Endevco's VC accelerometers, in addition, are particularly well suited to measuring low-level accelerations before or after being subjected to a violent shock event (measuring low-level vibrations on a space launch vehicle after surviving the launch event; acceleration of a missile on a railcar before it impacts a truck in a transportation safety test). PR accelerometers are ideal for medium-level, lower-frequency shock events like automotive crash testing (200 - 2,000 g's), where a DC response is necessary. We do also offer a PR accelerometer (7270A) to measure high-level shock events. It can measure shocks as high as 200,000 g's and has a mounted resonant frequency in excess of 1 Mhz.

Out-of-Plane Vibrations
I need to measure out-of-plane vibrations of a rotative structure (~wheel). Since wireless accelerometers are not available, I would like to use a slip ring system. A slight motion of the wire and electro-magnetic effects (speed: 200 km/h) are to forecast. What about the resulting measurement noise? Can the motion damage the connection with the accelerometer? Could built-in electronics or particular accelerometer types help

Moderator's Response:
An ISOTRON accelerometer would be the best candidate for your test. ISOTRONs have low-impedance outputs that make their signals less susceptible to the sources of noise that will plague your application. They also are suitable in the sense that they are capable of measuring dynamic phenomenon only. All accelerometers have some degree of transverse sensitivity--meaning the centrifugal accelerations that your test object will develop can contaminate your output signal. The design of ISOTRON (internal-electronics piezoelectric) accelerometers is such that a constant transverse sensitivity will decay, or bleed away. The piezoelectric crystals utilized within ISOTRONs have a finite resistance that allows charge to bleed from one side to the other--the very reason they will measure dynamic phenomena only. DC accelerometers (piezoresistive, variable-capacitance), on the other hand, will not allow the measurement of a transverse acceleration to decay. If you have a DC accelerometer with a 1% transverse sensitivity and it is subjected to 100 g's of centrifugal acceleration, it will return a signal of 1 g from this transverse acceleration. You will not be able to distinguish this g from the handful that you will probably be measuring out-of-plane. As an alternative to using slip rings, you may want to consider a telemetry unit. Companies like Wireless Data Corporation (+ www.wirelessdatacorp.com) and Accumetrics (+ www.accumetrix.com) offer telemetry units that will, apparently, interface directly with accelerometers. There are a multitude of slip-ring manufacturers one might use. If you go this route, you'll probably want to use a product with a brushless design. Mercotac (+ www.mercotac.com) might me a good place to begin a search for such a slip ring.

Tangential Acceleration
We want to determine tangential acceleration of the mass at the end of a pendulum as it strikes a test specimen. The g range of interest is 0 to 20. However, there would be the possibility of 200 to 500 g's after the test as the pendulum hits a stop. What product(s) would you suggest? We have never used accelerometers. What instrumentation do we need? What would the ballpark cost (range) be of such a system?

Moderator's Response:
The most appropriate accelerometer for your test is probably the 7290A-30. It is designed to measure up to 30 g's and can withstand the 200 to 500 g's of shock that will occur after your measurement of interest. In fact, it can survive a shock as high as 10,000 g's. In addition to the accelerometer, you will also need a signal conditioner and, perhaps, a data acquisition system. Endevco does have a signal conditioner to offer, but I'm afraid that we currently have no data acquisition system to offer. The signal conditioner provides the proper power for the accelerometer and offers variable gain, filtration, zeroing capability, and a display for its output. A data acquisition system will allow you to capture your test data digitally and, therefore, makes it possible to manipulate your data (integration to velocity and displacement, frequency analysis, digital filtering, etc.) with the appropriate software. The Endevco 136 signal conditioner is an excellent companion for the 7290A-30 accelerometer.

Site Phase Measurement Difference
Planning a system that provides ability to match vib phase measurements across test sites, each site with unique signal processing (charge amps, phase analyzer, etc); system to be tested will have consistent 1/rev tach signal and accelerometers. Is it possible to use charge generator (with charge phased to tach signal?) to determine site phase measurement differences?

Moderator's Response:
I don't see any reason you couldn't use a charge signal phased to a tachometer signal. You can feed the tachometer signal in parallel to your phase analyzer and something like our 2947B-2 calibration capacitor. The calibration capacitor, in conjunction with the tachometer voltage pulse, will simulate the charge output of a piezoelectric accelerometer. Instead of using a tachometer pulse, you can always use an AC voltage. If you have access to a function generator, using an AC voltage will allow you to test your system at a variety of frequencies. This could be an advantage, considering that phase shift is frequency-dependent.

Cable Attachment
My production uses piezoelectric accelerometer model 222C to measure resonance frequency of my product using "Ping" test. Currently we attached a set screw to the accelerometer so that we can fix it to the our product. Many times the delicate wire at the connecter cable 3093 tends to break at the tip (connects to accelerometer). We then have to cut short the cable and extend the fine wire to the tip of the connecter. These results in the cable getting shorter and shorter till the point we need to get a new cable. I believe the manual handling of screwing the accelerometer to our product causes stress to the wire. Is there a way to prevent it?

