Working principle of temperature measuring instruments

Working principle of temperature measuring instruments

1.1 Temperature measuring instrument
Temperature is a parameter that characterizes the degree of hotness or coldness of an object. It cannot be measured by direct comparison like mass and length. We can only measure it by other physical properties of the substance related to temperature, such as the volume, density, viscosity, hardness, conductivity, etc.

1.1.1 Thermal resistance temperature instrument
The principle of thermal resistance thermometer is to use the property that the resistance of a conductor or semiconductor changes with temperature. The main advantages of thermal resistance thermometer are: high measurement accuracy and good reproducibility; large measurement range, especially at low temperatures; easy to use in automatic measurement and also convenient for long-distance measurement. Similarly, thermal resistance also has defects, such as poor accuracy in high temperature (greater than 850℃) measurement; easy oxidation and corrosion resistance.

The relationship between thermal resistance and temperature can be described by a quadratic equation:

: Resistivity, Ω·cm t: Temperature, ℃

a, b, c: constants (determined by experiments), units are Ω·cm, Ω·cm·℃－1, Ω·cm·℃－2

At present, the materials used for thermal resistors are mainly platinum, copper, nickel, etc. These materials are used mainly because the ratio of their temperature and resistance in the commonly used temperature range is a linear relationship. Here we mainly introduce platinum resistance thermometers.

Platinum is a precious metal with very stable physical and chemical properties, especially strong oxidation resistance. It is easy to purify and has good processability. It can be made into very fine platinum wire. Compared with copper, nickel and other metals, it has higher resistivity and high reproducibility. It is an ideal thermal resistor material. The disadvantage is that the resistance temperature coefficient is small, it tends to become brittle when working in a reducing medium, and the price is also relatively expensive. The purity of platinum is usually expressed by Baidu resistance ratio:

W(100)=R100/R0

R100: Resistance value at 100℃ R0: Resistance value at 0℃

According to the IEC standard, the platinum resistor with W(100)=1.3850 and an initial resistance value of R0=100Ω (R0=10Ω) is the industrial standard platinum resistor. The resistance wire of the platinum resistance thermometer with R0=10Ω is relatively thick and is mainly used to measure temperatures above 600℃. The resistance and temperature equation of the platinum resistor is a piecewise equation:

Rt=R0[1+At+Bt2＋C(t－100℃)t3] t at -200～0℃

Rt=R0(1+At+Bt2) t If

we solve this equation at 0～850℃, we can know the temperature value according to the resistance value. But in actual work, we can check the thermal resistor graduation table to determine the temperature value according to the resistance value.

According to the standard, platinum thermal resistors are divided into Class A and Class B. The allowable temperature error of Class A is ± (0.15°C + 0.002|t|), and the allowable temperature error of Class B is ± (0.3°C + 0.005|t|).

The thermal resistors used on site are generally armored thermal resistors, which are composed of thermal resistor body, insulating material, and protective tube. The thermal resistor body and the protective tube are welded together, and the insulating material is filled in the middle, so that the thermal resistor body can be well protected, impact-resistant, shock-resistant, and corrosion-resistant. As shown in Figure 1 on the right.
Platinum thermal resistors are available in two-wire, three-wire, and four-wire systems. The two-wire system has a large error in measurement and is no longer used. Now the industrial use is generally three-wire, and the laboratory use is generally four-wire. Here we mainly introduce the wiring of the three-wire platinum thermal resistor. As shown in Figure 2 below, the three-wire platinum thermal resistor is connected in parallel with the a end of the resistor to realize the three wiring terminals a, b, and c of the resistor. In this way, the resistance of the measuring wire itself introduced by the b wire can be compensated by the c wire, so that the influence of the lead resistance error introduced by the lead resistance not changing with the temperature is greatly reduced. The three-wire platinum thermal resistor used in Qinshan Phase II has a variable resistance bridge in the secondary instrument. According to the different ranges of the matched platinum thermal resistor, the platinum thermal resistor in the bridge of the secondary instrument can be fine-tuned to make more accurate measurements.
1.2 Thermocouple temperature instrument
Thermocouple thermometer uses the thermoelectric effect to measure temperature. Thermoelectric effect: when two conductors of different materials form a loop, if the junction temperatures at both ends are different, a certain amount of current will be generated in the loop. The size of this current is related to the conductor material and the junction temperature. One of the two nodes is the T end, the measurement end, and the other is the T0 end, the reference end, as shown in Figure 3 on the right. In actual measurement, the millivolt signal generated by the thermocouple should be measured with a more precise millivolt meter or I/O card.

