Agilent Technologies 34970A manual Sources of Error in Thermocouple Measurements

Chapter 8 Tutorial

Measurement Fundamentals

Sources of Error in Thermocouple Measurements

Reference Junction Error A thermocouple is typically formed by welding or soldering two wires together to make the junction. Soldering introduces a third metal into the junction. Provided that both sides of the thermocouple are at the same temperature, the third metal has little effect.

Commercial thermocouples are welded using a capacitive-discharge technique. This technique is used to prevent overheating of the thermocouple wire near the junction and to prevent the diffusion of the welding gas and atmosphere into the thermocouple wire.

A poor weld or bad solder connection can also cause errors in a thermocouple measurement. Open thermocouple junctions can be detected by checking the resistance of the thermocouple. A resistance measurement of more than 5 kΩ typically indicates a defective thermocouple. The 34970A contains a built-in, automatic thermocouple check feature. If you enable this feature, the instrument measures the channel resistance after each thermocouple measurement to ensure a proper connection. For more information on using the thermocouple check feature, see page 107.

Diffusion Error Diffusion in a thermocouple wire is the process of changing the alloy type along the wire itself. Atmospheric particles can actually diffuse into the metal. These changes in the wire alloy introduce small voltage changes in the measurement. Diffusion is caused by exposure to high temperatures along the wire or by physical stress to the wire such as stretching or vibration.

Temperature errors due to diffusion are hard to detect since the thermocouple will still respond to temperature changes and give nearly correct results. The diffusion effects are usually detected as a drift in the temperature measurements.

Replacing a thermocouple which exhibits a diffusion error may not correct the problem. The extension wire and connections are all subject to diffusion. Examine the entire measurement path for signs of temperature extremes or physical stress. If possible, keep the temperature gradient across the extension wire to a minimum.

352