Cold-end compensation method for thermocouple temperature measurement

Thermocouple is a commonly used temperature sensor. It uses the thermoelectric effect and measures the temperature based on the thermoelectric potential generated by the temperature difference between the hot and cold ends. It has the advantages of high measurement accuracy, simple structure, and easy use. It has been widely used in temperature measuring instruments. The common cold end compensation method has a complex structure, high noise, and poor linearity, which will have a great impact on the measurement results.

New method for cold end compensation of thermocouples

Principle

This method measures the cold end temperature by PT1000. After A/D conversion, the MCU transmits it to the host computer to convert the resistance value into a voltage value through software and add it to the voltage of the thermocouple. Then, the compensation block is used to eliminate the influence of the cold end temperature change, thereby performing compensation. Thermocouple temperature measurement cold end compensation method

Design of compensation block

The main device for cold end compensation in this method is an aluminum block with good thermal conductivity.

Universal thermocouple cold end compensation method

Principle of bridge compensation method

Among them, R1, R2, and R3 have equal resistance values ​​and are made of manganese copper with a temperature coefficient close to zero, that is, their resistance value does not change with temperature changes, and Rt uses thermal resistor PT1000, which is in the same temperature field as the cold end of the thermocouple, and its resistance value changes with temperature changes. When the temperature rises, the resistance value increases. When the cold end temperature is zero, R1=R3=R2=R1, which can make the output of the bridge zero. If the cold end temperature rises, the thermoelectric potential of the thermocouple will decrease and cause measurement errors. However, at this time, the resistance value of PT1000 will also increase with the temperature increase, then the compensation bridge loses balance, and the output value is not zero. The change value of the bridge output is equal to the change value of the thermoelectric potential of the thermocouple, and the two change in opposite directions, then the two cancel each other out so that the total output value does not change with the change of the cold end temperature.

Experimental data recording

During the experiment, a millivolt voltage generator was used to simulate the thermoelectric potential of a K-type thermocouple. After the A/D conversion was completed on the circuit board, it was uploaded to the host computer through the MCU, and the host computer converted the A/D value into temperature and displayed it.

This method has very high requirements on the accuracy of R1, R2, and R3, and the noise and temperature drift of V+ must be small and the stability must be high. In order to meet the experimental requirements, the bridge current must be a suitable value, and the debugging is difficult. When performing multi-channel measurements, it is necessary to arrange multi-channel devices, and the structure is relatively complex.

Three through holes are punched in sequence and equidistantly on the horizontal center axis of the rectangular aluminum block, and cut along the horizontal center axis. In the subsequent wiring process, the two compensation wires are pressed into the left and right through holes, and the middle through hole is pressed into the thermal resistor PT1000. In the process of pressing, in order to ensure the uniformity of heat conduction, the diameters of the thermal resistor and the compensation wire must be consistent and fully in contact with the compensation block, and the insulating materials must be the same.

Compensation circuit design

The thermocouple is connected to the compensation block in the instrument box through the compensation wire and then connected to the circuit board in the box through the Cu wire. The compensation block and the cold end of the thermocouple are in the instrument box. PT1000 is used to measure the temperature To in the instrument box, and Tc is the ambient temperature outside the instrument box.

Is there any effect of connecting a section of copper wire in the compensation wire of the thermocouple?

1. The middle of the thermocouple compensation wire can be soldered with tin!

2. If the compensation wire is not long enough, it needs to be replaced with copper wire, and the "intermediate conductor law" must be followed!