[volt-nuts] AC Voltage Measurement Standards

[Stephen Grady]

Todd and anyone else would is interested,

Measuring a Thermal Converter against another Thermal Converter is a bit of
a black art. The main problem is thermal converters are a square law device
that is if you change the input voltage by a factor of 2 the output voltage
will change by a factor of approximately 4. Now I say approximately because
for most thermal converters like the Fluke 540B, A55, Ballantine and Holts
the actual factor can be anywhere from 1.4 to 1.8 due to losses in the
thermal converter. In the literature you often see this factor refer to as
the N factors. Each thermal converter will have its own N factor which must
be measured to make sense of the measurements.

It even becomes more difficult in that the AC-DC difference of a thermal
converter is defined as (Vac-Vdc)/Vdc where Vac and Vdc are the inputs to a
thermal converter which give an equal output from the converter. Also Vdc is
the mean of the forward and reverse DC voltages. The problem is that when
you have two converters connected in parallel you cannot balance both
converters AC and DC inputs to produce equal voltage out of the converters
at the same time because each converter has its own AC-DC difference and its
own N factor.

It anyone is interested I can send them a technical paper that describes
this process and the appropriate math to use but I cannot send it to the
list due to copyright issues.

Kind Regards,

Stephen Grady
Sydney Australia


-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of Todd Micallef
Sent: Thursday, 10 July 2014 2:36 AM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] AC Voltage Measurement Standards

I would like to know more about your setup. Which source(s) are you using
for the input and which nanovoltmeter(s) are you using to transfer the 10V
TVC to your other converters? I remember you asking on PMEL forum about the
accuracy of using a 34420A nanovoltmeter. I did not see a response as to
whether you opted for an alternative like Keithley 2182(A) or a low noise
preamplifier connected to a 3458A.

I have done some reading about how NIST transfers their calibrations using
two TVC's in parallel and I am guessing that is what you are doing.

http://www.nist.gov/pml/div684/acdc/tcs.cfm

To transfer the accuracy up/down to other TVC's at different rated voltages
appears to be a difficult task since they typically need at least half the
rated max voltage to be within spec. It would be similar to starting with a
SR104 standard and transferring its value through a set of SR1010 and
SR1050 resistors using an ESI 242.

I have a few AC sources, and I would like to be able to verify my TVC's
without sending all of them out for cal. Ballantine quoted $600+ per TVC and
I haven't checked what Fluke would charge for each A55.

Todd


I personally did the following: I got a Ballantine 1605A transfer
> voltmeter. This is comparable to the 792A in a way, except it was much 
> cheaper. It is automatic, much easier to use than the Fluke 540 and 
> goes up to I think 100MHz. This can be used for percision calibrations 
> as a working standard. The calibration of this meter as well as others 
> (e.g. the 3458A in its AC mode) I am doing with a set of thermal
converters (0.5V to 100V).
> One of which (10V) has been externally calibrated up to 30MHz, cal of 
> the others are derived from it. That way I am deriving everything from 
> a very precisely (few ppm) calibrated 10V TVC. Overall, this saves 
> cost on the calibration side, allows for high accuracy and measurement
speed is good.
>
>
>
>
>
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