Fluke 742A-1 Resistance standard drift - Page 1
Forum section: electronics / general

author: jfphp
author_rank: Regular Contributor
timestamp: October 31, 2020, 10:27:52 am
content: Hello,Funny question : I have an old Fluke 742A-1 Ohm resistance standard, bought on Ebay years ago, stamped 1.000001.3 Ohm SN6290002. Apparently it is from 1995 with a "primary standard laboratory" label dated 1995 and a Fluke warranty starting 1995 and expiring in 2000. It is just coming back from Fluke cal center with the value 1.000663.6. Drift specification 8ppm/year : 663/8 = 83 years or 663/25 = 26.5 ppm/year... There is something wrong : abuse ? V limit exceeded ? Any idea ? Thank you.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 31, 2020, 12:36:15 pm
content: A damage due to excessive current is well possible. The drift specs are valid when treated well.Drift is often not linear. So one can not really divide the numbers. Usually the 8 ppm/ year would be for the early years, getting better with time, not worse.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 31, 2020, 04:18:44 pm
content: A defect is the most probable explanation.See how the 742A-1 is built:https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg891672/#msg89167220X 20.01 Ohm (+/-0.25%) resistors in parallel, plus one additional, individually produced, trimming resistor in the kOhm range to initially get near 1.000000 Ohm.Latter is faintly visible to the left.A heavy abuse by overload is possible, but that would affect 20 resistors at once, which is less probable, than that only this kOhm resistor failed to open.If you have the guts to open the case, and you find this trimming resistor to have about 1500 Ohm, then you identified the root cause.As the stability is mainly determined by these 20 low Ohm resistors, your new calibration value is not beautiful, but the resistor itself serves very well as a standard.FrankPS: Hopefully, it's not a bad solder joint, which creates an intermittent connection.

author: martinr33
author_rank: Frequent Contributor
timestamp: October 31, 2020, 05:10:45 pm
content: Using Dr. Frank's numbers, the nominal resistance of the 20 x 20.01 Ohm resistors in parallel is 1.0005 Ohms (a 2001 Ohm resistor in parallel adjusts this value to 1 ohm).This value seems close enough to your reading that I would suspect the adjustment resistor to be broken or disconnected. The .25% tolerance on the 20 ohm resistors could account for the rest.So as you are in calibration, I would say let it ride and use the new value.

author: jfphp
author_rank: Regular Contributor
timestamp: October 31, 2020, 05:51:40 pm
content: The funniest thing is that I made an unofficial cal in november 2018 with a 5700, another 742A-1 and a 3458A --all in cal-- in a french cal lab in order to verify before official and costly cal the integrity of the 742A-1 !!! All items were at 23°C... and my 742A-1 was since 2 days in the lab. The 5700 was supplying 100 mA to the 2 in series 1 Ohm standards on the current binding posts and I mesured the voltage drops at the two resistors, the calibrated one and mine, on the sense binding posts. Mean of 10 mesurements was 1.0000688, standard deviation was 5.25 E-7 and uncertainty 2 sigma 6µOhm.Fluke cal is only a comparison between their 742A-1 and mine through a 8508A at 10 mW.Explanation?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 31, 2020, 09:08:02 pm
content: what's your question, now?Have you measured 68ppm deviation only, or did you unintentionally put a wrong zero inside the number, so your measured deviation was 688ppm in reality?

author: jfphp
author_rank: Regular Contributor
timestamp: October 31, 2020, 10:05:43 pm
content: My question is a two stage one :1. Today return from Fluke +663 ppm printed value (yes, 663). I presume Fluke made no error. Explanation of such a drift in comparison with value in 1995 ? Some answers.2. Own mesurement with in cal instruments in a different process in 2018 +68ppm (yes 68 : computation checked several times...). Could process (constant I of 100mA into 2 cal standards, one in cal and mine and V across each) explain such a difference ? Fluke uses a silly direct comparaison with a 8508A...

