[time-nuts] Voltage standards

[Will Matney]

Bill,

What I was getting at, on the military equipment, was that generally this
equipment has more testing done on it than for the normal industrial
consumer. I would assume that Fluke was working with the military here to
make the equipment pass the rigorous tests of the military, and as you
stated, the Navy helped in furthering the designs. From my understanding,
and from reviewing military bids, they wanted testing done on everything to
the point it would cost an kings ransom to make. That's why I never did
fool with bidding on government contracts. Generally, when a contract is
won, the manufacturer will build a certain lot of the equipment to the
specs of that order. I know of one company who was building differential
voltmeters, who lost to Fluke over not being able to meet the government
specs. They tried for an appeal, saying the government was biased, but they
lost over something to do with the filtering on the front end, if I
remember correctly. In your case, I would say that the Navy's calibration
program was wanting to work with Fluke to better the products for the
future.

You mentioned the HP 6920, but there is another for the analog and 3-1/2
digit meters, well to 4-1/2 digits really, and that was the Fluke 760A, if
you didn't mind lugging around all the weight of the unit. I have one of
these also, and it is ex-Navy. The HP 6920 turns up on eBay more than the
Fluke now, as about all of the 760A's were ex-military units a few years
back, and they were gobbled up. The one I bought was freshly calibrated at
the time. However, anymore, I use artifact standards for most of my
calibration, especially on resistance, and use a round-robin transfer on
voltage. Current calibration is done easily, by setting the load resistor
tolernace with an artifact standard, and applying an accurate voltage.

For frequency, I am using a GPS controlled DOCXO standard and a rubidium,
like most of us now. I never did invest in a cesium standard.

On the boards that are available, I prefer the Geller. Use them as soon as
you buy them, or soon after you build the board, and have him calibrate it.
If you build one up, make sure to use a battery system so that the thing
will run powered up from the time you ship it for calibration, and then
back to you. I could see using one to calibrate my Fluke, and then using
its stability for long term precision. The Geller unit would make a good
transfer unit to a traceable standard at Fluke.

Best,

Will
*********** REPLY SEPARATOR  ***********

On 5/26/2011 at 5:33 PM WB6BNQ wrote:

>Will,
>
>I beg to differ with you on the military statement.  The Navy's Primary
LAB
>received 100 of the 731's that specifically were not hand picked, as you
>suggest.  Out of that 100 a number of them were used for a vast array of
testing
>including destructive testing.  A few units had problems right at the
start, but
>the majority held up very well.  The purpose being to define the value of
the
>item regarding the Navy's mission and support there of.  I know all this
because
>a close friend of mine was the LAB person who did the project.  His
>responsibility, at the LAB was maintaining the Navy's primary voltage
reference.
>
>From this large test base came a number of improvements my friend
suggested to
>Fluke that were incorporated in later production units.  I guess it would
be way
>more accurate to say Fluke used (wisely) the LAB as a "beta tester" of
their
>product which was brand new at the time.  Gaining the Navy's acceptance
was a big
>deal in the scheme of things in those days.
>
>The quality of the Fluke equipment was outstanding and, no, the military
did not
>hand pick.  The same went for the HP equipment and Tektronix.  Simpson was
>another cornerstone with their 260 VOM as the Navy had a ton of those.  So
was
>Triplett who had some very good VOM's in the day.
>
>A very good instrument to grab when they show up is the HP 6920 Meter
>calibrator.  Yes it is old but it was specifically built to do VOM's like
the
>simpson and individual meter movements as well.  It outputs DC and AC
voltage (60
>Hz) (zero to 1000) and current (zero to 5A) and is quite handy to have.
>
>As for the "small transfer boards" you refer to, I completely understand
what you
>mean.  While they may serve a purpose for the masses, their advertising is
a bit
>misleading to put it kindly.
>
>
>Bill....WB6BNQ
>
>
>
>Will Matney wrote:
>
>> Bill,
>>
>> Yes, Fluke did cherry pick on what they bought, or paid to have Motorola
to
>> do it. HP does the same when purchasing voltage references, but they
both
>> do burn them in themselves after they receive them. The duds are used in
>> less tolerant meters, etc. That is what Fluke wrote about doing in an
app
>> note of theirs. Also, the military gets the pick of them all, and is the
>> reason I try to buy military surplus that's in good shape. My Fluke 730A
is
>> ex-Navy.
>>
>> Actually, back in 2008, this subject was discussed here on Time Nuts,
and
>> was the reason why I made this post. At the time, nobody had crossed the
>> Motorola part numbers, and just called them the part from Motorola. It
took
>> digging out an old Motorola manual from 1968 to find them. I am hoping
it
>> will help others if they have an older Fluke transfer standard they need
to
>> repair, as there are parts still available by a few dealers. However, to
>> get any kind of accuracy, the parts will have to be burned in at least
>> around, or over, a three month period.
>>
>> I also wanted to note that the small transfer boards being offered, by
>> several, should be looked at by the way they are calibrated, if they are
to
>> have any sort of precision.
>>
>> Best,
>>
>> Will
>>
>> *********** REPLY SEPARATOR  ***********
>>
>> On 5/26/2011 at 4:18 PM WB6BNQ wrote:
>>
>> >Hello Will,
>> >
>> >I think you meant to post this on the "Volt-Nuts" list ?
>> >
>> >The reference parts used in the Fluke 731/732 are not "off" the shelf
>> parts, they
>> >are selected.  While they are referred to as transfer devices, if
working
>> >correctly, they exceeded their specs versus time on the 10 volt port.
I
>> base
>> >this on the history of over 50 units that were tracked by Navy's
Primary
>> Lab, as
>> >these were used to feed the second level labs in the Navy's system.
>> >
>> >Bill....WB6BNQ
>> >
>> >
>> >Will Matney wrote:
>> >
>> >> Hello all,
>> >>
>> >> I was reading some old posts about voltage standards from Dr. Kirby,
and
>> >> some others, and one that was mentioned was the older Fluke
standards. I
>> >> did some research on this, going back to the Fluke 730A transfer
>> standard,
>> >> which I have one of, and found out what the voltage reference and the
>> >> differential amplifier was from an old Motorola parts book dating
from
>> >> 1968.
>> >>
>> >> The voltage reference in the Fluke 730A was most likely the Motorola
>> part
>> >> number: MCA1914N or the MCA1924N in a can package. Both are rated at
6.8
>> >> Vdc, but there is a difference in the maximum voltage change between
the
>> >> two. The MCA1914N is the tightest, with a change of only 0.005 volts
>> from 0
>> >> to 75 deg C. The MCA1924N is made for a broader temperature, with a
>> change
>> >> of only 0.010 volts from -55 to 100 deg C. These are not an oven
zener,
>> but
>> >> a voltage reference. Motorola first produced these using two diodes
and
>> a
>> >> transistor sealed in a can, literally. They used a temperature
>> compensated
>> >> zener diode, consisting of a zener, and a regular diode, turned with
>> their
>> >> cathodes facing, and connected together. These were placed inside the
>> can,
>> >> along with a transistor across from them, then sealed. They used this
>> same
>> >> reference transistor on up through several transfer standards,
including
>> >> the 732A.
>> >>
>> >> The Fluke 730A did not use an op-amp after the reference, but used a
>> dual
>> >> transistor in a can, and created their own comparator amp. Motorola
made
>> >> several of these along with Fairchild. There are two possibilities
here
>> in
>> >> the parts. I think they used a MD918A or MD918B, which are dual NPN
>> silicon
>> >> annular transistors, with a Vceo of 15 Vdc, and an Ic of 50 mA. The
>> MD918
>> >> series was to be used for "differential amplifier applications
requiring
>> a
>> >> matched pair of transistors with a high degree of parameter
uniformity
>> >> under varying environmental conditions".
>> >>
>> >> The other possibility on the dual transistor was a 2N2914, which was
a
>> >> "dual npn silicon annular transistor, especially designed for
low-level,
>> >> low noise, differential amplifier applications, featuring very high
>> Beta,
>> >> guaranteed from 10 uAdc to 1 mAdc, and excellent noise
characteristics".
>> >>
>> >> Next, the Fluke 731B used the same voltage reference, but it did not
use
>> >> the dual transistor amp. Fluke changed this to an op-amp, which turns
>> out
>> >> to be a LM308. This op-amp was then used in the newer models after
the
>> >> 731B. Also, this op amp used a few of the same resistor values, and
>> >> circuitry, as the two transistor amp.
>> >>
>> >> Now, a word about "voltage transfer standards". They are not meant to
be
>> as
>> >> stable as a true primary voltage standard, as they are meant to be
used
>> to
>> >> "transfer" a voltage from a known source to other equipment being
>> >> calibrated. The transfer standard is made to be recalibrated in one
year
>> >> increments or closer. Tight tolerances in the short term, in one year
or
>> >> less, are all that is needed. The Fluke 730A, 731B, and 732A and B
are
>> all
>> >> transfer units. The small voltage transfer standard boards, now sold
on
>> >> ebay, are meant for calibration every six months, and are not meant
to
>> be a
>> >> long term primary standard. They should be calibrated from another
>> transfer
>> >> standard, which has been calibrated from a primary voltage standard,
>> such
>> >> as a Josephson type, by Fluke, or the NIST. The ones calibrated by
using
>> a
>> >> DMM, no matter who made it, are not calibrated correctly. To
calibrate a
>> >> board correctly requires the use of a voltage transfer standard, that
>> was
>> >> calibrated by a primary voltage standard, and a very sensitive null
>> >> meter-detector such as a Fluke 845A, etc.
>> >>
>> >> Hope this helps the ones who had a few questions.
>> >>
>> >> Will
>> >>
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>> >
>> >
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>
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>
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