Moderator's Response:
It sounds to me as if threading the accelerometer into the mounting surface is overstressing the 3093-12 cable assembly. Perhaps you are unaware, but the 3093-12 is detachable. It can be removed from the 2222C accelerometer. There is a small wrench (Endevco part # 16205) that fits onto the miniature connector that has been threaded into the 2222C's case. This wrench is included with the purchase of every 2222C accelerometer. My suggestion is to remove the cable before you fasten the accelerometer to your mounting surface and replace it when the accelerometer is fastened. By the way, how have you attached a set screw to your 2222C? I hope you’ve only used an adhesive. If you’ve tapped the case of the 2222C, your accelerometer has probably been damaged. The 2222C is designed to be used as an adhesively mounted accelerometer only. There are a variety of temporary adhesives that can be used, including: cyanoacrylate, Petro Wax, double-sided tape, and hot-melt glue. For the advantages/disadvantages of each type, feel free to download Technical Paper 312 from the Endevco web site.

Remote Charge Convertor
I need to confirm the proper installation and functionality of multiple piezoelectric accelerometers on a space vehicle. Each setup will involve ~10 accelerometers without the charge amplifiers available(!), a ?tap test?, and some confirmation on a remote computer. Is there a less expensive method other than to provide my own charge amps for each accelerometer? Can I interface charge converters to standard analog input signal conditioning modules instead? Or, is there a better (and less expensive) way?

Moderator's Response:
Ideally, you would be able to subject your accelerometers to a known acceleration level with a portable shaker like the 28959F and verify that the outputs from the charge amplifiers are what they should be. Since you don't have charge amplifiers available to you, maybe you can use, as you've suggested, ISOTRON signal conditioners with remote charge converters. Remote charge converters that you can use include the 2771B and 2771BM1. Also commonly used for such an application is the 2944.1 T-Junction Calibrator. The calibration method that utilizes the T-Junction Calibrator requires that you have access to charge amplifiers also, but you should be aware of this tool for future use. The T-Junction Calibrator allows for passive voltage-insertion calibration of measurement systems that utilize piezoelectric transducers. The voltage-insertion calibration method is a simulation technique that introduces a voltage into the ground side of the accelerometer output signal cable between the accelerometer and its associated charge amplifier to simulate the self-generating output of the transducer. It is a convenient method to verify that there is nothing wrong with the accelerometer’s cabling and that the charge amplifier is performing as it should. It doesn’t ensure that the accelerometer is working properly, but it can underscore severe accelerometer damage. For more information about T-junction calibration, have a look at the 2944.1’s Data Sheet and send an e-mail to + applications@endevco.com with your postal address, requesting Endevco Technical Paper 216. I suppose the most basic test you can perform is a capacitance test. If you know the capacitance values for your accelerometers and associated cable assemblies, you can verify if either has been severely damaged with a meter that reads capacitance. The value your meter returns should be the sum of a cable assembly’s and accelerometer’s capacitance.

Wireless Radio Transmission
We need to take a vibration reading at a point that isn't accessible to wired hookup. Also it needs to be a temporary magnet accelerometer. We were thinking wireless radio transmission, instead of the wire, since we can't use wire in this particular circumstance. Is there any product available for this purpose?

Moderator's Response:
I repeat the response given to an entry below submitted on November 26, 1999 titled "Wind and vibration measurmement of metal structure.": At this point in time, it is necessary to have cables between your accelerometers and signal conditioners/power supplies. In the not-too-distant future, however, wireless accelerometers will be available. The best compromise you can achieve today is to use a telemetry system. A telemetry system will accept the output of your measurement system and transmit it to a remote point via radio frequencies. If you're lucky enough, the telemetry system will incorporate the electronics necessary to power your transducers and condition their signals. If you're not lucky, you will need to first interface your accelerometers with signal conditioners. You will then need to feed the output of your signal conditioners into a telemetry system. Aydin-Vector is reputable manufacturer of telemetry systems. As far as mounting an accelerometer magnetically goes, our 50M1 ISOTRON accelerometer has an integral high-strength magnet on its mounting base and our 2988M7 adapter allows accelerometers that have a 10-32 tap to be mounted magnetically. A majority of our accelerometers have a 10-32 tap in their mounting bases.

Motorcycle Helmet Impact
I want to measure the RMS values of acceleration along the three directions to a motorcycle helmet impacted on a steel anvil at a velocity of 7 m/s by a guided free fall from a height. Also required is the impact velocity just before impact. Could you suggest suitable accelerometer with associated measuring assembly?

