4 wire resistance measurement using Compact DAQ - How to connect?

[TS-T1000]

I like to find out the correct way to connect the wires for 4-wire resistance measurement. I am using Compact DAQ. Thanks

[Ima U]

Hello,

Please take a look at this helpful Developer Zone article on resistance measurements.  There is a section on 4 wire resistance measurements.  In order for me help you specifically I will need you to answer the following questions.

1.  Which Compact DAQ module are you using?

2.  Are you using a DMM?

[TS-T1000]

Thanks for the reply. I am using 9219 for resistance measurement. I don't think I am using DMM. Thanks once again.

[JaredA]

Go into MAX or the DAQ Assistant and setup a DAQmx Task. If you're in MAX (Measurement and Automation Explorer), right click on Data Neighborhod and select Create New. Select NI-DAQmx Task. From there, select Acquire Signals > Analog Input > Resistance. Select the 9219 module and the appropriate channel(s) you want to connect up to. The next window will be the channel configuration window. By default, 4-wire mode should be selected. At the bottom of that screen, you should see "Connection Diagram". Click on that and you will see a graphical depiction of how to make your connections.

For instance, by following the above instuctions I see the following:
EX+ = pin 3
EX- = pin 5
CH+ = pin 4
CH- = pin 6

Here's a white paper in the NI-DAQmx Getting Started Series to assist you:
http://zone.ni.com/devzone/cda/tut/p/id/5396

JaredA

[TS-T1000]

I have managed to wire up the 4-wire for resistance measurement; however, I don't get any resistance reading from my DUT. It just saturate at 10.5k-ohms. What has gone wrong? As for the DUT, it is a wire that is heated resistively, and I am trying to measure the change in resistance of the DUT during heating. Am I right to say 4-wire measurement has 2 wires measuring the cuurent across the DUT while the other 2 measures the voltage across? I have tried measuring a resistor & the correct reading was indicated. But I just could not obtain a reading for my DUT.

[Ima U]

Hi TS-T1000,

From my understanding, you asked two questions:

1.  How does a 4 wire resistance measurement work?

 

For precision measurements with resistances (ohms) below 100 kΩ, 4-wire works more reliably and conveniently than 2-wire. 4-wire requires 4-wire switching and more cabling; however, you may decide the tradeoff is acceptable, depending on the accuracy versus complexity requirements of your system.
The following figure shows a 4-wire resistance (ohms) measurement, including lead and switching resistance:



Current (amps) is forced through the source leads (HI, LO). As a result, a voltage develops across the resistance (ohms) under test. By measuring the voltage directly across the resistor using the sense leads, the voltage drop of the interconnects (R_LEAD) is ignored.

2.  Why does your resistance measurement of your DUT only result in a saturate of 10.5 Kohms?

 

 According to page 21 of the NI 9219 Operating instructions and Specifications, the highest actual measurement the 9219 can take in 4 wire mode is 10.5 kΩ.  My guess is that your DUT is outside of the specifications for the module.  Have you used a DMM to measure the resistance of your DUT?   Is the resistance of the DUT below 10.5 kΩ? 

[AnalogKid2DigitalMan]

I have a hunch you will not be able to measure the resisitance of your DUT with the current approach:

You say the DUT is a wire that is heated resistively, thus  I presume that you are passing a current through it to heat it up at the same time you have your four wire connection hooked up and trying to measure resistance. You cannot measure the resistance while the heating current is being applied.

You could add a shunt to scale heating current to a voltage (Idut=Vshunt/Rshunt) that you measure in addition to measuring the voltage across the DUT. Both inputs will have to be configures as differential inputs. Depending upoon the current, you may need to amplify the voltage across the shunt, since you want shunt to have a low ohmic value (shunt is also known as a current sensing resistor)

To calculate the DUT resistance, use ohms law again Rdut=Vdut/Idut.

[TS-T1000]

Thanks for replying.

[TS-T1000]

hello everyone, its me again with another question. i measured the voltage of the DUT and managed to obtain some results. however, i need to know the current passing through the DUT, is there anyway to measure this current? read from the manual that for current measurement, the CompactDAQ can only handle 2mA, but the current that may be passing through the DUT may be as high as 0.7A to 1A! is there any way to measure the current in this case? thanks once again.

[Ima U]

Hello,

From my understanding, you would like to know if it is possible to measure current from your DUT even though the currents that you will be measuring greatly excede the input current specification of your 9219.  The best suggestion I have is for you to use a DMM to measure the current in your DUT.  The second suggestion I have for you is to put a known resistor in series with your DUT a measure the voltage across the known resistor.  You can then calculate the current from the known resistor value and the measured voltage value.  Make sure the resistor is accurate enough for your application.