7Tutorial
286 Keysight 34970A/34972A User’s Guide
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/34972A 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 137.
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.
To Next Page
Loading...
