How to calibrate a DMM, i.e finding the correct trim pot? - Page 1
Forum section: electronics / testgear

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 07, 2014, 11:31:41 pm
content: I have a decent Klien MM2000 DMM, which I bought to use at work since it is 1000v rated. I find myself using it more and more on my bench though. I have been working on a Low Ohm meter, basically I need one to get other projects done and don't have an extra couple 100 dollars!I noticed that my klien meter was having problems measuring 1 ohm resistors, but I was unsure if it was the meter or the resistor. Today I found a bunch of precision resistors in my junk, a 22ohm, a 51 ohm, a 19.9k and a 90k. I figured the meter was just haveing problems with the low ohms resistors, as the 22 ohm measured 23.5, and the 51 ohm measured 52 ohms. But even the 19.9k and 90k are off, I get 19.95k and 90.2k. These resistors are all Dale or Vishay +/- .1 or .2%. My 30 dollar mastech meter make closer measurements even in the low ohm range!Can I just open the thing up and adjust the trimmers? How do I determine which trimmer to adjust? I also have precision current and voltage sources on the way to make sure the rest of the meter is calibrated properly. Now that I am starting to work with shunt resistors, and millivolts/amps on a regular basis Id really like to trust my $120 meter.

author: Hideki
author_rank: Frequent Contributor
timestamp: May 07, 2014, 11:53:20 pm
content: First calibrate your spelling. It's Klein, not KlienThen head on over tohttps://www.eevblog.com/forum/reviews/klein-tools-mm200-multimeter/and see what trimmers to adjust.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 08, 2014, 12:19:14 am
content: It is unusual to find trimmers for the Ohms range on the PCB.The DC mV and V ranges can be adjusted if required and if you have a reliable voltage reference.A tighter voltage calibration should improve the accuracy of your Ohms readings.Don't expect sterling performance from such a meter, especially dealing with low resistance shunts.You may need a 4-wire Kelvin measurement set-up or a proper milliohm meter.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 08, 2014, 12:36:39 am
content: Sorry about the spelling, lol. So a 4.096 voltage reference reads at 4.11. Would I calibrate this to 4.10 or 4.09? I guess what im asking is if a meter rounds according to numbers out of its boundary?

author: Wytnucls
author_rank: Super Contributor
timestamp: May 08, 2014, 12:50:00 am
content: It is a 4000 count meter. The calibration should be done at 1/2 scale or close to full scale:For the 4V range, 2V or ~3.7V.For the 40V range, 20V or ~37V.Adjusting the 40V range at the bottom of the scale 4.096V is not recommended.If your initial readings are within the manufacturer's specifications, there is little benefit in adjusting anything, as drift will occur again within a few weeks.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 08, 2014, 06:36:07 am
content: Quote from: Hideki on May 07, 2014, 11:53:20 pmThe 200 (two hundred) and 2000 (two thousand) are different models.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 08, 2014, 11:59:02 am
content: In case you still want to go ahead with tweaking your meter, here is the PCB. All the pots are labeled.https://www.eevblog.com/forum/testgear/klein-tools-mm2000-multi-meter-review/msg15912/#msg15912