General > General Technical Chat
off-zero analog meter because of static charge on its face...
<< < (2/2)
Ian.M:
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.
drussell:

--- Quote from: wetbox on February 08, 2021, 09:36:10 pm ---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...
--- End quote ---

If you have any of those clothes dryer softener sheets available there (like Bounce or whatever,) try wiping the plastic lens with one.
wetbox:
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?
basinstreetdesign:
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.   ;D
TerraHertz:
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.

Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod