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Shanghai Automation Instrumentation Co., Ltd.
Address:No.191 Guangzhong Road (West) Shanghai
Tel:86-21-59189606
Fax:86-21-60917975
Website:www.sc-saic.com/english
E-mail: tom.saic@gmail.com

Application and principle of thermocouples
Release time:2016-09-08   Clicks:820

Thermocouple of Shanghai Automation Instrumentation Co., Ltd. is one of the industry's most commonly used temperature sensing element. Its advantages are:

① high accuracy. Because thermocouple in direct contact with the measured object, is not affected by the middle of the media.

② wide measuring range. Common thermocouple can be measured from the edge Continued -50 ~ + 1600 ℃, some special thermocouple to the lowest measurable -269 ℃ (such as Gold and Iron nickel-chromium), up to + 2800 ℃ (such as tungsten - rhenium).

③ simple structure, easy to use. Thermocouples are usually composed of two different metal filaments, and without size restrictions beginning with the outer protective casing, very convenient to use.

1. Thermocouple Fundamentals

Two different conductor or semiconductor materials A and B welded together to form a closed loop. When the two conductors A and B of the persistent presence of the temperature difference between point 1 and 2, the force arises between the two, so that in the loop forming a current magnitude, a phenomenon known as the thermoelectric effect. Thermocouple is to use this effect to work.

2. Thermocouple types and structure formation

Category (1) thermocouple

Common thermocouple and standard thermocouple can be divided into two major categories of non-standard thermocouple. The call refers to the state standard thermocouple standards for the relationship between the thermoelectric power and temperature, tolerance, and a unified standard thermocouple indexing table, it has its support of the display instruments are available. Non-standardized thermocouples on the scope or magnitude and not standard thermocouple, generally there is no uniform indexing table, mainly used for the measurement of certain special occasions.

Standard thermocouple China from January 1, 1988, all the thermocouple and thermal resistance according to IEC international standards, and designated S, B, E, K, R, J, T thermocouple seven kinds of standardization for our unified design type thermocouple.

(2) thermocouple thermocouple structure in order to ensure a reliable and stable work, its structure requirements are as follows:

① welding electrodes consisting of two hot thermocouple must be firm;

② two hot electrodes to each other should be well insulated to prevent short circuits;

③ compensation wire thermocouple is connected to the free end of the convenient and reliable;

④ protection tube should be able to ensure adequate thermal isolation electrode and harmful media.

3. Thermocouple cold junction temperature compensation

Since the thermocouple materials are generally more expensive (especially the use of precious metals), and the distance measurement points to the instrument very far in order to save the thermocouple material, reduce costs, usually compensation wire thermocouple cold end (free end) extends to the relatively stable temperature control room, connected to the meter terminals. It must be noted, the role of thermocouple wires extending from the hot electrode only, so the cold end of the thermocouple is moved to the control room instrumentation terminals, which in itself does not eliminate the effect of temperature on the cold junction temperature measurement, can not afford compensation. Therefore, the need to use other correction methods to compensate for the cold junction temperature t0 ≠ 0 ℃ temperature measurement on time.

When using the thermocouple wire must be noted that the model match, can not pick the wrong polarity, compensation wire thermocouple connecting terminal temperature should not exceed 100 ℃.
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