传感与测量 Engineering

A thermocouple uses the thermoelectric voltage of a junction between two metals, offering a wide range up to thousands of degrees Celsius, fast response, rugged construction and low cost, but low sensitivity, a need for cold-junction compensation and modest accuracy. A resistance thermometer such as a platinum RTD infers temperature from metal resistance, giving high accuracy, good stability and excellent linearity from about minus two hundred to six hundred degrees Celsius, at higher cost, slower response and with lead resistance to consider. Furnaces use thermocouples, while laboratories and precise process control usually use platinum RTDs.

An orifice plate or Venturi tube is placed in the pipe; as fluid passes the constriction its velocity rises and static pressure falls, and the pressure difference across it is roughly proportional to the square of velocity. Taking the square root of the measured differential and multiplying by a calibration coefficient gives the flow rate. The method is simple, has no moving parts and tolerates high temperature and pressure, but it causes appreciable pressure loss and its relative error grows at low flow where the differential is tiny.

Range is the span from minimum to maximum that a sensor can measure properly, beyond which it saturates or is damaged. Accuracy describes how close the reading is to the true value, usually expressed as a percentage of full scale and covering linearity, repeatability and thermal drift. Resolution is the smallest change it can distinguish. High resolution does not imply high accuracy: an instrument may display hundredths of a degree while its overall error is still half a degree.