The processing method of the cold junction of the thermocouple

Thermocouple cold end processing methods and precautions thermocouple cold end processing method of compensation and compensation matters needing attention

in order to solve the above problem, thermocouple cold junction temperature constant, far heat transfer electric potential signal, get any cold end temperature of thermoelectric potential, in the practical application of thermocouple to the thermocouple cold end treatment and compensation. There are several common cold end processing method.

1, the cold end temperature correction method ( Also known as calculating correction method) When the thermocouple cold junction temperature 0 ℃, higher but stable when t0, can be obtained modified indicating value of the instrument, the cold end at 0 ℃ thermoelectric potential E ( t, 0) Through the check grade table to determine the measured temperature t.

2, the cold end temperature of the cold end temperature is the thermocouple cold end put in constant temperature controller, when the temperature 0 ℃ constant temperature device, this method also known as the freezing method or ice bath method. In order to prevent it short out, this approach is to do a good job of insulation. Because the method USES to freezing point slot device such as a constant temperature and relatively trouble, more measurement used in laboratory, industrial measurement is generally not used.

3, thermocouple compensation wire extension thermocouple compensation wire extension method ( Also called extension hot electrode method) In order to make the thermocouple cold end temperature remains the same, can prolong the thermocouple, the cold end extends to the region of relatively stable temperature, away from the influence of the temperature being measured. In order to reduce the cost of the precious metal thermocouple extension, to a certain temperature range and can be used to extend the thermocouple thermoelectric properties of similar cheap metal instead of precious metals, the realization of the cold end.

in order to ensure the accurate migration cold side effect, the following points should be paid attention to when using the thermocouple compensating conductor:

( 1) Different thermocouples with different thermocouple compensation wire, thermocouple, thermocouple compensation wire when using must form a complete set.

( 2) Pay attention to the using of the thermocouple compensating conductor temperature range, if the demand is higher measuring temperature, ensure the thermocouple compensation wire and thermocouple junction temperature under 100 ℃.

( 3) Has positive and negative thermocouple compensation wire, pay attention to when using the polarity can't wrong.

( 4) Thermocouple compensation wire just moving the cold end, and cannot be fixed the cold end, therefore, if the new cold end temperature fluctuations, still need to adopt other constant cold end temperature cold junction compensation method.

( 5) Because of thermocouple compensating conductor thermoelectric properties and corresponding thermocouple thermoelectric properties not identical, therefore, using thermocouple cold junction compensation conductor migration can introduce certain measurement error.

( 6) Guarantee thermocouple compensating conductor equals the two junction temperature hot electrode connection.

4, instrument zero calibration instrument zero correction method ( Also known as the instrument of mechanical zero adjustment method) If the thermocouple cold junction temperature t0 constant, form a complete set of display instrument and zero adjustment function, can be directly to the instrument zero adjustment to the thermocouple cold end temperature, equivalent to in the thermocouple thermoelectric potential joined a thermoelectric potential (E t, 0) 。 At this point, the input instrument thermoelectric potential equal to E ( t, t0) + E ( t0,0） = E ( t，0） , the temperature of the display instrument for practical measuring temperature t.

5, compensation bridge compensation bridge ( Also known as the cold end temperature compensator) This is the most commonly used one kind of the cold end compensation method, especially in the cold end temperature fluctuation is larger. Use of unbalanced bridge of unbalanced voltage to compensate for cold away thermoelectric potential changes with the temperature change.

when using the cold end temperature compensator should pay attention to the following:

( 1) The thermocouple and cold end temperature compensator must form a complete set.

( 2) When the cold end temperature compensator access temperature measuring system is, the cathode can't wrong.

( 3) Display instrument of zero to adjust to the balance point temperature of the cold end temperature compensator.

( 4) Output voltage due to the cold end temperature compensator is not identical with corresponding thermocouple thermoelectric properties, thus, a bridge within the scope of compensation temperature compensation in only two points fully compensated, other point cannot be fully compensated, can introduce certain measurement error.

6, software processing method in computer systems, software processing method should be adopted for cold junction compensation. Use of the other temperature sensor to the cold end temperature t0 into computer, according to the preset program automatically compensate the cold end temperature.

in practical thermocouple temperature measurement circuit is often combine a variety of compensation mode, maximum minus side xiao leng to measure the effects of temperature fluctuations.