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

Robert Atkinson robert8rpi at yahoo.co.uk
Sat Dec 15 21:09:25 UTC 2012

Hi Charles,
I have a couple of Fluke and HP null voltmeters. One of the Flukes has a FET chopper board in it. It was apparently fitted as part of a repair by Fluke, but I have no data on it. The HP 419 battery packs contain standard sized cells and can be recelled. There are 20 of them so it gets expensive. An alternative is to replace them with a shunt regulator (2 x12Vzener diode) and use mains only. You need the shunt to generate a virtual earth that is provided by the battery center tap in the original design.

Robert G8RPI.

 From: Charles P. Steinmetz <charles_steinmetz at lavabit.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com> 
Sent: Saturday, 15 December 2012, 10:28
Subject: Re: [volt-nuts] Fluke 335A versus HP 740B
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,


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