Reason on DMM resistance inaccurate the longer it's used - Page 1
Forum section: electronics / testgear

author: abdulbadii
author_rank: Frequent Contributor
timestamp: December 09, 2021, 11:01:02 pm
content: What's logical and technical reason on DMM resistance meter become inaccurate/incorrect ?After adjustment, a capability given by DMM, it gets correct but then long time later become inaccurate/incorrect again then after adjustment it's correct and so on.Knew it, it's mosly due to the less battery power as the more being used, but need more other knowledge to complete the understanding.And how is this adjustment capability working technique ?

author: alm
author_rank: Super Contributor
timestamp: December 09, 2021, 11:42:07 pm
content: What kind of DMM are we talking about?In general, if you look close enough, nothing will truly stay stable. A resistor will slowly change in value due to processes like diffusion and stresses in the material that relax. Same with semiconductor junctions that form voltage references. This tends to be less of a problem in 3.5 digit DMMs than 6.5 digit DMMs because it's much easier to make something 0.1% stable vs 0.002% stable. This applies to all ranges of a DMM, but the ranges that involve fewer parts, like DCV, are usually more stable than ranges that involve more parts, like resistance, current or ACV.Linked to this, the uncertainty in the data sheet is trying to model this process. If we know your current location, it might be accurate down to 10 meters if we use GPS. However, if we knew your location 1 hour ago, and we know you only travel by foot, we only know your location within a say 10 km radius. After 24h, worst case you might have walked 240 km, but if we assume you don't walk in a straight line, but randomly change direction, it might be something like 50 km (sqrt(24) * 10 km). If you determine your position again via GPS, you'll know it within 10 meters again. This is why the longer ago it was calibrated, the less certain we are of the value, and the more likely it is that it has drifted. Doesn't mean it's necessarily inaccurate. The Fluke 7x 3.5 digit DMMs are often still in specification after decades. Clearly whatever drift they have on this timescale is well within the specified uncertainty.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 10, 2021, 01:35:27 am
content: Many factors as the earlier thread is written, we can call that "aging"But if you do precise measurements, you need a temperature / humidity controlled room, theses factors can affect logging and measurementsLead test qualities too etc ...For voltages measurementsSay the old venerable hp Agilent 34401a at 6.5 digits has a good lm399 reference in it, the more it ages the more it will get stable, a calibration will correct and set the correct ''read values'' in the tolerance ranges it need to be, if its outside of theses ranges the calibration will failSame for the higher venerable 3458 at 8.5 digits ... with an ltz 1000 reference etc ..Any precise meter i would say over 5.5 digit will need to ''age'' and stabilize over time, and a proper calibration will correct any drift.My 3x ''old'' 34401a haven't drifted much and stayed in the tolerances ranges, i had re-done all the power supply capacitors before sending them to be calibratedI have many 0.035% precision resistors, and they do sligthly drift over time, and the storage conditions may affect themFor resistances measurement, it can depend of the parts quality and stability to generate the proper currents it need to test the resistors, a 4 wire mode will get better resultsNormally on any good meter you can nullify residual leads "impedance" values, and do a resistance measurement.You have Precision and Resolution, you have to understand what it means, many threads here had a debate lolll

author: radiolistener
author_rank: Super Contributor
timestamp: December 10, 2021, 04:27:30 am
content: most of all probes are degraded.I'm using Brymen gold plated probes, and their resistance after 4 years is more than 1 Ω.When I pull the wires, it gets better to about 0.10-0.20 Ω.With cheap chinese wires I got 0.02 Ω.

author: bob91343
author_rank: Super Contributor
timestamp: December 10, 2021, 06:23:12 am
content: For low resistance it's necessary to use 4-wire measurement techniques.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2021, 09:48:05 am
content: A change in battery valtage is usually the least problem and part of the normal uncertainy specs. The change in the readings and increasing uncertainty is more due to aging of resistors, OPs (offsets) and possibly buildup of surface contaminations that can lead to leakage currents.The lower end meters rarely give seprate specs for short time (e.g. 90 days) and long term (2 years), because the uncertainty is large to start with and nobody needs ths short time specs for a cheap meter.How much the parts actually age is difficult to estimate and this may vary between units. So the specs (especially the part due to long term drift) are only a more or less conservative prediction, nothing very accurate. So it is not uncommon that it later turns out that the parts were much more stable, or in rare cases also that aging may be faster. This is a reasone why it needed the repeated calibration.In most cases a calibration does not include a new adjustment. With lower cost meter a later adjustment is rarely done: If the readings have drifted so much to be out of specs, this is usually due to a defect and only an adjustment would not make the meter reliable again. The procedures for adjustment are often also no public in the lower cost meters.In older meters the adjustments was with mechanical trimmers. In newer meters the adjustment is usually digital by scale factors in the software or digital pots.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 10, 2021, 12:27:40 pm
content: Quote from: abdulbadii on December 09, 2021, 11:01:02 pmYou have an old meter with adjustment pots. You have a resistor that you are using for your standard. You are adjusting the pots so the meter reads the value of this resistor. And you continue to adjust the meter's pots over time to try and keep the meter reading the same value.Start with which meter, what are the specs for the resistor and how much drift are you seeing?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 10, 2021, 12:45:19 pm
content: One place I worked bought early model UNI-T DMMS.They had two delightful faults:-(i) They would turn themselves OFF in the middle of testing something. Apparently, the "power saver" function was just a timer!(ii) Similarly to the OP, sometimes, over time, they could give very incorrect resistance readings.In one case, on a faulty new device being tested, a particular resistor was suspected, but on testing it with the UNI-T, it appeared to be close to the correct value, but I was suspicious, because the colour code looked wrong, although it was hard to see.On a hunch, I grabbed my private Fluke 77 & retested the resistor------the value was about twice what it should be.It turned out that UNI-T didn't flag a flat battery till it was virtually dead.As the battery dropped well below the correct voltage, the resistance range started to give silly readings.The other guys were deeply into denial---my scabby old Fluke 77 couldn't be right!

author: Fungus
author_rank: Super Contributor
timestamp: December 10, 2021, 02:35:55 pm
content: Quote from: abdulbadii on December 09, 2021, 11:01:02 pmI'm not sure that's universally true. There are many, many multimeters that are still accurate after 30 years (and counting...)Multimeters are affected more by ambient temperature, humidity, and even altitude. Look in the manual for the meter and it should tell you the environmental limits for the meter's specified accuracy.Quote from: abdulbadii on December 09, 2021, 11:01:02 pmHere's the calibration procedure for a Fluke 87V:https://www.manualslib.com/manual/1505585/Fluke-87-V-An.html?page=37#manualYou can find similar procedures for most "professional" meters.Quote from: radiolistener on December 10, 2021, 04:27:30 amThat's what the "rel" button is for.