This article describes five common industrial thermometers: liquid-expansion, solid-expansion, pressure-type, thermocouple, and resistance thermometers.
01 Liquid-expansion thermometer
Liquid-expansion thermometers are based on the thermal expansion and contraction of a liquid. The most common form is the liquid-in-glass thermometer, which uses the volume change of a liquid in a glass tube as temperature changes. It consists of a liquid reservoir (bulb), capillary tube, scale, and a safety bulb. Typical liquids include mercury, alcohol, and toluene.
02 Solid-expansion thermometer
Solid-expansion thermometers use two materials with different coefficients of thermal expansion. Common types include rod-type thermometers, which generally use a solid material with a relatively large expansion coefficient, and bimetallic strip thermometers, whose sensing element is made by firmly bonding two metal strips with different expansion coefficients. Solid-expansion thermometers have simple and reliable structures but relatively low accuracy.
03 Pressure-type thermometer
Pressure-type thermometers are mechanical instruments that determine temperature by measuring the pressure change of a working medium in a sealed volume as temperature rises. The working medium can be a gas, liquid, or steam. Pressure-type thermometers are simple, reliable, and have good vibration resistance and inherent explosion resistance, so they are often used in aircraft, automobiles, and tractors, and can also provide temperature control signals. However, they have poor dynamic performance and significant reading hysteresis, making them unsuitable for rapidly changing temperatures.
04 Thermocouple thermometer
Thermocouples are widely used temperature sensors in industrial applications. They consist of two conductors of different compositions (called thermocouple wires or thermoelements) joined at their ends to form a circuit. When the junctions are at different temperatures, an electromotive force is generated in the circuit; this phenomenon is called the thermoelectric effect, and the generated voltage is called thermoelectric EMF. Thermocouples measure temperature by using this principle.
05 Resistance thermometer
As temperature rises, the resistance of a conductor or semiconductor changes. Because there is a single-valued functional relationship between temperature and resistance, measuring temperature by using that relationship is known as resistance thermometry. The conductor or semiconductor used for temperature sensing is called a resistance thermometer.