Moderator's Response:
An accelerometer commonly used in such an application is the 7267A. It is a triaxial piezoresistive accelerometer whose individual-axis subassemblies are half-bridge. To complete the bridge, customers often use two fixed resistors in a connector that is designed for just such a purpose. The 7267A meets The Society of Automotive Engineers' specifications for anthropomorphic dummy instrumentation. Other single-axis accelerometers appropriate for your test that can be mounted on available triaxial mounting blocks include: the 7264, 7264B, 7264C, 7231C, and 7270A. In the near future, Endevco will also be introducing the 7269. The 7269 is a miniature (0.5 gram) triaxial piezoresistive accelerometer that will be available in two versions: one to measure up to 500 g's and one to go up to 2000 g's. Keep your eyes peeled for it. As far as measuring the helmet's velocity just before impact goes, you may want to consider two approaches. You can use a high-sensitivity DC accelerometer (like our 7290A) as either a trigger to measure time or as an acceleration output that can be integrated to obtain velocity data. If you choose the former, you can just measure the time it takes for the helmet to reach impact from its drop position on an acceleration-vs.-time curve. If the helmet is truly in freefall, it will only be subject to gravitational forces. As such, you can use the following simple formula to compute velocity before impact: v = gt, where "g" is the acceleration of earth's gravity (9.807 m/s/s) and "t" is the time of freefall until impact. A three-channel DC signal conditioner that is suitable for use with all of the accelerometers mentioned above is the 136.

Polysilicon Pressure Sensor
What is the future of Endevco silicon pressure sensors and what are their respective part numbers?

Moderator's Response:
Endevco is one of the companies that first developed silicon pressure sensors. A major breakthrough came when Endevco introduced the sculptured diaphragm design which almost doubles the sensitivity of the sensor. The main advantage a silicon diaphragm affords is a very broad linear frequency response. The 8511A series of pressure transducers, for example, utilize silicon diaphragms with resonant frequencies in excess of 1 MHz. This high of a resonant frequency allows the 8511A to have a linear (5%) frequency response from 0 Hz to approximately 200 kHz! A related secondary advantage of micromachined silicon diaphragms is the ability to miniaturize a pressure transducer. Pressure transducers with stainless steel diaphragms are typically larger in overall size. As noted above, the Endevco sensors have the ability to measure both static (0 Hz) and dynamic pressure. This characteristic also allows for the measurement of fast pressure events that have a somewhat long time history. Since these transducers can sense down to 0 Hz, they are often called “DC responding” devices. The main disadvantage of silicon-diaphragm pressure transducers is their incompatibility with water. The diaphragms can tolerate about one week's exposure to water, or media containing water, but will eventually be permeated. Once water gets through a diaphragm, it will short-out the diffused Wheatstone bridge and any associated temperature-compensation circuitry, rendering the transducer temporarily in-operational. Silicon pressure transducers can be used in applications involving water, but they must be removed periodically and subjected to temperatures that will evaporate any water that has begun to permeate the diaphragm.

Non-linearity Interpretation
I would like to know how to interpret the non-linearity specification in your catalogs of the models 2224C, 2262C, 2262CA, 2270, 2272 and 7221 Accelerometers. Are they evaluated to end point, best fit, zero-based best fit, etc.?

Moderator's Response:
I suppose that one might describe our accelerometers' amplitude linearity curves as "best-fit." We simply subject an accelerometer to successive levels of acceleration and measure the respective non-linearities. The 2224C accelerometer, for example, has an amplitude linearity spec. of "1% per 250 g's, 0 to 1000 g's." To determine this specification, we subjected a 2224C to various levels of acceleration up to 1000 g's. At 250 g's, we noticed that the accelerometer's sensitivity was exaggerated by 1%. At 500 g's, we noticed it was up by 2%. At 750 g's, we noticed the sensitivity was overstated 3%, etc.,etc.

Accelerometer Mounting
I've noticed that mounting the accelerometer directly onto the specimen or via a metal fixture introduces significant noise into my results. A colleague of mine recommended a type of composite material which is supposed to be able to reduce the amount of noise in the results. Do you have any recommendations for the material for a fixture?

Moderator's Response:
It sounds like you've encountered a ground loop. A ground loop can rear its ugly head when the accelerometer/cable/signal-conditioner system is grounded at more than one point. This can happen, for example, when a case-grounded accelerometer is mounted to a conductive surface that happens to have a different potential than the point at which the low-side of the signal is grounded at the signal conditioner. This will cause current to flow to/from the grounding point at the signal conditioner through the cable's shield to/from the accelerometer's case, which is tied to the mounting-surface grounding point. This ground loop, as you've witnessed first-hand, will distort your output signal. There are a variety of ways to deal with ground-loop issues, the most common of which is an isolated mounting stud. If the accelerometer you're using is case-grounded (if there is continuity between the outer part of the connector and mounting surface) and has a tap to accept a stud, then an isolated mounting stud should solve your problem. Other means of eliminating ground loops include: isolated adhesive mounting adapters, isolated signal-conditioner inputs, case-isolated transducer designs, and non-conductive mounting adhesives. For help in selecting an appropriate isolation method for you, feel free to contact us at + applications@endevco.com. In your message, make sure you let us know what accelerometer you're using and be sure to describe your application a little bit.

Smart Sensors
I want to get more information on your smart sensor technology. Where can I find a white paper on this subject?

Moderator's Response:
We do have a literature packet that can be mailed to you if you send your request, along with your mailing address, to + applications@endevco.com.

