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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?
Comments: To: David Beierl <dbeierl@ibm.net>
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"


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