Calibration procedure for Fluke 720A Kelvin-Varley Voltage Divider - Page 1
Forum section: electronics / general

author: vsmith
author_rank: Regular Contributor
timestamp: October 25, 2019, 01:24:33 pm
content: Hi all.I own a working condition Fluke 720A, which I want to use as my primary voltage ratio standard in my shop. It's spec of 0.1 ppm absolute linearity would be a factor of 10 better than my best DMM, a Datron model 1082. In fact, I want to use the 720A to calibrate the basic linearity of the 1082 (there is a trimpot and selected resistor used to achieve this)So, reading and attempting to perform the calibration procedure in the 720A "Instruction Manual" (PN 294058, March 1969, Rev2 9/74) have come across a few areas that I am confused about. I am hoping that my fellow calibration "nuts" who have experience with the 720A could help answer my questions.My primary area of confusion concerns the procedure for setting the trimmers for each step of the first three decades of the divider. The procedure in the manual breaks this up into two parts (decade A and B with front panel trimmers is covered in section 2-24 on page 2-4 and decade C with internal trimmers is covered beginning with 4-19 on page 4-13). There is also a procedure for calibrating the internal whetstone bridge (which is really a pre-requisite to the decade calibration procedures) found on 4-25, page 4-15.The first thing I would like help with is that I think there are errors in the procedure for the C decade. In paragraph 4-27, d., I think the reference should be turning the B decade switch to the Blank position rather than the C decade switch. Also, paragraph 4-27 should reference trimmer R-1130 as decade C-0, rather than decade C-9. (It's the only way I got any useful results to the procedure!)The more mystifying question for me, is that the procedures for calibrating the bridge are very self referential (circular) in that the calibration of the front panel Bridge Balance control using the trimmers R203 (10K standard) and R205 (4007.6 ohm standard) seem t allow an infinite number of solutions which would result in the bridge being balanced for values near (but different than) the nominal standard values. I am wondering if this is because the actual range of adjustment is so narrow that the effect on linearity is negligible, provided that every resistance in each decade is set to equal every other resistance?I would like to hear from the group about your experiences following this procedure and calibrating the 720A. I decided not to try to post the relevant pages of the manual here, hoping that interested commenters could download the manual (it is available from Fluke's support website) and follow along with my references.As a final comment, I was able to calibrate my 720A (with the two changes I mention above) and as far as the Datron 1082 is concerned, the linearity of the 720A is as it should be, with errors less than about .4 ppm when using a 100 volt input and the 10 volt range on the 1082, and comparing setting the same ratios using different ranges on the 720A.What has your experience been?Thanks!Vic

author: amspire
author_rank: Super Contributor
timestamp: October 26, 2019, 06:25:00 am
content: You probably want to look at the errata for the manual:http://download.flukecal.com/pub/literature/720A____imeng0206.pdfIt answers at least one of your concerns I think.Also, if you are going to use the 720A to check the Datron, it should be fine on the 10V range. You probably know this, but the 100V and 1000V ranges on the Datron will significantly lower the 720a output voltage due to the 10M input impedance.

author: vsmith
author_rank: Regular Contributor
timestamp: October 26, 2019, 09:17:22 am
content: Hi Amspire. Thanks for that feedback. I will look at the errata link you posted!And yes, I am aware of the loading limitations for the output of the KVD. I have to admit that occasionally I forget and ignore those limitations, and then find myself on the wrong path with whatever work I am trying to do...but with the Datron, I have been staying on the 10V range since that is where the basic linearity of the AtoD circuit is adjusted.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 26, 2019, 12:27:41 pm
content: You may want to take a look at this thread:https://www.eevblog.com/forum/metrology/calibrating-dvms-with-a-kelvin-varley-divider/msg1081254/

author: vsmith
author_rank: Regular Contributor
timestamp: October 27, 2019, 12:06:46 am
content: Hi SilverSolderI can definitely live with a 250 nanoVolt error. The Datron spec is 2ppm error on the 10 volt range. (20 uV)Having calibrated the 720, and spending the better part of a day tweaking the 1082 linearity, I have it reading within 1.5 ppm of nominal from 2-19 volts, which is much better than when I acquired it.It would be interesting to "borrow" a 3458 to check each data point and see how well the 720 is actually doing! I don't know of any other way to check the 720 linearity.Has anyone else actually self-calibrated their 720? What were your experiences?

author: guenthert
author_rank: Frequent Contributor
timestamp: October 27, 2019, 01:10:39 am
content: Quote from: amspire on October 26, 2019, 06:25:00 amI think if one were concerned about the loading of the KVD, one would use a low impedance adjustable voltage source and set it to the same (compared using a null meter) voltage as the output of the KVD and feed the voltage of the low impedance source into the DVM.

author: vsmith
author_rank: Regular Contributor
timestamp: October 27, 2019, 07:11:16 am
content: Threeroomlab:Thank you for pointing me at that thread! I love reading these accounts of meticulous and precise measurement techniques. I think this is a large part of why I enjoy acquiring (and rehabilitating) the metrology equipment I am able to afford, and trying to achieve the optimum performance it is capable of. That thread is also at my "ideal level" of discussion, meaning that it is just a little beyond my knowledge and pushes me to think and learn more. I swill need to read it over a few more times, but yes, it is very interesting to me in the work I am doing with the Datron 1082.In passing, and not to get off the present topic, I find the 34401a (at least the 20 or so I have had experience with) to rather dramatically roll off in terms of linearity right around the 10% of full scale mark. This makes it unsuitable (in my experience) as a transfer standard for 10:1 voltage ratios.

author: vsmith
author_rank: Regular Contributor
timestamp: October 27, 2019, 07:16:55 am
content: Guenthert:Thanks for that feedback. Yes, that should remove any loading effects. The problem for me is that adding yet another source of drift to worry about (I am already using a null compared voltage source to drive the KVD, so that I can achieve measurements from 20 volts down to zero and required for testing the Datron's linearity). My sources are Fluke 342a types, which I used to think of as quite stable, but am learning that when you enter the arena of ppm, everything becomes non negligible!