Low ohms measurements

[grumpydoc]

So, one of the items that I acquired as a "quick repair & re-sell on ebay " last week was a Seaward PrimeTest 100 PAT tester.

The repair was, indeed, quick - just a new rubber button moulding and quick clean of the PCB contacts but I would like to check the calibration before re-sale. Unusually for a 2nd hand acquisition it has a cal cert up to November but I had to break the seals to repair it.

One of the functions is earth continuity and the magic pass/fail value is around 0.1\$ \Omega \$. The stated accuracy of the Seaward is 5% (+2 digits).

So I would like to measure an 0.1\$ \Omega \$ resistor with sufficient accuracy to act as a standard for the Seaward (and a range of, say, 0.05 to 0.5 ohms). 0.5% would do it but 0.1% would be better.

I have at my disposition two Keithley 2015's and one 2000, none are formally calibrated but I have no reason to believe that they are out of cal with the exception of resistance on one of the 2015's which is definitely off they all agree closely (and within expected uncertainty) on DCV and DCA and two agree very closely on ohms. In fact using my DMM Check 5V reference (which, sadly, has drifted a bit high) they are all within 2 counts on volts. Getting at least one of these meters formally calibrated is on the "To Do" list bit has not yet graduated to the "Done" list.

The first thoughht was just to do a 4-wire measurement directly on Ohms - that is not so great however. The uncertainty on the lowest range (100\$ \Omega \$) is 40ppm of range or 4milliOhms. That is 4% of the value I'm trying to measure so barely any better than the Seaward. The reading drifts rather a lot as well - perhaps because with the 1mA test current we're trying to measure just 100\$ \mu\$V

So I did a manual 4-wire measurement with my bench PSU running about 100mA test current (10mV across the resistor). That gives a value of 0.996\$ \Omega \$. The uncertainty mainly comes from the DC Amps range of the Keithley which has an accuracy of 500ppm of reading + 800ppm of range or 1300ppm total - i.e 0.13%. There is a further 350ppm uncertainty in the voltage measurement so the overall uncertainty is about 1650ppm (0.165%)

I also have a Voltcraft LCR-300 - on the 1kHz series resistance measurement this shows the value as 0.099\$ \Omega \$. This has 0.3% accuracy but only 1milliOhm resolution so effectively limited to 1%.

So, some questions for the panel

1. Is that good enough to check the Seaward

2. Are my figures correct for the accuracy I will get from the above reading

3. Short of buying a 2001 (lowest range is 20\$ \Omega \$ with 72ppm of reading and 7ppm of range) is there a relatively inexpensive way to do a little better (say 0.1% or 0.05%).

[SeanB]

Just buy however many 1% resistors as you need to put in parallell to get that. I would guess 47 4R7 1% 0.3W metal film resistors, all soldered to a simple flat PCB with a slot either ground or etched into the middle, to make a simple 2 wire ( or 4 wire if you want, but for a PAT with a 2 wire measurement pretty moot anyway) 0R1 1% resistor. You need a lot of resistor, simply because the test current will be around 6-25A AC, and that is a lot of power for a metal film resistor to handle, so a lot of resistors is needed.  Will be marginal with 47 resistors, they will be horribly overrun, but as the test is normally short duration they will survive short term, though for longer testing 100 10R 1% 0.6W metal film resistors on the big copper heat spreader will survive, you will just need a big soldering iron to solder all of them on, or get solder paste and a lot of 100 1206 SMD resistors instead on the boards and reflow it instead.

[HighVoltage]

5% is not really high accuracy .
I would just buy a few 0.1 Ohm power resistors and find the one that is closest to spot on.
Your method of testing if clearly sufficient for this task.

[grumpydoc]

Well, I can come up with an 0.1\$ \Omega \$ resistor to 0.1% - it doesn't even need to be that high power. Whereas "real" earth continuity testers do sometimes use 25A test currents the PrimeTest 100 is a hand-held, battery powered tester and only uses a current of 200mA for the test so 0.1\$ \Omega \$ will only dissipate 4mW and 0.2\$ \Omega \$ will only dissipate 8mW.

The issue is that the resistance is measured between the earth pin of a BS1363 plug inserted into a test socket on the instrument and either a 4mm banana socket or the earth pin of an IEC socket on the instrument - thus there needs to be additional wiring with a minimum length of about 30cm (to get from one socket to the other). 30cm of 1.5mm² flex has a resistance of about 4milliOhms, plus any connections - it would be easy enough to add something in the region of 20milliOhms to the resistance seen by the meter.

To be fair the 5% is a bit of a red herring. As I said it's 5%+2 digits and the resolution is 10milliOhms so the 2 digits is actually 20% at 0.1ohms. That said I do need to know whether a reading of 0.14 for an 0.1ohm resistor is because the meter is seeing 0.12ohms and is in spec or because it is seeing 0.11ohms and is out of spec.Thus I would like to measure the test resistor plus associated wiring.

I'm actually reasonably happy with the "manual" 4-wire measurement and think that it is good enough for checking the tester. I just wanted a bit of a sanity check.

[HighVoltage]

I am not familiar with this brand of tester but I would think that it is totally accepted if you have a 0.1 Ohm resistor and in addition 30cm of 1.5mm² cable. This is not a precision instrument.

[MK]

I was asked to calibrate a PAT tester many years ago and had to refuse a certificate as it difted out of spec each time I tried to make a measurement at the rated current... At low currents it was fine and in spec.

[grumpydoc]

I am not familiar with this brand of tester but I would think that it is totally accepted if you have a 0.1 Ohm resistor and in addition 30cm of 1.5mm² cable. This is not a precision instrument.

True but it has a spec, and there is an IEE code of practice so it probably should meet the spec, such as it is.

It just turned out to be slightly more tricky than my original thought of - I'll just grab one of those 0.1ohm 1W resistors from the pile, measure it accurately on the Keithley and see how well the Seaward matches.