Based on the thermocouple thermometer table cold junction compensation method

Thermocouple temperature sensor is a kind of common, is the use of thermoelectric effect, and according to the hot and cold end temperature difference of corotating coarse electromotive force measuring temperature, and has high measurement precision, simple structure, convenient use. The thermometer has been widely used in the table. Common cold junction compensation method due to its complex structure, large noise, linearity error can cause great influence on measuring results.

1。 1 bridge principle of compensatory method

as shown in figure 1, in which R1, R2, R3 resistance equal, made of manganese bronze with zero temperature coefficient approximation, namely the resistance varies with temperature, and Rt with PT1000 thermal resistance, with the thermocouple cold end in the same degree of RML field, its resistance changes with the temperature, temperature, resistance to increase when the temperature of the cold end is zero R1 = R3 = = R1, R2 can make the output of the bridge is zero, if the cold end temperature, makes clear the thermocouple thermoelectric potential hole, and measurement error, but the P value of the 1000 will also increases with temperature rise, the compensation bridge out of balance, the output value is not zero, the change in bridge output equals the thermoelectric 4 Yu thermoelectric potential variation, and the change in the opposite direction, the two cancel each other out make the size of the total output of changes over the cold end temperature.

1。 Record the data

2 used in millivolt voltage generator simulation experiment of K type thermoelectric lm thermoelectric potential, on the circuit board in error of A/D conversion, through the MCU, upload the upper machine by the upper machine will be A/D conversion value for temperature and explicit

this method of R1, R2, R3 accuracy requirement is high, and the noise of V +, temperature drift, higher stability, to meet the experimental requirements need to make the bridge current to A suitable value, high difficulty of debugging. When the multichannel measurement, need to decorate a multiplex device, structure is more complicated.

2 new method of thermocouple cold junction compensation

2. 1

the method principle of the cold end temperature is measured by the PT1000, through A/D conversion, the MCU to the upper lateral resistance capacity Cut inch software called calculating voltage value added to the thermocouple voltage eliminated by compensation block on the impact of the cold end temperature change, thus to compensate.

2。 2 the design of compensation block

this method for cold junction compensation of the main device is a piece of good performance of thermal conductivity of aluminum in cuboid aluminum piece on the horizontal axis of isometric type 3 hole, in turn and cut along the horizontal axis. To aid in the connection process after about two root pressure compensation conductor should be as two hole, the hole in the middle of the press in PT1000 thermal resistance. In the process of the pumped in order to guarantee the uniformity of heat conduction, heat resistance and the diameter of the compensating conductor should be consistent and taste with the repair piece full access, insulation material to be the same.

2。

3 compensation circuit design as shown in figure 3, thermocouple for compensation wires to the compensation of the instrument in the block before by Cu wire connection in the circuit board. Compensation and thermocouple cold end in the meter box. PT1000 is used for measuring the temperature in the cabinet To the Tc is the temperature outside the meter box.

due to program design requirements, in has not received compensation circuit in PC display temperature T1 Tr is A place of real temperature and the temperature in the cabinet Tb, namely the T1 = Tr + Tb. Instrument in the actual use of the temperature in the oven temperature changes to avoid the influence of the change of the temperature in the cabinet of the actual measuring temperature, access the compensation circuit design.

when the temperature in the cabinet, can make 1 PC display temperature butyl increased with the temperature in the cabinet, after joining compensation circuit, compensation in the heated evenly in the cabinet, compensation and PT1000 are in the same temperature field on both ends of the wire, compensation conductor voltage is produced to offset the impact of the cold end temperature changes, to ensure the measured value is not affected by the change of temperature in the cabinet, is only related with the box outside ambient temperature, namely the T1 = Tr + Tc.

2。 Record the data

4 experimental process and experimental process similar bridge method, millivolt voltage generator was used to simulate the K type thermocouple thermoelectric potential, on the circuit board in error of A/D conversion, through the MCU, upload the upper machine by the upper side of diao, let A/D conversion value for temperature and display.

you can see from the table above, the compensation method has high accuracy, the error is within 10 c and good linearity, in only when the multiplex measurement on the compensation block hole can add a few group, simple structure can meet the demand of industrial application.

3 to fill a try block method and bridge method of data analysis

according to the data in table 1 and table 2, the input voltage as the abscissa, error value of ordinate respectively to make the error of the two methods under different temperature curve, as shown in figure 4 and 5.

curve can be seen in the figure 5, bridge method of linearity is poorer, because the input and output characteristics of the thermocouple and compensate the output characteristic of bridge which is a nonlinear feature and do not overlap, therefore, within the scope of compensation only if the two curves intersect points corresponding to the cold end temperature compensation error can be compensated completely, no, the other parts of the cold end temperature can only be compensation, compensation error exists. In the actual use of need to use more complicated circuit to reduce the nonlinear error.

from figure 4 compensation method of block 4 curves can be seen, the maximum error is not more than 10 c and linear is good, can meet the measurement requirements more accurately.

4 the complimentary described in this article, based on the thermocouple thermometer table cold junction compensation method of circuit is simple, stable, low noise. The multi-channel measurement on the compensation block just play a few more in TLA compensation wire into the can, on the premise of cost control ensures the accuracy of measurement, technical indicators.