Metrahit 24S DMM strange behavior on milivolt range - Page 1
Forum section: electronics / repair

author: duzycinek
author_rank: Contributor
timestamp: October 14, 2025, 08:38:38 pm
content: Hello, I got Metrahit 24S with fluctuating issue. I know it should fluctuate a bit, but this is way too much. It even changes range automaticly from 300mV range to 3V range. At manual 300mV range it shows O.L. If I short probes it show exact 0mV, also while measuring known mV value it shows it correctly. Anyone have idea what is causing this behavior? Videos available in google drive link below. Looking forward to hearing from you!https://drive.google.com/drive/folders/1s2JpwPZaW7DS6vmQBtLZVfhsHjab7n-V

author: 2N3055
author_rank: Super Contributor
timestamp: October 14, 2025, 08:50:48 pm
content: Does it have high impedance in 300mV range?Datasheet says >20MΩ

author: duzycinek
author_rank: Contributor
timestamp: October 14, 2025, 09:33:45 pm
content: Yes, it should have, but no idea how to verify it.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 14, 2025, 09:54:49 pm
content: go in manual range, select the one you need to check use another multimeter to check the 24s between ohms/dc/ac inputs and groundif you get 20 Megs ohms, all seems normal, 20 megs is an high impedance ... many meters are in the 10 or 11 megs of input impedance ion that range ?ex: my brymen 857 gives 12 megs, a fluke 179 in 600 ohms range does not like to be probed lollex brymen is stable until i get near the probes and fluctuates up to 40mv, some spike at 70mv, 1 feet away and drop to zero, the probes are under 1 inch. Further away say 12 inches, seems more sensitive / prone to fluctuations ?if it fluctuates does it have probes connected, have you tried no probes, having probes but not shorted out means you have antenna effects, parasite capacitance, the input charge little by little

author: duzycinek
author_rank: Contributor
timestamp: October 14, 2025, 10:33:34 pm
content: Thank you. In AC mode 300mV range Brymen 829s shows 10.15MOhm. In DC mode 300mV range Brymen shows O.L. I know about antenna effect but it shouldn't be more stable? Brymen at mV range shows from about 0.0 to 20mV and Metrahit most of the time have O.L in 300mV range so more than 300mV.

author: 2N3055
author_rank: Super Contributor
timestamp: October 15, 2025, 07:32:40 am
content: Quote from: duzycinek on October 14, 2025, 10:33:34 pmOk so that is the issue. That is common for High impedance meters. See, input has very high resistance, and internal circuits are never fully balanced, so there is very small input current. There is some capacitance in parallel with input too.So what happens is that since there is no resistance to load input current of meter, it charges that capacitance and voltage rises. Your meter shows increasing voltage, positive or negative, depending on direction of internal input current. So it measures that rising "phantom" voltage, and once that voltage reaches high enough, it will autorange up. This will then put 10MΩ in parallel with input, that will rapidly discharge this voltage. Then it will autorange down, where it will start charging again. Relaxation oscillator.To prove this, connect a few µF capacitor to the input by its own. Nothing else. It should start doing this same thing but slowly so it can be seen more easily. And by measuring time to charge known capacitor you can measure what input bias current is...All this means it is normal that high impedance meter behaves this way when nothing is connected. What is unusual is that on my Metrix (and on bench meters) you can choose if you want HiZ or 10 MΩ, and here it seems you cannot.

author: mqsaharan
author_rank: Regular Contributor
timestamp: October 15, 2025, 08:10:59 am
content: Quote from: duzycinek on October 14, 2025, 08:38:38 pmHi duzycinek,There is nothing to worry about as is already explained by 2N3055.If you want to see fluctuating issue, watch this Dave's video:https://youtu.be/LCFEonQDkQU?t=2875I have two multimeters from Uni-T, a UT70B and a UT195DS. Both are based on Cyrustek chipsets. Ut70B is more crazy in mV range close to what is shown in the video linked above because it is driving the display directly and UT195DS is comparatively less crazy like your multimeter in mVDC range because its readings come through a microcontroller and a separate display driver chip even though it is much faster than the older UT70B.Does your meter invert positive and COM leads in resistance mode? I mean if COM becomes positive and V/Ohm becomes negative in resistance mode? If it does, I am somewhat sure it is using one of Cyrustek's multimeter chips inside.

author: duzycinek
author_rank: Contributor
timestamp: October 15, 2025, 09:28:48 pm
content: Hello, after connecting to input 1uF cap it is fluctuating very little at about 25mV and after connecting 4,7uF cap it is fluctuating very little at about at about 61mV. Also changing cap polarity is changing displayed value to same but negative value. So u both are right i guess and there is everything okay with meter. Thank you for your help guys!

author: coromonadalix
author_rank: Super Contributor
timestamp: October 16, 2025, 01:24:22 pm
content: my 28s does the same thing, higher with probes and almost nothing with no probesnever get into OL