Chapter 3
Signal Connections
3-10
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National Instruments Corporation
input range with a gain of 500 gives the best resolution. Set the jumpers on
the MIO board for differential input, ±10 V input range.
Note
Set jumper W1 on the AMUX-64T to select the temperature sensor, and connect
the thermocouple to CH1 and CH33. Connect a resistor between CH33 and GND
for the bias current return path.
Set the jumpers on the MIO board for single-ended input, ±10 V input range. Set
jumper W1 on the AMUX-64T to select the temperature sensor, and connect the
thermocouple to CH1.
Two software compensation procedures for this example follow.
Procedure 1 is more accurate, but procedure 2 is faster and requires less
computation.
Procedure 1
1.
Read the voltage from the temperature sensor (channel 0). If you are
using NI-DAQ, you can use the
AI_Read
and
AI_Scale
functions to
do the reading. This voltage is 10 mV/°C, so the gain should be either
1 or 10 (10 for the best resolution). Multiply the voltage by 100 to get
the AMUX-64T temperature in degrees Celsius. For example, if the
reading is 0.25 V, the AMUX-64T is at 25 °C.
2.
Translate the reading into the voltage for a J-type thermocouple at that
temperature using either a look-up table or an NIST polynomial.
Notice that the polynomials required here are the inverses of those
given in Table 3-3. For your reading of 25 °C, you would have
1.277 mV.
3.
Read the voltages on any thermocouple channels. If you are using
NI-DAQ, you can use the
AI_Read
and
AI_Scale
functions to read
each channel. For the example given, assume that you get a reading of
9.39 mV on channel 1.
4.
Add the voltage from step 2 to the voltage measured in step 3. You then
have:
1.277 + 9.39= 10.667 mV
5.
Translate the result into a temperature using either a look-up table or a
polynomial such as one from Table 3-3. For example, assume that the
reading from the J-type thermocouple is 10.667 mV. By applying the
third formula in the Linearizing the Data section and using the
coefficients from Table 3-3, you can calculate that the temperature
is 198 °C.
