Method for Calibrating Thermocouples Using a Dry-Block Calibrator

Method for Calibrating Thermocouples Using a Dry-Block Calibrator

During operation, the hot junction of a thermocouple is subject to oxidation, corrosion, and high-temperature recrystallization of its constituent materials. These factors alter the thermocouple's characteristics, thereby introducing measurement errors. When the measurement error exceeds the permissible tolerance, the thermocouple must either be replaced entirely or repaired by cutting off the degraded section of the hot junction and re-welding it; in the latter case, the repaired thermocouple must undergo cleaning and successful verification before being put back into service.

Typically, thermocouples are calibrated using either a portable dry-block calibrator or a dedicated thermocouple verification system. Within a verification system, thermocouples designed to measure temperatures below 300°C are calibrated using a constant-temperature bath, whereas those designed for temperatures exceeding 300°C require the use of a thermocouple calibration furnace.

Calibration furnaces (dry-block calibrators) require a specific temperature field profile within the chamber; ideally, they should feature a constant-temperature zone of approximately 300 mm in length, and the temperature fluctuation during readings must not exceed 0.2°C per minute. Typically, the temperature at the calibration point is adjusted by varying the voltage via an autotransformer, with precise control maintained through the use of thyristors and automatic temperature control instrumentation. Furthermore, the accuracy class of the associated potentiometer used for measurement must be no lower than Class 0.05.

Procedure for Calibrating Thermocouples Using a Dry-Block Calibrator:

1. During calibration, place the hot junctions of both the thermocouple under test and the standard thermocouple (of Type S) into the calibration furnace to compare their respective measurement data. When verifying Type K or Type E thermocouples, they must be enclosed in quartz sheaths; subsequently, use nickel wire to bind them together with the thermocouple under test before inserting them into the isothermal zone of the calibration furnace. To ensure temperature uniformity between the hot junctions of the standard and test thermocouples, place the hot junctions of both devices into two separate holes within a solid nickel block; then, insert the nickel block into the isothermal zone of the furnace to conduct the verification.

2. Once the thermocouples have been inserted into the isothermal zone of the calibration furnace, the furnace opening must be sealed using asbestos rope. The standard insertion depth for thermocouples is typically 300 mm; for shorter thermocouples, this depth may be reduced appropriately, provided it remains no less than 150 mm. Position the cold junctions of the thermocouples within an ice-point bath to maintain a reference temperature of 0°C. When the furnace temperature reaches a range of ±10°C relative to the target calibration point—and provided the rate of temperature change does not exceed 0.2°C/min—measurements of the thermocouples' thermoelectric potential may be taken.

3. At each calibration temperature point, readings of the thermoelectric potential for both the standard thermocouple and the thermocouple under test (using the portable dry-block calibrator) must be recorded in a specific sequence: Calibration Point 1 → Point 2 → ... → Point n → Point 2 → Point 1. Furthermore, a minimum of four readings must be taken at each point. Subsequently, calculate the average value for each point, consult the relevant thermocouple reference table, and finally, determine the temperature error value by comparing the results.