Keysight DAQ970A Guia Del Usuario página 228

4 Measurement Tutorials
T/C Type Post (+) Lead
T Copper
U.S. Blue
British White
DIN Red
Japanese Red
French Yellow
Constantan = Copper-Nickel; Nicrosil = Nickel-Chromium-Silicon; Nisil = Nickel-Silicone-Magnesium;
N/A = Not Available
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 DAQ970A/DAQ973A 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.
Diffussion 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.
Shunt Impedance
The insulation used for thermocouple wire and extension wire can be degraded by high temperatures or corrosive
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.
228
Neg (-) Lead Temperature Range
Constantan -200 °C - 400 °C
Red
Blue
Brown
White
Blue
Probe Accuracy
±0.5 °C - 1 °C
Keysight DAQ970A/DAQ973A User's Guide
Comments
Withstands moisture.
Has a copper lead.
Low temperature
applications.