measure temprature from thermistor using 6225

[liucga]

Hello,

I am using LabVIEW 8.6/DAQmx with a M-series 6225 DAQ board,  connected to a CB_68LPR terminal board.  I would like to mesure the temperature from a thermistor.

First I connect the two leads of the themistor to AI6 (J25) and AI GND (J24) since I have assigned AI0 to Ai5 to six voltage signals.

When I tried the temperature measurement example, if I configured the channel ai6 as "Iex Thermistor 2-Wire" and choose internal excitation source, I got the error message "Measurements: Device to which the sensor is attached does not have an avaible internal excitation source". But the requirement of that example says that PXI-6225 can run the example. So here comes my questions : Is my thermister connection to the terminal board wrong? Which device should provide the excitation source, the DAQ board or ther terminal board, if I really want to choose internal source?   

Then I tried to use DAQ Assistant to configure the channel ai6 as "Vex Thermistor 2-Wire", I got the error message "Measurements: 2-wire resistance configuration is incompatible with voltage excitation", no matter if I choose "Internal" or External" excitation source. But generally 2-wire configuration is good for thermistor. So, is my thermistor not the type Vex Thermistor? Then, what kind of thermistor is Vex Thermistor? The datasheet of my thermistor doesn't say anything if it is Vex or Iex type.  

Thanks in advance,

Caizhen

[nayer · Accepted solution]

Hello Caizhen

PCI-6225 doesnot have an internal excitation source, but you can use one of the AO line as an excitation if you do not have the an external voltage source, in that case you will still have to configure your task as external excitation and four wire measurement and in the program you can take care of AO task where you can specify what exciatation you need. The two wire measurement is not supported in Vex-themistor task. The connection diagram is shown in Measurement & Automation Explorer (MAX) in your DAQmx Task.

Another  possibility is you can measure resistance of the thermistor and can map it to the temperature in two wire configuration, but you will still need to have external current excitation as NI-6225 does not have built in current source which is what it is doing in Iex-thermistor task.

Keep us posted on the updates.

Thanks

nAyer

[liucga]

Hi nAyer,

Thank you very much for your reponse. I've tried to use AO1 as the external excitation source. It is kind of working.  

Caizhen

[nayer]

Hello Caizhen,

It is good to hear that it is working. Keep us posted on the latest updates

Thanks

nAyer

[liucga]

Hi nAyer,

Sorry to bother you again. I am monitoring some voltage signals using the same terminal board and same 6225 board too. I am not sure there is any crosstalk between the temprature channel and other voltage signals.

I use DAQ Assistant to configure all the channels on the block diagram. I try to measure the temperature of ice water, which should be close to 0C. The temperature measurement is way off (about 5C) if I configure other voltage channels first. The temperature measurement changes to 2C if I configure the temperature channel first.

How come the measuremnt changes when I just switch the order of the channels?

I tried diffent acquresition mode (different, RSE, NRSE). It didn't help.

Any ideas or suggestions would be deeply appreciated.

Caizhen

[nayer]

Hello Caizhen,

The reason it is doing is because of the ghosting between channels. Follow this Knowledgebase: How Do I Eliminate Ghosting From My Measuerments which explains in detail why you are seeing that difference and how you can eliminate it. Keep us posted in case you have any other questions.

Thanks

nAyer

[liucga]

Hi nAyer,

Thank you very much for your response. The resistance of the thermistor is 30K at 25C. No wonder the measurement makes more sense if the temperature channel is the first one.

Caizhen