Thermocouple principle, parameters, classification, etc. introduced
Thermocouples are one of the most commonly used temperature sensing devices in the industry.
The advantages are:
① high measurement accuracy. Because the thermocouple directly with the measured object contact,
not subject to the impact of intermediate media.
② measuring a wide range. Commonly used thermocouple from -50 "+1600 ℃ can be continued
measurement, some special thermocouple can be measured to a minimum -269 ℃
(such as gold-nickel-chromium), up to +2800 ℃ (such as tungsten - rhenium).
③ simple structure, easy to use. Thermocouple is usually composed of two different metal wire,
and not subject to the size and the beginning of the restrictions, the outer protective sleeve,
with very convenient.
1. Thermocouple temperature measurement of the basic principles
Two different materials of the conductor or semiconductor A and B welded together to form a closed
loop, as shown in Figure 2-1-1. When there is a temperature difference between the two attachment
points 1 and 2 of the conductors A and B, an electromotive force is generated therebetween,
thereby forming a current of a large magnitude in the circuit. This phenomenon is called a
thermoelectric effect. Thermocouple is to use this effect to work.
2. Thermocouple type and structure of the formation
(1) the type of thermocouple
Commonly used thermocouple can be divided into standard thermocouple and non-standard thermocouple
two categories. The standard thermocouple is called the national standard provides the
relationship between the thermoelectric power and temperature, allowing the error, and a unified
standard table thermocouple, which has its supporting display instrument available.
Non-standardized thermocouple in the use of the scope or magnitude are less than the
standardization of thermocouples, generally there is no uniform indexing table, mainly for some
special occasions.
Standard thermocouple China from January 1, 1988 onwards, thermocouple and thermal resistance of
all international standards according to IEC production, and specify the S, B, E, K, R, J, T seven
standardized thermocouple for China's unified design Thermocouple.
(2) thermocouple structure In order to ensure reliable and stable operation of the thermocouple,
its structural requirements are as follows:
① thermocouple composition of the two hot electrodes welding must be strong;
② two hot electrodes should be well insulated from each other to prevent short circuit;
③ compensation wire and the free end of the thermocouple connection to be convenient and reliable;
④ protective sleeve should be able to ensure adequate separation of the hot electrode and harmful
media.
3. Thermocouple cold junction temperature compensation
Because thermocouple materials are generally more valuable (especially when using precious metals),
and the temperature measurement point to the instrument are far away, in order to save the
thermocouple material, reduce costs, usually using the compensation wire to the cold end of the
thermocouple (free End) to a relatively stable temperature control room, connected to the
instrument terminal. It must be noted that the role of thermocouple wire compensation only play
the role of extension of the hot electrode, so that the cold end of the thermocouple to the
control room of the instrument terminal, which itself can not eliminate the temperature change of
the cold end of the impact of temperature compensation. Therefore, the need to use other
correction methods to compensate for the cold side temperature t0 ≠ 0 ℃ on the temperature
measurement.
In the use of thermocouple compensation wire must pay attention to the model match, the polarity
can not take the wrong, compensation wire and thermocouple junction temperature can not exceed
100 ℃.
4. Thermocouple performance and parameters
(1). Thermal response time
The time taken for the thermocouple output to change to 50% of the step change when the temperature
changes step by step is called the thermal response time, expressed as τ0.5.
(2) Nominal pressure
Generally refers to the working temperature of the tube can withstand static external pressure
without cracking, in fact, the nominal pressure not only with the protection of pipe material,
diameter, wall thickness, but also with its structural form, installation method, placement depth
and Measuring the flow rate and type of media and so on.
(3). Minimum thermocouple placement depth
Shall be not less than 8 to 10 times the outer diameter of the protection tube (except for special
products)
5. Thermocouple protection tube
The role of protection tube is to protect the temperature sensing element, so that it is not in
direct contact with the measured medium to avoid or reduce the erosion of harmful media, flames
and airflow erosion and radiation, and mechanical damage, but also play a fixed and support sensor
temperature The role of the component. 321,304 and 316 (316L) series of stainless steel protection
tube is the most widely used materials, usually used in the 900 ℃ below, some products can also
be used to 1150 ℃; 900 ℃ above the general use of non-metallic material protection tube.
6. Thermocouple structure classification
Thermocouple according to the composition of the structure is divided into thermocouple temperature
wire, armored thermocouple, thermocouple assembly.
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