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Old 25th Dec 2020, 9:16 pm   #1
The Philpott
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Default Mystery liquid formed in ammeter 'glass'

A simple moving coil 0-3mA ammeter, i am concerned about liquid which has formed between what appears to be the two laminations of the plastic window.

Unfortunately the plastic is shaped around the zeroing screw so even if i could get the plastic out without damage (it is glued in) sourcing a glass could be tricky.

Does anyone know: What the plastic is? What the liquid is?

The inside of the meter looks unaffected thus far, but i'm still not comfortable with fluid being present!

Dave
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Old 25th Dec 2020, 10:33 pm   #2
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Default Re: Mystery liquid formed in ammeter 'glass'

Is it actually liquid? I'd suspect it's more an optical-interference effect caused when the 2 layers of the 'glass' are less than a wavelength-of-light apart.

[If you've ever looked at sheet-mica you get a similar effect where the 'leaves' of the mica are so close together that the air-gap is less than a wavelength of light]
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Old 25th Dec 2020, 10:49 pm   #3
high_vacuum_house
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Smile Re: Mystery liquid formed in ammeter 'glass'

I have seen this before on a couple of old moving coil instruments.

Basically there are 2 sheets of glass of identical dimensions which are then glued together with a transparent plastic type glue to form a laminated unit.

I presume this would increase the toughness of the meter glass compared with a single piece or at least keep the glass together should it be broken.

What has happened here is over time the glue has started to de-bond from the 2 pieces of glass from the outer edge. Probably caused by moisture penetration over time. This then causes the effect you can see here. I wouldn't be worried about it as it is just cosmetic and would not affect the instrument.

I think that the window is far more than likely to be glass than plastic as plastic would cause static charges being built up when touched or rubbed (which would give attraction and repelling forces to the pointer) and would affect the accuracy and repeatability of the instrument

Christopher Capener
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Old 26th Dec 2020, 1:06 am   #4
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Default Re: Mystery liquid formed in ammeter 'glass'

Thanks. It's definitely liquid as pressing from both sides makes the droplets spread out, then reform as pressure is released.

Perhaps you are correct Christopher- it doesn't sound like glass when tapped-but in retrospect this could be a damping effect caused by the laminated construction.

(Note- the first image shows the droplets)

Dave

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Old 26th Dec 2020, 10:16 am   #5
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Default Re: Mystery liquid formed in ammeter 'glass'

If the 2 pieces of glass have become delaminated it might be possible to remove at least the inner layer, clean the outer one and re-assemble, a single layer fo glass will be enough.
If you do break the glass I should be able to make a replacement from a template.

Peter
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Old 26th Dec 2020, 11:22 am   #6
John M0GLN
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Default Re: Mystery liquid formed in ammeter 'glass'

Quote:
Originally Posted by The Philpott View Post
Thanks. It's definitely liquid as pressing from both sides makes the droplets spread out, then reform as pressure is released.
Dave
I've seen air bubbles behave like that between 2 layers of glass, you press and they spread out and return when the glass resumes it's previous position. Would it be possible to gently warm it with a hair dryer and see if you can evaporate it if it is a liquid?

John
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Old 26th Dec 2020, 11:35 am   #7
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Default Re: Mystery liquid formed in ammeter 'glass'

If you get some 2mm picture glass (I keep it from broken clip frames etc) you can mark it up with a thick marker, place it underwater in a bowl, and cut it with scissors (or tin snips)! Something about the water stops the cracks running everywhere very fast and smashing the sheet. The edge will be rough and crunchy, but this will get you near enough to finish it on a belt linisher or maybe even a sanding pad on a drill (sand in-line with the glass sheet, not across it or it will break). I've done several meters / speedos etc this way - just glue the sheet in once you're done. cheers, M
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Old 26th Dec 2020, 9:33 pm   #8
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Default Re: Mystery liquid formed in ammeter 'glass'

Thanks for comments. I opened up the meter once more and had a closer look; the window is secured by a thin (but wide) split ring- material unknown but the same black colour as the bakelite. This split ring is firmly glued in.

I selected meths as a solvent on the basis that it will mix happily with any water present, and act as solvent if the contamination is oil based. I suspected there was an oily element to the fluid since some iridescence/rainbowing was visible.

Curious development- on soaking the meter shell in meths the glue securing the window and it's split ring was completely unaffected BUT.. the contamination became more crystal clear and slightly less yellowed, yet still slopping around in the void. After some time for evaporation the fluid actually solidified, leaving no liquid present (which, after all, was the main purpose of attacking this job.) It seems the meths seeped in and did something good- we'll see if it lasts.

This meter has been messed with and re-purposed before- note the gloss black paint over factory legends at the bottom, and also (virtually invisible) is the professionally deleted legend D.C.MILLIAMPERES over the scale arc.

Dave
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Old 29th Dec 2020, 5:39 pm   #9
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Default Re: Mystery liquid formed in ammeter 'glass'

Update-

Optically less satisfactory after a few days so soaked once more in meths for good measure, and tackled the split ring (which DOES seem to be bakelite-esque) which came out easily with a blade, albeit a couple of fragments broke off from the ends.
Separated the two thin glasses with a blade, exposing the flexible centre laminate complete with puddles of hardened glue. I shall just reassemble using the two glasses with the remainder of the split ring and some small blobs of araldite here and there.

