Chapter 5 Using the SCXI-1125
5-2
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thermocouple junction voltage and temperature, this voltage conversion
(linearization) is best done through software.
NI-DAQ has built-in scaling for most thermocouple types. In NI-DAQmx,
you can create a thermocouple task or global channel. In Traditional
NI-DAQ (Legacy), you can create a thermocouple virtual channel.
If you choose to not let the driver scale the voltage readings for you in
software, you must do several conversions by using conversion coefficients
that reflect the voltage-temperature relationship for the type of
thermocouple and CJC being used. Complete the following steps to
accurately determine thermocouple temperature:
1.
Read the voltage from the CJC sensor and convert this voltage to a
temperature.
2.
Convert this temperature to the corresponding voltage for the
thermocouple type in use.
3.
Read the input voltage from the thermocouple.
4.
Add the two voltages.
5.
Translate the resultant voltage into the thermocouple temperature
reading.
You have completed the steps to get the true temperature reading from the
thermocouple junction.
National Instruments software ADEs have useful conversion functions for
CJC. In LabVIEW, virtual channels with the CJC channel invoked or the
Convert Thermocouple Reading VI are used. In C, use the NI-DAQ
function,
Thermocouple_Convert
. In C, you might also need to use the
function
Thermistor_Convert
, if your terminal block uses a thermistor
to perform CJC. For more information about CJC, refer to your software
ADE user documentation.
To calculate the temperature accuracy of your SCXI-1125, you must
consider several factors. First, the type of sensor and the temperature range
you expect directly affects which gain your SCXI-1125 module uses for
voltage readings, thereby directly affecting the resolution with which you
can read temperature. After determining the range necessary for your
application, you can apply the measurement accuracy specifications of the
SCXI-1125; such as offset error, gain error, and noise to determine how
these will affect your temperature measurement. Next, you must consider
the accuracy of your cold-junction sensor and incorporate this into the total
temperature error of your reading. Finally, the accuracy of the DAQ device
you use must be factored in to determine your overall system error.
