[volt-nuts] Fluke 335A versus HP 740B
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Sat Dec 15 10:28:31 UTC 2012
>I have a Keithley 155 Null Detector how does that rate?
The three usual suspects are the Fluke 845AB, the HP 419A, and
Keithley 155. I have one of each, and they are all good meters. The
received wisdom is that the Fluke is the one to have. However, in my
view, the Keithley is the best of the bunch. It is a bonus if you
find one with the rare 1554 AC power module (the 1554 mounts to the
rear panel and allows AC operation -- otherwise, it is battery-only).
The main problem today with the HP is that it uses impossible-to-find
batteries. Not only are the original batteries unobtainable, I have
yet to find a satisfactory replacement strategy. It also does not
have a +/- 1 uV range, although I do not count that as a major fault
since thermocouple noise in the measurement setup frequently prevents
taking full advantage of the 1 uV range.
The Fluke's batteries (sub-C NiCd cells) are readily replaced (though
not inexpensively, if you get the best cells). Keithley used four, #
246 9 V carbon cells, which can easily be replaced with common 9 V
alkalines or 9 V primary lithium cells that simply plug into the
The HP and Fluke both use photocell choppers. Fluke published
pre-release information indicating that they had designed a FET
chopper for later production, but I have never seen an 845 with a FET
chopper or a schematic of the FET chopper. (Does anyone here have
either?) The Keithley was designed with a MOSFET chopper from the start.
Not only is the Keithley the most modern design and the most likely
to remain reliable, it also performs the best in my lab. I have had
fewer ground loop and shielding problems with it than with the Fluke,
and it has less noise and less drift. It is not enough better that
most people should sell their Fluke to get a Keithley just for the
performance difference (reliability may be another story), but -- IME
-- it does perform better.
They are all good meters, but IMO the Keithley is the best of the
three. If I had only one, that is the one I'd want.
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