[time-nuts] multimeter

[Bob Albert]

 That is not unusual.  Back then, all VOMs and most VTVMs pumped substantial current through the unknown.  When I became aware of that I started paying closer attention to how measurements were made.
In general, 100 mA is typical short circuit current on low Ohms range.  Some, such as the Triplett 630, go even higher.  (I suggest you measure the short circuit current on low Ohms range on your most-used meter before you burn out somehting.)
So you are right to caution people to avoid thinking of test gear as magic boxes.
Another thing that came to my attention is the fact that the current through the unknown will be different for different values ot the unkown.  Not so much a factor with the 410B, but most VOMs rely on batteries and the terminal voltage will change as the unknown value changes, making the calibration poor for low resistance.
For instance, if center scale is 15 Ohms and you short the leads to zero the meter, then measure a 15 Ohm resistor, it will read less than 15 Ohms due to the slight increase in battery voltage at half current.  It gets worse as the battery ages and for higher resistance on that range.  The crowding of calibration at the high resistance end of the scale should not be an invitation to estimate high resistance either, since the mechanical meter zero becomes a factor.  Pointer 'stiction' can cause errors as well.
Bob
    On Sunday, March 24, 2019, 5:01:49 PM PDT, Ben Bradley <ben.pi.bradley at gmail.com> wrote:  
 
 I have a comment regarding "classic equipment." About 20 years ago I
got an HP 410B VTVM, was pleasantly surprised that it still appeared
to be within the stated accuracy, but was astounded to find the
short-circuit current (measured the current range on a modern DMM) on
the 1-ohm scale was 100mA. This is from memory from back then, so I
should double-check it, but it was still a lot.

This brings to mind the importance of knowing the ins and outs of
equipment, and that you may not want to, for example, check the
polarity of a sensitive transistor with this unit's resistance scale.

On Sun, Mar 24, 2019 at 12:11 PM William H. Fite <omniryx at gmail.com> wrote:
>
> Fluke is the yellow one. Agilent is the orange one. Both are excellent. You
> made a fine choice.
>
>
>
> On Sunday, March 24, 2019, Jim Palfreyman <jim77742 at gmail.com> wrote:
>
> > Hi All,
> >
> > Thanks so much everyone for your comments. I especially liked "get the
> > orange one". Because that's what I did get (I think it's orange - could be
> > yellow).
> >
> > I was deliberately vague (apologies) but I just wanted a broad response.
> > Which I got.
> >
> > I've settled on the Fluke 87V. It's on it's way.
> >
> > Jim
> >
> >
> > On Sun, 24 Mar 2019 at 12:00, Bob Albert via time-nuts <
> > time-nuts at lists.febo.com> wrote:
> >
> > >  I have a cheap Chinese clamp-on power line ammeter I use occasionally.
> > > Its lowest range is 20 A I think but I built a 10:1 expander (very
> > > simple).  I also built a line separator so I can grip just one line.
> > > Bob
> > >    On Saturday, March 23, 2019, 5:00:23 PM PDT, Jim Harman <
> > > j99harman at gmail.com> wrote:
> > >
> > >  jim77742 at gmail.com said:
> > > > I think I'm in the market for a new digital multimeter.
> > > > Could I have some recommendations?
> > >
> > > >
> > > > It all depends on whether you are checking for a bad power supply or
> > > testing the linearity of a 24 bit DAC.
> > >
> > > For the former, I find a built-in clamp type AC/DC ammeter with a
> > > reasonably sensitive range (40 A or less) is very handy. My current
> > > favorite is the Mastech MS2108, from Amazon.
> > >
> > > -Jim Harman
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