Turning off excitation current out of SCC-RTD01

[JG001]

I have a strain gage which has four wires instead of two. The reason I have four wires is because at each pad, I have a chromel and an alumel wire to create a thermocouple. This way I can measure the temperature right at the gage (two TC per gage).

I need to measure the resistance of the gage as I change the temperature. I have a SCC-RTD01 module that I can use to measure the gage resistance (I've done this actually and it worked). But since this module puts out a 1mA excitation current, my thermocouple reading is wrong (because current flows through at least one of the wires creating a voltage that is not the pure thermocouple voltage).

Is there any way I can turn the excitation current off from LabVIEW? Or, does anyone have any other suggestion? Thanks.

[enderWiggin10]

Hi JG001,

The current is required for the resistance to be measured using the SCC-RTD01.  The current is driven at a constant level and then the voltage drop over the RTD is recorded by the device as well.  This is then converted to a resistance using Ohm's Law and then converted to a temperature using the equation as specified by the RTD manufacturer.  With that said, if you turn off the current, you will not be able to measure the resistance of the RTD.

Here is the manual for the SCC-RTD01 device.  You can see on page 5 that a 2-wire RTD requires the connection to be made between AI+ and EX+ and well as EX- and AI-.  Does this help?

Best,

[JG001]

This really doesn't help.

I guess I need to be more clear - yes, I know that I need the 1mA excitation current to measure the resistance. But I also need to turn off this excitation current for a little while so that I can measure the thermocouple. So, half of my goal is reached (reading the resistance). But the other half requires zero excitation current to flow through the resistor.

So, I'm asking the question again - Is there a way to turn this 1mA excitation current off on the SCC-RTD01 module from LabVIEW?

Thanks.

[enderWiggin10 · Accepted solution]

Hi JG001,

Thank you very much for clearing up the issue, I now understand your request fully.  Really the fact of the matter is that the excitation is broken out to its own dedicated pins; it cannot be turned off, and this is found in the SCC-RTD01 manual. You can also verify this by inputting a 0 mA in your task for the excitation and you can find that it is not a supported value, only 1mA is a supported value, and it will throw an error as shown below.  I have also included some screenshots from the manual that describe this.  If the excitation could be turned off, 0mA would be a supported value for the current excitation level.  I hope this clears up any confusion.  A possible solution would be to use a relay to turn this off, perhaps the SCC-RLY01.

[JG001]

Thanks. I suspected the same. I guess I'll use a MUX to get the job done.