Ac Noise Effects; Thermal Emf; Measuring Temperature With Rtds And Thermistors And Measuring Resistance - National Instruments NI 435 Series User Manual

Measuring Temperature with RTDs and Thermistors
and Measuring Resistance
© National Instruments Corporation

AC Noise Effects

The NI 435x rejects AC voltages as specified 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 signal source, the
NI 435x, and its accessories away from strong AC magnetic sources, and
minimize the area of the loop formed by the wires that connect the signal
source with the accessories. Choosing the notch filter frequency of 10 Hz
provides 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

Thermoelectric potentials or thermal EMFs are voltages generated at the
junctions of dissimilar metals and are functions of temperature. Thermal
EMFs in the source generating the signal can introduce errors in
measurements that change with variations in temperature.
To minimize thermal EMFs, use copper wires to connect the signal to the
NI 435x accessory. Avoid using dissimilar metal wires in connections.
Also, minimize temperature gradients in the space enclosing the signal
source, the NI 435x, and its accessories.
RTDs and thermistors are essentially resistors whose resistance varies with
temperature. Therefore, measurement techniques for RTDs, thermistors,
and resistors are quite similar. All techniques involve exciting the resistor
with a current or a voltage source and measuring the resulting voltage or
current, respectively, developed in the resistor.
With the NI 435x, you can excite the resistor with the built-in precision
current source and measure the resulting voltage. When using virtual
channels with LabVIEW, set the measurement mode to Resistance or
RTD. When using VirtualBench-Logger, set the measurement mode to
Resistance, RTD, or Thermistor. When using the NI 435x instrument
driver, set the measurement mode to Resistance. These modes return the
measurements in units of resistance (ohms) by dividing the measured
Chapter 2
2-13
Operating the NI 435x Device
NI 435x User Manual