Hi Claudia,
The specifications for the 9217 provide the absolute accuracy for measuring Pt 100 RTDs (both 3- and 4-wire) on page 14, so there is no need to calculate it [1].
Since the 9219 is more flexible in the kinds of measurements it can make, absolute accuracy for every use case isn't provided. Instead, NI gives gain and offset error values to use for calculating the absolute accuracy [2].
Please follow this logic to calculate the absolute accuracy:
(Absolute accuracy) = (Input Reading) * (Gain Error + Temperature Drift) + (Offset Error + Temperature Drift)
Reading the "4-Wire and 3-Wire RTD, Pt 100" line in the table on page 24, the gain and offset error for a module between 20 and 30 degrees Celsius is ±0.1% of input reading for gain and ±2400ppm of range for offset. The temperature drift comes into play only when the module is outside of the normal operation temperature, but for the Pt 100, the gain drift is ±15ppm and the offset drift is ±60ppm (see page 25).
So for a module at room temperature, 23 degrees Celsius, and a Pt100 at 100 degrees Celsius using the 505 Ohm input range, the calculation goes:
At 100 C, the RTD would measure 138.5 Ohms if its temperature coefficient was 0.00385 Ohm/Ohm/°C, so
Absolute Accuracy = (138.5 Ohm * 0.1%) + (2400ppm * 505 Ohm)
= 0.1385 Ohm + 1.212 Ohm
= 1.3505 Ohm
divide by 0.385 Ohm/°C, and
Absolute Accuracy = ±3.5 degrees Celsius
[1] NI 9217 Operating Instructions and Specifications
http://www.ni.com/pdf/manuals/374187c.pdf
[2] NI 9219 Specifications
http://www.ni.com/pdf/manuals/374473b.pdf
I hope I can help you with this information.
Best regards
Suse
______________________________
Certified LabVIEW Developer (CLD)
