How sensitive is the BM869s in its upper resistance range compared with other meters?
Started by using the resistive decade box to determine the thresholds for the upper range. Use the center as the test resistance. Using my terminated HP34401A signal generator and placing it in series with the DMM and the decade box. Using a 50Hz sinewave, increase the voltage until the DMM is unstable. Reduce the voltage until it recovers.
The newer Brymen BM869s with the latest firmware has thresholds of 540K and 470K ohms. Setting the decade box to 505K, the meter is unstable at as low as 27mVRMS. Test current was around 0.85uA.
The Fluke 189 that my friend gave to me, has thresholds of 560K and 470K oms. Using a test resistance of 515Kohms, the meter was unstable as low as 740mVRMS. Test currents are around 1uA
My UNI-T UT181A after being damaged, repaired and modified has thresholds of 600K and 550K. Using 575Kohms, the meter was unstable as low as 680mVRMS. It has an odd behavior where the longer it sits with the AC wave applied, the chances are it will have a problem. For example, I can apply 1V and the meter will read the correct value for a short time, then bounce between ranges.
The Yokogawa TY720, again damaged and repaired but no mods to meter. Thresholds were 560K & 490K. Testing at 535Kohms, I could test up to 3.52 Volts RMS and the readings were stable. It of course effects the reading but it never hunts.
The Brymen BM786 with the last firmware I received has thresholds at 660K and 600K. Using a test resistance of 630K, the was unstable as low as 380mVRMS.
How good is good enough?
The longest standard leads I have came from Keysight. They are 1.4 meters long. If I attach these to the decade box and run them aside a 120V 60 lamp cord for a few feet, I get about 20mV. It I use their entire length, about 30mV. If I twist them the full length with the line cord, 10 turns total, about 43mV. I'm not suggesting I would use a meter this way but it puts some numbers on the board.
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I was curious about the Gossen Ultra or what ever Gossen's marketing now calls it (PN# M248B). Never damaged but a lot of Netic and copper foil added for shielding. This meter has some pretty major problems but can throw up some very impressive data after I added all that shielding to it.
I tried two tests with it. First threshold was 310K and 270K. The next was 3.1Meg and 2.7Meg. I ran it at 290K and 2.9Meg. In both cases, I could go to 7.1VRMS and saw no effect. It's stable down to the last digit! That's 290.771Kohms BTW. I doubt the shielding I added helped at all with this testing other than I can get near the meter without it going unstable.
Clip of the Gossen with up to 35V superimposed.
The thresholds are correct but the data was not! Repeating the test with the proper connections, at 270K the range was still stable up to 3.5VACRMS. At 2.7M with 3.5V applied, again the range was stable but the values are off by a fair amount and the reading vary by about 40Kohms. .
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So for fun, using the prototype 121GW which was damaged, repaired, modified, aligned using hand picked parts. Version 1.57 firmware. Thresholds are 550K and 383K. Using a test resistance of 467Kohms, the meters range is mostly stable up to 1VRMS. The displayed resistance varies a lot. Even with 18mV applied, the reading is not very stable.
I tried it with the Production #1 121GW FW 2.02 as well and it seems about the same.
When I first ran this test, I realized I had connected it up wrong when I ran the Gossen. No wonder it was so stable!! Should have rechecked the obvious. Corrected above.