Vocal Measurement
We would like to be able to measure the mechanical properties of the human vocal fold. The vocal folds are like a pair of tiny lips about 15 mm long and several mm wide, with mass

Moderator's Response:
To do so, you'll need a very small accelerometer. Have a look at our 25A, 25B, and 2250A-10. If you find any of these suitable, a couple of things you'll need to consider are which adhesive to use and that the transducers mentioned are not hermetically sealed. I've never come across a temporary adhesive that is designed to be used on a mucous membrane, so I'm afraid I can't recommend something there. If you're careful to not allow any bodily liquids to get into these accelerometers, they'll perform well for you. If liquids are allowed to penetrate these accelerometers while they're powered, however, they can be damaged. It's not easy for liquids to get into these accelerometers, but it's certainly not impossible.

MEMS Sensors
I assume you are an expert in MEMS sensors. Is there a MEMS technology sensor that measures absolute angle of rotation to resolutions of 5 arc minutes or less? Such a device would be used in servo control of a rotating axis.

Moderator's Response:
There are transducers that can resolve as little as 5 arc minutes. In fact, there are transducers that can resolve as little as 0.1 arc seconds. They are called "inclinometers" and are available from a variety of manufacturers. I should mention that most DC accelerometers (by "DC" I mean "having a linear response down to 0 Hz") with a reasonably high sensitivity (> 200 mV/g) can be used as inclinometers. Because they have a DC response, these types of accelerometers can measure constant, or static, accelerations like gravity. When their axes of sensitivity are aligned with the direction of gravity, they output a voltage proportional to 1 g (9.807 m/s/s) of acceleration. When their axes are not aligned with the direction of gravity, they output a voltage proportional to the product of the cosine of the angle of misalignment and 1 g. Because the discussed are linear accelerometers, you would have to do some math to determine the angle your accelerometer has tilted. Although the math is simple enough, you may prefer to use a device that returns an output directly proportional to angle of rotation. I only mention the use of a DC linear accelerometer in case you already own one.

Cable Connections
In my application I measure the output of various Piezoelectric Accelerometer. They are assembled with coaxial cable for 10 meters, and we have to connect them with differential cable (6917-C) for 40 meters more, What is your opinion about this breakpoint?. Could we make other breakpoint in the path without damage a lot the measure?. And on the other hand, my installation is ready with differential low noise cable (6917-C) due we use the differential input of your 6634B amplifier. Which is the best way to assemble the connection in the breakpoint (in the pins) . Is this OK, or is better to use the SE input?

Moderator's Response:
If you are using a differential piezoelectric accelerometer, it is best to use a continuous length of differential low-noise cable. If you are using a single-ended piezoelectric accelerometer, it would be wise to use an uninterrupted length of coaxial low-noise cable. The way you are configuring your cables can be a recipe for disaster. Perhaps I'm misunderstanding you, but it sounds as if you're splicing low-noise coaxial cable to low-noise twisted-pair (differential) cable. Low-noise cable utilizes a conductive material around the dielectric(s) that serves to dissipate triboelectric charge that would otherwise wind-up as noise in the output signal. Triboelectric charge results from friction between materials utilized in cable construction when the cable is flexed. This conductive material, or low-noise treatment, must be removed entirely from any splice. Removing it entirely can be a tricky enterprise. If even microscopic particles remain, a leakage resistance can be developed between a center conductor and shield. This leakage, needless to say, can render your data useless. In addition, it's a bad idea to have a break in a low-noise cable's shield or to have its conductors exposed to the environment. The output transmitted through low-noise cables is usually of the high-impedance variety. As such, it is very susceptible to a variety of noise sources: electrodynamic fields, electrostatic fields, humidity, etc. Having a discontinous shield or exposed conductors may invite these unwelcome noise sources. I'm guessing you are using differential piezoelectric accelerometers. If so, you should attach them to your 6917C cable assemblies and the differential input of the 6634B signal conditioner.

Electrical Output Units
Hi! Could you correct if something goes wrong in next calculations. Let's give 50 g shock to Model 23 accelerometer. It gives out 0.40 pC/g so the total output is now 50 g * 0.40 pC/g = 20 pC. Model 133 Signal Conditioner convert's charge to volts by formula 0.303 mV/pC, so the result is 0.303 mV/pC * 20 pC = 6.06 mV. There is also constant gain = 3 so the output is now 6.06 mV * 3 = 18.18 mV. If I want to boost the signal to hit 5 V I have to gain it about 5 V / 18.18 mV = 275. Now when the 50 g shock hits the accelerometer, the output of signal conditioner is 5 V ? And when the shock comes of opposite direction (-50 g), the output of signal conditioner is -5 V.

Moderator's Response:
If you want to have 50 g's equate to 5 V (5000 mV), there's a much easier way to do it. Take your full-scale output (5000 mV) and divide it by the number of g's you want it to represent (in this case, 50 g's). The number you get by dividing 5000 mV by 50 g's is, of course, 100 mV/g. Just enter 100 mV/g into the output-scaling step when you configure your 133 and you'll have an output from the 133 of 100 mV/g (5000 mV / 50 g's). It doesn't even matter what you enter on the input-sensitivity step just prior to the output-scaling step. The model 133 will automatically adjust the gain so your system sensitivity will become 100 mV/g.

