off-zero analog meter because of static charge on its face... - Page 1
Forum section: electronics / general

author: wetbox
author_rank: Newbie
timestamp: February 08, 2021, 09:36:10 pm
content: Today I received an almost mint HP 6827A with both front panel indicators way off zero, and very responsive to the position of my fingers! After 'fixing' one of the two meters by breathing on it, I made a short clip of the other - let's see if I can attach it as a zip file.The clear plastic can easily be recharged to reintroduce the off-zero by rapidly wiping the meter face with a finger, and the same works for several of my other instruments.I guess it is to be expected on a relatively dry winter day, but I never thought of waving my finger by an analogue meter to verify that static charge on its face is not biasing the reading...VID_20210208_163949_s.zip

author: TimFox
author_rank: Super Contributor
timestamp: February 08, 2021, 10:56:49 pm
content: Back in the day, I often saw that effect on sensitive d’Arsonval meters (say, 50 uA and below full-scale) during the winter with very low indoor humidity. When necessary, I would lightly rub the clear plastic surface with a moist tissue paper before making the measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2021, 11:05:37 pm
content: A have an older (1980s) russian analog multimeter, that was essentially non working for some 2 year due to static charge. The needle nearly stuck at a point near 70% FS, causing extreme nonlinearity (could still use 0-20%). Waiting did not solve the problem - I finally opend it and removed the charge from the inside surface with some water. It now works again.It does not take a dry winter day - just some wipe with wool.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 09, 2021, 02:08:25 am
content: A long time ago I found someone's topic regarding restoration of similar HP equipment with meters like that. His solution was to gently wipe the surface with clean pencil soft lead shavings. This left a static dissipative layer that was good enough to drain any static buildup on the surface. It was thin enough to be invisible. Unfortunately I can't find that topic any more.

author: coppice
author_rank: Super Contributor
timestamp: February 09, 2021, 02:26:28 am
content: I used to own a large TMK 100k ohm per volt (10uA full scale) analogue multimeter which could be quite susceptible to static charges altering the reading in very dry conditions. When it takes so little current to move the pointer, its not too surprising other spurious forces can influence it too. It was a lovely instrument for its time, despite this effect.

author: Ian.M
author_rank: Super Contributor
timestamp: February 09, 2021, 04:11:58 am
content: I'd be inclined to try wiping it with a lint-free cloth moistened with a dilute solution of fabric softener. You can also get antistatic lens cleaner suitable for plastic lenses.

author: drussell
author_rank: Super Contributor
timestamp: February 09, 2021, 04:26:00 am
content: Quote from: wetbox on February 08, 2021, 09:36:10 pmIf you have any of those clothes dryer softener sheets available there (like Bounce or whatever,) try wiping the plastic lens with one.

author: wetbox
author_rank: Newbie
timestamp: February 09, 2021, 09:44:50 pm
content: I will make the face of the meters dissipative. I'm just surprised that I have never seen in the manuals any reminder about removing static charge before adjusting the zero of the indicators. Actually also before making a reading. From several comments it may have been common knowledge back when these were more common than today. Now I feel younger. Yay!Then again, they could have made the paint or whatever is on the needle dissipative. Wouldn't that make it insensitive to any charge on the lens?

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 09, 2021, 09:55:30 pm
content: I have had to deal with this, too. I used the meter movement and part of the housing with the glass from a Simpson 260 V-O-M. The glass front of the Simpson 260 was gone so I cut some Plexiglas to fit, polished it good and screwed it in. Putting it back onto the meter caused the needle to wave around and be generally weird. This puzzled me until I realized that the plastic had become charged with static electricity and this charge drove the needle all over the place. I eventually figured it out and used my air cleaner which has ION generation as an option. Turning that on, and passing this air over the meter face fixed it perfectly.

author: TerraHertz
author_rank: Super Contributor
timestamp: February 10, 2021, 02:02:56 am
content: I've experienced this effect many times with analog meters.It's not directly dependent on the current sensitivity of the movement, since the 'interfering force' is attraction between the (effectively) grounded metal needle, and charged areas on the meter cover plastic. But the charge-induced offset will be inversely proportional to the helical spring elasticity modulus. Since more sensitive meters tend to have 'lighter' springs, they do tend to deflect more electrostatically.It happens more with plastic-faced meters than with glass, because plastics tend to transfer charge to other materials rubbed across them, more than glass does. But glass does too.Once there's a charge on the meter face surfaces (inside or outside) the needle will also move as you wave a hand/finger around nearby, since your wet-salt-bag body is shifting around the electrostatic field lines originating from that electrostatically charged plastic area. Thus varying the force on the needle.Generally such charge accumulations are on the surfaces of the plastic, and can be easily removed with a breath of moist air, or anti-static coating. But I've always wondered - like any good insulator, plastic can hold internal charge concentrations, that are trapped and no amount of surface wiping will eliminate them. A good conductive coating will shield the electrostatic field originating from an internal charge, but lacking that... I wouldn't be surprised if internal trapped charges result in permanent non-linearity errors in some analog meters.You can see evidence of internal trapped charge in the dust patterns that accumulate over time on the insides of some injection-molded plastic cases. Especially when the plastic is white. As the hot plastic is squeezed through the passages of the molding machine and into the mold, the flowing plastic acquires volumes of electrostatic charge. Which become trapped in the cooling molded item. Then dust is attracted by the permanently present electrostatic fields, and shows up as dusty 'flow lines' on the item surface. You can see how they reveal plastic flow radiating from the injection molding points.