Programmable Open-Thermocouple Detection - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
NI 4350 User Manual

Programmable Open-Thermocouple Detection

To detect open or broken thermocouples, switch on open-thermocouple
detection on that channel. Then, if the thermocouple breaks, the voltage
on that channel will rise rapidly above 100 mV, at which point you can
conclude that the thermocouple is open.
Notice that when open-thermocouple detection is on and the floating
thermocouple is not broken, a very small amount of current is
injected into the thermocouple. It is approximately 125 nA when
ground-referencing is also on. If the thermocouple is very long, this
injected current can cause an error voltage to develop in the lead
resistance of the thermocouple that is indistinguishable from the
thermocouple voltage you are measuring. You can estimate this error
voltage with the following formula:
error voltage = resistance of the thermocouple • 125 nA
For example, if you use a 100 ft long, 24 AWG J-type thermocouple
with a resistance of 0.878 Ω per double foot, the error voltage generated
is approximately 11 µV, which corresponds to about 0.2° C. If this error
is too large for your measurement, you can reduce the error by reducing
the thermocouple resistance. Do this by reducing the length of the
thermocouple or lowering the AWG of the wire (use a wire of larger
diameter). Alternatively, you can switch off the open-thermocouple
detection to eliminate the current injected into the thermocouple.
AC Noise Effects
Your NI 4350 instrument rejects AC voltages as specified in NMR in
Appendix A, Specifications.
voltages are large compared to the DC voltages, or if the peak value
(AC + DC) of the measured voltage is outside the input range, the
NI 4350 instrument may exhibit additional errors. To minimize these
errors, keep the thermocouples and the NI 4350 instrument and its
accessory away from strong AC magnetic sources and minimize the
area of the loop formed by the thermocouple wires connected to the
accessory. Choose the notch filter frequency of 10 Hz for the best AC
noise rejection. If the peak value of the measured voltage is likely to
exceed the selected input range, select the next higher input range.
Thermal EMF
When using thermocouples, any thermal EMFs other than those at the
hot-junction (where the thermocouple measures the test point
However, if the amplitudes of the AC
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