Accelerometer spiking under cryogenic conditions

Spurious transient outputs of a piezoelectric accelerometer have been observed when the accelerometer is attached to a surface undergoing rapid cool-down to a cryogenic temperature. Such conditions occur in cryogenic propellant rockets on assemblies such as feedlines and turbopumps. Under exposure to liquid oxygen, surface temperatures can drop to near −300°F in a matter of minutes. The occurrence of spurious output depends on the internal construction of the accelerometer and on the effectiveness of thermal isolation provided by the mounting for a susceptible accelerometer. Specifically, an Endevco Model 2271AM20 charge-mode, compression-type accelerometer (CTA) has been observed to produce intermittent spurious output spikes during flight of a launch vehicle, during ground test firing of a rocket engine and in laboratory experiments. It is suggested that this spiking, also known as the pyroelectric effect, can be attributed to charge buildup due to thermal effects in the piezo-electric crystal stack. In the case studies, sporadic jumping of this charge across a gap at the outer edge of the plating on the crystal stack face is believed to cause momentary saturation of the charge amplifier. A replacement Endevco Model 7722 shear-type accelerometer (STA) using a different type of crystal arrangement as well as a different crystal material proved to eliminate these spurious charge jumps in a series of laboratory tests. This paper presents the results of experiments conducted to evaluate both the CTA and the STA. The effectiveness of different mounting configurations, used to provide thermal isolation sufficient to eliminate the spikes from the CTA, was evaluated. It is also suggested that observations of a particular type of nonzero mean signal in the time domain and broad, elevated, low-frequency content in the frequency spectrum can be useful in detecting the presence of pyroelectric-induced transients, even if the spikes are visually buried within high-amplitude vibration data.