[volt-nuts] AC calibration

[Bill Gold]

There is one more thing that enters into this discussion and that is
"reversal errors" on the DC.  The complicates the transfer somewhat.  AC is
always going "+" and "-".  DC is in one direction so you have to then
reverse the voltage to the Thermal Transfer Standard and then take the
average of the two readings.  That is why there is a "Reversal" switch on
the 540B.  When you are using a fixed voltage High Frequency Thermal
Converter you need an external DC reversal switch in addition to other
equipment.  You also need an AC/DC transfer switch so that you don't have to
disconnect the AC source and then hook up the DC source manually.  See the
540B again.

All Thermal Transfer Standards have some "reversal" error.  This is
controlled by the internal construction of the unit and exactly where the
glass isolation bead is located on the heating element.  The thermocouple
converter used in the 540B is selected to have a very low reversal error,
but always will have some error.  The error is fixed so you can approximate
a DC measurement once you have characterized the particular converter.  I
can't remember now but I think there can be up to around .05% reversal
errors on some converters, while the ones selected for the 540B are under
.01%.

Read the FLUKE "Calibration: Philosophy in Practice" for further
information.

Bill

----- Original Message ----- 
From: "Dave M" <dgminala at mediacombb.net>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Monday, August 25, 2014 11:38 AM
Subject: Re: [volt-nuts] AC calibration


> Well, you sort of answered your own question.  The equipment is called a
> Thermal Transfer Standard, but instead of thermistors, it uses a
> thermocouple.  Look at the manual for the Fluke 540B
> (http://bama.edebris.com/manuals/fluke/540b/) and you'll see how it's
done.
> Basically, the AC source is input into the transfer standard, and the
> standard's internal reference voltage is adjusted for a null on the
> galvanometer.  Leaving the reference voltage setting alone, a DC voltage
is
> input into the unit, and the DC source is adjusted for a null on the
> galvanometer.  At that point, the AC voltage source is equal to that of
the
> DC voltage source.
>
> Ther are thermocouple-type thermal converters used for RF voltage
> measurements with the transfer standard.  They aren't cheap, and you have
to
> have a converter for each range of voltages that you need to measure.  The
> thermal converters used with this type of transfer standard isn't great
(50
> MHz or so typical), but their accuracy far surpasses that of the
thermistor
> type sensors.
>
> There are other brands and models of thermal transfer standards, but I
have
> a Fluke model 540 and a few thermal converters.  That's why I referred you
> to the manual for it.
>
> Cheers,
> Dave M
>
>
> pa4tim at gmail.com wrote:
> > Is there a way to link an AC voltage to a DC source for compare. I
> > can check my calibrators (like a Fluke 332, 760 , 731 and a Philips)
> > against standardcells. But for AC I can not do that. I have two AC+DC
> > TRMS 7,5 digit meters but the last calibration was 2 years ago.
> >
> > My idea is in theory simple. It is based on the thermal converters
> > used in RF powermeters. Two resistors, two high resolution
> > temperature meters. AC on the first en DC on the second. If both are
> > the same temperature the AC voltage is the same as the DC voltage.
> > But I'm sure some people here have done this in the past. I would
> > like to use it for 50 to 100 kHz (or less) and something like for 1V,
> > 10V and 100V (and use several resistors/heaters.)
> >
> > Or mabey there is an other way to convert AC (for RF it can be done
> > with lightbubs but I never tryed that)  I do not mind if it is slow
> > etc, I like this sort of experiments. You can learn a lot from it.
> >
> > Fred, pa4tim
>
>
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