author: TiN
author_rank: Super Contributor
timestamp: October 31, 2020, 11:56:45 pm
content: QuoteDid you cancel TEMF in this setup? It can cause large error for 1 Ohm measurements.Using 8508A in substitution comparison for 2 Ohm range would have transfer uncertainty about 6ppm.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 01, 2020, 10:39:18 am
content: At first, what TiN already indicated, have you used an Offset Compensation method?That would require that you cancel the thermal offsets by either reversing, or zeroing the measurement current (100mA in your case)If yes, then about 68ppm shift between 1995 and 2018 would be fully inside reasonable drift specification, i.e. 3ppm/yr.The additional 600ppm shift could then best be explained by this failing trim resistor, which would have about1679 Ohm => open the case and simply look inside!Maybe this reference encountered a mechanical shock during the transport.Frank

author: jfphp
author_rank: Regular Contributor
timestamp: November 01, 2020, 11:49:00 am
content: Thank you to all for the interesting answers. Just a last question : to open the 742A, are the screws on the front panel or the rear one (under the plastic sheet) ?

author: chekhov
author_rank: Regular Contributor
timestamp: November 01, 2020, 12:25:25 pm
content: Answer is at the same teardown topic:https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg2936136/#msg2936136But are you ready to void your calibration ?

author: martinr33
author_rank: Frequent Contributor
timestamp: November 02, 2020, 05:07:13 am
content: 1) The 8508A at 10mW should still be capable of full resolution, on the 100mV range. So the calibration should be good. However - to Dr. Frank's point, a ppm at 100mV (which is the reading across a 1 ohm resistor at 0.1A, 10mW) is only 0.1uV. That's low enough to see thermoelectric effects. But not enough to explain a 10x delta, assuming you are using decent cables.2) If you use freeze spray on the label, get it really cold - iced up - the adhesive hardens and losses its stickiness. The label will peel (almost lift) off and you can reuse the adhesive afterwards. So you can get inside the box and keep the labels. You can also do it in stages, to avoid getting the unit too cold. Plus, let it equalize before you open it.However - I would recheck against your friend's unit before anything else, as opening a unit like this is a drastic step. The calibration difference sounds too great. The test would also tell you if something has gone wrong with the internals.

author: jfphp
author_rank: Regular Contributor
timestamp: November 12, 2020, 11:30:29 am
content: END !The last but not the least, as I received the 742A-1 I made a simple test with a Keysight 3458A coming back from cal (8 october 2020) in a cal lab environnement (23.5 °C, 50% humidity, T internal 37°1 C, same as during cal, after ACAL) and I got 01.00006/7 (less sensitive ohm range on the 3458A is 10 Ohms). Specs for 90 days : 15 + 5 ppm.Mail sent to Fluke (France with cal in the Netherlands) explaining the whole story and answer today after a long delay : " There was indeed a typing error in the certificate, and this has been corrected.Find attached the corrected certificate ".1.000 066 4 !!!!!!!I have no word to express my disgust : I have paid almost $1000 and no excuse, not any partial refund. You can imagine the consequences for somebody silly trusting a big well known company. I apologize to all those I have disturbed by this topic and kindly responded, because I too trusted this company. I invite all the fans of metrology and ... all the professional metrologists to think about this lamentable story...

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 12, 2020, 03:05:24 pm
content: Errare humanum est.And that's why calibration labs use more than one standard and do regular cross checking.And that's why the history is important.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 12, 2020, 05:31:03 pm
content: A calibration report should come with the instrument, having all data acquired and written down in an automated way, to avoid manual errors.I always recommend to make all measurements with Statistics, because a measured value is only valid with the incorporated error , like the Standard Deviation.This way, you also get 1..2 additional digits for the mean value, and might resolve 1μOhm / 1ppm. StD might also be around a few ppm

author: felixd
author_rank: Regular Contributor
timestamp: June 24, 2025, 03:14:22 pm
content: Any update on the story?How Fluke reacted in the end?

author: coppercone2
author_rank: Super Contributor
timestamp: June 24, 2025, 03:52:30 pm
content: they reacted with fi fi fo thum

author: zlymex
author_rank: Frequent Contributor
timestamp: June 25, 2025, 02:02:24 am
content: The upward drift of 742A is common.Fluke hermetic wire-wound is poor and out of date, they have abandoned the use in most of their equipment.Take a look atTiN's test here, five 742A-1 drifted +8.6ppm, +38.0ppm, +43.7ppm, +44.6ppm, +54.5ppm.I have a friend using 742A-1 as the reference and need periodical calibration, the value for last year is +23.5ppm. This cannot be tolerate anymore and now he is looking for replacement.