[Solved] Health check for DMM and specially for Brymen BM235 - Page 1
Forum section: electronics / testgear

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 07, 2017, 01:25:47 pm
content: Hello!Is there a health check for DMM and specially for Brymen BM235? I'm getting some weird readings and I would like to discard if the DMM is faulty.Thanks in advance!

author: rstofer
author_rank: Super Contributor
timestamp: April 07, 2017, 03:46:42 pm
content: Absent a DMM calibrator, you just need to get creative. Borrow another meter and make comparisons.Measure the voltage of a 1.5V battery. You won't get 1.5V, probably, but you shouldn't get 100V either.Now that you know the battery voltage, measure the current through a 1k resistor. You should get 1.5 mA, not 350 mA.Repeat the tests with any DC wall wart you have laying around. Measure DC voltage and DC current.Now find an AC wall wart and try again including AC current through the 1k resistor. You might want to stay away from measuring the mains.Sometimes weird measurements are caused by weird circuits, not issues with the meter.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 07, 2017, 03:57:26 pm
content: In addition to the good tips from rstofer, make sure the DMM cables are good. Intermittent connections inside the cables are a source of frustration and more than once caused me to suspect the meter was at fault.

author: BMack
author_rank: Regular Contributor
timestamp: April 11, 2017, 05:53:58 am
content: Quote from: rsjsouza on April 07, 2017, 03:57:26 pmThis should ALWAYS be the first thing to check. You should be in the habit, when checking resistance/continuity, to touch your leads together first. You never know when a lead will crap out and it will waste your time when troubleshooting.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 11, 2017, 03:26:28 pm
content: Thanks for your answers. I'll keep in mind all of them.All began with Segal's law: "A man with a watch knows what time it is. A man with two watches is never sure."I've got two different readings in a very simple circuit with two different DMMs (BM235 vs Mastech 830), so I used a third one (BM079 clamp meter) to verify which of them was correct and all Brymen match.Conclusion: I'm not completly sure if the cheapest DMM is faulty, I guess it is not accurate enough. Now I have this new annoying feeling that I need a new DMM.

author: ProBang2
author_rank: Frequent Contributor
timestamp: April 11, 2017, 04:31:12 pm
content: The cheap DMM don´t must be necessarily faulty.Consider: Every measurement influences the circuit under test.E.g. the measurement of voltage (in parallel) creates a voltage divider. The DMM and the part under test acting like two resistors in parallel. This changes the voltage drop at the testpoints. The effect is bigger with the 830 DMM, because it has only 1 Mimpedance, versus other DMM´s, which have usually 10 Mimpedance (some ones even more).The measurement of current (in series) is influenced by the different, so called burden voltages of different DMM´s.

author: rstofer
author_rank: Super Contributor
timestamp: April 11, 2017, 04:51:28 pm
content: Cheap meters can be amazingly accurate:https://www.eevblog.com/forum/testgear/aneng-an860b-multimeter/msg1159254/#msg1159254No, the Aneng doesn't come close to the accuracy of my Fluke 189 but I can buy 20 of the Anengs for what the 189 cost 13 years ago.Measurements is an interesting topic. We want accuracy to 6 decimal places but when we write the results, we round to 2 or 3. When I went to college, we used slide rules. At best, we could only calculate to two or three digits and if the expressions had a lot of factors, we would soon be down to just a single or perhaps two digits of accuracy. On some slide rules, the cursor had a magnifier so we could keep as many as 2 digits a little farther into the calculation.And we made it to the Moon and back...

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 11, 2017, 05:05:58 pm
content: Quote from: ProBang2 on April 11, 2017, 04:31:12 pmI usually forget what you say "Every measurement influences the circuit under test" because I don't completely understand yet some basic principles."One of the realities of electrical testing is that you can't measure something without changing it in some way."http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/burdenvoltage

author: Fungus
author_rank: Super Contributor
timestamp: April 11, 2017, 05:10:04 pm
content: Quote from: HoracioDos on April 07, 2017, 01:25:47 pmTHISis why you need two or three multimeters. If you only have one, this situation will eventually arise. Even if it's a Fluke.A second/third meter will give you confidence in your main meter. Even cheap ones will do.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: April 11, 2017, 05:27:04 pm
content: Quote from: rstofer on April 11, 2017, 04:51:28 pmI read your post about the Aneng last weekend when I was looking for a new cheap DMM. It's still bouncing in my mind.Quote from: rstofer on April 11, 2017, 04:51:28 pmWith due respect to the inevitable/obvious differences, (I don't know the correct english expresion) as an IT consultant I found myself many times fighting with customers asking them why they want measurements with 2 decimal places and later they make decisions based on an integer.

author: Fungus
author_rank: Super Contributor
timestamp: April 11, 2017, 06:04:34 pm
content: Quote from: rstofer on April 11, 2017, 04:51:28 pmThere were slide rules with 5-digit accuracy, eg. the Gilson Atlas. This was an 8" diameter circular slide rule (already equivalent to a 24" slipstick) which had a secondary spiral scale going down to the center of the disk. You did the calculation once on the outer ring, giving three digits of accuracy. You then repeat the calculation on the spiral track. Because it's a spiral you get multiple results under the cursor but you know which one is the right answer because you already did a calculation to three decimals in the first step. The spiral is over 50 feet long so it's as accurate as a 50 foot slide rule.I've got a couple of them somewhere.If you need more than that, pull out your seven digit log tables*.Get off my lawn.(*) Yes, somebody actually sat down and hand-calculated 10,000 logarithms to seven decimal places in the early 1700s. Seven decimals, plus the extra 'PP' columns to get 100,000 logarithms by interpolation.And then ... some poor individual had to typeset them into a book ( 10^4 * 10^7 = 10^11 little lead numbers hand-placed in trays).Some light bedtime reading...