Variable Conversion Elements

This chapter discusses about outputs from measurement sensors that take the form of voltage signals that can be measured using the voltage indicating and test instruments. Bridge circuits are used very commonly as a variable conversion element in measurement systems, and produce an output in the form of a voltage level that changes as the measured physical quantity changes. This chapter discusses about devices that convert the measured quantity into a change in resistance and include the resistance thermometer, the thermistor, the wire-coil pressure gauge, and the strain gauge. The main device that has an output in the form of a change in inductance is the inductive displacement sensor. Devices that have an output in the form of a change in capacitance include the capacitive level gauge, the capacitive displacement sensor, the capacitive moisture meter, and the capacitive hygrometer. Current measurement is needed for devices such as the thermocouple-gauge pressure sensor and the ionization gauge that have an output in the form of a varying electrical current. It is often also needed in signal transmission systems that convert the measured signal into a varying current. Frequency is measured in units of hertz (Hz). Instruments that convert the measured variable into a phase change in a sinusoidal electrical signal include the transit-time ultrasonic flowmeter, the radar level sensor, the linear variable differential transformer (LVDT), and the resolver.