Won Seaward test gear on ebay for £35 - Bargain?

[RF Lighting]

My local electrical wholesalers have a calibration day once a year. Lots of local contractors use the service. The calibration guy does loads of business in one day, so can hugely reduce the cost compared to a one off calibration.

I paid £35+vat last year for my MFT.

Have a chat with your local wholesaler and see if they offer a similar service.

 

[Adam_151]

Also how quickly to test meters drift out of spec and become inaccrute in the real world?

As John says it doesn't really happen... although on some makes low batteries can cause the readings to drift a bit. Normally instrument failures result in no function or wildly inaccurate readings that stand out a mile...


however what I have noticed is lead breakdown causes the readings to steadily increase, perhaps adding 0.05 ohm onto your loop test. It is missed by the check box because of the mindset that you are testing the instrument so it'll be checked with the 13A plug lead, all fine, back on site the probe leads will be used which could be breaking down

 

[RF Lighting]

I'm on my third set of leads since I got my meter

 

[Adam_151]

I must have got upto about the fifth set on my previous loop tester.

Then the meter had to be replaced so technically back to no. 1 ... bloody thing cooked the ceramic load bank resisters and then just kept flashing up that it was unserviceable!

You still got your alphatek/ metrel ? Not sure I am a massive fan of their gear tbh... and their customer services dept is just...woeful!

 

[JohnW2]

however what I have noticed is lead breakdown causes the readings to steadily increase, perhaps adding 0.05 ohm onto your loop test. It is missed by the check box because of the mindset that you are testing the instrument so it'll be checked with the 13A plug lead, all fine, back on site the probe leads will be used which could be breaking down

Whilst I certainly agree that leads deteriorate, I'm not sure that I fully understand what you are saying. With my Fluke MFT, and I presume most other meters, one is meant to 'zero' the test leads (whichever ones are being used) prior to undertaking an EFLI (or 'continuity') measurement. This ought to compensate for any changes that have occurred in the resistance/ impedance of the leads die to their 'deterioration', shouldn't it?

Kind Regards, John

 

[RF Lighting]

Yep still running on the mighty metrel. It's never missed a beat. Just needs 4 C cell batteries every 6 months and that's it.

The trouble with deteriorating leads is it can affect IR readings, (appearing better than they are) continuity and loop impedance.

If you get an intermittent broken conductor, it can give you some really misleading readings.

 

[SimonH2]

Engineering a method of checking loop impedance measurements is less straightforward

The checkbox plugs into a non-RCD socket in your house, and itself has a socket which your tester plugs into, with a 1Ω resistor switchable into the earth path. It doesn't matter what your EFLI actually is, what matters is does it change by 1Ω with the resistor in circuit.

Well that will tell you that the gain is still correct, it won't tell you that the (IMO possibly more important) offset is still correct.

EG, you check and find the EFLI is (say) 0R2, and with the test resistor, 1R2. What if the meter offset is wrong ? That 0R2 might be (say) 0R8 - cutting your margins for OPDs operating in a timely manner.

So yes, the 1R test resistor will detect some faults, but it by no means "proves" the unit is operating correctly.

I don't have any non RCD protected sockets ...

That's OK - do the equipment check using L-N (so determining PSC) which doesn't trip an RCD. However, if the purpose of possessing the equipment is to do your own tests, you'll need to open the CU for some of them which gives you access to L&N upstream of the RCD(s).

 

[JohnW2]

Well that will tell you that the gain is still correct, it won't tell you that the (IMO possibly more important) offset is still correct. EG, you check and find the EFLI is (say) 0R2, and with the test resistor, 1R2. What if the meter offset is wrong ? That 0R2 might be (say) 0R8 - cutting your margins for OPDs operating in a timely manner. So yes, the 1R test resistor will detect some faults, but it by no means "proves" the unit is operating correctly.

That is all true. However, when one thinks about how the machine is presumably working (in some sense, measuring voltage across an impedance), I would think that it's pretty unlikely that there would be a 'systematic offset', such that it would display a non-zero result when there was no voltage across the impedance.

That's OK - do the equipment check using L-N (so determining PSC) which doesn't trip an RCD. However, if the purpose of possessing the equipment is to do your own tests, you'll need to open the CU for some of them which gives you access to L&N upstream of the RCD(s).

That's all true - but, as I said, most of these meters (I can't speak for the OP's - which is broken, anyway!!) have a 'non-trip' mode for measuring EFLI in the presence of a (30mA) RCD, anyway.

Kind Regards, John

 

[SimonH2]

However, when one thinks about how the machine is presumably working (in some sense, measuring voltage across an impedance), I would think that it's pretty unlikely that there would be a 'systematic offset', such that it would display a non-zero result when there was no voltage across the impedance.

Two problems there.
1 - it's not showing a non-zero result with zero voltage. It's measuring on a live circuit.
2 - Offset errors are not an insignificant problem - at least as "difficult" as scale errors.

At work we had (past tense, it's was crap, broke down bit by bit, and it's been replaced) a 16kVA UPS. Through SNMP it was possible to ask for various measurements - such as output voltage and current. The offset on the current measurement was (IIRC) around 2A - meaning it under read by the equivalent of around 1/2kW Once the load reached around 1/2kW then the scale was fairly accurate.

