Li-Cor Thermistor Measurement

[Shazlan]

Dear all,

I'm trying to measure temperature using a 3-wire thermistor from Li-Cor. The operation sheet is as attached. On the DAQ side, I am using an NI 9263 to give the 5V excitation and NI 9215 to measure the voltage output. A 'crude' wiring diagram is as attached. As a result, I didn't get the reading that make sense. Here are some questions I have:

1. First of all, are the wirings correct to begin with? In one of the diagram from NI's 'How-To' guide, it says that for voltage excitation, I need to connect a resistor (I assume a low-ohm resistor) between the +ve excitation and the black wire. Is it suffice if I were to use a long wire to act as a low-ohm resistor (around 2-10 Ohm)?

2. Do I need to connect the 'com' port on the AO module to the AI module? If yes, which port (+AI, -AI or com) should I connect to?

3. The operation sheet says that the conversion uses this equation:

Y = a0 + (a1)X + (a2)X^2 + (a3)X^3 + (a4)X^4 + (a5)X^5

However, in all documentations I found on the web, the equation is very different... I think it goes something like 1/T = A + B(lnR) + C(lnR)^3. LabVIEW's voltage to thermistor subVI also uses this equation. The question now is, how do I relate the two equations?

Please advice. Thank you.

Shazlan

[Wayne.C]

Shazlan,

First problem you have is that the the +5V supply is not referenced to the 9215.  Remember that the 9215 has built in isolation.  That means if you want to supply +5V from the 9263, you will need to connect the COM signal between the two modules.  This is not an ideal situation and you will need to carefully test to make sure you are not introducing noise or creating a ground loop.

Message Edited by Wayne.C on 03-16-2009 02:46 PM

[Shazlan]

Hi Wayne,

1. I've connected the COM for two modules but still does not get the value that I need. What I got instead is something like a sawtooth signal. I'll post a screenshot of it in a short while.

2. Now, assuming that I manage to get the voltage to be correct, how do I convert the voltage into meaningful data? The conversion equation that I found online is different with the equation that is provided by the manufacturer, as described in my previous post. Having different equation means that the constant provided are also different.

Please advice.

[lhaoxian]

Hi Shazlan,

1) Are you connecting 1 thermistor only? Could you let us take a look at the screenshot? Keeping the setup connected, if you use a DMM to tap across the AI0+ and AI0-, what voltage reading do you get?

2) The equation used in the LabVIEW VI "Convert thermistor reading.vi" relates the temperature to the thermistor resistance, whereas the equation supplied by the manufacturer relates the temperature to the raw voltage. Since they are approximating the temperature based on different variables, I think we should not compare them directly against one another; for your situation I believe it will be a better suggestion to use the equation from your manufacturer as it is based specifically for this class of thermistors.

Best regards,

Victor

NI ASEAN

Message Edited by lhaoxian on 03-26-2009 06:03 AM