Raw resistance data of a thermistor?

[samantha.arato]

I'm working with a two-wire 2252 Ohm thermistor from Omega and everywhere I've read on NI articles talks about requiring an excitation current to then convert the resulting measurement to voltage, and then using a VI to convert that voltage to temperature which automatically uses the 3rd order steinhart-hart eq.

When I had the thermistors calibrated i was informed that the 3rd order steinhart-hart eq would yield "mathematical residuals" of +/- 2.6 deg F (which is too large/I can't work with), so i was provided with 4th order polynomial coefficients in T(R) and R(T) form, and steinhart-hart 5th order coefficients T(R) and R(T).

Is there any way to read in the thermistor resistance values and simply dump them to excel? Why do thermistors need the excitation current for all this extra narrow processing in labview?

Thanks for all the feedback/help!

[GerdW]

Hi samantha,

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@samantha.arato wrote:

Is there any way to read in the thermistor resistance values and simply dump them to excel? Why do thermistors need the excitation current for all this extra narrow processing in labview?

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To measure any kind of resistance you usually need to source a current through that resistor and measure the voltage drop over the resistor. This is very basic stuff in electrical measurements!

So to read the thermistor resistance you need an excitation current and measure the voltage across the thermistor. Then you can calculate its resistance…

[samantha.arato]

I will reword my question as follows - why do i need to specify the excitation current for thermistors, when I have a block diagram made in LabVIEW for two RTDs and i dont have to specify the excitation current and i dont receive voltage data? Are the excitation current and voltage measurements converted to resistance "under the hood" in this case? I have resistance measurements directly exported to excel, and i've converted the values to temperature with 4th order polynomial coefficients (hence why i'm not configuring the RTDs with Callendar Van Dusen coefficients in LabVIEW) and they're in line with my control measurement.

[RTSLVU]

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@samantha.arato wrote:

I will reword my question as follows - why do i need to specify the excitation current for thermistors, when I have a block diagram made in LabVIEW for two RTDs and i dont have to specify the excitation current and i dont receive voltage data? Are the excitation current and voltage measurements converted to resistance "under the hood" in this case? I have resistance measurements directly exported to excel, and i've converted the values to temperature with 4th order polynomial coefficients (hence why i'm not configuring the RTDs with Callendar Van Dusen coefficients in LabVIEW) and they're in line with my control measurement.

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Um....Yeah there's a thing called Ohms Law (you should read up on it). It basically states there is a relationship between Voltage, Current, and Resistance. So by knowing any of the other two you can determine the third.

Therefore, by applying a known voltage (excitation) and measuring the current draw, you can calculate the resistance.

Then in the case of a Thermistor you can, convert that to temperature 

BTW: If you need better accuracy than +/- 2.6 degrees you need tom consider a better sensor than a thermistor. Heck even properly made or manufactured thermocouples are not that accurate