Note that for this latest operation (and in retrospect) i do not think the meths was necessary.

Dave
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Old 29th Dec 2020, 7:40 pm   #10
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Default Re: Mystery liquid formed in ammeter 'glass'

I personally would only use a single sheet of glass, with 2 sheets togeter even the slightet trace of liquid between could give rise to Moire fringe rainbow effects.

Peter
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Old 29th Dec 2020, 8:43 pm   #11
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Default Re: Mystery liquid formed in ammeter 'glass'

Hmm...and possibly more reflections.
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Old 29th Dec 2020, 9:09 pm   #12
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Default Re: Mystery liquid formed in ammeter 'glass'

There is some optically clear adhesive available on eBay, normally sold for repairing mobile phone screens, I have some here, I used it to bond the OLED display of an Apple watch to the digitizer glass, it doesn't set hard, it sets like a really sticky soft glue, but is totally clear (unless you get muck in it!). You just put a blob on one piece of glass, put the other piece of glass onto it, let it go to the edges then mop up any excess, then hit it with UV light to cure it. Could work for this?

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Lloyd
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Old 30th Dec 2020, 6:16 pm   #13
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Default Re: Mystery liquid formed in ammeter 'glass'

The comically thin bakelite split-ring has 3 flanges on it's outer edge, at 120 degree intervals.. these engage in slots in the meter shell, effectively trying to dictate the exact thickness of the window! A lot of bother to go to...all i can see that it achieves is to make the meter small by reducing the necessary width of the step where the glass sits. I have dispensed with this system and just fitted one sheet of glass with slow araldite - waiting the requisite 14hrs for it to cure- much preferable to rushing it with fast araldite, and helps precision with cocktail stick application.
I will also check the calibration before reassembly- there is what appears to be a magnet shunt inside. Quite a high quality item.

Dave
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Old 31st Dec 2020, 9:04 pm   #14
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Default Re: Mystery liquid formed in ammeter 'glass'

'Made in England by Ernest Turner at High Wycombe Bucks'
Basic resistance around 20 ohms. On experiment, 1100 ohms in series scales it up to 3 volts. No indication of how it was held in the panel, possibly a circumference clamp..? No witness marks are present.
Dave
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Old 31st Dec 2020, 10:01 pm   #15
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Default Re: Mystery liquid formed in ammeter 'glass'

I think with clamps; similar to some bigger, modern radial electrolytes...
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Old 1st Jan 2021, 2:22 am   #16
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Default Re: Mystery liquid formed in ammeter 'glass'

It looks like it once had a steel "U" bracket that fitted across two studs on the back.
I looked back to some photos of an elapsed time indicator that has such a bracket. Unfortunately I only took photos of the fascinating bits inside it at the time.
Encouragement will be needed to get me to dig it out again if you need a photo of the bracket.
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Old 1st Jan 2021, 11:02 am   #17
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Default Re: Mystery liquid formed in ammeter 'glass'

Dave,
I think it was possibly held in the panel with so -or similar- a circumference clamp...
Happy New Year!
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Old 1st Jan 2021, 2:47 pm   #18
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Default Re: Mystery liquid formed in ammeter 'glass'

Yes that style of meter was generally fitted with a 'ring clamp' on the back: attached is a pic of the clamp on a part-dismantled ex-Army charging panel.

The ring clamps the barrel of the meter, then there are 3 smaller machine-screws on 'ears' punched/bent out from the ring, that are used to tighten the ring against the panel. You can only see one of these screws in the photo - the others are 120-degrees away each side.
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Old 1st Jan 2021, 8:56 pm   #19
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Default Re: Mystery liquid formed in ammeter 'glass'

That must be the system used. (Can't be a conventional U clamp as the rear is similar to those you have pictured- there are no mounting studs, only threaded terminals)
Dave
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Old 22nd Feb 2021, 3:46 pm   #20
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Default Re: Mystery liquid formed in ammeter 'glass'

....I used a suitable capacitor clamp in the end..

Rather than start a new thread, there is a postscript which (could be) connected with the fluid trapped in this meter, so here it is-

Oddly, the 1.1kohm resistance i used to scale the meter to 3 volts had to be removed and a lower value inserted- in this case 1kohm. In other words, the meter had been reading high then later decided it would read low..

There's no obstruction to the movement and the magnet and other ferrous components are not corroded, however...the hairsprings are blackened and have a crystalline appearance. I am almost certain this is connected with the anomalous readings, as i have encountered it once before, on that occasion green crusted hairsprings were seen on an admiralty voltmeter which developed an over-read after being used a few times- but a completely non-linear over-read.

Pure speculation that in this case it might be connected with the fluid in the laminated glass, (more likely to be solder flux reaction) but i suppose if there's a moral it would be close inspection of the movement springs on any old PMMC meter that you intend to use in a project- even if it appears clean, free moving and OK in all other respects.

Dave
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