[volt-nuts] Fluke 720A Kelvin Varley Divider Questions
wb6bnq at cox.net
Tue Sep 21 00:28:15 UTC 2010
You need to re-evaluate some of your statements. Out of 10 volts, 1uv
is 0.1ppm. I also feel you should re-read, more slowly, the two Fluke
articles you referenced below.
In so doing, keep in mind that Fluke has several objectives in their
presentation. First is to live up to the ideals of being a top notch
metrology company. Second, is to sell their newest products. Third,
is to sell you on why you want to buy their newest products while not
severely detracting from their older products. In all cases,
specmanship can be your friend.
In the first Fluke article, while trying to demonstrate that the 720 is
worse, they conveniently stated that they did not do reversal on the
720 set up which would have improved the numbers. Yet they did so on
the 752. A small slight of hand that skews the analysis for one
reading the article. Would it have been enough to thwart their intent
? Well, we cannot be sure as we do not have the extensive LAB
environment that they have.
So on the one hand they try to out sell the 720. Yet in the second
Fluke article, they tend to contradict themselves by stating that the
720 is better than the 8505A DMM which they are trying to claim can do
the job. So, which is it ? Their justification ? The cost and
complexity of using the 720 and the required knowledge to use it
It is all specmanship at its finest ! I am not saying their new
equipment is a sham or that it is not worth having. Just that with
each new run of equipment, the sales department has to find ways of
marketing the product. Hopefully, the customer is getting what he pays
for in the end.
All of this specmanship race does not address whether or not you truly
need such a level of complexity. If you are trying to construct a real
(doing business that is, unless you are just filthy rich) top notch
calibration facility, then all of that equipment is needed, including
the 720. If you are just doing you own HOME arrangement, then a
different set of parameters apply. By the way, that Josephson Voltage
Standard is a few 100K dollars all by itself. Lacking such an item
makes it near impossible to prove all those specmanship numbers.
So, what we have not asked Charlie yet is what is his arrangement ?
Does he have a temperature and humidity controlled space worthy of
competing with Fluke ? OR, is he just fooling around at home ?
If he is just fooling around at home, even if he has SOME control of
his space, then he is kidding himself on the level he is going to
achieve. Not that he shouldnt try to do better all the time, but there
is a cost involved and going that extra millimeter might really be
Another thing to keep in mind about all those specmanship numbers is
they only apply if you have the same LAB conditions as the original
reference items were calibrated or verified in. The minute that
reference instrument leaves that environment, all bets are off.
Going back to Charlies original post, he states that this is for his
HOME LAB. So his interests are to do the best he can. He has
indicated that the cost of the 752 is too much, so we know either he is
not filthy rich or he is being very responsible with his means of
living. Truth be known, he would probably be finding a bush to live
under if the wife caught him going overboard.
So, the end result is the 720, and the 750 for that matter, would
satisfy his needs to a given level few truly need. Forget the
specmanship as metrology is all about knowing your limitations and
applying them properly.
One last point Frank, you mentioned a 10 to 1 ratio for the standard to
the Unit Under Test. While that is nice, more often then not that is
not achieved when you start playing around at the levels that Fluke
does, even in their own LAB. The same holds true for NIST, for in some
cases the ratio is down to 2 to 1 or even less.
"Dr. Frank Stellmach" wrote:
the 752A or a 3458A are able to calibrate the 5440B correctly, but
not the 720A.
What you are missing is the correct interpretation of the 720A
The 720A is specified "0.1ppm linearity error of input" for a
setting S of 1.1 ..0.1.
That means, if you apply 10V input voltage ("Input" is the crucial
word!), it gives an error of 1µV.
Related to the output (linearity!!!), that means the 0.1x divided
output of 1V is unsecure 1µV, i.e. 1ppm of output, or 1ppm of ratio.
That means, that the 1V reference point for calibration of the 5440B
is too unprecise, or "on the edge", as the reference should have
of the precision level of the UUT.
For the 100:1 transfer, 100mV range, the situation is even worse.
4ppm is the precision of the 5440B, plus EMF, but the 720A gives
4,6ppm on the 100:1 ratio only
For the high voltage range transfers, you also have to add the
nonlinearity of the thermal heating in the same manner (power
coefficient of linearity .. of input..").
The 1000V => 10V transfer is precise to 4+2,6ppm = 6,6ppm for the
720A (10W power dissipation!!), for 100V=> 10V it is 1,2ppm (on the
For high voltage, the 720A resistance is much too low.
The 752A is better suited for that, its divider resistors have 4
MOhm in the 1000V range, giving 250mW only.
This divider is specified for 0.5 and 0.2 ppm ratio (output)
accuracy, thermal effects included.
Low voltage transfers can be done even more precise, ca. 0.2 ppm for
a 10V => 100mV, and 0.15ppm for 10V => 1V transfers.
Those precision levels are at least 5 times better than the
specified accuracy of the 5440B ranges - good metrological practice.
For the beginning I recommend two articles where this aspect is
Here you can check my statement about the correct calculation of the
ratio error of the 720A:
And this famous one describes the problem of checking the ultra low
nonlinearity of the HP3458A, for which the 720A is not precise
enough, due to the same root cause.
See pages 14 and 23 for discussion.
Here it is stated also, that the definitions of nonlinearities are
not precisely given, what I can underline from painful experience.
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