[volt-nuts] Fluke differential meters

[J. Forster]

They are useful for testing power supplies for regulation. In fact, IMO,
they are better for that than a DMM.

Basically, you null the meter at light load, then apply a load and read
the voltage (drop) out directly. It's trivial to see 1 mV on 100 V.

YMMV,

-John

======================





> Joe wrote:
>
>>The first one is a Fluke 893A and is solid state.
>>      *   *   *
>>The second one is a Fluke 803B (I think. There is no label on it).
>>      *   *   *
>>In the long run, are these things worth having?
>
> What they are good for is making DC measurements of very high
> resistance sources without loading the source.  They contain an
> internal voltage reference, a voltage divider, and a null meter that
> compares the divided reference to the unknown.  Their accuracy
> depends on the accuracy of the internal reference and the voltage
> divider and the noise and offset of the null amp.  [The 893A and 803B
> also include both a normal TVM/VTVM mode and an AC rectifier, which
> are nothing special.]
>
> Separate, high-accuracy components -- a precision woltage source such
> as a Fluke 732A, a Kelvin-Varley bridge such as the Fluke 720A, and a
> null meter such as the Keithley 155, Fluke 845A, or HP 419A -- can be
> used in the null configuration to make voltage measurements about as
> accurate as most home-lab amateurs can aspire to.
>
> In the 803B and 893A, the voltage reference and divider accuracy and
> the null amp noise and offset are not as good as in the above
> instruments, so both the accuracy and resolution of measurements are
> correspondingly less.  Typically, the last decade or two provide
> resolution but not accuracy.
>
> These days, unless you are measuring DC sources with very high
> resistance, a good DVM can do the same job more accurately and with
> much less effort.  But they are cool pieces of instrumentation
> history, so if you are the curator type they may be worth having even
> if you don't use them much or at all.
>
> Best regards,
>
> Charles
>
>
>
>
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