Signal Conditioner
I have been unsuccessful in using a Columbia 6062 accelerometer with an Endevco 133 signal conditioner. I am using the 'microdot' input and setting the conditioner to 'CHRg' mode with O.O excitation current, and a setting of 1.5 pC/EU and 1000 for mV/EU. Am I missing something?

Moderator's Response:
Columbia's 606-2 should be compatible with our model 133 signal conditioner. It sounds like you're test set-up is correct--you should be connecting a low-noise cable to the Microdot (10-32) connector and the Input Select mode should be "Charge." If the vibration levels you are measuring are very low, however, I don't think you'll have much success with Columbia's 606-2. It has a nominal sensitivity of 1.5 pC/g and is, therefore, not well suited to measure low-level accelerations. If you were trying to measure 3 g's-peak, for example, you would be buried in the noise floor of the 133 or any other charge amplifier, for that matter. For the sake of posterity, I will list below the settings you should try for your 133 in order to have the best chance of getting an output using Columbia's 606-2. 133 SETTINGS Input Select: Charge Current Excitation: Off Sensitivity: approx. 1.5 pC/g (606-2-specific) Output Scaling: 10 or 100 mV/g HP Filter: Off LP Filter: Off Monitoring State: any of the choices available

Glue Solvent
I have an model 22 accelerometer mounted on metal with cyanolit, what kind of solvent can i use for soften this kind of glue?

Moderator's Response:
I'm not familiar with cyanolit. The most common accelerometer adhesive is cyanoACRYLATE. Acetone works well to soften cyanoacrylate, so I'm guessing it will be effective with cyanolit, assuming both adhesives have a similar molecular structure. The best combination of temporary accelerometer adhesive and solvent that we've evaluated is Loctite 430 (Endevco part # EHX399) and Loctite X-NMS (Endevco part # EHX398). Loctite X-NMS removes Loctite 430 more effectively than does acetone. Loctite 430 has an optimal temperature range between -18 ?C and +121 ?C.

Bi-polar or Unipolar Excitation
Dear Endevco, We're in the process of changing over to Megadac's TCS Software for windows. In the sensor setup it asks if our specific accel is bi-polar or unipolar excitation. Could you give us a clearer explanation concerning your 7265A-HS accel in this area? Thanks, RLU

Moderator's Response:
Endevco calibrates its piezoresistive accelerometers with unipolar excitation: a difference in potential between +10 VDC and ground. Theoretically, the accelerometers will operate exactly the same with bipolar excitation: a difference in potential, for example, between +5 and -5 VDC. Unipolar excitation is known as such because there is only one polarity involved--in the example above, the single polarity is "+." Bipolar excitation, as I'm sure you're guessing by now, involves two polarities: "+" and "-."

Output Voltage
Hi, I'd like to know what is meant by "or greater" in the description of the output voltage of the conditioner 4416B in order to check, whether my DAQ board (max Input Range +/- 10V) is capable of processing the signal coming from the conditioner. Thanks in advance, Lutz P.S.: Stated in the data sheet of Conditioner 4416B: "Linear Output Voltage...10V pk-pk or greater"

Moderator's Response:
The 4416B is designed to power and condition signals from ISOTRON accelerometers. ISOTRON accelerometers utilize a circuit design that produces a DC bias voltage once it is powered with a constant current (typically 4 mA) associated with a DC compliance voltage between, typically, 18 and 24. Any dynamic (AC) signal that an ISOTRON produces in response to an acceleration input "rides" on top of this bias voltage. The output of the accelerometer is limited by the compliance voltage available and the DC bias voltage that has been set-up. For example, if a typical ISOTRON accelerometer has a bias voltage of 11 VDC and a compliance voltage of 20 VDC, it can have an undistorted output of 18 V, peak-to-peak. It can have this output because the voltage available to ride on the bias voltage is as high as 20 V or as low as 2 V (lower than two volts may cause distortion). Most ISOTRON accelerometers are capable of this 18 V, peak-to-peak output but are rated conservatively, in case the signal conditioning in use cannot supply an adequate compliance voltage. Therefore, depending upon the bias voltage of your particular accelerometer and the compliance voltage of your signal conditioning (in this case, the 4416B--its compliance voltage will vary depending upon how completely it is charged), it is possible to have a voltage output from your ISOTRON accelerometer greater than 10 V, peak-to-peak. If you can limit most ISOTRONs' usage to within their specified acceleration ranges, you should not exceed an output of 10 V, peak-to-peak. For a clearer description of the above, send an e-mail to + applications@endevco.com requesting a copy of Endevco's ISOTRON Instruction Manual.

In-Line Load Cells
I am working on a project which involves using in-line load cells to measure a compressive force we are exerting on a test piece. The force is produced by a hydraulic ram. I would like to know how a load cell actually works so I can get a better understanding of the system.

