4 wire resistance readings are sometimes negative

[Gordon W]

Hello NI Forums,

This is my first time using a PXI-4072 in 4 wire resistance mode.  I am using the proper probes and things seem to function properly when I test the device on a 10k resistor.  However when I try to measure resistance across my DUT (which should have a max resistance of 0.050 ohm), I read -0.012ohm.  The 0.012ohm value seems reasonable for my DUT but I am not sure about why the reading came up as a negative value  Any ideas?

Thanks,

Gordon

[AnalogKid2DigitalMan]

Gordon:

Is the negative reading happening with the DUT by itself, unpowered? Are there any capacitors in the circuit that could be storing a voltage to cause an erroneous reading?

Are the probes connected properly at both ends in terms of polarity?

What reading do you get when you short all 4 probes together by themeselves?

[K C]

And: Do you enable the Offset Compensated Ohms setting ??

[Gordon W]

Hey,

I've gathered some of the information that was asked.  Here are some clues that may be helpful in figuring out what is going on,

- My DUT is a passive component and does not have any circuitry and is unpowered.

- Shorting the probes with "Offset Compensated Ohms" off, the reading varies between -0.0006 and -0.0020

- Shorting the probes with "Offset Compensated Ohms" on, the reading varies between 0.0001 and 0.0002

- Trying a different DUT than yesterday, with "Offset Compensated Ohms" off, the max reading is 0.0064

- Trying again, with "Offset Compensated Ohms" on, the max reading is 0.0012

All my measurements are taken with "Autozero" turned on, and "Filter" is turned on as well.  The measurement range is naturally 100ohms (the lowest selection).

I have checked my connections and I believe that they are correct.  The black probe's striped wire goes to the Voltmeter's negative socket, and unstriped goes to the Ammeter's negative socket.  Similarly goes for the red probe.

[K C]

Hi,

Most of your measurement are within the 90 days calibration limits    

WIth these kind of measurements (very low voltage) it is very hard to get things right.

You to look at the materials you use. Watch out for interference into the measuring leads.

Did you already tried to measure a small known resistor to see if that is OK.

What measurement do you really want to perform ?

[Gordon W]

I have to say that it's definitely reassuring that my readings are considered reasonable.  I'm using this in a semi-automated data acquisition system, measuring the resistance between the ground bolt and a connector on airplane landing gear - essentially checking for continuity, and making sure that the measured resistance is within customers' specifications.  As I've already mentioned, the landing gear will not be connected to any power source or sink during the test.  I was curious as to why I'm reading negative values.  It seems that "Offset Compensation Ohms" seems to get rid of a lot of the negative readings, I guess I'm going to read more about this feature to make sure its appropriate for this stuff.

Message Edited by Gordon W on 04-17-2008 03:31 PM

[K C]

Hi,

Maybe it is reassuring but that doesn't say that it is good !!

Still depends on what you want to measure. You talk about 'connector and ground bolt' it is possible that you are using the wrong method. On what standard should the test be based ?

[Gordon W]

Hey guys, sorry for the late reply

I'm not too familiar with measurement standards, but there are areas where the test procedure asks to measure between the frame and a particular bolt to make sure the resistance is under 7 milliohms. The test document also asks that the equipment to have accuracy of 2% of full scale.  If I'm using the PXI-4072 calibrated to the NIST standards.

In another topic, does anybody have any idea what negative readings mean, or are they just anomalies?

[nitad54448]

Hi,
negative resistance in a passive DUT, unpowered, no capacitors, etc..., means "free lunch" and there is no "free lunch". Fot me there is no such thing as negative resistance, even in a superconductor, you'll get zero resistance but NEVER negative.
For your application : this is in the error range, it is difficult to measure such low values of the resistance : you need a higher current source and/or a better voltmeter. The sense current for a typical device is depending on the range; I guess that for the lowest range of a regular device this is 1 mA to 100 mA or so. For 7 milliOhms, this means your voltmeter need to measure accurately 7 microVolts. Now, if you take your measurement with other than Cu wires or if the device is not made of Cu, Cd, gold, ...(low Seebeck coefficient materials), a temperature gradient will induce an error which can be much larger than the 7 microvolts (as an exemple, Nickel alloys may have 40 microvolts per degree). The solution is to take care of thermoelectric coefficient, better voltmeter or alternating the sense of the current  to get rid of the thermoelectric contribution (or use AC measurement ?).
Regards


Message Edité par nitad54448 le 04-27-2008 04:28 PM

[K C]

Hi,

I agree fully with Nitad.

If you are testing a ground connection it must be possible to test with a high current. Testing safety earth connections must be performed with at least 25A (Europe) With these high currents it is much easier to measure the voltage (resistance).

Currently I am looking for a resistance meter for crimp contacts and I can asure you it is not easy. They (the manufacturers) all say that their equipment can do the job better than the other one. Crimp contact resistance  must be measured with a voltage not higher than 20mV and a current not higher than 100mA. We are talking about a resistance of 0.5mOhm !!!

So it is possible to measure a small resistance with a small current but it is not easy. One little mistake can give you the reading you get. Go through the whole test setup again step by step. You already said that shorting the test clamps give a reading near zero. So it must be something from your DUT that is causing these false readings.