Belt and suspenders approach to cryogenic measurements
The terms “belt and braces” or “wearing belt and suspenders” are figurative terms used for carefulness or proceeding with an abundance of caution. Most certainly, this is the preferred approach when working with cryogenic liquefied gases that have boiling points below -150 °C (-238 °F). These liquids can be inert (nitrogen, helium, neon, argon, krypton) or flammable (hydrogen, methane, natural gas). Oxygen is a special case as some non-combustible materials will burn in the presence of liquefied oxygen. Typical hazards include burns, adhesion of skin to materials, asphyxiation, rapid pressure changes, explosions and flammability.
Meggitt’s Endevco “belt and suspenders” approach to cryogenic accelerometer design is to use self-powered, piezoelectric sensor technology with expanded temperature ranges (-492 °F/-269 °C) and external signal conditioning. As opposed to internal electronics, this is the more reliable approach as it avoids the potential sensor failure and resultant fiscal risks of project failure inherent in IEPE technology approaches.
Most certainly, the strength of IEPE designs is having on-board signal conditioning as this saves some time in initial setup and may initially be seen as the economical approach given the limited cost-effectiveness of the sensor and external instrumentation. But, as the saying goes, “Do it quickly and inexpensively, get it right the first time.”
Here are the five major issues one needs to consider when looking at IEPE accelerometers for cryogenic measurements:
1. Narrow temperature range: Some IEPE designs barely reach the minimum -150 °C temperature range leaving little room for temperature spikes. Further, the shallow temperature range poses the potential for an inexperienced buyer or technician to use a sensor labeled as “cryogenic” for an application outside its temperature range capability.
Typical IEPE products specify -196 °C as their lowest range while Endevco’s PE products typically specify – 269 °C as the lowest range.
2. Electrical connections: Solder alloys have typically been used in sensor design for components, cable connections and assemblies. However, at cryogenic temperatures, the Sn-based solders eventually transition from ductile to brittle. This leads to cracking and failure. Recent research has provided evidence that some solders and epoxies might be a good alternative in the short term but empirical evidence is still required to confirm long term performance.
No SN-based solders or epoxies are used for electronics and connections in Endevco’s cryogenic sensors.
3. Flammability: Providing power to a sensor and having associated power-conditioning circuitry on-board the sensor in a combustible environment poses a safety risk to personnel, instrumentation and materials/structures in the immediate vicinity.
No external power is supplied to Endevco’s PE cryogenic sensors while they are in the cryo environment.
4. Electronic failure: The good news about operating in cryogenic temperatures is that most degradation mechanisms such as electromigration, ion-induced drift and corrosion that cause failures at normal temperatures will be suppressed. However, the extreme cold, varying pressures and rapid temperature cycling introduce hot carrier induced degradation, material stresses and current/voltage spiking. All of these pose a high risk to both signal and structural integrity of the sensor.
No on-board sensor electronics are used in Endevco’s cryogenic products. This ensures reliable operation, accurate data and long term reliability. Also, as there are no on-board electronics, there are no mounting epoxies for resistors and capacitors that will fail (crack) after a few cycles in cryogenic temperatures.
5. Variable reinforcement: Occasionally, an IEPE accelerometer will perform beyond its specifications and reasonable expectations and give test personnel hope that they have made a breakthrough. This subsequently encourages them to expend greater amounts of energy and resources in trying to get this technology to work beyond its capabilities. By sometimes working and sometimes not working, a variable reinforcement schedule is set up like the reinforcement schedule that makes gambling addiction so difficult to cure.
The bottom line is that reliable and accurate cyrogenic vibration measurements involve risk. The smart way to minimize risk in this extreme environment and make the most efficient use of time/resources is to use piezoelectric accelerometers that ensure the safety of your team and bring your project to successful completion. With Meggitt’s Endevco solution, sensors do not use any epoxy, solder or interference fit as a means of construction due to their inability to withstand repeated cyrogenic temperature cycling. Meggitt’s Endevco engineers have many years of expertise with cyrogenic applications and work alongside committed customer service personnel to guide you through product selection, installation and data analysis.