Meter Mystery!

[kestrelmusic]

I'm working on a 1940s American valve tester and the meter is behaving very oddly. It's a pretty standard moving coil meter with around 80uA FSD, as far as I can tell. The clear plastic insert in the cover had come loose, and was very scratched and dirty, so I replaced it with a new piece cut from acetate sheet.

Without the cover, the meter reads perfectly. With the cover in place, the needle deflects all over the place, irrespective of whether there is any current - it does it even with the terminals shorted. I can only conclude that it's some kind of static, because if I wave my hand over it the needle moves. I tried replacing the original insert, but it does exactly the same thing. I am greatly puzzled!

[G1RAO keith]

Hi.

This does sound like static affecting the movement. Look for a glass supplier and get a thin piece cut, take your plastic one as a template.

Regards, Keith.

[Nickthedentist]

Definitely static. The "original" insert was probably a replacement too.

A similar effect can sometimes be had, I'm told, if you polish a record player's acrylic dust cover while it's playing a record, assuming the tracking force is low enough.

Nick.

[Pamphonica]

I think Servisol (and others) do an antistatic spray that can help a lot.

[Herald1360]

I came across exactly the same problem with some professional multimeters years ago at work. You can try going huurrrrr over the plastic window- this may discharge the static charge

Possibly operating a piezo gas lighter in its vicinity may also work- it will produce some ionised air. Some kind of alpha or beta emitter would probably work best but this isn't a serious suggestion!

[G6Tanuki]

It's electrostatic - the signal-level meter on my Racal RA217/MA323 suffers the same issue: I can rub the front and the needle will move to 1/4-scale even when it's not even switched on!

[m0cemdave]

A lot of meters with plastic covers suffer from this. Some of them seem immune, which leads me to wonder if they have a thin (conductive) antistatic coating applied.

I think it's what used to cause the pointers to collide and bend on those expensive dual-movement PPM's, before the manufacturers changed the pointer cross-section to make them stiffer.

[stuarth]

Try breathing on the glass/plastic cover. If the needle behaves properly with the cover steamed up - the problem is static. Household foam cleaners have anti-static properties, i.e. they leave a thin film behind which attracts moisture to give a slightly conducting layer - provided that you don't over-polish after applying the foam - just dry it off gently.

S

[Skywave]

Heathkit manufactured a VTVM with a front over for the dial made from plastic transparent material. It is notorious for the needle sticking due to static. I know one enthusiastic engineer who had a replacement meter front manufactured from glass and at considerable expense: the problem was not solved!
I suspect that if the rest of the meter case had been made from Bakelite or even metal, the problem would have vanished

Al.

[yampy187]

Hi.
Just a quick thought, Would a piece of glass from a cheap pound shop picture frame be thin enough to make a replacement glass? Provided you have the tools to cut it. It would work out cheaper than going to a glazier. I did this for an old Bakelite KWh meter and it worked a treat.

Jan

[MotorBikeLes]

Fifty years ago I came across this problem with temperature controllers. The Elliots (formerly Electroflo) and the Ether "Transitrols" would both give misleading readings if you touched the GLASS panel, sending the pointer up scale or downscale. Yes, a quick breath on the glass would return things to normality. Some of the Elliots were incorporated into a (rather dreadful) programme controller. The controlling cam mechanism was connected to the "setpoint" by about one metre of Bowden wire inner, and the setpoint reproducibility was so poor that I instructed the operator to carefully check the peak temperature against a Honeywell-Brown indicator with much better accuracy and scale readability. If it was below the required setting, a gentle tap was usually needed to jog a bit more movement of the setpoint. This of course would often send the indicated temperature off on a static induced journey, so the breathing on the scale was required. The bodges required to keep industry on track! The FSD of these was about 12mV from memory. (1230 Celsius with a Pt/Pt-Rh (13%) thermocouple.)
Les.

[kestrelmusic]

Many thanks for all these helpful thoughts. A gentle 'hahhhrrrr' solved the problem!...

[Herald1360]

Splendid!