[volt-nuts] Fluke 720A Kelvin Varley Divider Questions

WB6BNQ wb6bnq at cox.net
Sun Sep 19 22:05:03 UTC 2010

Hi Charles,

Well, it is OK as a general purpose null meter.  However, it does not match up to
the specs and operational experience of Fluke 845A (all variants).  Particularly
with regard to leakage, noise and stability.  The Fluke 845A goes down another
range to 1 uv full scale.  The Fluke 845A is 10 megOhms on the 1 uv through the
100 millivolt range, whereas the hp is only 100K on the 3 uv to 3 millivolt
ranges.  It has sturdier construction.

The input resistance becomes important as the two nulled signals become "UN"
nulled.  For instance if you were comparing a Standard Cell against a very stable
Reference Power Supply, any off nulled signal means you are either sucking
current from or forcing current into the Standard Cell.  A BIG NO-NO !  The
difference from 100K to 10M means that with the HP you would have 100 times more
current flowing than with the Fluke.  Standard Cells do not like to have any
current flow as it upsets their stability and makes them unreliable.  Clearly,
the Fluke 845A has superiority in this case and was the reason for its inception
back in the Standard Cell days.  Interestingly, no one has made a meter to match

If you have a choice, definitely go for the Fluke.  Otherwise, and if the meter
is working, it is an OK item.


"Charles P. Steinmetz" wrote:

> Bill wrote:
> >You're kidding, right ?
> I've never used one, and I'd be interested to know what you (and
> others) think its failings are.
> Best regards,
> Charles
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