High temperature accelerometer materials
Early in Endevco's history, it was realized that there was a need for high temperature accelerometers to meet the needs of engine and rocket motor testing. As early as 1957, Endevco was producing and conducting research on accelerometers with temperature ranges in excess of 1000°F. Research continues today on new crystal technology and high temperature electronics.
This paper will describe Endevco's advanced devices used at extreme temperatures, their applications, and practical considerations for the user's consideration. Both piezoelectric (charge mode) and Isotron® (voltage mode) devices will be discussed.
High Temperature Accelerometer Materials
Currently, the state of the art for accelerometers with high temperature internal electronics is 347°F (175°C). In order to exceed this temperature, charge-mode accelerometers must be employed. An "ordinary" charge-mode accelerometer has a high temperature rating from 300°F (150°C) to 500°F (260°C).
The design and manufacture of very high temperature devices requires the use of special crystals that can operate and provide the required performance (stability, frequency response and reasonable charge sensitivity). Of equal importance is the selection of case materials, connector components and specially designed cables.
Endevco's P-15 material (Lithium Niobate) provides the highest temperature accelerometers that can operate up to 1400°F (760°C). Care must be taken in the design of these very high temperature accelerometers since material strength decreases, insulating materials deteriorate and most crystal materials will be damaged.
High Temperature Accelerometers and Accessories
When operating at very high temperatures, a user can expect to use hardline cable in the hot areas. Special charge amplifiers are required, since the sensing crystal resistance is considerably lower at high temperatures. For high temperature operation, the Endevco model 2721B charge amplifier is offered.
The highest temperature accelerometer is the Endevco model 6240M10 in a convenient, small package with a single-ended output. Many high temperature accelerometers up to 482°C (900°F) are radiation hardened. Differential, high temperature accelerometers provide for high noise immunity and are used in many engine test applications. Differential sensors have operation temperatures up to 482°C (900°F).
Endevco models 6240M10 and 6233 single-ended accelerometers
with integral hardline cable and high temperature differential sensor.
High Temperature Isotron Accelerometers
Accelerometers with internal electronics (Isotron, IEPE, etc.) have become popular since they eliminate the need for special cables and charge amplifiers and offer high noise immunity. One of their major disadvantages is the operating temperature range. While sensing materials are available for measuring acceleration at high temperatures, the integral electronics still present high temperature limitations.
As a result of electronic research conducted by Endevco, high temperature limitations and sensitivity have been greatly improved. The maximum temperature of IEPE devices is generally limited to 257°F (125°C). Higher temperature versions have been available, but recently Endevco announced a a newly developed accelerometer with an upper temperature of 350°F (175°C) and sensitivities of 10mV/g and 100 mV/g.
This new higher temperature device, the model 67, will improve automotive engine testing by providing longer life and higher temperature operation than current competitive devices. For combined environments testing, thermal stress screening, HALT, HASS and ESS applications, users may now go to higher temperatures without the need for remote charge converters and expensive low noise cable.
Endevco model 67 high temperature Isotron accelerometer
with 10 and 100mV/g capability in a triaxial configuration.
Low Temperature Operation
Operating accelerometers at very low or cryogenic temperatures requires specialized devices. This section will review the entire low temperature spectrum of Endevco accelerometers.
IEPE (Isotron) accelerometers and even charge mode piezoelectric accelerometers exhibit a lower temperature limit of about -67°F (-55°C). Some general purpose charge mode accelerometers exhibit excellent operational characteristics at temperatures below -67°F (-55°C), making them desirable for flight testing and use in environmental test chambers. Consult individual data sheets for the specified temperature ranges of these devices.
Endevco is known as the pioneer in the area of cryogenic and very wide temperature range accelerometers. An example of a wide temperature range accelerometer is the Endevco model 2272 which has an operating temperature range form -452°F (-269°C) to 500°F (260°C)! While the 2272 features a relatively high sensitivity of 13 pC/g, it does produce some pyroelectric effects at cryogenic temperatures, especially with variable temperatures. These wide range accelerometers are made from Endevco's P-10 material and exhibit extreme stability over long periods of time.
The ultimate in cryogenic accelerometers are the Endevco models 7722 and 7724. These accelerometers will operate at temperatures as low as -300°F (-184°C). Unlike the wide temperature range accelerometers described above, their maximum operating temperature is limited to 347°F (175°C). The major advantage of this cryogenic series is the fact that they exhibit no pyroelectric effects at cryogenic temperatures.
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