9
temperature-to-voltage routines for this conversion. You can also use standard look-up tables or polynomials
listed in National Institute of Standards and Technology (NIST) NBS Monograph 175.
Finally, add the voltage, V
TC
(T
cjc
), to your measured thermocouple voltage, V
MEAS
, to yield the compensated
thermocouple voltage, V
TC
(T
TC
).
Nonlinear Data
After determining the compensated thermocouple voltage, you must convert this voltage into temperature
units. The voltage-temperature relationship of thermocouples is very nonlinear. To convert the nonlinear
voltage readings into temperature values, you can use either look-up tables or the following polynomial
equation:
T = a
0
+ a
1
x + a
2
x
2
... + a
n
x
n
where T is the temperature in degrees Celsius, x is the thermocouple voltage in volts, and a
0
through a
n
are
coefficients that are specific to each thermocouple type. National Instruments software provides thermocouple
linearization functions that use this polynomial equation.
Low-Voltage Signals
Thermocouples generate low-voltage signals, typically in the millivolt range. For example, a J-type
thermocouple outputs –8.1 mV at –210˚ C and 21.8 mV at 400˚ C. Therefore, you must amplify the signal to
accurately read and digitize it. The SCXI-1102 has programmable gain amplifiers that can be programmed for
a gain of 1 or 100. This amplification yields an input voltage range of
±
10 V or
±
100 mV. This gain can be
combined with gain on the DAQ board or module for higher amplification. For example, an AT-MIO-16E-2
configured for a gain of 5 and an SCXI-1102 configured for a gain of 100 yields an input range of
±
20 mV.
Noisy Signals
•
Low-voltage signals are susceptible to noise corruption. Thermocouple wire acts as an antenna and picks
up stray electromagnetic signals in the environment. The most common sources of stray electromagnetic
waves are power lines, electric motors, and computer monitors. Poor grounding of your system also
produces noise. Use the following methods to diminish the effects of noise on your thermocouple.
•
Use a shielded cable from the SCXI chassis to the plug-in DAQ board and apply extra shielding to your
thermocouple wire.
•
Use lowpass noise filters to attenuate high-frequency noise. The SCXI-1102 includes 1 Hz lowpass filters
on every channel to maximize rejection of 50 Hz and 60 Hz noise.
•
Use the SCXI-1102 programmable gain instrumentation amplifiers (PGIA) to amplify the signal and
increase the noise immunity before the signal leaves the SCXI module.
•
Make sure you have only one point of ground for your thermocouple circuit.
•
In extreme cases, you may find it helpful to average your thermocouple voltage readings to improve noise
rejection. For example, acquire 100 samples from a single thermocouple and use the average value of all
the samples as one data point.
Summary of Contents for SCXI SCXI-1102
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