What are my options for mounting an accelerometer at or above 250°F?
There are several options for mounting devices to each other at that temperature. Screw mounting an accelerometer is always best. There is no problem handling temperature if properly torqued (see relevant product data sheet for correct mounting torque), and the component is easily removable for recalibration.
If screw mounting is not practical, then we must look into permanent and semi-permanent adhesive methods. It is important to note that removal of any vibration transducer should not be done by "snapping" it off or using a blunt force device, like a hammer, to remove it. Almost without exception, this approach will destroy the device. Soften the bond with an approved solvent prior to gently removing the device.
Cyanoacrylate (Zip-Grip®, Eastman® 910, Super-Glue, etc.) is typically good to about 250°F and is, depending on the compounding, usable up to 340°F. Both surfaces must be very flat and clean of all contaminants prior to mounting. This compound makes a stiff bond that has good frequency response. Removal involves using a chemical softener, then gently prying the transducer loose. Be aware that some of the removal chemicals can harm some transducers and most are not environmentally friendly.
For higher temperatures, epoxy and ceramic compound glues are necessary. In general, epoxy is not as stiff as the other adhesives, so it will not transmit higher frequencies (5 kHz and above). Epoxy can be either electrically insulative or conductive depending on its composition. Its use is limited to the lower end of the 250°F and above range. Removal can take quite a bit of time, since epoxy softening chemicals work slowly. The transducer can be removed by gently prying after softening the epoxy. Removal of epoxy to achieve a clean surface is very difficult.
Ceramic glues are good for the 250°F to 2000°F range and require very clean, flat surfaces to work. They are considered to be permanent adhesives, requiring destruction of the cement to loosen the transducer. which obviates the ability to provide regular calibrations for the transducer. Attempts at removal frequently result in the destruction of the accelerometer. Despite their drawbacks, ceramic glues are the only useable adhesive for extremely high temperatures. Some adhesives in this class have high metal content. Resbond 940SS has the same expansion coefficient as stainless steel and is a good fit for transducer-to-stainless steel bonds.
This discussion is by no means a comprehensive treatment of all the possible high temperature bonding methods available for vibration monitoring. It is intended to provide some helpful hints and areas of concern. Endevco provides vibration monitoring accelerometers, and pressure sensors for use in extreme conditions. Please contact an Endevco Applications Engineer at firstname.lastname@example.org for the right device to monitor your high temperature machinery.