I haven’t used a pressure sensor for some time, so when I received a news announcement from Meggitt Sensing Systems that included a resonant frequency for the Endevco model 8530C series sensors, I wondered how a resonant frequency related to pressure. These sensors use a piezo-resistive sensor in a 4-arm strain-gauge bridge diffused into a sculpted silicon diaphragm.
The sensors have three ranges–15, 50, or 100 psia that correspond to a full-scale output of 225 mV. The specs also include this information: “…high resonant frequencies of 180 kHz (15 psia), 320 kHz (50 psia) and 500 kHz (100 psia).” For information about Meggitt’s sensors, visit: www.meggittsensingsystems.com.
So I called and talked with Bruce Lent, an applications engineer in the Measurement Group at Meggitt. Bruce explained that in dynamic-pressure measurements–where pressures can change rapidly–the resonant-frequency spec helps equipment designers determine the uppermost frequency response of the sensor. Dynamic measurements can involve pulsating fluids and pressures created in explosions or shock tubes. According to Lent, you can use 20% of a sensor’s resonant frequency as the frequency-response limit for the given sensor. That’s a rule of thumb worth remembering. So, a sensor in this series with a 500 KHz resonant frequency can operate with fluid pulses with a maximum frequency component of 100 KHz.
I dug deeper into piezo-electric pressure sensors and found an “Introduction to Piezoelectric Pressure Sensors” from Dytran Instruments. That document includes a frequency vs.magnitude graph that shows at 20% of a sensor’s resonant frequency, its sensitivity increases by +5%. At 30% of the resonant frequency, sensitivity has increased to 10%. Thus for a sensor that produces, say, 20 mV/psia, and has a resonant frequency of 100 kHz, the output increases to 21 mV/psia at 20 kHz and to 22 mV/psia at 30 KHz. The slope between the +5 and +10% points on the graph looks linear, so you could correct for small changes as needed. You can find the Dytran document at: www.dytran.com/img/tech/a5.pdf.
If you plan to measure pressure changes with the highest frequency component at 20 kHz, remember you must sample at a rate faster than 40 ksamples/sec., the Nyquist limit. I recommend sampling at 80 ksamples/sec. to 100 ksamples/sec. Sampling at or between those rates should give you plenty of useful data without any aliasing artifacts. –Jon Titus