Discussion on the calibration method of thermometer
A stable DC voltage generator or thermocouple temperature calibrator can input the required calibration temperature point thermoelectric potential of the thermocouple thermometer through digital buttons. The simulated thermoelectric potential value is monitored by a high-precision digital multimeter as the main standard to adjust the output thermoelectric potential of the thermocouple temperature calibrator; At this point, the high-precision thermocouple thermometer being calibrated obtains the digital information of the signal through amplification, filtering, and A/D conversion sampling (or amplification, filtering, and A/D conversion sampling on an A/D chip). Finally, the collected digital information is calculated into the corresponding temperature value through software using a microcontroller (or embedded system). At the same time, the cold end temperature automatically measured by the cold end measurement circuit is compensated for by the compensating thermoelectric potential generated by the temperature difference between the measuring end and the freezing point end of the compensating wire.
(1) The necessity of cold end compensation
Due to the wide range of applications of the cold junction compensation method, most temperature secondary instruments used with thermocouples are equipped with a cold junction compensation system, and the detection of such instruments is an important part of temperature secondary instrument detection. According to the verification regulations of JJG617-1996 "Digital Temperature Indicator Regulator", the method for detecting the basic error of digital temperature secondary instruments used with thermocouples is to connect the compensating wire of the tested instrument to the freezing point device, and then connect it to the signal source with a copper wire. The input DC voltage signal is used to measure the error of the instrument. Considering the following aspects: although the compensating wire has a correction value at 20 ℃, the expanded uncertainty of its correction value itself is U=0.3 ℃ (k=2); The gradual change in performance and instability of thermocouples caused by stress changes due to material oxidation and bending; The difference in correction values caused by temperature differences does not necessarily mean that the cold end temperature is 20 ℃, and the correction values are uniformly based on the correction data at 20 ℃, which may result in certain errors. Using direct search for compensation values to eliminate errors introduced by compensation wires.
(2) Cold end compensation method
In order to reduce the error caused by compensating wires, it is attempted to use a constant temperature bath that can adjust the temperature around 20 ℃ instead of a freezing point sensor. The temperature setting deviation of this constant temperature bath is not high, and the uniformity must be ensured to be no greater than 0.10 ℃. Set the temperature of the constant temperature bath to T0=20 ℃, short-circuit one end of the compensation wire into the constant temperature bath, and directly connect the other end to the tested instrument (high-precision digital multimeter instrument). If the tested instrument has zero position calibration function, calibrate the zero position first. After the cold end of the tested instrument remains constant, measure the potential value Δ e at this time and convert it into a temperature value Δ T according to equation (1):
In the formula: - the differential potential value of the thermocouple at 20 ℃.
Adjust the temperature of the constant temperature bath to T1, wait for the bath temperature to remain constant, and then measure the potential value Δ e. Repeat the above steps until Δ T is as close to zero as possible, that is, the temperature of the constant temperature bath is consistent with the cold end temperature of the tested instrument (usually Δ T<0.10 ℃). At this time, use the tested instrument to read the reading td. Use a standard platinum resistor to read the temperature t0 of the constant temperature bath. According to the verification regulations, the tested instrument is tested using Δ e instead of the correction value of the compensating wire at 20 ℃. This can improve the accuracy of the standard device and achieve the verification of high-precision thermocouple temperature measuring instruments.
