NTC sensor with extended steinhart-hart constants

[Dejmek]

Hello.

I have a problem with NTC temperature sensor measuring.

Sensor's manufacturer (Vishay) in his catalog mentioned four-number constant values (A,B,C,D) for extended steinhart-hart constants, but Vex (Iex) VI express can't handle with it and constantly gets -273,15 K.

Is there any solution? Or have anyone some experiences with it?

[Jennifer O]

Hi Dejmek,

Thank you for visiting the Discussion Forums. I am not too sure what type of sensor you are working with.  Perhaps you could link the datasheet?  I assume you are working with a thermistor and you are trying to measure temperature using a DAQmx task which is configured for an AI Thermistor task with voltage or current excitation. 

The result that you are getting seems to point to the signal being out of range.  You will want to verify that you have all your wires connected correctly.  Make sure that you have specified an adequate range for the measurements you expect.  Additionally verify your excitation source.  What hardware are you using to measure from the sensor?  What are you using as your excitation source?  If you are using external current excitation and measuring a voltage with a data acquisition (DAQ) board, you can try measuring the voltage directly to see if it changes when different temperatures are applied to the sensor.

Do you have another sensor available or a known voltage source that you can try measuring to verify that your measurement device and sensor are working correctly?

I would be happy to provide further suggestions if the above does not help.  Could you please share with us the models of both your sensor and your DAQ device?

Regards,

Jennifer O
Applications Engineer
National Instruments

[MagGuy]

I have kind of same problem. I measure/calculate the A, B and C-parameters and use Iex in the Express VI. It doesn't work. I also get a constant value. But on the other hand if I make the calculations with Steinhart-Hart equations and the above found coefficients my self in LabVIEW, then everything is ok. So why doesn't the Express VI work?

[TMC-NI]

Hi,

I'm not sure what express VI you are referring to.  However, there's a standard VI called Convert Thermistor Reading.vi that may allow you to have the same functionality.  The behavior is described in the following KB: How Do I Use the Convert Thermistor Reading.vi With Various Thermistor Types?

[MagGuy]

Hi Tolga

I meant the DAQ assistant VI.

Yesterday problem was solved (too easy but still easy to miss): thermistor must be connected in differential mode, I didn't realise that before. Now I will always use the tab 'Connection Diagram' in DQA assistant to be sure of appropriate connection for different cases.