Context: I'm writing about an old industrial training facility used for a petrochemical process.

In this training facility, there is a 3-phase separator. On that separator is a pressure measurement. My teacher, having limited knowledge of the facility in question, told me that the sensor was "ceramic".

I searched for ceramic pressure measurement sensors and found two principles. At least, I think they are two different principles, however I suspect they may be the same principle but explained differently.

My current (mis)understanding:

The capacitive principle: A ceramic membrane is exposed to the process pressure. This process pressure will cause it to bend, which will increase its capacitance (probably due to an increase of proximity to an electrical contact?). The higher the pressure, the thicker the membrane needs to be, lest it break. Due to the increasing thickness requirements, this measurement principle only gets less and less accurate as the pressures get higher.

The piezoelectrical principle: A ceramic membrane is exposed to the process pressure. This pressure causes it generate a small but measurable voltage, due to its molecular structure. The voltage's magnitude is translated into a measure of the pressure's magnitude.

Now, I suspect that these two principles may actually be talking about the same principle, though perhaps focusing on different aspects of the principle. The reason for my suspicion is that I don't know a lot about electricity, and it seems unlikely to me that people would be using ceramic materials for pressure measurement for a while without noticing that the material had a piezoelectrical property that could make it usable for presumably better pressure measurement.

Now, my teacher said that it is most likely the capacitive principle being used in the facility in question, but he said the piezoelectrical principle is most common nowadays (the facility is quite old). So, it seems like my teacher thinks the two principles are distinct, but I'm a bit unsure.

  • $\begingroup$ Welcome to engineering.SE! I think your post misses a part in the end, you can press the edit button on the bottom to add that still. $\endgroup$ – OpticalResonator Oct 28 '20 at 12:51
  • $\begingroup$ @OpticalResonator Thank you, I have edited it now. $\endgroup$ – A. Kvåle Oct 28 '20 at 12:54
  • $\begingroup$ Yep, MEMS piezoresistive strain-gage type pressure sensors are now widespread due to low cost, higher sensitivity, the possibility for automated wafer level trimming and calibration, and integrated signal processing. $\endgroup$ – Pete W Mar 28 at 20:52

A capacitive measurement device measures how close the sensor element (or process material) is to the detector plate. A flexible ceramic membrane separates the process material from the detector plate, and as the pressure changes, the shape of the membrane changes, permitting the sensor element to move closer or cover a larger area. This is a mechanical sensor with a capacitive detector.

I think that the main advantage of capacitive measurement is that you can connect it as a tuned circuit. Control systems were built around frequency detectors and A/C circuits.

I don't think that people "didn't notice" that you could do "better measurement" using piezo-resistive or piezo-electric sensors. Cost and technology have changed. Sensors that only measure changes in pressure are more difficult to use: sensors that have to endure high-temperature or corrosive environments are difficult to construct. Piezo elements have to push against something and be mounted on something: capacitive detectors only have to change shape.

I'm sure that capacitive sensors still have some advantage is some application I don't know about. The only one I know about is that capacitive sensors go into systems that were designed for capacitive sensors.

  • $\begingroup$ Sensors that depend on integration for readings will not work wery well in long running continious operations. They are fine for batch processing though. $\endgroup$ – joojaa Oct 29 '20 at 15:38
  • $\begingroup$ Okay, thank you for your answer! Interesting to see that ceramic material is used in two different principles of measurement. $\endgroup$ – A. Kvåle Oct 29 '20 at 20:37

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