Date: Fri, 27 Aug 1999 10:45:26 -0700
Reply-To: Coby Smolens <cobys@WELL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Coby Smolens <cobys@WELL.COM>
Subject: Re: Which Voltmeter to use?
In-Reply-To: <4.2.0.58.19990827100126.0712b540@pop01.ny.us.ibm.net>
Content-Type: text/plain; charset="iso-8859-1"
I don't dispute that there must of necessity be delay between arrival time
of the input signal and the display update, which must always be playing
catch up. What I'm interested in is whether the input signal is constantly
monitored, or read, in real time, so that a sudden glitch like a 0 V. signal
lasting, say 300 MS, will be captured and held for later retrieval with the
unit in Min-Max recording mode. It is my understanding (and I don't pretend
to clearly understand the electronics - I just use the thing) that this IS
the case with fairly sophisticated DMMs like the ones we use in the shop.
So the Fluke 88 User's Manual spec sheet says the following:
Min-Max Recording
NOMINAL RESPONSE ACCURACY
100ms to 80% Specified accuracy +/- 12 digits for
changes > 200ms in duration
1 Sec Same as specified accuracy for
changes > 200ms in duration
It is apparent you have a deeper working knowledge of these things than I
do - do these specs support what I'm thinking?
Coby
-----Original Message-----
From: David Beierl [mailto:dbeierl@ibm.net]
Sent: Friday, August 27, 1999 7:06 AM
To: Coby Smolens
Cc: Vanagon List
Subject: RE: Which Voltmeter to use?
At 02:51 8/27/99 -0700, Coby Smolens wrote:
>I'm gonna check this in the literature tomorrow - I seem to remember that
>although the readings you see on-screen on a good DMM, whether numerical
>output or simulated analog (the little bar graph thingy that approximates a
>moving needle) are "averaged", the min-max feature works differently. Since
>it doesn't have to produce real-time displays it records actual minimum and
>maximum readings and holds them in memory until one requests them. Thus, if
>the V. drops to zero, even for a thousandth of a second, that fact will be
>captured by the machine. Or have I been bamboozled?
Back when I was building these things (before the bar-graph thingy
appeared) they used a technique called "dual-slope integration" where they
use the input signal to charge a capacitor for a specified time, then
measure the time it takes to discharge it at a constant rate and display
the count as (voltage, pressure, weight, whatever). The display processing
itself was essentially instant, but it only got data to display every
couple seconds. With that technique there is no such thing as an instant
measurement. But they may be doing it differently now...
david
David Beierl - Providence, RI
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
