21 Sep
2010
21 Sep
'10
8:16 a.m.
On 9/21/10 11:07 AM, dwight elvey wrote:
That's excellent! For every rule, there is an exception. ;)
Date: Tue, 21 Sep 2010 10:17:41 -0400
From: mcguire at neurotica.com
To:
Subject: Re: Multimeter recomendations
On 9/20/10 8:02 PM, Shoppa, Tim wrote:
This is not always true. I have a cheap meter that I bought
for about $8 at HSC. It has always been dead-nuts-on.
It even has a socket for testing transistors ( never use that ).
It has calibration pots if needed but I've never needed it.
I've used it in all kind of realistic temperatures ( those I
can withstand ) and it hasn't had a problem.
I don't use it to calibrate my supply output ( for that I use
a 5 digit Fluke ). As a handy hand held meter that I
could always not worry about breaking, it has worked quite well. In terms of used Flukes, I highly recommend any
used 77-series unit. You can critique the designs because of what you saw as failings of
your Fluke 85, but believe me, the LCD display on any Fluke LCD DMM will be far more
durable than the faceplate on my Simpson 260 or Triplett 630.
At the very low end, in the $3 to $5 range there are "disposable" DMM's
available at discount stores here in the US. They are not a joy to use but they do mostly
work.
I've evaluated five or six of these. The accuracy and stability of
these things, especially in the face of temperature variations, is utter
crap. Don't ever adjust a +5V supply in a PDP-11 using one of these. ;)
Some of these were tested as being 15% off, measuring nearly six volts
from a (very tightly calibrated) 5V supply!
Buying cheap multimeters (or any other tool, really) is a false
economy. My daily-use handheld DMM is an original Fluke 77 (no suffix)
that I stole from work so long ago that I don't mind mentioning it here.
;) I've had it for almost twenty years, and it was well-used when I got
it. It works as if it were brand new. I don't think I've ever seen such a meter
being 15% off unless
it was broken to start with.
That's kinda my point. ;)
When you say, some of these were tested, what
exactly does
that mean? Which brands and were they calibrated or just
off the shelf?
Just off-the-shelf. I don't recall the brands offhand; they're in a
box somewhere. I got curious about cheapie DMMs after a discussion with
a colleague many years ago, and I bought a bunch of them. They cost no
more than $20/ea. I tested them against my lab standards (I have
NIST-traceable voltage and resistance standards here) and did some very
basic "stick it in the freezer for five minutes then retest, hit it with
a hair dryer and retest" type of thermal stability testing. The results
were less than stellar.
-Dave
--
Dave McGuire
Port Charlotte, FL