[volt-nuts] AC calibration

[acbern at gmx.de]

Dave,

the title is: 
Thermal Voltage Converters and Comparator for Very Accurate AC Voltage Measurements
by E.S.Williams.

Adrian


> Gesendet: Montag, 25. August 2014 um 19:32 Uhr
> Von: "Dave M" <dgminala at mediacombb.net>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] AC calibration
>
> Adrian,
> Do you have a link or title for the NIST paper that you mentioned?
> 
> Dave M
> 
> 
> acbern at gmx.de wrote:
> > fred,
> > generally you raise a good point, I had the same issue of calibrating
> > an ac voltage to a high level of accuracy. you need this e.g. to
> > validate the self.cal of a 3458a or other precison stuff like the
> > 8506a0.   
> > 
> > what i would recommend to do if you want to keep costs down is:
> > in a nutshell, get a thermal converter in the lowest range you need
> > and a second one on range above. build a set of resistor range
> > extenders (rf type with appropriate connectors and housings) to
> > expand the range to where you need to be max. get one of the thermal
> > converter calibrated (the higher one usually, and you need to havr 
> > good cal lab, should be <10ppm accuracy) and use it to calibrate the
> > rest. generally, up to 20khz, the accuracy is some 20 ppm anyway for
> > thermal converters! at higher frequencies, due to reflections and
> > stray capacitance/inductance influences, the accuracy decreases. the
> > resistor range extenders though, if build up correctly, only have a
> > few ppm impact (there is a paper from nist on that, but this is only
> > typical). you can calibrate all converters to the one you got
> > externally calibrated. do some research in the web, when you do the
> > calibration, you need to determine the so-called constant N. then do
> > an ac, dc+, ac, dc-, ac measurement between the the two and establish
> > the deviation, also establish the error propagation. the end result
> > will be a set of highly precise (low inaccuracies9 thermal converters
> > good enough to calibrate a 3458a an better devices. if you want to
> > spend the money, you could also buy a set of converters/range
> > resistors (with/without a 540), that typically is a few k altogether,
> > while a single device sometimes is available for below 100 bucks. you
> > need to have a stable 7.5 digit nanovoltmeter though for the
> > measurements of the tvcs (34420a or 2182 typically ) and precision
> > (stable) dc and ac sources. but in the end, all you need is a single
> > calibrated thermal converter.                        
> > 
> > adrian
> > 
> > 
> > 
> >> Gesendet: Montag, 25. August 2014 um 18:38 Uhr
> >> Von: "Dave M" <dgminala at mediacombb.net>
> >> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> >> Betreff: 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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> 
> 
> Dave M
> 
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