The most common type of flush pressure transmitter is the one with piezoresistive sensors. The actual sensor element is a silicon measuring element embedded in oil. This oil, usually a silicone oil, acts as a pressure transmission medium and is protected from the measured medium by a separating diaphragm with a characteristic wave structure. Only the separating membrane encounters the medium, but it must be very thin so as not to influence the measured value.
The advantage of piezoresistive sensors is their high sensitivity, thanks to which low measuring spans and higher accuracies are possible. They are suitable for measuring absolute pressure and relative pressure with measuring spans of less than 100 mbar pressure up to several 100 bar. The separating diaphragm can be made of different materials and even coated if desired. The standard version is usually made of steel 1.4435 (AISI316L). Alternatively, there are variants in corrosion-resistant materials such as nickel alloys or titanium. Piezoresistive sensors are well suited for homogeneous, rather pasty media and for liquids that contain no or only small, soft solids. Hard particles can damage the very thin separating membrane, which is only approx. 20 - 70 micrometres thick, and thus lead to an irreversible signal offset. If the membrane is not only deformed but even punctured, the oil of the pressure sensor leaks out and can contaminate the entire system, depending on the application. Another disadvantage of piezoresistive sensors is the lower long-term stability of the silicon under elevated temperatures. This can lead to signal drift.