Error Message: Measurements: 2-wire resistance configuration is incompatible with voltage excitation

[tacky]

Hello,

I am trying to use LabVIEW 9 to measure temperature with thermistors.  I have the SCXI-1000 chassis, with the SCXI-1102B module.  The terminal block I am using is the SCXI-1308. 

After speaking with NI engineers several times, I have been able to construct a voltage divider circuit so that the thermistor changes correspond with voltage changes and the the circuitry works because we have tested it with a multimeter and the voltage changes accordingly when the temperature changes.

With the circuit working properly, I wanted to test the current data acquisition program that I have in LabVIEW.  When I created the task in the Measurement and Automation Explorer and set up the thermistors, I used a Vex (voltage excited) 2-Wire thermistor, since there is an external voltage source powering the circuit.  After attempting to save the task, I received an error message stating "Measurements: 2-wire resistance configuration is incompatible with voltage excitation".  When I changed the configuration to 3- and 4-wire, the save went fine.  Why is this an error message if I have a properly setup voltage-excited, 2-wire thermistor?  Will it matter if I change the configuration to 3- or 4-wire with a 2-wire thermistor?

Thanks in advance!!!

Shari

[Sarci1]

Hi tacky,

I believe you are seeing this error because the acquisition has been set up as a voltage excitation and not a current excitation.  Because thermistors are resistive devices your system must provide a current excitation source to measure voltage across the device.  If you change your task type to current excitation you will be allowed to select the 2-wire mode without error.  I encourage you to read the Developer Zone Tutorial Measuring Temperature with an RTD or Thermistor to learn more.

Lynn