[volt-nuts] Fluke 720A Kelvin Varley Divider Questions

WB6BNQ wb6bnq at cox.net
Mon Sep 20 06:54:32 UTC 2010

Hi Charlie,

First off you are using the right email address to send to the list.  The "reply to" function of your email program should automatically put that same address in when replying to.

My experience with the 720 is that it holds quite well, assuming that it is not abused.  Besides, you can easily re-cal it at any time.  There is no reason why you could not use the 720 for calibrating the 5440B as far as I can see.

As for the 752, you could use the 750 instead and those are around and would, or should, be cheaper.  They perform the same function as the 752, just a slightly older model with yet more capability than the 752.


"Myers, Charlie" wrote:

> Thanks to Greg, Bill, Frank and all the others for your valuable comments and insight on the Fluke 720A reference divider.
> Frank, I will attempt to answer your questions about calibrating the 5440B.  I have the 5440B manual which states, "the recommended calibration procedure uses the Fluke 732A Direct Voltage Reference Standard and the Fluke 752A Reference Divider to establish the necessary voltage standards which are then compared to the calibrator with the Fluke 845A Null Detector."
> I would really prefer to get my hands on a Fluke 752A because it would provide me the required highly precise 10:1 and 100:1 divider ratios that would, in combination with the Fluke 732A's 10 V output, provide me highly accurate standard voltages for all the required cardinal points of 0.1V, 1V, 10V, 100V, and 1,000V.  Unfortunately, the 752A does not seem to be readily available on the used equipment market (e.g., EBay, Craigslist, etc.) and the Fluke 720A is available.  While the Fluke 720A may be overkill for providing only two precise divider ratios of 10:1 and 100:1, it appears to be the only option that is readily available.  So, I understand your statement that I "don't need the 720A for calibrating a 5440B", but I do not understand your statement that I "can't use a 720A".  Maybe I am not understanding a subtle point that you are making and I sincerely would like to understand it if that is the case.  I think I can use the 720A to provide the required precision 10:1 and 100:1
> divider ratios specified in the 5440B calibration procedure since I can't get a 752A.  Am I missing something?
> In response to Greg regarding the initial question I asked in my first post, you are correct about the performance and drift specifications of the Fluke 720A.  I think I didn't ask my real question correctly.  Given the Fluke published specification, what is this group's actual real-world experience with the Fluke 720A in terms of the stability of its ratios and its linearity over time.  To make my question clearer, I will use a real life example.
> Fluke states that the 732A voltage reference must be powered on continuously after calibration in order to maintain its uncertainty specifications.  Apparently, if the power is removed, the reference goes out of calibration because of hysteresis effects that cause it not to come back to the original calibration point after the power has been removed and the temperature of the oven has cycled.  I can say that in real life, at least with my two 732A standards, this is not the case.  When I cycle the power on them (allowing the oven to cool down) and re-apply power, after about 12 hours they come right back dead-on to their original calibration voltage.  So, in the case of my 732A standards, the specifications may be the theoretical outer bounds of performance but the reality is that my actual performance is substantially better and I do not have to re-calibrate them if the power and the oven temperature cycles.  I was wondering if the actual experience of the group with the 720A has
> been similar.
> One other procedural question.  What is the proper procedure for replying to a Volt Nuts message string?  I do not see any "Reply to message" function in the emails I receive.
> Thanks again for everyone's most helpful and insightful comments.  Keep the knowledge flowing!!
> Charlie
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