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Thermocouple principle, parameters, classification, etc. introduced
Dec 24, 2017

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.


Add: No. 16, Road 3, Sanjiang Industry Zone, Shengzhou, Zhejiang, China

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Email: boris@benoelectric.com

Contact:Charles Yu     

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