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
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@dell.com
To: volt-nuts@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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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@dell.com
To: volt-nuts@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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Bill wrote:
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
Why not the HP 419A?
Charles
Charles,
Your kidding, right ?
Bill....WB6BNQ
"Charles P. Steinmetz" wrote:
Bill wrote:
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
Why not the HP 419A?
Charles
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Charles,
{ Corrected my English. }
You're kidding, right ?
Bill....WB6BNQ
"Charles P. Steinmetz" wrote:
Bill wrote:
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
Why not the HP 419A?
Charles
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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
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
A more general question would be:
Is it possible to design a chopper stabilised null detector using modern
components with a performance equivalent to the 845A?
Whilst the noise of most chopper stabilised opamps is much larger than
that of the 845A, perhaps something like the approach used in:
http://cds.linear.com/docs/LT%20Magazine/LTMag-V16N01-11-FETAmp-Williams.pdf
could be used.
Since the low noise preamp is AC coupled a lower noise input using a
single ended JFET could be substituted for the one using the LSK389.
Bruce
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
it.
If you have a choice, definitely go for the Fluke. Otherwise, and if the meter
is working, it is an OK item.
Bill....WB6BNQ
"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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Hi Charles - The HP 419A DC Null Meter seemed adequate for purpose back in
the seventies, and might still be useful today if you could resurrect one.
The problem today is that I think probably most (if not all) 419s have gone
to the graveyard because the choppers all eventually went bad. But IMO the
HP 419A wouldn't be worth the trouble because the Fluke 845 was a better
box; so I concur with Bill.
Best,
Greg
----- Original Message -----
From: "Charles P. Steinmetz" charles_steinmetz@lavabit.com
To: "Discussion of precise voltage measurement" volt-nuts@febo.com
Sent: Sunday, September 19, 2010 2:57 PM
Subject: Re: [volt-nuts] Fluke 720A Kelvin Varley Divider Questions
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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To unsubscribe, go to
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