Three Common Thermocouple Calibration Methods
During the use of thermocouples, the hot junction gets oxidized and corroded, the material recrystallizes at high temperatures, altering the thermoelectric properties, and subsequently, measurement errors become increasingly significant. To ensure a certain level of temperature measurement accuracy, thermocouples are regularly calibrated to measure changes in thermoelectric potential.
Thermocouple calibration typically involves three methods: the direct comparison method, the unipolar method, and the differential method.
The direct comparison method, also known as the bipolar method, involves directly comparing standard thermocouples and thermocouple calibration thermocouples through a temperature thermocouple calibration furnace. This novel method is simple, easy to operate, and allows for calibrating different types of thermocouples within the same furnace; the measurement end of the thermocouple is not attached to the same isothermal surface within the furnace; the number of measurements is minimal, typically two to four times. A drawback is that the free end of the thermocouple requires a 0° reading; otherwise, calibration is necessary. When the standard model differs from the thermocouple to be calibrated, due to a larger potential difference at the same temperature, special attention should be paid to the galvanometer during operation to avoid damage. Since the furnace temperature directly affects the thermocouple calibration results, strict control of the furnace temperature is required, usually not exceeding 1°C.
Single-point method is a thermocouple calibration technique that involves placing a standard thermocouple and another thermocouple, known as the thermocouple, in an oven after they are bonded. It measures the differential electromotive force between the standard and the identical thermocouple. The advantages are quick readings and simple calculations; during calibration, the oven temperature is not strictly controlled, allowing ±10℃ fluctuations. Free forging only needs to be done at room temperature. The drawbacks include that the standard and the thermocouple to be calibrated must be of the same type, and the measurement end must be securely connected, otherwise errors are likely; due to the small measured potential, high quality requirements are placed on the electrical measuring instruments, switching devices, and connection wires; the wiring is relatively complex and requires a polarity switching switch.
The millisecond method involves reversing the standard thermocouple and the thermocouple to be calibrated in series and placing them into a temperature calibration furnace to directly measure the electromotive force difference within this time period. The advantages are quick readings, direct reading of the difference, simple calculation, easy wiring, and readings are half that of the unipolar method. The free end of the thermocouple does not need to be restricted to 0℃. During calibration, the temperature control within the furnace is not strictly required, allowing ±5℃ fluctuations. The disadvantages include that the standard and the thermocouple to be calibrated are of the same type; when measuring the differential electromotive force, the positive terminal of the thermocouple to be verified is connected to the positive terminal of the potentiometer; as the measured values are also small electromotive forces, there is a high requirement for the quality of electrical measuring instruments, switching gears, and connecting lines; and the galvanometer must not be damaged during operation due to the sudden change in large electromotive force measurements when the temperature of the calibration furnace changes to measure micro-differential electromotive force. It is recommended to use thermocouples for calibration standards above 660℃. Standard platinum rhodium 10 platinum thermocouples can meet the requirements. It should be noted that the thermocouple protective tube should be carefully selected during use, as its outer diameter is less than 1mm. Both quartz tubes and alumina can meet the requirements for size and temperature resistance.
The length of the thermometer sensors used to measure the axial temperature field in the dry kiln should be less than 5mm. For thermocouple thermometers, the measuring end is approximately a point, which meets the requirement. For resistance thermometer sensors, the wound type is generally larger than this size. Therefore, film resistance thermometers are commonly used for temperature measurement, with the thermometer not exceeding 250°C. Resistance thermometers used to measure the temperature difference between holes, temperature fluctuations, and load characteristics do not require calibration, as long as the performance remains relatively stable, provided their outer diameter meets the aforementioned requirements.
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