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 k W typically indicates a defective thermocouple. By enabling this feature, the instrument measures the channel resistance after each thermocouple measurement to ensure a proper connection.
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 stretch 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 that exhibits a diffusion error may not always correct the problem.