Moderator's Response:
If you've ever seen a strain gage in action, you understand how a load cell works. Just like strain gages, load cells utilize resistive elements whose values change when stressed. The changes in resistive values are directly and conveniently proportional to the physical quantity of interest for those who use load cells--force. A common configuration for a load cell's resistive elements is the Wheatstone bridge. A stable DC voltage is applied between the northern and southern points of the bridge (Imagine the shape of a two-dimensional diamond, each of the four sides containing a resistive element.). The output of the bridge is the difference in potential between the eastern and western points. When unstressed, and if each of the resistive elements is equivalent in value, the difference in potential between the eastern and western points is zero. When stressed, the values of half or all of the resistive elements will change. Some decrease, some increase. These changes create a difference in potential between the eastern and western points and this is the transducer's output. What I've described is difficult to imagine. It would be much easier if I could show you a diagram. Endevco's pressure transducers function on the same principle I've just described--piezoresistivity. If you send an e-mail with your postal address to + applications@endevco.com, we can send you IM8500. IM8500 is our piezoresistive-pressure-transducer instruction manual. It does a much better job of describing what I've just attempted to.

Subwoofer Driver Acceleration Measurement
My plan is to mount a accelerometer to the cone of a subwoofer driver to measure it's acceleration. My question is which accelerometer should I use that will have enough sensitivity and be light enough to not affect the speakers performance?

Moderator's Response:
You should consider using either the 2250A/AM1-10 or the 256-100. If you can accomodate the mass of the 256, 3.2 grams, you should use it. It has a nominal sensitivity of 100 mV/g--ten times that of the 2250A/AM1-10. The higher sensitivity will afford you a more favorable signal-to-noise ratio, a goal common to any test utilizing instrumentation. Use the 2250A/AM1-10 (0.4 grams) if you decide the mass of the 256 is too much.

Piezite & Quartz characteristics
The Piezo electric effect is employed in solid state "gyros" used to enhance yaw stability of radio-controlled model helicopters. They have replaced electro- mechanical gyros & are subjected to vibration & shock, from engines & ground impact respectively. For proprietary reasons, it is difficult to learn how they work. Is Piezite a quartz or ceramic crystal? And can you speak to the differences in shock vulnerability.

Moderator's Response:
PIEZITE is a ferroelectric ceramic and quartz is a naturally occurring piezoelectric crystal. Quartz and PIEZITE have similar mechanical characteristics, although quartz is a little bit stronger because it is monocrystalline. PIEZITE is polycrystalline and contains grain boundaries along which stresses can have more damaging effects. Think of grain boundaries kind of like microscopic cracks in a material. The main advantage of PIEZITE over quartz is increased sensitivity. For a given mechanical stress, most PIEZITE materials will yield a higher charge output.

Shock Limitation
We want to use 2220D accelerometers for vibration measurement. However, prior to vibration testing, our hardware will be submitted to shock test (without measurement by 2220D). The question is : will the sensors withstand the shock ? Your data sheet gives 5000g for shock limit, but is it an operational limit ? Or is it the maximum shock level that the sensor can withstand before damage ? We expect shock levels up to 6000 g : can we mount the sensors before the shock test without fearing damage during the shock ? Can this shock modify the sensor calibration ?

Moderator's Response:
The 2220D is rated to withstand 5,000 g's of shock, corresponding to a half-sine pulse of about 15 milliseconds. If a 2220D is subjected to more than a 5,000-g transient input or a half-sine pulse with a duration less than 15 milliseconds, it might become permanently damaged. At Endevco, we generally publish conservative specifications. Our specifications oftentimes reflect a factor of safety. The 2220D, therefore, may be able to survive your 6,000-g shock. I am only comfortable, however, with citing the published 5,000-g shock limit as the maximum transient input the 2220D can survive. If you can get away with it, the 2220D has a sister accelerometer that you might consider using instead. The 7250A is an ISOTRON (voltage-mode) piezoelectric accelerometer that has a form factor virtually identical to that of the 2220D. The 7250A can withstand up to 10,000 g's but, because it incorporates miniaturized electronics, has a more narrow usable temperature range (-55 to 125 °C).

Wind Measurement
We have a 18 meters high metal structure standing in a very windy place. We should make vibration measurements which are concurrently synchronized with wind measurement, in other words we should know what the vibration is in known wind. Do you have any solutions to problem like this? Because we can't use wires, do you have solution where wireless data is sent between vibration measurement unit and the data collection computer?

Moderator's Response:
At this point in time, it is necessary to have cables between your accelerometers and signal conditioners/power supplies. In the not-too-distant future, however, wireless accelerometers will be available. The best compromise you can achieve today is to use a telemetry system. A telemetry system will accept the output of your measurement system and transmit it to a remote point via radio frequencies. If you're lucky enough, the telemetry system will incorporate the electronics necessary to power your transducers and condition their signals. If you're not lucky, you will need to first interface your accelerometers with signal conditioners. You will then need to feed the output of your signal conditioners into a telemetry system. Aydin-Vector is reputable manufacturer of telemetry systems.

