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 tem-
perature 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 min-
imum.
Shunt Impedance
The insulation used for thermocouple wire and extension wire can be degraded by high temperatures or cor-
rosive atmospheres. These breakdowns appear as a resistance in parallel with the thermocouple junction.
This is especially apparent in systems using a small gauge wire where the series resistance of the wire is
high.
Shielding
Shielding reduces the effect of common mode noise on a thermocouple measurement. Common mode
noise is generated by sources such as power lines and electrical motors. The noise is coupled to the unshiel-
ded thermocouple wires through distributed capacitance. As the induced current flows to ground through
the internal DMM, voltage errors are generated along the distributed resistance of the thermocouple wire.
Adding a shield to the thermocouple wire will shunt the common mode noise to earth ground and preserve
the measurement.
4Measurement Tutorials
192 Keysight DAQ970A User's Guide
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