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: [email protected] [mailto:[email protected]] 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. > > > > > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
