Electrical methods for measurement systems are cheaper and accurate compared to old mechanical methods. Electrical transducers can be calibrated and replaced easily.
A piezoelectric material is one in which an electric potential appears across a certain surface of a crystal if dimensions of the crystal are changed by application of a mechanical force or pressure.
The figure shows a conventional, simple type of piezoelectric transducer. A piezoelectric crystal is inserted in the transducer and there is a pressure port which is the pressure inlet of the transducer.
The pressure that should be measured is applied to the pressure port and the same pressure will fall on the force summing member. The crystal inside will change the dimension due to the pressure. A potential difference will generate across the crystal due to crystal’s property. The potential difference produced will be proportional to the pressure applied.
Strain gauge (piezoresistive gauge):
When a strain gauge metal conductor is stretched or compressed its resistance change on account of change in length and diameter. This property of the strain gauge is known as piezoresistive effect.
A strain gauge wire is connected to the branch of a balanced Wheatstone bridge so that the change in the resistance make a deflection in the voltmeter. The voltmeter reading is proportional to the resistance change in the strain gauge and which is directly proportional to the applied pressure.
Capacitive cell work on the principle that the distance between electrode will change the capacitance of a capacitor.
Here one electrode of the capacitor is made fixed and the other movable.
When a differential pressure is applied across the capsule the silicone oil will be pressurised more on one side than the other. The sensing diaphragm will move away from one fixed capacitor plate and nearer to the other. The difference in capacitance is measured by an electronic amplifier. This measurement shows the differential pressure.
The frequency of vibrating wire changes when the tension applied to it change.
The vibrating wire is stretched and connected at two ends, one of the ends is fixed and other is a movable diaphragm. The tension of the vibrating wire is changed by the pressure applied to the diaphragm.
The electronics unit has a driving coil (D.C) to make the wire resonate and a sensing coil (S.C) to measure the resonant frequency. This changes the pressure applied to the diaphragm into an electrical signal output.