The difference between bimetal thermometers and thermocouple thermometers

Thermocouples  and bimetal thermometers 

The thermocouple can convert the temperature signal into a thermoelectromotive force signal, and convert it into the temperature of the measured medium through an electrical instrument (secondary instrument).

The basic principle of thermocouple temperature measurement is that two different material conductors form a closed loop. When there is a temperature difference between the two ends, a voltage will be generated in the loop, and the magnitude of the voltage is related to the temperature. According to the different metals (graduation number) used, the measurement range is distributed in the range of -200~1800℃.

Thermal resistance is one of the most commonly used temperature detectors in the medium and low temperature region (-200~500℃). Its main features are high measurement accuracy and stable performance.

The thermal resistance is based on the thermal effect of the resistance to measure the temperature, that is, the resistance of the resistor body changes with the temperature. Therefore, as long as the resistance change of the thermal resistance is measured, the temperature can be measured.

Bimetal thermometer is a field detection instrument for measuring medium and low temperature (-80℃-+500℃).

The main component of a bimetal thermometer is a bimetallic sheet composed of two or more metal sheets stacked together and wound into a circular curved shape. Two different metals have different degrees of expansion when the temperature changes, and they expand when heated. When the pointer is driven to rotate, the working instrument will display the temperature value corresponding to the thermoelectric potential.

Bimetal thermometer is a tool that we often use in our daily work, but many people don't know the difference between his and other thermometers. So, today let us understand how to distinguish the difference between bimetal thermometer and ordinary thermometer. I also hope that you will not be confused with other thermometers when using bimetal thermometers in the future.

Bimetal thermometers work on the principle that two different metals expand differently when the temperature changes. The main component of an industrial bimetal thermometer is a multi-layer metal sheet composed of two or more metal sheets stacked together. In order to improve the sensitivity of temperature measurement, the metal sheet is usually made into a spiral shape. When the temperature of the multi-layer metal sheet changes, each layer of metal expands or contracts by an unequal amount, causing the spiral coil to roll up or loosen. Because one end of the spiral coil is fixed and the other end is connected with a pointer that can rotate freely, when the bimetallic sheet feels the temperature change, the pointer can indicate the temperature on a circular graduation scale. The temperature measurement range of this instrument is 200~650℃, and the allowable error is about 1% of the two steps of the scale. This thermometer is similar in use to a rod-shaped liquid-in-glass thermometer, but can be used under conditions that require higher mechanical strength.

Glass thermometers are made according to the physical properties of working liquids such as mercury, alcohol, and toluene, which have obvious thermal expansion and contraction. The most used in industrial boilers are mercury glass thermometers and electric contact mercury thermometers. Mercury glass thermometers are composed of temperature measuring bulbs, thin expansion tubes and rulers. Generally, there are two types: internal standard type and external standard type (also known as rod type). The scale division of the internal standard mercury thermometer is engraved on a milky white glass plate placed behind the expansion tube. The plate is enclosed in a protective glass enclosure together with the bulb. Usually used to measure feed water temperature, return water temperature, economizer outlet water temperature, and air temperature at the inlet and outlet of air preheater. The external standard mercury thermometer has a thicker glass tube, and the scale divisions are directly engraved on the outer surface of the glass tube, which is suitable for measuring the temperature of liquids and gases in the laboratory. Insert two wires into the thin expansion tube of the mercury thermometer. When the temperature reaches the rated value, the mercury will connect the wires, and the control system will be driven by the action of the current, or the signal device will issue an audible and visual alarm.