Ac Noise Effects; Thermal Emf - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device
NI 435x User Manual
that channel rises 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. The value of the current is approximately 125 nA when
ground-referencing also is on. If the thermocouple is very long, the 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 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 the measurement, you can reduce the error by reducing the
thermocouple resistance or by lowering the length of the thermocouple or
gauge 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

The NI 435x rejects AC voltages as specified in normal mode rejection
(NMR) in Appendix A, Specifications. However, if the amplitudes of the
AC voltages are large compared to the DC voltages, or if the peak value
(AC plus DC) of the measured voltage is outside the input range, the
NI 435x may exhibit additional errors. To minimize these errors, keep the
thermocouples, the NI 435x, and its accessories 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 introduce error other than
those at the hot junction (where the thermocouple measures the test point
temperature) and at the cold junction on the accessory.
To minimize thermal EMFs, use wires made of the same material as the
thermocouple when extending the length of the thermocouple. Also,
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