How does Fluke get below 5ppm in their Kelvin-Varley divider? - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 05:40:33 pm
content: I am having a hard enough time sourcing/trying to build a Hamon divider with 5 PPM resistors in the chain. How in the world do they get this thing to guaranteed <= 0.1 ppm?

author: ltz2000
author_rank: Regular Contributor
timestamp: October 30, 2015, 06:19:50 pm
content: The 720A has a built in bridge for "self calibration" e.g. trimming the resistor chains.And that is 0.1 ppm referenced to input, not output. For example 1:10 division -> 1 ppm.

author: pelule
author_rank: Frequent Contributor
timestamp: October 30, 2015, 06:29:21 pm
content: Have a look to the report from the famous Gelller Labs (unfortunately closed December 2013,http://www.gellerlabs.com/)Read the article athttp://www.gellerlabs.com/752AJunior.htmAlso follow the link to Conrad Hoffmans nice report"An Easy to Build 0.1X and 0.01X Resistive Divider"http://conradhoffman.com/HamonResistor.htmlThis may give you a better feeling, how to get such a high precisionsBRLudger

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 06:34:40 pm
content: Quote from: ltz2000 on October 30, 2015, 06:19:50 pmIf there are trimmers in the chain, that is an even greater opportunity to introduce high PPMs...

author: TimFox
author_rank: Super Contributor
timestamp: October 30, 2015, 06:44:13 pm
content: Yes, but the ppm's that we are discussing here are "accuracy" of the ratio, which can be adjusted by making appropriate resistors equal to each other, not necessarily to a standard value.The trimmers are a very small part of the total resistance for each resistor.

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 06:47:07 pm
content: Quote from: TimFox on October 30, 2015, 06:44:13 pmThe thing is that even from the same batch (i.e., reel or even strip of 50), resistors with the same 5 ppm rating drift differently. After all, it's +/-5 PPM in reality.So, after you sit there and find say three matching resistors in value (even to 6 places, which I've managed with a large enough stock). Good luck on your chances that they also drift in the same way. Then, even if you do win the lottery in both value and drift, they have to also drift the same (proportionally speaking) as your high valued resistor(s) forming the big leg of your divider and you are in "fat chance in hell" territory.I guess it comes down to painfully time consuming hand selection for matching (but non-equal, i.e., value adjusted) drift characteristics across all parts that will be used...

author: ltz2000
author_rank: Regular Contributor
timestamp: October 30, 2015, 07:54:40 pm
content: main resistor + padding resistor (if needed) + trimmerQuote from: SharpEars on October 30, 2015, 06:47:07 pmThe trimmers are there for compensating the (aging) drift.If you adjust the resistor chain just before use, the resistors drift nowhere in that probably 10 minutes needed for your measurement.I would put the effort in matching the temperature coefficients of the resistors rather than the absolute value.And if your goal really is sub ppm or even ppm performance, there are so many other things to think about. Isolation resistances, wiring arrangements, thermal design etc.

author: SharpEars
author_rank: Unknown
timestamp: October 30, 2015, 07:57:48 pm
content: Quote from: ltz2000 on October 30, 2015, 07:54:40 pmThe thing is that for calibration, the currents going through the box are only hundreds of microamperes. Still, I get enough drift to constantly have to adjust the thing.

author: eas
author_rank: Frequent Contributor
timestamp: October 30, 2015, 11:51:14 pm
content: What have you done to control and correct for other sources of error in your system?

author: SharpEars
author_rank: Unknown
timestamp: October 31, 2015, 12:48:39 am
content: Quote from: eas on October 30, 2015, 11:51:14 pmSadly not much, but on the other hand, those other factors are all present without the resistor network in play and don't effect the measurement. That is, if by other factors you mean temperature, probes, wiring, etc...

author: dacman
author_rank: Frequent Contributor
timestamp: October 31, 2015, 01:33:41 am
content: The main MSD resistors are in an oil tank inside. If the resistors are made at the same time and out of the same batch of material, then they will have the same aging rate and temperature coefficient by default (i.e., Fluke manufacturer's their own critical resistors). The 720A manual is available at the Fluke website and it explains the theory behind the Kelvin-Varley divider and Fluke "enhancements" for adjustability.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 31, 2015, 09:47:41 am
content: Quote from: SharpEars on October 30, 2015, 05:40:33 pmAt first, FLUKE uses precision wirewound resistors in all of their precision divider and resistance chains (720A Kelvin Varley divider, 752A Hammon divider, 332/335 calibrator).These have <=1ppm T.C. by using special alloys like Evanohm. These keep their T.C. over lifetime. A hermetical sealed, presumably oil filled housing cares for small ageing.Metal film resistors change their individual T.C. over ageing.Simply using all resistors from "one batch" does never work!Fluke instead always determines the resistors individual T.C.s.Then they create 'matched sets' for one decade, by either pairing two resistors of opposite T.C. for one setting (like in the 33x calibrators), and additionally by matching the residual T.Cs. of the whole decade.So they either achieve near zero T.C.s (absolute), or matched T.C.s with near zero variation (relative) over a decade or over a whole divider chain.The oil bath inside the 720A does NOT improve the T.C., nor the aging.It only keeps all resistors of the highest decade on the same temperature, so that you won't get any thermal unbalance caused either by room temperature changes, or by power dissipation heating (at high input voltages).In the 752A, there is no such oil bath, so they matched the T.C.s to 0.05ppm, and additionally (obviously) the combined self heating effects to a minimum of about 0.3ppm (see manual of the 752A, chapter about error analysis)The most critical resistors can always be trimmed to equal values on the order of < 0.1ppm.That requires, that the absolute resistance values of the highest decades have to be accurate (absolutely) to about 50ppm.The whole set of one decade can then be trimmed to equal resistance by one small trimming resistor and one trim pot per each constituent of this decade.As latter resistors are orders of magnitudes smaller than the PWW resistor, (due to the initial accuracy of the main resistors), their own drifts (T.C., ageing) do not contribute to the total.All these techniques allow combined stabilities in aspects of T.C., AND resistance or ratio matching AND ageing on the order of 0.1ppm.Frank

author: builder
author_rank: Newbie
timestamp: February 24, 2016, 01:26:04 am
content: Hi Frank and others,I'm a late comer to this blog so I hope your able to see this post.Briefly, I have a Fluke 895A DC differential voltmeter which I have completely overhauled. That's another story but I was dreaming about how to calibrate its MSD (most significant divider, the only one that is adjustable). I would like your comment on this possible internal method. Set X55555. This makes a bridge which should have zero potential between the K-V output and the mid point of the two bridged MSD resistors. The bridge unbalance can be measured by adjusting the LSD if one wants (exactly how to use this info I'm not sure). To calibrate the MSD start at X=0 leave the lowest resistor, R0, alone and balance the bridge by adjusting R1. Set X=1 and repeat.I'm concerned that errors would accumulate with such a method. Would value your opinion. Regards, Builder

author: Dave Wise
author_rank: Regular Contributor
timestamp: January 25, 2020, 12:09:40 am
content: Even later here, but it might be useful to somebody sometime.Builder, I'm sorry to say your technique is not usable. First, you would be using the untrimmed ratios of decades B, C, D, and E to trim decade A. You could make decade A consistent, but unless your untrimmed BCDE is perfect, dial A will be wrong by the same percentage. Second, the 895 uses a 0-100 slider for decade E so mid-scale is "50" not "55".Regards,Dave Wise