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

[WB6BNQ]

Hi Charlie,

As Greg points out the 720 is calibrated by the user.  You will need a very good
null meter for the job.  The only one worth a damn is the Fluke 845, so you
should hunt around for one of those in good shape.  There are two versions, a
rack mount and a portable.  The portable IS better then the rack mount because it
has higher isolation; i.e., leakage.

The portable is battery operated, so if you can inspect the unit before buying,
make sure to check that the batteries have not leaked inside.  The original
batteries are NiCd that, by today's standards, are fairly low current rating.  I
replaced my worn out batteries with the much higher capacity 2700 ma/hr NiMh
batteries.  However, you will need to keep the switch in the charge mode as these
batteries have a higher self-discharge rate.  The original charging current level
in the 845 is low enough to not hurt the NiMh and essentially trickle charges
them.  If it is a concern, you can always lower the charge rate.

However, if you can only find the rack mount (not battery operated by the way),
don't pass it up either as it will do the job.

The only gotcha to keep in mind is there is an older series and newer series.
The outside difference is the color scheme.  The older are a dual gray color and
the newer has a very light pastel green color on the face plate.  Internally, the
newer series has a different photo-chopper block that is way more easily
reparable.  The older series would be very hard to fix if the older chopper block
is bad as they are not interchangeable.

If I may suggest studying the manual on the 720 and the 845 to become familiar
with them before buying.


Bill....WB6BNQ


Greg Burnett wrote:

> Welcome Charlie,
>
> According to Fluke's specs, the 720A's stability of linearity is 0.1ppm/30
> days. ...and +/- 1.0ppm of input/yr for dial settings of 1.1 to 0.1. So,
> because the 720A can be self-calibrated by the user (you), I'm guessing
> that, based on your requirements, you'd want to do another self-calibration
> every so often.
>
> If you buy a 720A on the used market, be careful that its switches are OK
> and that it doesn't have any oil leaking out of any of its resistors.
>
> Best,
> Greg
>
> ----- Original Message -----
> From: "Myers, Charlie" <Charlie_Myers at dell.com>
> To: <volt-nuts at febo.com>
> Sent: Sunday, September 19, 2010 12:36 PM
> Subject: [volt-nuts] Fluke 720A Kelvin Varley Divider Questions
>
> Hi to everyone,
>
> I am new to this group and have a keen interest in precision and accurate
> voltage references and metrology.  I am using a pair of Fluke 732A Reference
> Voltage Sources for my voltage standards and a Fluke 5440B Direct Voltage
> Calibrator in my home lab. I am thinking about acquiring a Kelvin Varley
> voltage divider for a number of lab uses including scaling of my 10 volt
> standards so that I can calibrate my Fluke 5440B and, in turn, various DMMs.
> The most precision DMM I own is an HP 3457A 6 ½ digit multimeter which can
> be extended to 7 ½ digits by adding the contents of the 7th digit register
> to the displayed reading.
>
> My question is how stable are the Fluke 720A dividers over time, i.e., if I
> have one that is in calibration (+/- 0.1 ppm accuracy and linearity) will it
> tend to stay that way over the course of years assuming that I don't apply
> excessive voltage, dissipate excessive power, or otherwise abuse the
> divider?  Or will it need to be calibrated on an annual basis to keep it
> within reasonable specifications?  I don't require +/- 0.1 ppm accuracy but
> I would like to maintain somewhere between +/- 0.5 ppm accuracy and  +/-
> 1ppm accuracy over time.
>
> All advice and comments are welcome.  For me, this is a new area of learning
> and adventure.
>
> Thanks.
>
> Charlie
>
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