Measurement Computing CIO-EXP-GP User Manual
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12-bit A/D improves from 2.4 millivolts per bit to 0.24 millivolts per bit. If you needed to measure a
change of 1 millivolt, you would need an amplification of 10.
In order to match your signals with the input range of the A/D board, you should do a similar calculation
and set switches on the CIO-EXP-GP for the required gain. Remember to make sure that the settings in
InstaCal match the switches on the DAS and CIO-EXP-GP boards.
When using RTD’s, the expected output from the sensor should be calculated and the gain of the
CIO-EXP-GP set accordingly.
To select the best gain for RTD type, base resistance and temperature range, consider that RTD
resistance changes with temperature, but the magnitude of the change also changes with temperature.
RTD type determines the ‘slope’ of the ohms vs. temperature curve. The most popular type has an
‘alpha’ of .00385, known as the European standard. Its value is .00385 ohms per ohm per °C.
The Universal Library and InstaCal support six different RTD types. Please call if you do not see the
RTD you are interested in listed here.
Material
‘alpha’
Platinum
0.00392 American standard
Platinum
0.00391
Platinum
0.00385 European standard (Most popular, OMEGA’s standard also)
Copper
0.00427
Nickel/Iron 0.00581
Nickel/Iron 0.00527
To determine which gain to use, you must know the maximum temperature the RTD will be used to
measure, and thus the maximum resistance value of the RTD. Here is a table for platinum:
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-GP (the voltage your DAS board will convert into a
number) is:
V
OUT
= I
EXC
* R
RTD
* GAIN
Normally, the CIO-EXP-GP supplies 1 mA of excitation current. The choices for standard gains are 1,
10, 25 and 100. (Higher gains are possible but are not generally practical for RTD applications.)
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
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