Working principle and classification of temperature transmitter - part 1

Because there are many kinds of temperature sensing elements, there are many signal output types. In order to facilitate automatic detection, unified provisions are made for the signal output of various temperature sensors, that is, a unified 4 ~ 20mA signal. In order to unify the output of various temperature sensors into 4 ~ 20mA signals, a temperature transmitter is used. The temperature transmitter is used to turn various input resistance and potential signals into a unified current signal of 4 ~ 20mA, which is the origin of the temperature transmitter.

After the temperature transmitter completes the acquisition of the measurement signal, it is transformed into a unified 4 ~ 20mA current signal for output. At the same time, it also plays an isolation role.

Classified according to the working principle, it is mainly the difference of thermal elements,

There are: thermocouple, thermistor (metal), and semiconductor thermistor

The integrated temperature transmitter organically integrates the temperature sensing element (thermal resistance or thermocouple) with the signal conversion and amplification unit to measure the temperature of liquid, steam and other gaseous media or solid surfaces within the range of - 200-1600 ℃ in various processes. It is usually used with display instruments, recording instruments and various control systems.

Characteristics of temperature transmitter

The temperature sensor produces resistance or potential effect under the influence of temperature, and generates a differential voltage signal after conversion. This signal is amplified by the amplifier and then transformed by voltage and current to output a 4-20mA current signal corresponding to the measuring range.

Thermocouple is generally used for medium and high temperature measurement, while thermal resistance is mainly used for low temperature measurement. What to adopt? See the following introduction for details:

Thermocouple

Thermocouple is one of the most commonly used temperature detection elements in industry. The working principle of thermocouple is based on seeback effect, that is, the two ends of two conductors with different components are connected into a loop. If the temperatures of the two connecting ends are different, the physical phenomenon of thermal current will be generated in the loop. Its advantages are:

① High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.

② Wide measurement range. Common thermocouples can be continuously measured from - 50 ~ + 1600 ℃, and some special thermocouples can be measured as low as - 269 ℃ (such as gold, iron, nickel and chromium) and up to + 2800 ℃ (such as tungsten rhenium).

③ The utility model has the advantages of simple structure and convenient use. Thermocouples are usually composed of two different metal wires, which are not limited by size and beginning. There is a protective sleeve outside, which is very convenient to use.

1. Basic principle of thermocouple temperature measurement

The conductors or semiconductors A and B of two different materials are welded together to form a closed loop. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, electromotive force is generated between them, so a large and small current is formed in the circuit. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work.

2. Type and structure formation of thermocouple

(1) Type of thermocouple

Common thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose national standard specifies the relationship between thermal potential and temperature, allowable error, and has a unified standard graduation table. It has its supporting display instrument for selection. Non standardized thermocouples are inferior to standardized thermocouples in the scope of use or order of magnitude. Generally, there is no unified graduation table, which is mainly used for measurement in some special occasions.

Standardized thermocouples since January 1, 1988, all thermocouples and thermal resistors have been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, e, K, R, J and t have been designated as thermocouples of unified design in China.

(2) Structural form of thermocouple in order to ensure the reliable and stable operation of thermocouple, its structural requirements are as follows:

① The welding of the two thermoelectrodes constituting the thermocouple must be firm;

② The two thermoelectric poles shall be well insulated from each other to prevent short circuit;

③ The connection between the compensation wire and the free end of the thermocouple shall be convenient and reliable;

④ The protective sleeve shall ensure that the Thermoelectrode is fully isolated from harmful media.

3. Temperature compensation of thermocouple cold end

Since the materials of thermocouples are generally expensive (especially when precious metals are used), and the distance from the temperature measuring point to the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to the control room with relatively stable temperature and connect it to the instrument terminal. It must be pointed out that the function of thermocouple compensation wire is only to extend the Thermoelectrode and move the cold end of thermocouple to the instrument terminal in the control room. It itself can not eliminate the influence of cold end temperature change on temperature measurement and has no compensation function. Therefore, other correction methods need to be adopted to compensate the influence of cold end temperature t0 ≠ 0 ℃ on temperature measurement.

When using thermocouple compensation wire, attention must be paid to the matching of model, the wrong polarity shall not be connected, and the temperature at the connecting end of compensation wire and thermocouple shall not exceed 100 ℃.

Thermal resistance

Thermal resistance is the most commonly used temperature detector in medium and low temperature areas. Its main characteristics are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.