The working principle of various thermometers

- Mar 22, 2018 -

According to the use of the purpose of the difference, has been designed to produce a variety of thermometers. Its design is based on: the use of solids, liquids, gases affected by temperature and thermal expansion and contraction of the phenomenon; under the constant volume conditions, the pressure of the gas (or vapor) is transformed by the difference of temperature, the effect of thermoelectric effect, the change of resistance with temperature, the influence of thermal radiation, etc.

1. Gas thermometer: more hydrogen or helium as a temperature-measuring material, because hydrogen and helium liquefaction temperature is very low, close to absolute 0 degrees, so its temperature range is very wide. The thermometer is of high precision and is used for precision measurement.

2. Resistance thermometer: is divided into metal resistance thermometer and semiconductor resistance thermometer, are based on the resistance value with the temperature change this feature. Metal thermometer mainly used in platinum, gold, copper, nickel and other pure metal and rhodium iron, phosphor bronze alloy, semiconductor thermometers are mainly carbon, germanium and so on. The use of resistance thermometer is convenient and reliable, and has been widely used. Its measurement range is 260 ℃ to 600 ℃.

3. Thermocouple Thermometer: It is a kind of temperature measuring instrument widely used in industry. Made by the phenomenon of thermoelectric phenomena. Two different wires are welded together to form the working end, and the other ends are connected with the measuring instrument to form the circuit. The working end is placed at the measured temperature, the working end and the free end temperature is not at the same time, will appear electromotive force, so there is current through the loop. By measuring the amount of electricity, you can determine the temperature at another place by using the known temperature. It is suitable for two kinds of material with large temperature difference, and is used for high temperature and low turbidity measurement. Some thermocouple can measure up to 3,000 ℃ high temperature, some can measure close to absolute 0 degree of low temperature.

4. Pointer thermometer: Is the shape of the instrument panel thermometer, also known as the thermometer, used to measure room temperature, is made of metal expansion and contraction principle. It is made of bimetal as a temperature-sensing element to control the pointer. Bimetallic plates are usually riveted together with copper and iron sheets, and the copper sheet is on the left and the iron sheet is on the right. Because the effect of copper expansion and contraction than iron significantly more, therefore, when the temperature rises, the copper piece pulls the iron piece to the right to bend, the pointer in the double metal piece is driven to the right deflection (pointing to the high temperature);

5. Glass tube Thermometer: glass tube Thermometer is the use of thermal expansion and contraction of the principle to achieve temperature measurement. Because the expansion coefficient of the temperature medium is different from the boiling point and the freezing spot, our common glass tube thermometer mainly includes: kerosene thermometer, mercury thermometer, red fountain pen water thermometer. His advantages are simple structure, easy to use, relatively high measurement accuracy, low price. The disadvantage is that the measurement of the upper and lower limits and the accuracy of the glass quality and temperature measurement medium. And can not be far from the transmission, fragile.

6. Pressure thermometer: Pressure-type thermometer is the use of closed containers in the liquid, gas or saturated steam after heating to generate volume expansion or pressure changes as the test signal. Its basic structure consists of three parts, Wenbao, capillary and indicator table. The advantages of pressure thermometer are: simple structure, high mechanical strength, not afraid of vibration. The price is low and no external energy is required. The disadvantage is: the temperature range is limited, generally in-80~400℃;

7. Mercury thermometer is a kind of expansion thermometer, the freezing point of Mercury is-38.87 ℃, boiling point is 356.7 ℃, used to measure 0--150℃ or 500 ℃ within the range of temperature, it can only be used as an on-site monitoring instrument. It is not only simple and intuitive to measure the temperature, but also can avoid the error of the external remote thermometer.