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thermistor

I am struggling with what I think should be a relatively simple problem.  I am trying to read a temperature with a 10K ohm Vishay thermistor.  I picked this thermistor because there were sample VI  codes and a link to the data sheet on the NI website.  In the samples, the user utilized the DMM to take readings and apparently it worked fine.  I am using a PCI 6259 card with a SCB-68 interface.  I obtained the ABC values for the thermistor from the data sheet.  When I use the Vex Thermistor DAQ my temperature values are not at all correct.  I seems that I have a scaling factor issue but as I mentioned I obtained the values from the data sheet.  I wired the R1 resistor as shown in the wiring diagram supplied by NI. 

My Vex is 5 volts which I am tapping off of pins 8&9 from the SCB-68 interface and the R1 value that I was using ranged from 1K to 10K.  I am confident that my wiring is correct because I am getting a signal.

 

Can anyone tell me If I am missing something or at least let me know what common mistakes are?

 

Thanks in advance.

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Hi Boom,


The +5VDC provided on your SCB-68 is not a sufficient source of providing excitation voltage to your RTD.  These voltages are to be used as references, not to source power.


For thermistor measurements, you'll need to provide an excitation voltage which would only be available with external circuitry such as the signal conditioning chassis (SCXI), or with our smaller signal conditioning platform: SCC. 

 

The SCC-68 or SCC-2345 is a housing to plug in our premade circuit modules which then interface to your board. For thermistor, you'd use the SCC-RTD01 module, as long as you consider the options outline in Can I Make Thermistor Measurements with the SCC-RTD01?

 

I might also recommend you check out our Working with Thermistors and RTDs tutorial if you haven't already.

 

Regards,

Paul Davidson
National Instruments
Product Owner - ni.com Chat
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Thanks for responding Paul-D.

 

Can I just use a Constant Voltage DC power supply as my external excitation voltage?

 

Boom

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You can use your own external excitation, but you will need to make sure it is a stable source.  Variations could throw your readings off.  Below is a screenshot from a DAQmx Vex Task Configured with a PCI-6259 and a SCB-68 showing how you could connect external excitation.  Here's how I created it.

 

Thermistor 6259 Connection.png

Paul Davidson
National Instruments
Product Owner - ni.com Chat
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Hi Paul-D and everybody,

 

I jump on this thread because I would greatly appreciate receiving advise on the following issue:

 

I need to measure temperature in the range 15 °C to 40 °C and to accurately resolve 0.1 °C changes. My thermistor has nominal resistance of 22 kOhm at 25 °C and resistance of 14 KOhm at 37 °C. Dissipation constant in air at 25 °C is 0.3 mW/°C - but the thermistor will be used in water-based solution. I have a NI6224 DAQ board with a SCB-68 connector and Labview 7.

 

How can make my NI6224 work with my thermistor with minimal cost? 

 

In particular:

the NI6224 has no analog output. Can I use a digital output as a current source? I would say no because digital output current is 24 mA, which would make 24 mA times 14 KOhm >> 10 V, which is the input range of the board. Is that correct?

 

Could I buy a NI 9265 module + chassis + cables etc to provide current excitation to the thermistor, and connect its two leads to a voltage differential input of the NI6224 board? Is there any cheaper option with other NI or third-party equipment? 

 

I thank you in advance for your time and your help 

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Hi groomit,

 

I would advise you to also post your questions to a new thread as this will increase visibility of your issue and will allow for better tracking on NI's end.

Nadine H.
Applications Engineer
National Instruments
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