Thermocouple temperature measurement why with cold end temperature compensation

Thermocouple temperature measurement why with cold end temperature compensation?

A: Because on the scale, the cold end of the temperature is zero degrees Celsius. The cold end temperature measured on site is generally not zero degrees Celsius, so the cold end compensation should be carried out.

Introduction to thermocouple cold end compensation:

In theory, thermocouple measurement is based on the standard measurement of the cold end at zero, but the appearance of the general measurement is under room temperature, because the cold end is not zero, resulting in the reduction of thermoelectric potential, so that the measurement is not accurate, presenting errors. The compensation method is the cold end temperature compensation.

Thermocouple temperature measurement requires that the temperature of the cold end (the measurement end is the hot end, and the end connected with the measurement circuit through the lead is called the cold end) adhere to the same, and the size of the thermoelectric potential is proportional to the measurement temperature. If the temperature of the cold end (environment) changes during measurement, the accuracy of severe measurement will be affected. In the cold end to adopt a certain way to compensate for the cold end temperature changes caused by the impact is called thermocouple cold end compensation.

The cold end compensation of thermocouple generally uses a bridge composed of thermal resistance in series at the cold end. Three arms of the bridge are standard resistors and one arm is made of (copper) thermal resistors. When the temperature of the cold end changes (e.g. increases), the thermoelectric potential generated by the thermocouple changes (decreases), and the resistance value of the thermal resistance in the series bridge changes and the voltage at both ends of the bridge changes (increases). If the parameters are well chosen and the wiring is correct, the voltage generated by the bridge is exactly the same as the thermoelectric potential changes with temperature, and the total output voltage (potential) of the whole thermocouple measurement circuit exactly reflects the measured temperature value.

1. Cold end thermostatic method

General thermocouple calibration cold end temperature to 0℃ as the standard. Therefore, the cold end is often placed in a mixture of ice water to maintain a stable temperature of 0℃. Under laboratory conditions, the cold end is generally placed in a test tube containing insulating oil, and then placed in an insulated container filled with a mixture of ice water. The cold end is maintained at 0℃.

2. Cold end temperature correction method

Because the thermocouple temperature indexing table is in the cold end temperature at 0℃, and it is matched with the use of the measurement circuit or display appearance is calibrated according to the contact curve, so the cold end temperature is not equal to 0℃, you need to be corrected on the appearance indication value. If the temperature of the cold end is higher than 0℃ but stable at T0 ℃, the measured thermoelectric potential is less than the division value of the thermocouple. In order to obtain the true temperature, the central temperature law can be used, namely, the following formula is used to correct: E(t,0)= E(t, T1)+ E(T1,0)

3. Compensation wire method

In order to make the thermocouple cold end temperature adhere to the stability (0℃), the thermocouple can be made very long, so that the cold end away from the working end, and together with the measurement surface placed to the constant temperature or temperature shake relatively small local. But this method makes the device inconvenient to use and may consume valuable metal materials. Therefore, a connector called a compensation wire is generally used to extend the cold end of the thermocouple. This wire has the same thermoelectric properties as the connected thermocouple within a certain temperature scale (0 ~ 150℃). If the thermocouple is made of cheap metal, its own material can be used as compensation wire to extend the cold end to the temperature stable place.

Matters needing attention in the application of compensation wire:

(1) Selection of compensation wire

Compensation wire must be correctly selected according to the type of thermocouple used and the occasion used. For example, the type K couple should be selected for the compensation wire of the type K couple, depending on the service situation, the operating temperature scale should be selected. General KX operating temperature is -20 ~ 100℃, wide scale is -25 ~ 200℃. General error ±2.5℃, precision ±1.5℃.

(2) Contact connection

And thermocouple terminal 2 contacts as close as possible, try to adhere to the temperature of the two contacts common. As far as possible, the temperature of the connection with the outer terminal is the same. Where the outer cabinet has the fan, the contact should be protected from blowing the fan straight to the contact.

3) Use length

Because the signal of thermocouple is very low, for microvolt level, if the distance used is too long, the attenuation of signal and the interference of strong electricity in the environment coincidence, enough can make the signal distortion of thermocouple, resulting in the measurement and control of temperature is not accurate, in the control of severe temperature shaking.

According to our experience, it is better to control the length of thermocouple compensation wire within 15 meters. If it exceeds 15 meters, it is recommended to use temperature transmitter to transmit signals. Temperature transmitter is the temperature of the corresponding potential value into dc current transmission, strong resistance to disturbance.

(4) the wiring

Compensation wires must be routed away from power lines and disturbance sources. Where crossing cannot be avoided, it is also possible to cross rather than parallel.

(5) Shielded compensation wire

In order to improve the resistance to disturbance of thermocouple connection line, shielded compensation wire can be used. The effect is better when there are many disturbance sources on site. However, the shielding layer must be strictly grounded, otherwise the shielding layer not only did not play a shielding effect, but increased disturbance.

4. Compensated bridge method

Compensating bridge method uses the potential generated by unbalanced bridge to compensate the thermoelectric potential changes caused by cold end temperature changes of thermocouple. Compensation Bridges are now standardized.

The unbalanced bridge (or compensating bridge) is composed of resistors R1, R2, R3 and RCu.

Meantime R1 = R2 = R3 = 1; Rs is made by winding manganese copper wire with very small temperature coefficient; RCu is a compensating resistance made by winding copper wire with large temperature coefficient. At 0℃, RCu=1; The value of Rs can be calculated according to the type of the selected couple.

The bridge is connected in series in the thermocouple measurement loop, the cold end of the thermocouple and the resistance RCu feel the same temperature, and the bridge balance is adjusted at a certain temperature (generally 0℃), so that R1=R2=R3=RCu.

When the cold end temperature changes, RCu changes with the temperature, the bridge balance is destroyed, an unbalanced voltage △U occurs, this voltage and thermoelectric potential superposition, together into the measurement surface.

The value of Rs is selected appropriately, but the bridge imbalance voltage △U can basically compensate for the thermoelectric potential change caused by the cold end temperature change within a certain temperature scale.

In this way, when the temperature of the cold end is changed, the appearance can still give the correct temperature indication.