Measurement Computing CIO-EXP-RTD16 User Manual
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For 100 ohm RTD, alpha = .00385:
Temp (°C)
Resistance (ohms)
-200
18.49
-100
60.25
0
100.00
100
138.50
200
175.84
300
212.02
400
247.04
At a temperature of 400°C, the maximum resistance is 247.04 ohms
The equation for the voltage out of the CIO-EXP-RTD16 (the voltage your DAS board will convert into a
number) is:
V
OUT
= I
EXC
* R
RTD
* GAIN
Normally, the CIO-EXP-RTD16 supplies 1 mA of excitation current. The choices for standard gains are
1, 2, 4, 7, 8, 14, 28, or 56.
Thus, if you want to measure temperature in the range of -200 to 400°C with the RTD listed above, the
maximum voltage output would be:
V = 0.001 * 247.04 = 0.24704
With a gain of 14, the DAS board will read 3.459 volts. If the gain is 28, the output is 6.917 volts. The
DAS board would have to be set to the 0 to 10 volt range.
If you are limiting your range of interest to -200 to 100°C, a common range, the calculations are:
V = 0.001 * 138.50 = 0.1385. Gain of 28 = 3.878V. Gain of 56 = 7.756V. In this case, a gain of 28 and
a range of 0 to 5 volts would be best (0.517 to 3.878 volts would be the temperature range
−
200 to 100).
A 12-bit A/D converter would be using 67% of its range of 4096 counts, or a total of 2752 counts, or
divisions of 300°C. The converter would be able to resolve to 0.109°C. That is more than enough
converter resolution even though you are not using the full range of the DAS board in this example.
If your DAS board has 16 bits of resolution, the DAS board would resolve to 0.006°C. This is far in
excess of the accuracy of the RTD.
The stages of gain you choose are not only dependent on the RTD you choose, but on the range of
temperature you are measuring. Use the equation above to fine tune the CIO-EXP-RTD16 circuit to your
advantage, then be sure to update the InstaCal program so the Universal Library linearization routines
will operate properly.
5.4 SETTING THE GAIN
Set the first and second stage gains according to the base resistance of the RTD, the RTD type and the
temperature range you intend to measure. The base resistance is the resistance of the RTD at zero
degrees Centigrade. The following table is a “quick reference” guide of recommended gains.
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