Principle And Use Of Thermometer

 Depending on the purpose of use, a variety of thermometers have been designed and manufactured. The design of the basis is: the use of solid, liquid, gas by the temperature of the thermal expansion and contraction of the phenomenon; in constant volume conditions, the gas (or steam) pressure varies due to different temperatures; thermoelectric effect; Changes in temperature, and the effects of thermal radiation.



In general, any physical property of all matter, as long as it changes with the temperature and monotonous, significant changes can be used to mark the temperature and made of thermometer.



Various thermometers work principle


  1. Gas thermometer: multi-purpose hydrogen or helium as a temperature measurement material, because hydrogen and helium liquefaction temperature is very low, close to absolute zero, so its temperature range is very wide. This thermometer is highly accurate and is used for precision measurements.


  2. Resistance thermometer: metal resistance thermometer and semiconductor resistance thermometer, are based on the resistance value changes with the temperature of this feature made. Metal thermometer mainly used platinum, gold, copper, nickel and other pure metal and rhodium iron, phosphor bronze alloy; semiconductor thermometer mainly carbon, germanium and so on. Resistance thermometer is easy to use and has been widely used. Its measurement range is -260 ℃ to 600 ℃ or so.


3. Thermocouple thermometer: is a widely used industrial temperature measurement equipment. The use of thermoelectric phenomenon made. Two different wires are welded together to form a working end, and the other ends are connected to the measuring instrument to form a circuit. Place the working end on the measured temperature, the working end and the free end of the temperature is different, there will be electromotive force, which has a current through the loop. By measuring the amount of electricity, using the known temperature, you can measure the other temperature. It is suitable for large temperature difference between the two substances, mostly for high temperature and low turbidity measurements. Some thermocouples can measure up to 3000 ℃ high temperature, and some can measure near the absolute zero of the low temperature.


4. High temperature thermometer: refers to the temperature used to measure the temperature above 500 ℃ thermometer, light thermometer, color temperature thermometer and radiation thermometer. High temperature thermometer principle and structure are more complex, no longer discussed here. The measurement range of 500 ℃ to 3000 ℃ above, not suitable for measuring low temperature.


5. Pointer thermometer: is like a dashboard of the thermometer, also known as cold summer table, used to measure room temperature, is the principle of metal thermal expansion and contraction made. It is a bimetal as a temperature sensor, used to control the pointer. Bimetallic pieces are usually riveted with copper and iron pieces, and the copper sheet is on the left and the iron sheet is on the right. Since the thermal expansion and contraction of copper is much more pronounced than that of iron, when the temperature rises, the copper pulls the iron sheet to the right and the pointer is deflected to the right by the bimetallic sheet (point to high temperature); , The temperature becomes low, the pointer in the bimetal driven by the left to the left (pointing to low temperature).


6. Glass tube thermometer: glass tube thermometer is the use of thermal expansion and contraction of the principle to achieve the temperature measurement. As the temperature coefficient of expansion of the medium and the boiling point and the freezing point of the different, so our common glass tube thermometer are: kerosene thermometer, mercury thermometer, red pen water thermometer. His advantage is simple structure, easy to use, the measurement accuracy is relatively high, low prices. The disadvantage is that the measurement of the upper and lower limits and accuracy by the glass quality and temperature of the nature of the restrictions. And can not be far away, fragile.


7. Pressure thermometer: pressure thermometer is the use of closed containers of liquid, gas or saturated steam heated after the volume expansion or pressure changes as a test signal. Its basic structure is composed of three parts: warm bag, capillary and indicating table. Pressure thermometer has the advantages of simple structure, high mechanical strength, not afraid of vibration. Low price, no external energy. Disadvantages are: temperature range is limited, generally in the -80 ~ 400 ℃; heat loss response time is slower.


8. Mercury thermometer: mercury thermometer is a kind of expansion thermometer, mercury freezing point is -38.87 ℃, the boiling point is 356.7 ℃, used to measure 0-150 ℃ or 500 ℃ within the range of temperature, it can only be used as a supervisor meter. Use it to measure the temperature, not only relatively simple and intuitive, but also to avoid the external remote thermometer error.