Multimeter, testing and reliable readings.

[dishman]

I bought a seemingly good quality multimeter. It has a good range of basic features with a good level of protection.

It's more something to learn with and practice using, but I did not want to get something dirt cheap, nor something with features I did not need nor would ever use.

I am wondering how I know if some readings I am getting are reflective of accuracy issues with this multimeter, or my own lack of skill (thus far) in using it.

I got the amazon commercial 90DM610. It has the additional on temperature probes over the one in this review. They are made for Amazon by another company that has a good reputation. They seem to get very good reviews for what you pay.

Anyway, my confusion comes from a few readings I have taken recently. The readings I get seem to fluctuate a lot, to the point where it makes me suspicious there is an issue.

For example, I was recently fault finding on a motor commutator, measuring the resistance between each bar.

While it sometimes seems to settle on between 0.5 -1 ohm, it can fluctuate wildly before that. Often starting at 20ohms + and slowly going down. Even the slightest movement can cause fluctuations in the reading. Sometimes it does not seem to settle at all.

 

[JohnW2]

....... Anyway, my confusion comes from a few readings I have taken recently. The readings I get seem to fluctuate a lot, to the point where it makes me suspicious there is an issue. .... For example, I was recently fault finding on a motor commutator, measuring the resistance between each bar. .... While it sometimes seems to settle on between 0.5 -1 ohm, it can fluctuate wildly before that. Often starting at 20ohms + and slowly going down. Even the slightest movement can cause fluctuations in the reading. Sometimes it does not seem to settle at all.

With any meter, when measuring 'low' resistances, one is at the mercy of the quality of the connections between the probes and whatever is being measured. Even if one 'clips the probes together' (so the reading should be 'zero ohms') it's quite possible to get a reading of at least an ohm or three until one wiggles the probes around a bit. That's why I'm always a bit uncertain about some of the very low resistances which electricians often routinely measure, record and seemingly 'have faith in'.

If measurements gradually change and then 'settle down', despite your not 'touching anything', that can be due to capacitance/.capacitors associated with whatever is being measured.

Kind Regards, John

 

[Jackrae]

Measuring commutator resistance will generally give very inconsistent results. As you move the rotor you generate an emf which will affect the resistance reading and as you bridge adjacent 'bars' the resistance varies. Similarly any variation in contact area or pressure from the brushes will vary the reading. ie too many variables to get a consistent reading.

The only way to ensure you have an accurate meter is for it to be provided with a calibration certificate issued by a reputable (in country) test laboratory. Anything supplied without a reputable certificate and bought at a low price is probably a reasonable device but of questionable accuracy.

If you want to check its ability to measure resistance then buy some 1% resistors and see what the meter reads.

 

[dishman]

I was wondering if moving the commutator could be inducing some kind of interference.

I may remove it from the housing and see if it reduces the variance.

Just to point out, I was not looking for accurate measurements per say. I was following a fault finding guide.

The guide suggested that while the values did not matter in themselves, a damaged bar would show an inconsistent result compared to the general value the others gave.

I was confused as I was seemingly getting wildly different values all the time.

I can actually see which bar is damaged, so I was more trying to see how that may manifest itself in a reading.

 

[ericmark]

Reading below 5Ω we tend to use a low ohm meter that uses at least 200 mA for the test.

As to amature we use a growler which is a coil on a laminated former, it induces voltage into the winding of the armature, and also if winding is shorted the magnetise increases so a hack saw blade hovering over the armature also helps.

 

[plugwash]

That's why I'm always a bit uncertain about some of the very low resistances which electricians often routinely measure, record and seemingly 'have faith in'.

It's interesting isn't it, electricans "low ohm" meters seem to produce far more stable measurements of low resistances, than I have ever achieved with a multimeter (and I'm not limiting myself to cheap handhelds here).

I suspect the higher test current is what is making the difference and that the contact behaviour is non-linear.

 

[JohnW2]

It's interesting isn't it, electricans "low ohm" meters seem to produce far more stable measurements of low resistances, than I have ever achieved with a multimeter (and I'm not limiting myself to cheap handhelds here).

It's certainly 'interesting', but I doubt that it's anything to do with the meters.

I have several multimeters (varying from the very cheap to the expensive), and also a Fluke MFT (which I presume qualifies as an "electrician's 'low ohm' meter"), and all behave similarly in this respect. If I have a length of cable with a resistance of, say, about 0.2Ω, if I fiddle with the probes I can usually get a reading of at least 'an ohm or two' - and that is as true with the MFT as with any of the multimeters.

The saving grace is that, provided it is correctly calibrated, what none of these meters can do is produce a reading which is less than the true resistance, and when an electrician measures a 'low resistances' it is almost always the case of "the lower the better". The readings will therefore 'err of the side of safety', and I suspect that what most of them do (as I do) is fiddle with the probes until they get the lowest possible reading, safe in the knowledge that it cannot be 'too low'

I suspect the higher test current is what is making the difference and that the contact behaviour is non-linear.

Test current could well be a factor. My MFT seemingly uses 210 for the lowest resistances, but th documentation of multimeters rarely indicates the current used (and I've never bothered to measure\). However, they nearly all use 9V worth of battery(ies), so all have the potential use use similar currents.

Kind Regards, John

 

[Harry Bloomfield]

If measurements gradually change and then 'settle down', despite your not 'touching anything', that can be due to capacitance/.capacitors associated with whatever is being measured.

Also true if there are capacitors in circuit, charging up and/or long cables, when measuring high resistance values/insulation.

 

[Harry Bloomfield]

Test current could well be a factor.

Test voltage too - the higher the voltage, the better is can cut through the murk on the points of contact.

 

[JohnW2]

Also true if there are capacitors in circuit, charging up and/or long cables, when measuring high resistance values/insulation.

I'm not sure where your '"Also" comes from - since that's exactly what I wrote, isn't it?

Kind Regards, John

 

[JohnW2]

Test voltage too - the higher the voltage, the better is can cut through the murk on the points of contact.

Hmmm. A fine theory - but if you tried to put anything other than 'a very low voltage' across a 'very low resistance', then something rather 'interesting' might happen! I reality, I think it would just be self-defeating since, unless something 'went bang' first, the battery voltage would probably fall to near-zero!

Kind Regards, John

 

[Harry Bloomfield]

I'm not sure where your '"Also" comes from - since that's exactly what I wrote, isn't it?

Kind Regards, John

Not is what I read.

 

[Harry Bloomfield]

Hmmm. A fine theory - but if you tried to put anything other than 'a very low voltage' across a 'very low resistance', then something rather 'interesting' might happen! I reality, I think it would just be self-defeating since, unless something 'went bang' first, the battery voltage would probably fall to near-zero!

Yes, you might get a more accurate low reading. John - just how do you think the higher current is caused to flow through a low resistance, except by increasing the test voltage? Remember Ohms law?

 

[JohnW2]

Not is what I read.

Eh?

Kind Regards, John

 

[JohnW2]

Yes, you might get a more accurate low reading. John - just how do you think the higher current is caused to flow through a low resistance, except by increasing the test voltage? Remember Ohms law?

Sure, but (because of Ohm's Law), if the resistance being measured is very low, then you are not going to get enough voltage across it to "cut through the murk on the points of contact" whilst still limiting the current to an acceptable level.

With a resistance to be measured of, say, 0.2Ω and a current limit of, say 200 mA (the most one could probably expect to get from the sort of batteries in a meter), the voltage (across whatever was being measured) could only be 40 mV, which is not really enough to 'cut through' anything, is it?!

Kind Regards, John