Temperature instruments' working principle

Thermometer is a tool that can accurately judge and measure temperature. It is divided into pointer thermometer and digital thermometer. According to the difference of use purpose, a variety of thermometers have been designed and manufactured.

Rosemount temperature transmitter's working principle

According to the difference of use purpose, a variety of thermometers have been designed and manufactured. Its design basis includes: using the phenomenon of thermal expansion and cold contraction of solid, liquid and gas affected by temperature; Under the condition of constant volume, the pressure of gas (or vapor) changes due to different temperature; Thermoelectric effect; The resistance changes with the change of temperature; Influence of thermal radiation, etc.

1. Gas thermometer: hydrogen or helium is mostly used as temperature measuring material. Because the liquefaction temperature of hydrogen and helium is very low and close to absolute zero, its temperature measuring range is very wide. This thermometer has high accuracy and is mostly used for precision measurement.

2. Resistance thermometer: it is divided into metal resistance thermometer and semiconductor resistance thermometer, which are made according to the change of resistance value with temperature. Metal thermometers are mainly used for pure metals such as platinum, gold, copper and nickel and rhodium iron and phosphorus bronze alloys; Semiconductor thermometers mainly use carbon, germanium, etc. Resistance thermometer is easy to use and reliable, and has been widely used. Its measurement range is - 260 ℃ to 600 ℃.

3. Thermocouple thermometer: it is a temperature measuring instrument widely used in industry. Made of thermoelectric phenomenon. Two different metal wires are welded together to form a working end, and the other two ends are connected with the measuring instrument to form a circuit. Put the working end at the measured temperature. When the temperature of the working end is different from that of the free end, electromotive force will appear, so there is current through the circuit. Through the measurement of electrical quantity, the temperature of another place can be measured by using the temperature of the known place. It is suitable for the measurement of high temperature and low turbidity between two substances with large temperature difference. Some thermoelectric couples can measure high temperatures up to 3000 ℃, while others can measure low temperatures close to absolute zero.

4. Pointer thermometer: it is a thermometer shaped like an instrument panel, also known as a thermometer for measuring room temperature. It is made of the principle of thermal expansion and cold contraction of metal. It uses bimetallic sheet as temperature sensing element to control pointer. Bimetallic sheet is usually riveted together with copper sheet and iron sheet, with copper sheet on the left and iron sheet on the right. Since the thermal expansion and contraction effect of copper is much more obvious than that of iron, when the temperature rises, the copper sheet pulls the iron sheet to bend to the right, and the pointer will deflect to the right (point to high temperature) driven by the bimetallic sheet; On the contrary, when the temperature becomes low, the pointer will deflect to the left (pointing to low temperature) driven by the bimetallic sheet.

5. Glass tube thermometer: the glass tube thermometer uses the principle of thermal expansion and cold contraction to measure the temperature. Because the expansion coefficient of temperature measuring medium is different from the boiling point and freezing point, our common glass tube thermometers mainly include kerosene thermometer, mercury thermometer and red pen water thermometer. Its advantages are simple structure, convenient use, relatively high measurement accuracy and low price. The disadvantage is that the upper and lower limits and accuracy of measurement are limited by the quality of glass and the properties of temperature measuring medium. It cannot be transmitted far away and is fragile.

6. Pressure thermometer: the pressure thermometer uses the volume expansion or pressure change generated by the heating of liquid, gas or saturated vapor in the closed container as the measurement signal. Its basic structure is composed of temperature packet, capillary and indicator. The pressure thermometer has the advantages of simple structure, high mechanical strength and no fear of vibration. Low price, no need for external energy. Disadvantages: the temperature measurement range is limited, generally - 80 ~ 400 ℃; Large heat loss and slow response time.

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