RTD temperature measurement with either NI 9216 or NI 9226?

[Cowboy12]

Hi all,

We will be using 4-wire RTD probe for temperature measurement. The RTD can be either 100 ohm or 1k ohm nominal resistance. For this we can either use NI 9216 (for 100 ohm RTD) or NI 9226 (for 1k ohm RTD). I have did some quick analysis so far:

	NI 9216 (100 ohm)	NI 9226 (1k ohm)
Excitation current	1 mA	0.1 mA
Measurement accuracy	0.15 °C	0.15 °C
Self-heating power generated in the RTD (I^2 R)	100 mW	10 mW

We are more inclined to use NI 9226 (with 1k ohm RTD). As you might notice in the above table, this arrangement has less RTD self-heating power (and hence less error).

Our only concern with this RTD 1k ohm is that some people are claiming that it might be more prone to receive external noise compared the 100 RTD (in an environment that may have High power motors, variable frequency drive…etc.). Which we don’t understand why because we don’t have strong electrical engineering background.

Is this claim correct? Is it because 1mA gives higher S/N ratio than 0.1 mA? What if we use NI 9226 but with high resolution mode, will that overcome the external noise. (As I can see in the data sheet, the High Resolution mode has the built-in filtering provides at least 85db suppression of power line frequency and harmonics).

[jordans · Accepted solution]

Hello Cowboy12,

The electromagnetic interference will be dependent on the voltage of the signal rather than the current. Any interference from the environment will be voltage, so the signal to noise ratio will be a voltage to voltage ratio.

From what you have said, I think either of these devices will work well for your application. The main consideration would be the expected interference from the environment, power lines, large motors, etc. The following link explains some of the main considerations when considering electromagnetic interference in your system. 

Field Wiring and Noise Considerations for Analog Signals
http://www.ni.com/white-paper/3344/en/

Good luck on your project!

[Cowboy12]

Many thanks Jordans for sharing this. It was helpful.