The manufacturer "excused" such inaccuracy by stating that it's "not a precision measuring instrument"

 

[JohnW2]

However, when one thinks about how the machine is presumably working (in some sense, measuring voltage across an impedance), I would think that it's pretty unlikely that there would be a 'systematic offset', such that it would display a non-zero result when there was no voltage across the impedance.

Two problems there. ... 1 - it's not showing a non-zero result with zero voltage. It's measuring on a live circuit.

I worded that very badly. I should have written something like "...such that its measuring circuitry would think that there was a non-zero voltage across the impedance when there was no such voltage".

2 - Offset errors are not an insignificant problem - at least as "difficult" as scale errors.

No argument with that, as a generalisation - indeed, as you said before, they can often be the worse of the two main types of systematic error. It obviously depends on how the circuitry is arranged, but I was suggesting that I would think that offset errors would be rare in terms of the sort of measurement we were talking about - it would be broadly comparable with the situation in which a DVM gave a non-zero voltage reading when there was no pd between its probes - which is certainly not something I've ever seen happen.

Kind Regards, John

 

[SimonH2]

... it would be broadly comparable with the situation in which a DVM gave a non-zero voltage reading when there was no pd between its probes - which is certainly not something I've ever seen happen.

But how do you know you've never had a situation where you've had a non-zero voltage under reported ? An offset error doesn't necessarily mean "zero reading for non-zero input", it can be the other way round. For DC you'd be likely to notice a negative reading for zero input, but for AC you wouldn't generally see a negative reading.
That's what we had with our UPS - zero reading for zero load, and if you didn't "check" against a known load then you'd not know that it needed a very non-zero load to get a non-zero reading. I tested it before installation using fan-heaters as dummy loads. So it was fairly obvious when a 1kW load was reported as about 0.5kW, and 2kW as about 1.5kW. Has someone just plugged it in (with a typically unknown total load) then they've not have seen the error.

 

[JohnW2]

... it would be broadly comparable with the situation in which a DVM gave a non-zero voltage reading when there was no pd between its probes - which is certainly not something I've ever seen happen.

But how do you know you've never had a situation where you've had a non-zero voltage under reported ?

On the contrary, I know that I have had (faulty or cra**y!) meters which (by comparison with others) have under-reported non-zero voltages, with both AC and DC. However, in the absence of a non-zero reading (negative, for under-reported results, positive for an over-reported one, in the case of DC) when a zero voltage was applied, this cannot really have been due to a fixed/constant offset error.

An offset error doesn't necessarily mean "zero reading for non-zero input", it can be the other way round. For DC you'd be likely to notice a negative reading for zero input, but for AC you wouldn't generally see a negative reading.

As you say, with DC there is no problem at all - if there is a fixed/constant offset error, then there will be a reading (positive or negative, as the case might be) with zero applied voltage. However, even when measuring AC, it is likely that one would, again, get a non-zero reading with zero applied voltage if their were a fixed/constant offset error (don't forget that most of the electronics will be working with DC, even for AC measurement).

I'm not sure what you are thinking would happen if their were a 'fixed/constant offset error' in an AC voltmeter ... say there was a fixed offset of exactly +10V. The closer the true applied voltage got to zero, the closer would the displayed voltage get to 10V. Even if the true voltage were, say, 0.00000001V, if the meter was up to it, it would display 10.00000001V - but are you suggesting that the reading would suddenly 'jump' from 10.00000001V to zero if the applied voltage was reduced from 0.00000001V to exactly zero? I think you are probably thinking about something more complex than a simple fixed/constant offset error.

Kind Regards, John

 

[SimonH2]

I'm not sure what you are thinking would happen if their were a 'fixed/constant offset error' in an AC voltmeter ... say there was a fixed offset of exactly +10V.

And what if there's an offset the other way ? I'm thinking in terms of a system that's been designed on the basis that there can't be negative AC voltages, so then anything under 10V would be displayed as 0, 11V would display as 1V, and so on.
As I say, that's effectively something I've observed - albeit with a different type of kit.

The thing is, without knowing the details of the internals (and software) of a device, there's no way of knowing exactly how it would behave. So while doing the "measure - add known difference - measure again" technique will give some confidence that the system isn't completely fubar, it can't guarantee to catch all possible errors.

 

[JohnW2]

I'm not sure what you are thinking would happen if their were a 'fixed/constant offset error' in an AC voltmeter ... say there was a fixed offset of exactly +10V.

And what if there's an offset the other way ? I'm thinking in terms of a system that's been designed on the basis that there can't be negative AC voltages, so then anything under 10V would be displayed as 0, 11V would display as 1V, and so on.

That's obviously a possibility (as you go on to say, depending upon the 'internals/firmware) - I'm just saying that I doubt it would often arise in practice. As I said before, most of the processing will inevitably be done at DC, so the sort of constant offset error in that processing you're talking about would result in negative 'answers' from that processing. You're suggesting that ('since AC can't be negative), it would display such results as zero. That seems surprising to me - I would probably expect an 'error message' (present the moment the meter was switched on, with no volts across its probes), rather than a zero reading. However, as you go on to say, we just don't know for sure ...

The thing is, without knowing the details of the internals (and software) of a device, there's no way of knowing exactly how it would behave. So while doing the "measure - add known difference - measure again" technique will give some confidence that the system isn't completely fubar, it can't guarantee to catch all possible errors.

Agreed. That's why I was slightly guarded when I wrote:

... it does give a reasonable degree of reassurance that the meter is probably giving correct answers.

Kind Regards, John