The analysis of the cold junction compensation of thermocouple perfect optimization method is introduced

4 thermocouple cold junction compensation

4.

1 thermocouple cold junction compensation analysis according to the type ( 2) , the thermocouple cold junction compensation must be made to obtain actual temperature corresponding to the thermoelectric potential of measured medium, according to the thermoelectric potential to calculate the real value. Obviously, the accuracy of the cold end temperature will affect the measurement precision of the thermocouple, if the cold end temperature measurement error is bigger, the cause of the thermocouple temperature measurement error is bigger. Cold junction compensation at present there are three kinds of commonly used methods.

( 1) Direct measurement method of the cold end temperature

the cold end temperature directly USES the temperature sensor, temperature measuring instrument table generally adopts thermal resistance measurements of the cold end temperature, then use type ( 2) The actual temperature of corresponding thermoelectric potential, in the end to the use of linearization method, to obtain the actual temperature value.

( 2) Bridge compensation method

bridge compensation method is to use the voltage produced by the unbalanced bridge to compensate for thermocouple is not due to the cold end temperature 0 ℃ when the thermoelectric potential changes value. Using compensation bridge, should pay attention to the following: (1) different index number of thermocouple with and the compensation of the thermocouple is the bridge;

(2) to compensate the bridge connected to the thermocouple, power source and the measuring instrument, to wiring is correct, especially the positive and negative power supply is the reverse; (3) of the unbalanced bridge output voltage varies with the voltage of dc power supply, so the dc power supply voltage to constant [ 5] 。

( 3) Integrated chip

cold junction compensation integrated chip of cold junction compensation mainly include precision thermocouple amplifier and thermocouple AD converter.

for example AD8495 is an integration of the cold junction compensation precision thermocouple amplifier, internal with fixed gain amplifier, to provide 5 mV / ℃ output, the amplifier with high common mode rejection performance, can restrain the length of the thermocouple wire may pick up the common-mode noise. For K type thermocouple for the calibration, only suitable for K type thermocouple measurements.

MAX31855 cold junction compensation is an integration of the AD converter, the output device 14 signed data, SPI interface, the temperature of the converter resolution of 0. 25 ℃, with thermocouple open circuit detection function, and the thermocouple to the GND or VCC short circuit detection function. Each chip only for a certain type of thermocouple measurement, its flexibility is not enough.

4。 2 thermocouple cold junction compensation method

cold junction compensation bridge compensation method is used to realize, its flexibility is insufficient, the different thermocouple need to match with the thermocouple is the bridge, and unbalanced bridge output voltage with the voltage of the dc power supply, dc power supply voltage to constant, therefore, to the accuracy requirement of the dc power supply is higher, based on the above shortcomings, the current bridge compensation method is seldom used.

cold junction compensation of thermocouple acquisition chip has the advantage of integrated cold junction compensation, the chip is small in size, with a thermocouple on short circuit detection function, but the conversion accuracy is determined by the chip itself, only made for a certain type thermocouple cold junction compensation calibration, system cannot modify the thermocouple acquisition by software type. Using temperature sensor to collect the cold end temperature cold junction compensation, temperature sensor can choose high precision, high reliability, low temperature drift of the sensor, acquisition to the cold end temperature and then converted into a certain type of thermocouple at the cold end temperature of thermoelectric potential, system USES type ( 2) Cold junction compensation is achieved, and the system can be set by the program thermometer table to collect a certain type of thermocouple, therefore, good flexibility, high precision, the method can be used for a variety of types of thermocouple signal acquisition. 5 conclusion

the place on put together is narrated, secondary element end thermocouple signal acquisition precision mainly depends on three factors, including thermocouple signal collection circuit, the linearization of the thermocouple signal and the thermocouple cold junction compensation. Improper handling any factors will affect the precision of the thermocouple signal acquisition, through the above three aspects, the paper proposes the optimization method, can greatly improve the thermocouple signal to the secondary components acquisition precision, thus improve the thermocouple temperature measurement precision. With the high-speed development of integrated circuit, integrated cold junction compensation, high common mode rejection capability, high precision, strong anti-interference ability, can be configured to a variety of types of thermocouple measuring the AD converter will be more and more widely used in the future.