Detection and acceptance methods for thermocouples based on heat seeking

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How should a user conduct acceptance testing after purchasing a search-type thermocouple? The calibration standard for thermocouples that rely on heat detection is JJF1631-2017. Changhui Instruments introduces the testing and inspection methods for such thermocouples in accordance with this standard, to serve as a reference for those who are evaluating thermocouple products. 1. Surface quality of search-type thermocouples: Inspected by visual inspection and tactile feeling. 2. Length deviation of thermocouples with heat-seeking function: Check using a steel tape measure with a resolution of no more than ±5 mm or other measuring instruments with corresponding accuracy. 3. Outer diameter deviation of thermocouples with heat-seeking function: Measurement shall be carried out using a micrometer or other measuring instrument with a resolution of not more than 0.01 mm, and the inspection should be conducted in two directions perpendicular to each other at the same cross-section. 4. Continuity of the heating-type thermocouple wire: Test using a digital multimeter with a resolution of no more than ±1Ω. Thermocouple testing using the heat-seeking method: yunrun.com.cn/tech/2914.html 5. Thermoelectric potential performance and tolerances ① The thermoelectric potential performance and tolerances of heat-seeking thermocouples are determined using a comparative method; the test temperature should be 80℃ ； 200℃ ； Carried out at 300°C (above 300°C is subject to the customer’s requirements; no factory inspection is required). ②Testing instruments and equipment: a. The temperature field uniformity of the constant-temperature equipment should be less than 2°C; it can be a constant-temperature chamber or a tubular resistance furnace. b. The input impedance of electrical measuring instruments should be greater than 10 MΩ, with a maximum allowable error of no more than 0.05% ； The resolution should be less than 2 uV. c. The temperature standard can be a class 2 standard platinum-rhodium 10-platinum thermocouple or a class 2 standard platinum resistance thermometer. d. The reference terminal thermostat can be an ice point bath or a semiconductor zero-temperature thermostat. e. For the detection of the thermoelectric potential accuracy of search-type thermocouples, a constant temperature chamber is used; this chamber should have two outlets so that a certain length (<300 mm) of the thermocouple cable can protrude from each end. The method for setting up the test setup is shown in the following image: http://yunrun.com.cn/upload/202002/13/202002130213378676.png. In addition to the setup and wiring required for testing search-type thermocouples, a cold junction device and compensation wires are also needed. The actual testing procedures and data processing are carried out in accordance with the JJF1631-2017 standards. At this point, after the cables of the same batch are manufactured into finished products, an error element in the form of a compensation wire is added; therefore, the accuracy of the thermocouple designed for heat detection and that of the cable associated with it cannot be exactly the same. The error may increase or decrease. ③The uniformity test is conducted at 300°C. The calibration conditions and instrument requirements are the same as those for thermopotential accuracy calibration. Uniformity calibration must be carried out in a tubular resistance furnace, with the furnace temperature controlled at 300°C ± 1°C; the temperature stability should be 1°C per minute, and this condition must be maintained for 10 minutes. The other operations are the same as those for thermoelectric potential accuracy calibration. For whole cables or rolls, it is recommended to select three testing points at intervals of one meter each; for shorter cables, the interval distance can be reduced accordingly. The procedure involves dragging the cable in the direction of movement along with the compensation wire, and after testing the data at the three points, the uniformity of the thermoelectric potential is determined by subtracting the minimum value from the maximum value. http://yunrun.com.cn/upload/202002/12/202002121206477011.png 6. Maintain the integrity of the protective tube: Strip 15 mm of thermocouple wire (the core wire) from each end of the cable, place it in an oven and dry it at 80°C for more than 8 hours. Use an insulation resistance meter to measure the insulation resistance between the thermocouple wires as well as between the thermocouple wires and the metal protective tube. After sealing with insulating sealant and drying again, immerse the entire cable in room-temperature water for 24 hours, ensuring that no more than 300 mm of the cable’s ends are exposed. Again, use an insulation resistance meter or multimeter to measure the insulation resistance between the thermocouple wires, as well as between the thermocouple wires and the metal protective tube. 7. Minimum bending radius: After winding the thermocouple cable or thermocouple around a cylinder with a diameter 20 times that of its outer diameter for 5 turns, observe the surface of the cable using a 5x magnifying glass. Check the resistance value of the thermocouple wire using a multimeter, and determine the insulation resistance between the thermocouple wires as well as between the thermocouple wires and the protective tube using an insulation resistance meter. 8. Thermocouple calibration table for heat search 9. **Standard: JJF1631-2017 \"Specifications for the Calibration of Continuous Thermocouples\"