A tale of two meters - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: July 01, 2016, 12:29:21 am
content: At work I often use my meter for setting the charging voltage of these small battery chargers that are part of our stuff to 13.65 volts. The other day I brought my home meter in and compared it and there was about 0.15 volts difference! The big question is, which one, if either, was right… So I chucked together this $12 kit just for this situation. It puts out 2.5000 volts more or less exactly. The work meter is a 1999 max reading thing and the home meter is 3999 max. Adjusted the work meter to 2.50 volts reading and the home meter to 2.500 volts. All good, so I thought. Then I connected them both up to a 12 volts battery that was just sitting on the bench and the work one read 12.86 volts and the other (better) home one 12.91 volts. Arrrgh! That bothered my inner OCD enough that I opened up the work one again and tweaked it to the same reading as the home one. The bottom line was adjusting the work one to read 2.50 volts has only 1/10 the accuracy of adjusting the home one to read 2.500 volts but setting it to a now known but higher accurate reference voltage is better. All in a day’s work.------Upon further reflection, I have realised that the work meter was reading 2.50 volts on its 20V range and the home meter was reading 2.500 volts on its 4 volt range, but now that I have compared and set them at 12 odd volts it occurred to me that the home meter was calibrated on 4V range but (wrongly) expecting a dead-on reading on the 40V range. Sigh… back to square one.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2016, 01:30:37 am
content: Surprised. Most placed I have worked will cal the equipment with a NIST traceable standard, where it is then tagged and sealed.

author: Circlotron
author_rank: Super Contributor
timestamp: July 01, 2016, 01:46:55 am
content: It is in fact a SLA battery charger as part of a larger piece of equipment. The charging voltage is not temperature compensated so the difference between ideal and actual charging voltage is actually greater than the meter error. The batteries are replaced on a 12 month schedule, but it is a bit seat-of-the-pants around here, thus my efforts at progressively establishing some sort of order in this small company...

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2016, 01:48:58 am
content: ?Quote from: DiligentMinds.com on July 01, 2016, 01:36:46 am

author: johansen
author_rank: Super Contributor
timestamp: July 01, 2016, 06:52:52 am
content: i have spent a number of minutes tweaking the 4 resistors in a certain meter.. to tweak them a few percent of a percent to get them within line.i would not immediately junk a meter if its out by 1%. you get what you pay for.. but you also get what you bought based on hype. even some of the cheapest DMM's have better than 1% matched resistors, and the same dmm chip and circuitry as in a meter that costs 2-3 times as much.. (they are 6000 count meters.. to give you a hint.

author: Cerebus
author_rank: Super Contributor
timestamp: July 04, 2016, 10:55:32 pm
content: Quote from: DiligentMinds.com on July 01, 2016, 02:05:26 amMy 1998 manufactured, well used, ex-military Fluke 25 cost me under £40 GBP last year and it still greatly exceeds its claimed basic 0.1% accuracy. To be more precise, it is digit accurate on it's resistance range from 10 ohms to 99.9k ohms (I have a resistance box full of VHP102s to check it against) and has been within 1 mV (or the available resolution) on every accurate DC source < 100V I've been able to throw at it. You can get some VERY good kit for little money if you pick wisely.I actually bought the Fluke 25 to give me something bomb-proof to use on circuits where there was any risk of serious fault voltages/currents flying about. It's behaved itself so well, and has such a nice solid stand, that it's become my everyday electronics bench meter.