Calculating current on loop

[Farside1]

#1. First question misunderstood:

 > If 6250 can read RTD 4-20mA current input?

#2. Second question not answered:

 > Can the PCI-6250 measure current?

We have an Omega HX15 humidity/temp probe supplying it's own current loop,

and it's connected to terminal block as "Differential non-referenced".

New task configured in Measurement & Automation "Data Neighborhood".

My setup is SCB-68 connected to PCI-6250 and configured:

Ai0 > Omega RTD 1K platinum probe analog output into channel 0 as "RTD"

 - looks like it's taking test samples, but I am very much a beginner in SW

Ai1 > Omega HX15 Humidity probe analag output channel 1

 - no reading of samples when self test is run

I have a problem with the math, but could be I have to put a resistor

across the differential pairs and setup both Ai0 and Ai1 to take 1-5 volt reading?

IF PCI-6250 accepts current readings, please help calculate loop resistance:

Rmax = (24 volt supply - 7 volts)/.02 amps = 850 ohms?

I have 22 awg wire on loop, adding another 15 ohms?, should that total 865 ohms Rmax?

When I enter current max "20m" for 20 mA and 865 ohms total resistance, I get error message,

and it is expecting a different current total. Math is my weakest subject.

IF you can assist me with a recommended business partner?

I contacted Honeywell Sensotec but they do not make Humidity sensors.

Should I have to return the Omega probe and seek a probe that does not

supply it's own current loop. That seems like the problem.

Regards,

Phil Johnson

[Sandra_T]

Hello Phil,

The PCI-6250 is one of our M series data acquisition cards, with a maximum input range of +/- 10V. If you want to measure current with this card you will need to provide an external shunt resistor. The current will then be pasted through the resistor and the voltage measured is across the resistor you provide.

In your application, I noticed that you are using an OMEGA HX15 humidity/temp probe. I am not very familiar with this particular part. I took a look at the data sheet and it wasn't very clear, but it appears that this device is powered externally by a power source. There are two current outputs in the range of 4-20mA, one representing the temperature and the other representing the humidity. If you want to use the PCI-6250 to make current measurements you will need to use a shunt resistor. Using ohms laq, V=IR --> R=V/I, you can find a suitable value for your resistor so that the voltage is with the range of +/-10V. The V would represent the max input range +10V and the maximum current range 20mA., that would give you a required shunt resistance of 500ohms.

I would use a "differential" current measurement, with range of 0 -20mA range, with external resistor of 500 ohms.

Give this try. If you are still getting error, please reply with more specific questions.

Have a good day!

Regards,
  Sandra T.

Applications Engineer | National Instruments