Thermocouple temperature measurement
When two conductors of different materials, A and B, are welded together, a thermoelectric potential is generated at the cold junction (also known as the reference junction) whenever there is a temperature difference between the junction (the hot junction) and the other end of the conductors.
2025-06-05
Temperature measurement principle
When two conductors made of different materials, A and B, are welded together, a thermoelectric potential is generated at the cold end (also called the reference junction) if the temperature at the hot junction differs from that at the cold end. A thermocouple is a temperature sensor that exploits this phenomenon to convert temperature into an electrical potential. If the cold-end temperature of the thermocouple remains constant—say, at 0°C—the output thermoelectric potential corresponds one-to-one with the hot-junction temperature. A Temperature Transmitter measures the voltage difference at the thermocouple’s output and then converts this voltage into a temperature reading, thereby enabling temperature measurement.
Cold-Junction Compensation
The thermoelectric potential output by a thermocouple depends on the temperature difference between its hot junction and cold junction. In practical measurements, however, the temperature of the thermocouple’s cold junction often fluctuates. If this variation is not compensated, even when the hot junction remains at a constant temperature, changes in the cold-junction temperature will still alter the thermoelectric potential, preventing it from accurately reflecting the hot-junction temperature and thereby introducing measurement errors.
The principle of cold-junction compensation is as follows: When the hot junction of a thermocouple is at temperature T1 (with the cold junction at temperature T2), its thermoelectric voltage is measured as V1. Simultaneously, a temperature sensor (such as a Pt100) is used to measure the cold-junction temperature T2, and the corresponding thermoelectric voltage V2 at T2 is calculated (assuming the cold junction is at 0°C). The sum V1 + V2 represents the thermoelectric voltage that would be generated by the thermocouple if the cold junction were at 0°C and the hot junction were at temperature T1.
Method for Measuring the Accuracy of Thermocouple Input Transmitters
Due to the issue of cold-junction compensation, incorrect measurements are sometimes mistakenly attributed to poor accuracy of the Temperature Transmitter. There are two methods for accurately measuring thermocouple transmitters: one is the precise measurement method, and the other is the practical measurement method.
Precision Measurement Method
1. The compensation lead shall be of the type corresponding to the thermocouple being measured.
2. Use an mV signal generator to simulate a thermocouple signal.
3. Maintain the container in an ice–water mixture to ensure the cold end temperature remains at 0°C.
Based on the corresponding thermoelectric potential values from the thermocouple calibration table, a millivolt signal generator is used to simulate the thermocouple output signal. The resulting 4–20 mA current signal is then measured to calculate the transmitter’s measurement accuracy. This method can eliminate measurement errors caused by variations in cold-junction temperature; however, it cannot compensate for errors introduced by the compensation leads themselves. It is typically employed in laboratory‑based measurements.
Practical Measurement Method
A thermometer is placed at the input to measure the cold-junction temperature. The corresponding thermoelectric potential, ECJ, is then determined from the thermocouple calibration table. Using the thermocouple’s reference‑junction potential, EO, a mV signal generator simulates the output (EO − ECJ), thereby replicating the thermocouple’s output. The resulting 4–20 mA current signal is measured to evaluate the transmitter’s measurement accuracy. However, this method is subject to two sources of error—ambient temperature variations and inaccuracies in the thermometer’s cold‑junction temperature reading—which cannot be eliminated and thus introduce measurement errors.
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