HP 3457A DMM reads 0.550 Ohm over value

[Swake]

Hi,
Got a new to me HP 3457A today and performed a quick review. The resistance measurement in 2Wire reads about 0.550 Ohm higher than it should be in the lowest 4 ranges. In 4Wire mode it is 0.0180 Ohm too high.

How 'bad' is this? Can this still be calibrated or is it more of a repair job?

I didn't verify all other ranges yet, but DC Voltage and DC Current seem fine.

[Kleinstein]

An offset in the 2 wire mode is not a surprise, though 0.5 ohm is still high.
Getting the same offset in different ranges would not be a fixed voltage offset.
I see little chance to get such an error from an hardware fault - more like a really poor calibration.

Maybe also check the resistor values for possible other problems.

[Swake]

Thanks for that information.
I corrected the 4-wire measurement values in my initial post as I made an error in my notes.

[Swake]

I will attempt a calibration of 2-wire offset as that only needs a short from what I could read.

[bdunham7]

Just out of curiosity, if you leave it connected with a 4W short but measure in 2W mode, what do you get?  A agree that this may be an offset calibration issue since it seems unlikely that 0.55R has appeared internally where it wasn't before.  But I'm not sure how the error would have occurred.

[TizianoHV]

Maybe a bad relay? try changing function a few times (DCV - 2WRES) and see if you get consistent 0.55ohm.

[Swake]

The 2-wire measurement with all 4 short reduces the measurement with about 1,5 mOhm (=0.0015 as some might think this is a typo  ).
Removing the short it raises back up.

I'm using test leads for this, 40cm long only, but still, it might have an impact. Tomorrow I'll make a direct short out of 1.5mm² copper wire. Not that I think this might change anything as half an Ohm is a lot.

Unfortunately yes, the measurement stays consistently 0.55 Ohms over after having changed ranges, functions etc for a good part of the evening, did many resets too. Did not power it down since a couple of hours, that will have to wait till tomorrow.

[Kleinstein]

0.5 ohm is quite a lot for 2 wire ohm. So it could be a bad relay. The idea of switching the relay was good and any change would point to dirty relay contacts.
If stable it would be something the calibration could fix.

a residual resistance of 18 mohm in the 4 wire mode is a bit on the high side, but it depends on the range and thus test current. I would have expected better. It still depend on how the 4 wire short is done:
the correct way is having cables connected (e.g. stackable plugs) like dive - drive - sense - sense and not drive - sense - sense - drive (this would measure the plug resistance).

[bdunham7]

The 2-wire measurement with all 4 short reduces the measurement with about 1,5 mOhm (=0.0015 as some might think this is a typo  ).
Removing the short it raises back up.

I'm using test leads for this, 40cm long only

My thought was that someone set the 2W offset with a 4W short, which would not null out the meters internal resistance.  But using test leads for this is no bueno unless you want to calibrate it specially for a certain set of leads. 

[Swake]

Sure test leads are not ideal. But these do not explain half an Ohm.

I intent to wire the short as position A in the image.

(Edit: corrected the image)

[Kleinstein]

The correct connection is B. Both  A and C measure the resistance between the 2 inner contacts and thus not a proper 4 wire short.

[bdunham7]

Sure test leads are not ideal. But these do not explain half an Ohm.

Probably not, but it's not out of the question.  Anyway, a shorting link or short bare copper wire of decent size should answer the question.  Give it time to settle.

[Swake]

Allow me to disagree about position B.

The 'resistance' should be put between the 2-wire leads. De idea is to have those leads as close as possible to the 'resistance'. The 2 additional sense leads should be connected as close as possible to those leads.
Ideally all 4 leads have the same length. In this specific case as there are not really leads the 'resistance' is to be put vertically in the middle of the 4 posts and connected left and right, but since it is a single piece of rigid wire I can't see how to do this.

[bdunham7]

Allow me to disagree about position B.

Nope, no disagreement allowed!   

The point of a 4-wire short is not to measure its resistance, but to trick the meter into seeing absolute zero resistance, or as close to it as possible.  The way the expensive OEM shorts work is sort of the opposite of your diagram 'D'--they directly connect the two source terminals with one conductor and the two sense terminals with another and then connect those two conductors together with a third, typically smaller, conductor right in the middle.  So 'D' turned 90 degrees.

Your diagram 'B' will work well enough, but due to some specifics as to how these meters work, I would have the connecting loop part at the bottom rather than the top.  In practice that probably makes very little difference.

[Swake]

Ok, so I made the short as described in the calibration manual of the 3458A with 2.5mm² freshly stripped bent wire.
The DMM was kept switched on but untouched for the last 12 hours or so. Also waited 2 hours for the copper wire to come 'at temperature' before taking the measurements.

2-Wire mode reads 0.44432 Ohm
4-Wire mode reads 0.00022 Ohm

[Swake]

2-wire mode, 3 first ranges are over spec.

Code: [Select]

Range Reading
 30 Ohm 00.44432 Ohm
300 Ohm 000.4279 Ohm
  3 kOhm 0.000443 kOhm
 30 kOhm 00.00022 kOhm
300 kOhm 000.0002 kOhm
  3 MOhm 0.000001 MOhm
 30 MOhm 00.00001 MOhm


[Kleinstein]

0.22 mOhm in 4 wire mode is OK. That corresponds to 0.22 µV of offset voltage, which is within the expected range for thermal EMF and errors from the calibration.
The 0.44 ohm difference with 2 wire mode can be from a relay contact and also the front / rear switch.
If really stable to switching of front / rear and 2/4 wire mode the offset for the 2 wire modes could be corrected in the calibration.

[bdunham7]

OK, can you take the same 2W measurements with the wire only shorting the 2W terminals? 

It seems clear that you need to make sure that the offset is stable and doesn't change if you cycle the F/R switch or ranges.  If it is consistent, calibrate the 2W offset and it should be good.  If it is not consistent, then you get to have more fun.  The only nagging doubt I have is that with your readings, I'm not sure how it could have been miscalibrated like that.

[Swake]

Similar reading with 2-Wire mode with only 2 wires shorted with a piece of clean 2.5mm² copper wire.
00.45005 Ohm. 2 last digits jumping around a bit.

Will keep it on for a couple hours, not that I expect it to change anything.

Now that you mention rear reading maybe it is important to know there is a 44492A multiplexer card inserted. Not the usual posts. There is nothing connected to that card at the moment. Have switched over from F/R and the several channels several times. Absolutely no change in resistance reading on the front in 2-wire.

Have also power cycled the DMM just in case, but this didn't change anything.

Thanks for your help, it is very much appreciated.

[Swake]

Calibration done, all good, me happy

Code: [Select]

Offset test 2-Wire Ohms
Range Reading
30 Ohm 00.00022 Ohm
300 Ohm 000.0002 Ohm
  3 kOhm 0.000000 kOhm
30 kOhm 00.00000 kOhm
300 kOhm 000.0000 kOhm
  3 MOhm 0.000000 MOhm
30 MOhm 00.00000 MOhm
300 MOhm 00.00000 MOhm

Offset test 4-Wire Ohms
Range Reading
30 Ohm 00.00012 Ohm