Re: Freaky Amazing DMM?!


Lostgallifreyan wrote:

"Michael A. Terrell" <mike.terrell@xxxxxxxxxxxxx> wrote in
news:rOWdnQ1TB4W1kOHUnZ2dnUVZ_v7inZ2d@xxxxxxxxxxxxx:

Same applies to responses to Stewart, who is speaking from
professional experience.


My experience is a broadcast engineer, (The largest was at a 5 MW UHF
TV site.) industrial electrical work, and specialized electronics that
you'll never see, without going to the International Space Station.


And both extremly impressive too, yet apparently in disagreement.

I don't think that high an experience is needed to understand this anyway, I
learned it at 14 when an aging friend of the family taught me how to build my
own (and first) multimeter.


Some people work with electricity their entire life and never really
understand it. There are dozens of basic voltmeter types, and you need
to know enough about them to use them properly. Have you ever used a
frequency selective voltmeter? How about a field strength meter? A
true RMS voltmeter that will display a .01 dB change at up to 20 MHz?
How about a Vector Voltmeter, or a Differential Voltmeter? One of the
old HP AC VTVMs that read voltage well up into the VHF range? There are
lots of specialized meters out there.


What is wrong with pointing out that there are other, and better
tools for the job? I built a radio at eight years old, and really
didn't understand a lot about how it worked, but I did make it work. At
13 I was repairing radios, working part time in a TV shop. I went
straight from high school to working at a TV bench. I have a poorly
built Eico 1000 Ohm per Volt multimeter someone gave me. I'll redo all
the bad solder work some day, but I would never actually use it for
anything. I use a DVM for electrical work, and it is the meters from my
electronics bench. I've used it to troubleshoot things wire pullers
couldn't figure out with their meters. Instead of get a 'thank you', I
was told off. OTOH, they had spent half a day trying to decode a bundle
of wires in a crowded conduit. I identified and tagged all the wires in
less than 15 minutes.


An equivalent to the specialized voltmeter for wire pullers is the
cheap CATV field strength meter with a few LEDs to give a basic Go/No Go
test for cable TV installers. It takes 30 seconds to train someone off
the street how to use it.


Ohm's law.


And common sense.


And a bit of awareness of insulation strength when high volts are involved.


Er work with HV armored cable? You can't see the insulation, because
it is made like Heliax, with a polypropylene filled insulator. A real
bitch to cut, strip and terminate. Do it wrong and you have a
spectacular fire.


If you're using a low resistance input you might have to take it into account
for accurate measurements but on mains, the error is small, so it's worth
keeping inputs resistance low for meters dedicated to such systems, for
reasons plenty of posts have explained, so I won't flog that horse now.


Only for those who don't understand how to read a real meter. That's
most users, or they wouldn't make so many of the crappy things.


If you have strong insulation, you can probe an HV circuit without trouble,
just make sure you understand what the meter says. If a meter designed to tax
the system as lightly as possible says 83V it means 83V, the problem isn't
the meter, you just have to know enough to interpret the truth it tell you.
(Mike Terrell got this one right). If you also need to know current through
the same meter, you could do it by measuring small voltage across a part of
one conductor, then measuring resistance of that part after removing power.
Most current meters just do this internally anyway, but they 'know' the
resistance of their shunts so they calculate correctly anyway.


A clamp on meter is safer and more accurate to read the current in a
working circuit. They are fairly cheap these days, too.


So the question isn't who is the most experienced, it's who is right? And
take care, because if two people with real experience start arguing over
something as basic as Ohm's law, they'll do each other's reputation harm, as
well as making it hard for newcomers to trust what they read here.


The bigger problem is ignorant trolls who infect every thread,
spouting garbage. Unfortunately, these bottom feeders are growing in
numbers. :(


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