measuring temperature Via USB-6009?

[Hanieh]

How can I measure temperature with a thermistor Via USB-6009 using Labview 8.5?

I am trying to design an application using Labview 8.5, to measure temperature using a thermistor, a very simple circuit is used, basically R=100Kohms and Rt=100Kohms in series, the voltage is measured in the middle of the 2 resistors and this voltage goes through the USB-6009 where I can interface it with LABVIEW and get some reading on the screen.

I need some help with the application design, not sure where to start....

I read a couple of NI articles about temperature conversion and temperature reading, but nothing about the actual design.

Thanks

Hanieh

[devchander]

Yes it is possible

Find the example vi 'Continous acquisition therrmistor int-clk.vi' in Examples shipped with labVIEW

Go to Find Examples>>Hardware input output>.DAQmx>>Analog measurements>>Temeprature>>Cont Acq Thermistor Samples-Int Clk.vi

Read the documentation of this vi for more information

[Matthew_W]

 Hi Hanieh,

 The example that devchander is a good starting point if you know the characteristics of your thermistor. If you are simply looking for a LabVIEW application for your USB-6009 that will read the voltage across the thermistor and put it on a chart then take a look at the Analog Measurements->Voltage->Cont. Acq & Graph Voltage-Int Clk.VI.
 
 The thermistor example performs the scaling from voltage->temperature for you. The voltage measurement example will return a voltage, you will have to do the scaling manually or in the LabVIEW VI.

 Have a great morning,

 MatthewW
 Applications Engineer
 National Instruments

[Hanieh]

Thanks guys...I will look into it now

[Hanieh]

Hi again,

I ran the LABVIEW application that you guys recommended, Con Acq&Graph Voltage-Int Clk vi.

Now I connected my USB-6009 to my simple voltage divider circuit which has R=100Kohms and Rt=100Kohms, I read the voltage across the thermistor, it' s supposed to show 5V, it shows 5V on DMM, but on the labview screen  front panel it display about 3V.

Another problem I encountered, I need to convert this voltage to the appropriate temperature, which formula would work best for this application, I found a few temperature-voltage conversion formulas,

appritiate your help.

Hanieh

[devchander]

Are you using connecting signal in RSE or Differential mode??

In your program, check if you are reading in appropriate mode

[Matthew_W]

 Hi Hanieh,

 I would second what devchander has said about checking the Input terminal Configuration, as seen here:
 
 


For the conversion formula, you will need to either measure the voltage at several temperatures and come up with a formula, or use the manufacturer lookup table to determine the resistance at different temperatures and then calculate the voltages.

Have a great afternoon,

 MatthewW
 Applications Engineer
 National Instruments



Message Edited by Matthew W on 02-01-2008 01:27 PM

[ned_konz]

Or of course you could use Ohm's law and the Steinhart-Hart equation (or its variants), with published or measured values for your thermistor.

If you have a known series resistance and a known voltage supply, you know the current using Ohm's law.

You can measure the voltage across your thermistor and use Ohm's law to find the resistance of your thermistor once you know the current through it.

Then you can plug the Beta (which is specified at a particular temperature range) into the equation:

where:

R	Thermistor resistance at T (K)
T	Thermistor temperature (K)
Ro	Nominal resistance at To (K)
To	Temperature where Ro is measured
B	Thermistor material constant (Beta)