[volt-nuts] Fluke 335A versus HP 740B

[Charles P. Steinmetz]

David wrote:

>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 
existing connectors.

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.

Best regards,

Charles