Re: Repairing fuel level sender in minivan
- From: "Ted Mittelstaedt" <tedm@xxxxxxxxxxxxxxxx>
- Date: Sun, 16 Oct 2005 02:40:51 -0700
<treeline12345@xxxxxxxxx> wrote in message
news:1129416424.864582.194350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Ted Mittelstaedt wrote:
>
> > It was a calculated risk. After the bending procedure I tested it and
even
> > putting a fair about of lateral force on the float arm and moving it,
the
> > wiper
> > did not pull away from the rheostat. And it tested out good on the
> > ohmmeter while moving the arm (I used an analog ohmmeter of course)
> > no dropouts.
>
> I'll bite. Why was an analog ohmmeter necessary for testing the
> rheostat?
>
> Is there something unusual about the rheostat?
>
> Granted, it's easier with an analog to see a smooth, clean reading
> sweep so probably almost having to graph the data.
>
> If the rheostat is a small area, I see your point about the analog
> meter.
>
Has nothing to do with that. A DVM, even a graphing one, is not
anywhere near sensitive enough for a changing resistance like testing
a potentiometer (rheostat)
The problem with potentiometers (pots, in the trade) is as Glenn said
areas of the resistance material can wear out. This can be seen by
when you move the wiper arm of the pot (turn the handle, slide
the level, whatever) and you see the needle of the analog VOM move,
if the wiper hits any areas where the resistance material is missing, even
minute, tiny areas, there will be a slight almost unnoticable stumble
or drop of the needle as it is sweeping up the scale of the VOM.
You may not see this unless you wipe the arm back and forth several
times. Of course, it's arguable if for a fuel level guage you need that
kind of
sensitivity, but for an audio pot you certainly do.
A non-graphing DVM will of course be nothing more than a jumble of
numbers, and even a graphing one probably would not show that kind of drop..
And a cheap, small pocket analog VOM without a sensitive needle would
also probably not show it either. An oscilloscope could show it, of
course.
> Generally I reserve the analog for testing capacitors. My analog uses
> so much current for its testing that it can actually polarize delicate
> electrodes if I'm not careful. But it's a cheap analog meter though.
>
I have several DVMs also, my analog meter is an older Radio Shack model
back when RS actually sold decent analog VOMs, with a large meter in it.
Besides testing pots, it is also useful for testing diodes, as DVM's
generally don't
supply enough power to a diode to get it to work.
Unfortunately, the big problem with analog meters is that they are only as
good as the meter used, and it is expensive to make a large, sensitive
meter. Back in the olden days, even though a good multimeter was
expensive, an oscilloscope was far more expensive, so people were willing
to pay for an expensive multimeter. Today, scopes are cheap and the
few testing tasks that you can't use a DVM for and need an analog
meter for, a scope can do. So, people buy scopes and DVMs and can
do everything that an analog meter can do, so there's no market for
the good ones anymore.
Ted
.
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