Thermistor measurement using NI PCI-6036E and CB-68LP

[umie]

Hi everyone,

I am quite new to LabVIEW and DAQ and I am in an attempt of performing temperature measurement using the only existing hardware available which are NI PCI-6036E and CB-68LP. I was in the attempt of trying to perform the example of temperature measurement based on a book by Robert H. King "Introduction to Data Acquisition with LabVIEW", but as I go further I realized that the hardware I have is totally different that the one in the book, as the one in the book can perform direct resistance measurement from the thermistor.

I have learnt that there is a need of external excitation current needed but I am not sure of now to create one.

Is it true that I need to do a lot of external circuitry in order for me to realize the temperature measurement with only PCI-6036E and CB-68LP? I don't have a budget to buy SCXI or BNC-2120 accessory.

I bought a breadboard and a few resistors and connected to a voltage supply to build the external excitation but it failed miserably. 

I am using Windows XP PC running LabVIEW 2011. 

[johnsold]

You should be able to make a thermistor measurement with one additional resistor per thermistor. Use either an Anolog Output or the +5 V source for excitation. Connect the external resistor in series with the thermistor.  Measure the voltages at both ends of the external resistor. Then you can calculate the current in the thermistor and its resistance.

Lynn

[umie]

Hi Lynn,

Thank you for your reply. I was just wondering at which terminal should I connect the AI0 of the DAQ card. I did the connection accordingly, by placing the external resistor in series with the thermistor. I connected the +5V source to one end of the external resistor and at the same point I place a connection to the AI0 and at the other end of the thermistor to the ground. The voltage seems to be a constant 5V.

I wanted to monitor the change in temperature of the thermistor by monitoring the voltage variation. When I measure the voltage with an external multimeter there seems to be some changes in the voltage, from 4.51V to 4.54V when I place my thumb on the thermistor to give some heat. However, I couldn't monitor any variation with the VI on LabVIEW. Did I misconnected everything?

I build the VI solely using DAQ Assistant to measure voltage.

[johnsold]

Try connecting like this:

The thermistor current It = [V(AI 0) - V(AI 1)]/R1.

The thermistor voltage Vt = V(AI 1).

The thermistor resistance Rt = Vt/It = V(AI 1)*R1/[V(AI 0) - V(AI 1)].

The value of R1 must be selected to provide a suitable range for the thermistor voltages.  This depends on the temperature range and the changes in the thermistor resistance over that range.  There are too many possibilites for me to guess.  If you are only measuring the temperature of your thumb, increase the value of R1 by a factor of about 20-50.

Lynn

[umie]

Hi Lynn,

Thank you for your fast response and your detailed explanation and sorry for my late reply. I will try to do as per suggested. I currently have 10k thermistor with a 10k resistor. Perhaps I shall try to find a different resistor.

Anyway one more query if its okay. 😄

Can I use op amp do directly measure the voltage difference? I am not sure if it's possible to have two DAQ assistant to acquire the voltage and then perform the mathematical operations in LabVIEW.

[johnsold]

Check your connections.  If you have a 10 k thermistor and a 10 k resistor you should get about 2.5 V near room (or thumb) temperature.  The voltages you measured suggest a resistor closer to 100 ohms than to 10000 ohms.

You could use an op amp in a diffential configuration but you still need to make two measurements, one for voltage across the thermistor and one to determine the current through it.

Using the Omega Model 44006 (10000 ohms at 25 C) and a 10000 ohm series resistor, I calculate 2.7538 V at 20 C and 2.7033 V at 21 C.  That should be easy to measure without an op amp.

I do not use the DAQ Assistant but I think you should use one VI to measure both channels rather than to try to use two DAQ Assistants.

Lynn