# Ceramic pressure measurement - capacitive or piezoelectrical?

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 principles are in fact distinct, but I'm unsure. Any thoughts?

• Depends on which level you are comparing the principles. Bottom line both are depend on Quantum-mechanical effects and described by similar equations on that level. But from the engineering point of view these are different. Piezoelectric is exploiting the property of the piezo-electric materials which are generating electricity when "squeezed", and the capacitive one is exploiting the change in the capacitance of a capacitor when its geometry is changed. – Eugene Sh. Oct 28 '20 at 13:43
• What is a "3-phase separator" and how does it relate to your question? – Andy aka Oct 28 '20 at 13:44
• @Andyaka A 3-phase separator is a tank that separates oil, water and mineral gas (methane) from each other. It's relevant because the kind of pressure such equipment generates is relevant to what kind of pressure measurement one would use. According to Spehro's answer, piezo sensors are unsuitable for measuring steady pressure. I am not sure if the pressure in a 3-phase separator would fall within the category of steady or rapid change. – A. Kvåle Oct 29 '20 at 20:41