Freaky Amazing DMM?!

Arlowe wrote:


I don't want your job. You're the expert at stupid.
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krw explained on 21/01/2009 :

I don't test for live circuits with a DMM. I use test lamps or a low(er) impedance voltmeter. If you worked for me I would sack your arse if I caught you using a DMM to test for live.
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On Thu, Jan 22, 2009, 7:51pm snipped-for-privacy@gmail.com (Arlowe) wrote: krw explained on 21/01/2009 :
this immortal opus: krw used his keyboard to write :
says...> krw explained on 19/01/2009 :
on 16/01/2009, Paul supposed :
I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over* 100Mohm impedance in 400.0mV mode. Nothing new there, many DMM's have selectable "high impedance" or "HI-Z" modes on the mV range. e.g. the Fluke 87. I've looked at the specs of ~ 30 DMM's today, include a lot of fluke's, and never seen anything near 14Gohms impedance. Keithley has an electrometer that's probably higher. Most DMM's are around 10Mohms (not gigaohms) input impedance. Don't you think 14 gigaohms is a bit high? PL The evil thing about Voltmeters with very high impedance is they will read induced voltages that analog meters wouldn't. It makes a voltmeter useless for checking for live circuits in a crowded panel. A craftsman never blames tools for his failures. Hackers, on the other hand... If you work with electricity you had better know the limitations of your tools or you will find them...the hard way. That is certainly true (though perhaps your heirs are the ones who will find you), but doesn't modify my statement. In this case, the tool *can* be used as long as the one using it knows what he's doing. Ok....everything seems to pass right over your head.... I pointed out a limitation of a DMM and you seem to be inferring that I am somehow a hack who blames his tools... BTW> I am not a "craftsman" I am an electrcian.. You don't do what I do for as many years as I have without knowing what the fuck you are doing. What's wrong with using a hi Z voltmeter in a live panel? I do it more often than I really care for, and never have a problem. There isn't anything wrong with it. Analog meters are dead. High impedance digital meters are only a problem if there is a loose nut inside the panel, holding the leads. I don't test for live circuits with a DMM. I use test lamps or a low(er) impedance voltmeter. If you worked for me I would sack your arse if I caught you using a DMM to test for live.
=>Arlowe: Ye shouldest cover thine arse and prepareth for a loss if mine DMM, openend Ammeter would causeth thee to derecruit me., there is no law or work code regulation violated by testing a live circuit with the proper test equipment and procedure...some DMMs' come with a clip on carrying case to facilitate it being attached to chassis bodies etc. for hands free testing.I wouldnst' work for thee anyway, Troll.
The Mighty WontVolt
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snipped-for-privacy@gmail.com says...>

ps.com...
SO there *is* something wrong with a digital meter. I thought you sparkys were just telling me that the display type didn't matter.

You couldn't afford me. I understand electricity.
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It happens that krw formulated :

bullshit. you are just another wannabe...
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snipped-for-privacy@gmail.com says...>

oups.com...
Heavens no! I have no interest in becoming a sparky. Crappy work, low pay, and no fun.
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krw laid this down on his screen :

Only for cable monkeys. Find the right niche and you can name your price.
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snipped-for-privacy@gmail.com says...>

groups.com...
So even in a perfect niche it's still crappy work and no fun, but pays well (doubt it). Even your perfect niche is two huge steps down. No thanks.
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It happens that krw formulated :

So you know nothing about working in the field...and you accuse me of not knowing how to use my tools?
lol
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snipped-for-privacy@gmail.com says...>

legroups.com...
I know enough to have a better life.

No, I accuse you of not understanding electricity.
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krw formulated on Friday :

Just goes to show you know nothing about the field.

Well, I am accusing you of the same.
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snipped-for-privacy@gmail.com says...>

oglegroups.com...
Hardly, sparky.

You have demonstrated you total lack of knowledge here repeatedly.
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Nice to know someone round here knows what they are doing!
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krw wrote:

Actually it does cause false voltage readings even in circuits with no "loose nuts". You can connect the meter to a point in the circuit that is open somewhere, perhaps because of an open switch contact, and you get false voltage readings that can range from 10V to close to line voltage due to stray pickup of the wires. It causes electricians to sometimes misdiagnose problems, and it causes confusion. I often demonstrate this to my training classes so that they are aware of it.
This is an unavoidable and unfortunate side effect of high impedance digital meters. You need to understand it when you use them so that you can question and investigate readings that don't make sense.
Analog meters don't have this problem, but they are disappearing because the analog circuits can not meet the necessary safety standards.
Ben Miller
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Ben Miller wrote:

You mean that cheap analog VOM had low impedance inputs. There were millions of VTVM and FET input analog meters with a 10 Megaohm input impedance in use over the years, and a lot are still in use.
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Michael A. Terrell wrote:

Those are great on a lab bench, however very few were ever used for industrial power measurements. FET inputs are not notoriously rugged, although they could be used if desired. It is difficult to run an extension cord for a VTVM out on a factory floor, not to mention the smoke that can escape when you apply a grounded test lead on a power system measurement, assuming that you survive to talk about it :-(
You will not find very many analog meters with an IEC CAT rating, with good reason. The typical analog industrial meters such as the Amprobe RS series or the Simpson 260 series have resistor networks on the inputs, and very likely don't have the requisite creep and clearance distances in their construction. There are also problems with the Ohms circuits. One of the IEC requirements is that you can safely apply maximum rated voltage (ie. 600 or 1000 volts for example) to the meter inputs with the range and mode switches set to any setting. The meter doesn't need to continue working, just fail safely or do nothing. Many meters have fuses for the amp ranges, but analog ohms circuits can fail catastrophically if high voltages are applied to them. The manufacturers just can not justify the redesign of those meters to meet the safety standards.
Ben Miller
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Benjamin D Miller, PE
www.bmillerengineering.com
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Ben Miller wrote:

Some VTVM had a built in battery for portable use. They were rare, but they existed. They used subminiature tubes with a 1.5 or 3 volt filament like those in early hearing aids.

The 260 is a dinosaur, over price instrument in today's work environment. Even if it was modified and certified, it would be sold to a niche market and not worth the engineering costs.
The early FET input analog meters I've used would die if their was a tiny spark when probing. If they had a small choke on the input to filter the RF, they were fine, but it would void the certification.
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Ben@somewhere says...>

Of course not. The "loose nut" is the one interpreting the measurements.

Right. The typical sparky is clueless. This fact has been demonstrated here endlessly.

It certainly may be if you're clueless.

"Analog" has nothing to do with it.
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..
This is an oft used but seldom understood saying. The reason a craftsman supposedly never blames his tools is because a craftsman tends to buy quality tools in the first place.
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I thought it was to do with a craftsman knowing the limits of his tools, thus not blaming them for over-reaching.
Related to that, I find the best definition of a toolmaker is one who achieves greater accuracy than was provided, and makes a system to easily repeat that gain.
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