To fill up with oil....??

paulraymaekers

Hello,
the Meg at 200: handsets and batt compartment were filled up with oil... to resist the high pressure.

??has anyone idee how much the pressure inside increases??
I suppose not the whole pressure increase is transmitted inside the housing, as the housing also resists to deformation, and will take up part of the pressure, unless there is a 'flexible' part in the housing.

?? can LCD's and batteries take up high pressure increase??

regards
paul

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Scuba_Vixen

Oooooo , Good Question!

I've wondered about how this practice works out in reality myself.

I would assume that it's hard to not have at least some small air space still left, and that deformation of the cases would be on the order of a few cubic millimeters, as the air is going to compress with deformation and act as a resisting force proprtional to the amount of deformation.

If all it does is to absorb some deformation, then the electronics may only be subjected to a pressure much less than the actual ambient.

With a slowly rising pressure, I'd think the oil may migrate into any air spaces, as opposed to just crushing components. How that may effect the component values thereafter, (hence accuracy of the instrument) would need to be checked often.

It would seem, impirically, that lcds and batteries can withstand at least some marked pressure above atmospheric.

How much pressure above atmospheric, and for how long, is probably the big unknown.


Darlene

jradomski

Darlene,
When your carefule its quite easy to get all of the gas spaces out, this can create a small problem in that if the oil heats heated up it expands a bit and it will generally ooze out.. You usually need to put something inside the case that is slightly compressible so that you dont get the oil oozing out then giveing a space..

Most types of LCDs can withstand a significant pressure increase as long as its uniform.. Look at handsets like the Inspiration they are at AMBIENT...

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cameron

I know guys in saturation have issues with LCD screens... The screen is fine for the compression, but when you start to reduce the pressure the screen can deform. Portable hard drives in Ipods and the such also have issues...

Some guys get the extended warranty and just keep replacing the peice of gear... It dosn't say anywhere in the warrenty that you can't take the electronics to 200M...

jradomski

I don't think its the pressure (lcd screens) , Its probably the Helium..
Helium penetrates all kinds of materials with long exposures and sometimes doesnt come out as easily as it went in!

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

zbskii

Excuse my rookieness, but a quick calculation:
200m ~ 656 fsw
656fsw/33 = 19.9 ATA
(Since the handset is already at 1 ATA, we can ignore ambient pressure at the surface, since it's equal)
19.9 ATA * 14.7psi/ATA = 292.5psi

So the handset is experiencing about 20x the pressure at the surface, or 293 psi.

cameron

I was wondering about that joe... but I didn't think that the helium would make a difference.... then again I don't know squat about LCD screens!

Scuba_Vixen

I would think one of the issues with lcd screens would be the conductive rubber strips used to connect the glass enclosed display screen with the pcb that drives it. Under slowly increasing pressure, I should guess it can compress at a slow enough rate that everything stays in contact, if it can't expand, (off-gas) fast enough on ascent, I could see where it would loose contacts. Batteries can be amazingly durable.

From some experiments we did a few years ago, where we took auto headlamps wired to SLA type batteries, and let them drop from a depth of ~200fsw; .... The glass lamps held up to about 300' or so, and the battery was still keeping it lit nicely untill the implosion. (the light goes out) .... Moments later it sounds like a .45 shot without earmuffs. ..... We always laughed hysterically for the first 120' of ascent or so, untill the narcosis pretty well wore off.

It wasn't too long after those adventures that I built the fill station so I'd have mix.



Darlene



We also tried it with TV picture tubes and a couple cement blocks to sink 'em, ,.......... went off like depth charges at about 175'.....

AD_ward9

There are quite a few threads touching on filling with oil. The devil of engineering lives in the details. Joe's reply touches on an important one: it is worth emphasising this point:

1. Oils expand a LOT. Silicone oil expands about 0.1%/C, so if your equipment is left in the car in winter and summer, that would be a 70C difference. 7%.

2. Sorry to all scientists and engineers out there, pointing out the obvious, but to contain a 7% expansion requires the same pressure to be applied as to compress the material 7%. For oil across a wide surface, that pressure is absolutely enormous.

Result is unless the unit is designed for oil fill, then expansion will crack the case and allow oil to seep out (or bypass the seals), then as it contracts, it will suck in water (if underwater). Bottom line: oil filling units not designed for oil fill can cause a lot of grief.

To modify a unit for oil filling:
A. To fill SCUBA gear with oil ensure you have either a bellows or a diaphram, to allow the oil to expand. If you have a small unit, like a watch, you can do it by putting in a small air bubble and use a strong case, but for anything bigger the volume of the gas bubble you need is too big, and the pressures too large, for that to work.

B. You can reduce the amount of expansion by filling with plastic beads: the same material as the case, and then add the oil. Plastics compress more easily than oil and give you less volume of oil expansion to have to deal with. However, beads make it harder to fill with oil unless you fill under vacuum (the best way), and harder to drain the oil. You also have to contend with the fact that generally the walls of the housing contract because they are in contact with water, and the contents of the housing expand because there is something in there getting hot: electronics, a motor or whatever.

Cheers,

Alex

jradomski

On handsets that I have filled with oil I have used a strip of neoprene across the back, this has worked well (but made it more difficult to fill) and I haven never had any oozing (nor any bubbles afterward) since I started doing this..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.