According to IEC standards, the thermocouples currently used mainly include platinum-rhodium 10-platinum, nickel-chromium-nickel silicon, nickel-chromium-copper-nickel, platinum-rhodium 30-platinum-rhodium 6, etc. They all have stable thermoelectric properties, stable physical and chemical properties, not easy to be oxidized and corroded, small resistance temperature coefficient, high conductivity, the thermoelectric potential of the material changes linearly with temperature, the material has good processability and is easy to process. At present, Qinshan Phase II uses two types of thermocouples: platinum-rhodium 10-platinum and nickel-chromium-nickel silicon. Now the thermocouples are all armored thermocouples, and the structure is similar to the armored thermal resistor, which is also composed of thermocouple body, insulation material, and protection tube.

Figure 5: Intermediate temperature law

In the application of thermocouples, there are two basic laws: the intermediate conductor law and the intermediate temperature law.

Intermediate conductor law: As shown in Figure 4 on the right, the T0 end of the thermocouple made of materials A and B is disassembled and a third conductor C is introduced. Then the total electromotive force in the circuit is EABC(T,T0)=EAB(T)-EAB(T0), that is, when the temperatures at both ends of the third conductor are the same, it will not affect the electromotive force of the original thermocouple.

Figure 6: Thermocouple measurement

Intermediate temperature law: In a thermocouple circuit, the thermoelectric electromotive force when the two junction temperatures are T, T0 is equal to the algebraic sum of the thermoelectric electromotive forces when the junction temperatures are T, Ta and Ta, T0. EAB(T, T0) = EAB(T, Ta) - EAB(Ta, T0)

Using the intermediate conductor law and intermediate temperature law of the thermocouple, we can disassemble the T0 end of the thermocouple, connect it to the measurement millivoltmeter, and measure it at a distance. As shown in Figure 6. In order to lead the reference end of the thermocouple to the constant temperature T0 or the compensator, the thermocouple needs to be extended. In order to save precious metals or temporarily extend it, we often use compensation wires to extend it. The compensation wire is a wire made of a material with thermoelectric properties similar to the working thermocouple. It is used to extend the reference end of the thermocouple to the required place, and it will not introduce errors beyond the allowable limit to the working thermocouple circuit.

Thermocouple reference end processing: The electromotive force output by the thermocouple is a function difference of the temperatures of the two junctions of the thermocouple. In order to maintain the linearity of temperature and electromotive force, one junction temperature must be kept constant. The standard graduation table is based on the corresponding values ​​of electromotive force and temperature when the reference end is 0℃. In practical applications, the reference end will not remain at 0℃, so we use some methods to compensate. We will try our best to keep the reference end at a constant temperature Ta. After checking the standard graduation table and calculating, we can get the temperature value, E (T, T0) = E (T, Ta) + E (Ta, T0). Now there are secondary instruments that can automatically compensate for the reference end temperature, including potential automatic compensation method and bridge automatic compensation method.

1.1.3 Expansion temperature instrument
Expansion thermometers use the principle of thermal expansion of objects for measurement. The most common ones are alcohol thermometers and mercury thermometers. The upper and lower limits of this liquid expansion thermometer are limited by the vaporization and solidification temperatures of the liquid, and the minimum graduation can be 0.1°C. There are also thermometers that use solid expansion to measure temperature. The most common one is the bimetallic thermometer, which is made of two metals with different expansion coefficients into a spiral shape. One end is fixed. When it expands due to heat, due to the different expansion coefficients, the free end will have a certain angular displacement. This angular displacement is transmitted through the transmission amplification mechanism, driving the pointer to indicate the corresponding temperature. Another method is to use the pressure change caused by the thermal expansion or vaporization of the liquid in a sealed container to measure the corresponding temperature.

1.1.4 Radiation temperature instrument
When any object is heated, part of the heat energy will be converted into radiation energy. The higher the temperature of the object, the more energy will be radiated to the surrounding space. The radiation energy is expressed in the form of waves, and its wavelength range is extremely wide, starting from short waves, including X-rays, ultraviolet rays, visible light, infrared rays and electromagnetic waves. Generally, visible light and infrared rays are used for temperature measurement in engineering. Radiation temperature measurement is a non-contact temperature measurement, which can be applied to many occasions where temperature measurement is difficult. However, radiation temperature measurement is generally used at high temperatures above 900℃. However, technology is developing, and thermometers that use the infrared temperature measurement principle can measure low temperatures such as human body temperature.