building hobbiest grade cal standards - Page 1
Forum section: electronics / general

author: Dan Moos
author_rank: Frequent Contributor
timestamp: May 21, 2023, 03:18:03 am
content: I am repairing an hp 3468a bench meter. I already know the cal data is lost. (battery died)I have no need for a lab grade cal, I'd just like it to be in agreement with, say, my Siglent 3055.Lets take a resistance standard. Would it be reasonable to use a network of same value resistors in a series/parallel combination that on paper should equal the resistance of 1 of those resistors? Basically using many of them to create an average that is close enough? Using as many as is practical?

author: bdunham7
author_rank: Super Contributor
timestamp: May 21, 2023, 03:32:38 am
content: Quote from: Dan Moos on May 21, 2023, 03:18:03 amNo, that theory would require the resistors to have a standard-ish distribution centered exactly around the nominal value. In reality, they usually are not. So if you have 10 100R 1% resistors, they all very well might be 99R. Or, if you get lucky, they might have a standard distribution with a standard deviation of 0R2 and a center of 100R0. Do you feel lucky?

author: beanflying
author_rank: Super Contributor
timestamp: May 21, 2023, 04:40:42 am
content: There is a heap of info to be had in the Metrology section.Highly recommended starting point however is Conrad Hoffmans articleshttps://www.conradhoffman.com/mini_metro_lab.htmlFor a basic Resistance 'standard' then buying a few Vishay metal foil resistors at 0.005% isn't a bad place to start.

author: ebastler
author_rank: Super Contributor
timestamp: May 21, 2023, 07:55:35 am
content: Quote from: Dan Moos on May 21, 2023, 03:18:03 amIf you are happy with your Siglent 3055 as the "calibration reference", why can't you use any old resistor, as long as it is reasonably stable over time? E.g. take that nominally 10k resistor, measure it as 10.04k on the Siglent, then confirm that the HP gives you the same value (within tolerances). Why would you need a resistor with the exact nominal value?Just make sure that you don't touch the resistor in the process to avoid temperature-induced drift, i.e. mount it on a suitable base.

author: EPAIII
author_rank: Super Contributor
timestamp: May 21, 2023, 09:39:36 am
content: If you can go for a bit less accuracy, 0.05% resistors are very reasonable in price. Not exactly up to digital standards, but can be used for analog meters.https://www.mouser.com/ProductDetail/IRC-TT-Electronics/WIN-T1206LF-11-1001-A?qs=pD8CpA3T84ph5%2Fg8bDLJgw%3D%3DQuote from: beanflying on May 21, 2023, 04:40:42 am

author: beanflying
author_rank: Super Contributor
timestamp: May 21, 2023, 10:26:05 am
content: Quote from: EPAIII on May 21, 2023, 09:39:36 amYou can not really use or even consider using such 'low spec' resistors even to transfer from the Siglent to the HP. For a start the HP is in theory more accurate just on the resistance range than the Siglent for a start and even then the Siglent has a basic accuracy of 0.02% well below the SMD resistors you linked.Ideally you want to be an order of magnitude under the spec of the DUT to have a reasonable expectation that what you are seeing is what it actually is.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2023, 01:25:23 pm
content: For just comparing 2 meters one does not need super accurate or stable resistors. It still helps if the are reasonable low TC (e.g. < 50 ppm/K) so that temperature changes are not that critical and also self heating from the test current is not changing the resistance. Some meters measure some resistors (e.g. 1 K) with quite some power. With not so good resistors one can get reduce the error with a few repeats / cycles, but this can be tricky for the cal procedure to do the adjustment.For the voltage transfer one can often measure both meters in parallel and thus get away with a simple source, like batteries. It may still be tricky for 300 V.For the HP3468 to SDM3055 comparison part of the problem could be the different ranges (2 V vs 3 V).

author: EPAIII
author_rank: Super Contributor
timestamp: May 25, 2023, 08:14:26 am
content: He already said he would be happy with the accuracy of the Siglent. So I fail to see why any fixed resistance can not be used to transfer a reading from that to the HP he is repairing. Of course you would not be at the accuracy of the HP. But you could work in a climate controlled room and go back and forth between the two. Read the value on the Siglent and then on the HP. Then on the Siglent again and on the HP again. And back to the Siglent. If all five of those values match, +/- one least countof the precision level of the Siglent, then it is safe to assume the two match,within the stated accuracy of the Siglent, of course.Care should be taken to not change the temperature of the resistor being used. And good quality components should be used to connect them to the two meters. But it should be do-able.Doing this with several similar value resistors would add to the confidence level.Quote from: Kleinstein on May 21, 2023, 01:25:23 pm