Question

Why do many of your piezoelectric accelerometers have a tolerance specified on the sensitivity specification, while many do not? I often see a "typical" and "minimum" spec. What does that mean?

Answer

Your questions highlight the fact that Endevco offers two types of piezoelectric accelerometers: charge output (referred to as PE type) and voltage output (referred to as IEPE type or by Endevco's trade name Isotron®). Let me explain further how this relates to your question.

PE type accelerometers have no electronics built into them. In a sense, a PE accelerometer is strictly a mechanical device, making it difficult (but not impossible) to "dial-in" a particular sensitivity. With careful design and production controls on both the piezoelectric materials used as well as the sensor construction itself, Endevco can hold tight limits on the sensitivity. But to remain cost effective, it is easier to target a typical value, and ensure a minimum value (if a minimum is specified, we won't ship an accelerometer with a sensitivity below that value).

There are cases where a PE accelerometer does have a relatively close tolerance specified on its sensitivity (the model 6222S series is an example). However, these tend to be accelerometers intended for mission-critical applications, and the higher cost may be justified.

In contrast, IEPE type accelerometers do have electronics built in. With an electronic circuit built inside these sensors, during the manufacturing process, it is relatively easy to "dial-in" the sensitivity (within general electronics manufacturing tolerances). Thus, with IEPE type accelerometers, you often see a tolerance specified around the nominal sensitivity.

It should be noted that regardless of how the sensitivity is specified, or what type of accelerometer it is, Endevco always provides a calibration certificate that states the exact value. Further, Endevco offers several signal conditioners that allows the user to set the output scale factor to a nominal value, for further processing by a data acquisition system.