Acceleration/Deceleration
Could you please tell me what is the meaning of the portion of an acceleration versus time graph for an impact where the acceleration changes signs? That is, for a dropped weight, the acceleration plot indicates that the weight's velocity is slowing, but then it crosses 0 g and goes positive (or negative, depending on choice of sign), apparently accelerating faster than freefall?

Moderator's Response:
It means that the point to which you've mounted your accelerometer has begun decelerating, or accelerating in the opposite direction. Perhaps the best analogy to help one visualize this is a car speeding-up, then braking in a direct, or straight-line path. Imagine a driver gnashing an accelerator to the floor of a car from a dead stop. This will cause the car to accelerate (increase in velocity per time) in the forward, or positive, direction. If the driver keeps the pedal to-the-metal, the car will ultimately stop accelerating and reach a terminal velocity. Therefore, as the car nears its terminal velocity, it is accelerating less than it was in the beginning of its drag-strip excursion. On a graph showing acceleration versus time, this lessening in forward acceleration would be indicated by a curve with a negative slope, descending from a peak of maximum acceleration in the positive domain of the ordinate (y-axis depicting acceleration). When the car reaches its terminal velocity, the graph reads zero. It reads zero because there is no longer any change in velocity versus time. If, after the car reaches its maximum velocity, the driver begans to brake, the car begans to decelerate, or accelerate in the opposite direction. The opposite direction is indicated on the graph by the negative domain of the ordinate.

Product Compatibility
Is the charge convertor 2777 is compatible with 2772?

Moderator's Response:
The 2777 is NOT compatible with the 2772. The 2771A, on the other hand, IS compatible with the 2772. The 2772 is designed to power and condition the signals of voltage-mode (ISOTRON), single-ended piezoelectric accelerometers or single-ended remote charge converters like the 2771A. The 2777 is designed to accept signals from differential, charge-output piezoelectric accelerometers. The 2771A requires a constant-current power supply--4 to 20 mA associated with a DC voltage between 18 and 36. The 2777, on the other hand, requires only a DC voltage between 22 and 31--constant current is not necessary. You need the 2772 in order to power the 2771A. A 2772 is not necessary to power a 2777.

Calibration Certificates
How do I obtain Certificates of Conformity and Calibrations for supplied transducers. The only information I have is Type and Serial number.

Moderator's Response:
We can fax or mail you the original calibration certificate for almost any Endevco accelerometer you have. A Certificate of Compliance and Traceability comes with every product purchased, repaired, or recalibrated. The certificate reads as follows: -Endevco is certified to ISO 9001, EN29001, BS5750 Part 1 and ANSI/ASQC Q91-94. -Endevco certifies items supplied on the referenced shipment comply with all applicable Endevco raw material, processing, fabrication, workmanship configuration and performance specifications. Objective evidence of conformance, such as inspection and test data or other reports, is on file and available for examination. -This C of C certifies to the functionality of items that have been recalibrated or repaired to performance specifications. -The measuring equipment used in determining compliance to applicable specifications or for purpose of calibration has been certified to it's accuracy and is traceable to the National Institute of Standards and Technology and is in compliance to MIL-STD-45662A, ANSI/NCSL Z540-1 & ISO 10012-1.

Heart Sound Measurement
1)What kind of Piezoelectric sensor we should use in a contact (diaphragm) type microphone to measure heart sounds? 2)Can you explain the method of selecting the charge amplifier to get the desired frequency response. I am interested in 5-2000 Hz bandwidth.

Moderator's Response:
You’re going to need something small and sensitive. If what you’re doing is experimental, have a look at the 256-100 and 752-500. If you’re planning on incorporating an accelerometer into a product, consider the 55 and 55L OEM accelerometers. The 256 and 752 can be found on our “Products” page using the “Table of Contents” facility. You will find them under “ISOTRON Accelerometers.” The 55 and 55L can found through the OEM hyperlink on the home page. An inexpensive signal conditioner that will provide you with a frequency response from 1 Hz to 20 kHz is the 4416B. It also offers x1 and x10 gain settings.

Accelerometer Accuracy
I would like to know what influences the accuracy of an accelerometer. (for the purpose discribed below and I want to use an accelerometer and/or maybe a gyro to measure the path of travelling of a round measurement device in a river. What would you suggest me to use and how?

