Calibration and Uncertainty Evaluation of Short Thermocouples

Short type thermocouples (hereinafter referred to as thermocouples) have a short and compact shape It has the characteristics of being compact, occupying small space, and being able to coordinate with temperature measurement and control devices in terms of design, making it widely used. However, there is no existing calibration protocol for low-cost metal short thermocouples with lengths ranging from 200mm to 350mm. Therefore, we have developed a calibration method based on the actual use of thermocouples.
1, Selection of effective working area for verifying furnace temperature field
When calibrating thermocouples, the selection of the calibration furnace is a crucial step. We use a horizontal thermocouple calibration furnace with a total length of 300mm The inner diameter of the furnace tube in the furnace is 40mm, with a short-term maximum temperature of 1100 ℃. The maximum temperature occurs at the geometric center of the furnace axis, with a deviation of no more than 10mm from the geometric center. The uniform temperature field is within a length of no less than 30mm and a radius of 15mm, with a temperature difference of no more than 1 ℃ between any two points. According to thermocouple calibration, in order to achieve furnace loading accuracy, in the uniform temperature field of the verification furnace, the radius of the furnace tube cavity is taken as 12mm, and the deviation from the highest temperature center along the axis is not more than 5mm. This is the effective working area of the temperature field, and the temperature difference in this area is 1 × 5/30=0.17 ℃
2, Correction condition items and methods
During calibration, the standard instrument adopts a second-class standard platinum rhodium 10 platinum thermocouple (hereinafter referred to as the standard thermocouple). The electrical measurement system adopts a 6-digit digital multimeter, and its allowable basic error calculation formula is+(50 × 10-6 × measured value+35 × 10-6x range), with a minimum resolution of 0.uV and a range of 100mV. The parasitic potential of the multi-point conversion switch shall not exceed 0.41V. Check The parameters of the fixed furnace are determined based on the effective working area of the selected temperature field. In the middle of the school, the thermocouple must be tightly connected to its matching extension type compensation wire, with a total length of not less than 600mm. Its working state should be simulated within the temperature range of 300 ℃~1300 ℃, and a standard thermocouple should be used for overall calibration by comparison method.
Other calibration conditions, items, and methods shall refer to the corresponding provisions of the Verification Regulation JJG351-96 (hereinafter referred to as the Regulation).
3, Measurement uncertainty analysis
The measurement uncertainty of the low-cost metal short thermocouple calibration device at 1000 ℃ is listed in Table 1. Due to the fact that standard thermocouples are calibrated at three fixed points of zinc, aluminum, and copper, but when used, they are calibrated on a whole degree using low-cost metal thermocouples, the uncertainty on the whole degree is composed of two parts: the uncertainty on the calibration of standard thermocouples at the three fixed points of zinc, aluminum, and copper, and the uncertainty caused by the calculation formula on the whole degree.