Moderator's Response:
There are many factors that can influence the accuracy of an accelerometer. In regards to DC accelerometers (accelerometers having a linear response down to 0 Hz--most appropriate for tests similar to yours), the list of factors includes: non-linearity, hysteresis, transverse sensitivity, zero measurand output, thermal zero shift, and thermal sensitivity shift, amongst other things. All of these factors can create outputs that are in addition to the actual acceleration output. Many of these potential sources of error are often neglected if test conditions are not extreme because most accelerometers are designed to minimize these factors' effects. It is always helpful, however, to understand your test parameters and whether or not any of the aforementioned sources of error might be significant. If you'd like to learn more about each of the sources of error, or "noise," feel free to request the instruction manuals for our different accelerometer technologies--IM 101 (Piezoelectric Accelerometers) and IM 121 (Piezoresistive Accelerometers). These manuals will, by no means, make you an expert on accelerometer noise sources and how they affect accelerometer acurracy, but they are a good place to start. I can't see any possible way you can use just accelerometers for the test you've described. With accelerometers, you can, at most, account for three degrees of freedom--you can measure acceleration in three orthogonal axes. This will only account for one-half of the motion your sphere will experience. Your sphere, like most objects, also experience rotation about these three axes. You will need to account for these rotations in order to know in which directions your accelerometers' axes of sensitivities are oriented. My suggestion is to contact a company that deals with gyroscopes and how they are used for inertial navigation. Systron-Donner is such a company. British Aerospace, Systems and Equipment also offers solutions.

Automated Equipment and LabView
I am a test engineer who is looking for automated equipment compatible with LabVIEW applications. I need to build an effort tester to determine the stress applied to a lever by means of a linear displacement device (ie: infinite screw, pneumatic stepped actuator, etc). I have not found yet and instrument that would perform this function.

Moderator's Response:
Mechanical stress is by no means my expertise (psychological stress is something I'm much more familiar with), but I do have a suggestion for your test--consider using a load cell in conjunction with a laser displacement sensor. If your test configuration can accomodate a load cell, the load cell can provide you with tensile and compressive force information. If used, your load cell would likely have to be mounted in-line with a link that your lever is attached to. The load cell can help you determine the stress the lever is subjected to by providing you with the amount of force that has been applied to the lever through the link. Depending upon the geometry of the lever and link, and the lever's material, you can calculate the stress the lever is experiencing. You can then correlate this stress to the displacement of the lever by means of a laser displacement sensor. Laser displacement sensors are non-contact devices and, as such, should not require modification of your test set-up.

Cableless Sensors
I use accelerometers with low-noise cables for many years. These sensors are in movement in our application: we destroy the cables very often. My question is: do sensors without cables exists?

Moderator's Response:
Yes, sensors without cables do exist. They are, however, fairly uncommon. The application in which wirless sensors have found a particular niche, thus far, is in industrial monitoring. Wireless acceleration, pressure, and temperature transducers have been used in instances where they are an affordable alternative to expensive cable runs through hostile environments. The advantages gained by having no cables attached to a sensor are offset by the design compromises necessary to produce a wireless transducer: 1) wireless transducers are considerably more massive than their wired counterparts because their packaging incorporates additional electronics for telemetry and batteries to facilitate a stand-alone power supply; 2) sensor frequency response is often limited by a wireless system's throughput. Endevco understands the importance of wireless-sensor technology and is in the process of developing its own line of wireless transducers. We don't expect that wireless sensors will replace all of those with cables, but there is a significant need for wireless sensors that must be addressed. You can expect to see a considerable offering of wireless sensors in the marketplace within the next 5-10 years.

Minimum Voltage
I'm trying to determine the minimum amount of voltage need to checkout the microdot cables used to connect my accels to the data equipment. The standard we've used in the past has been 50Vdc and I need to know if a 9Vdc/2000Mohm(handheld meter) would be sufficient to ensure a serviceable cable.

Moderator's Response:
We test the insulation resistance of our 3090C low-noise cable assemblies with 100 VDC. We also apply 1200 VAC between the center conductor and the shield to verify that there is no reduction of insulation resistance at this high of a potential. It would be overkill for you to test our cable assemblies in this manner. I don't see a problem with you using a nine-volt source to check a 3090C's insulation resistance in the field. If a signifcant leakage resistance has developed (uncommon for our 3090C cable assemblies), it will be detected with your portable meter.

Radiation Sensitivity of Accelerometers
Where can I find a good source of information on the radiation sensitivity of accelerometers to pulsed radiation sources? My application involves collection of information shortly (microseconds) after exposure to an intense short (nanoseconds) radiation pulse.

Moderator's Response:
I'm not aware of any literature that addresses your application specifically. Endevco, however, has published a couple of tidbits of information that may help you indirectly. These two pieces of literature are Endevco Technical Paper 272 and Section IV.L from the Endevco textbook, SHOCK & VIBRATION MEASUREMENT TECHNOLOGY. From the description of your test, I anticipate that you will suffer two effects of radiation-induced error. These two effects, as stated in Section IV.L of SHOCK & VIBRATION MEASUREMENT TECHNOLOGY are: 1) "Spurious charge results from free secondary electrons which are absorbed by or ejected from conductors and crystal electrodes; 2) Pyroelectric properties of the crystal element result in charge proportional to gamma-ray and neutron heating." Perhaps the best way to approach your problem is to make your measurement with two accelerometers--one that works as it is supposed to and the other having no sensitivity to acceleration (The latter is achieved by assembling an accelerometer with an unpolarized piezoelectric crystal and is commonly known as a "noise monitor"). If possible with your data acquisition system, you can "subtract" the noise-monitor output from that of the normal accelerometer. Theoretically, the resulting waveform should represent the true acceleration signal.