Plastic/rubber - absorbing water

Hi

I'm working on a pump that is suspended in water and I need to add some electronics to it (not more than 1 meter water level)

My initial thought was to house the electronics in a sealed plastic/rubber enclosure, but someone pointed out that plastic/rubber might over time absorbe water and render the electronics un-usable

Anyone got details of the behavour of plastic enclosures exposed to long periods of total suspension i water?

Thanks

Klaus

I read in sci.electronics.design that Joerg wrote (in ) about 'Plastic/rubber - absorbing water', on Wed, 28 Sep 2005:

A bit of calcium peroxide wouldn't come amiss in there. With water, it gives off oxygen, thus pressurizing the enclosure, and the remaining calcium oxide (quicklime) absorbs the water to make inert calcium hydroxide (slaked lime). Remember, you read it here first.

Lots of people seem to want to sell you CaO2. You'd need either a solid lump or a pellet of compressed powder with an *inorganic* binder. Don't try compressing it with an organic binder!

One way to make electronics pressureproof is to fill the enclosure with oil and have a short piece of closed siliconhose sticking out for compression. At least that is done when the immersion is going to be hundreds of feets and more.

Rene

hmmm, isnt there air inside an IC package? Wont it crack when pressurised in an oil enviroment?

Just wondering, it seems deep ocean technology is in some ways more difficult than designing stuff to take pictures on mars

martin

In article , martin griffith wrote: [...]

Plastic packaged ICs are good to 1000PSI or more

Any semiconductor in a metal can will fail well short of 1000PSI

Capacitors will mostly survive 1000PSI but plastic ones change in value and any with a metal can deform. If a brand of plastic capacitor survives once, chances are all of them will.

Power transistors in plastic packages survive fine.

Most moderate cost connectors are no problem. A few really low cost connectors develop problems if they are cycled a few times and pressurized. Some of the more costly connectors such as the PT series will sometimes trap air and distort under pressure.

What ever you do clean the PCB well before you put it into the oil. Never-ever use "no clean" flux in this sort of application. The no clean stuff is supposed to trap the ionic stuff under a layer of plasticy stuff. The plasticy stuff will be slowly eaten away by the oil.

If you use an oil in and around Decane, a PCB will very slowly dry if taken out of the oil. This is important if you are going to work on it. Soldering temperatures are about the smoking temperature for many light oils. The carbon left behind causes troubles.

If you want to be eco-friendly, you can use peanut oil if you make doubly sure not to let water and air and peanut oil all get together in one place.

I read in sci.electronics.design that martin griffith wrote (in ) about 'Plastic/rubber - absorbing water', on Thu, 29 Sep 2005:

Yes, but it's just like taking pictures on Jupiter.

Thanks Ken, very informative, but the"water, air and peanut oil statement confused me" Is it explosive or something under high pressure?

martin

In article , martin griffith wrote: [...]

No. something green and slimy will take it for dinner. Air, water and mineral oil can also be eaten but the bugs that eat it seem fairly slow working.

Most green slimy things make chemicals that will attack your circuits.

No, Jupiter is very hot along with the very high pressure. Most materials get weaker fast as the temperature goes up.

Since the maximum depth is only 1 metre, you're probably looking at more of a long-term diffusion problem than a leakage problem. All polymers will absorb water, and osmosis will make sure that eventually the inside of an arbitrarily well sealed plastic case will become wet. Oil and pressure will prevent macroscopic leaks developing, but won't do anything much to prevent diffusion. Diffusion is driven by a concentration gradient, and the water concentration outside is going to be greater, oil or no oil.

If you want something to be really dry inside for a long time, make it out of metal. Even foil glued to the inside of a plastic box will make a big difference in the diffusion rate.

Cheers,

Phil HObbs

I read in sci.electronics.design that Ken Smith wrote (in ) about 'Plastic/rubber - absorbing water', on Thu, 29 Sep 2005:

It doesn't get hot until you get DEEP into the atmosphere, by which time the pressure is really immense.

So is there a "sweet spot" in the Jovine atmosphere (sea?) where the birds/ fish could live at a baby-bear type of temperature?

Paul Burke

I once read that there's no such thing as "on Jupiter", i.e., it has no well-defined surface - it's gas all the way down to the core, which, according to Carl Sagan, is "liquid metallic hydrogen." Well, I suppose there's a liquid/gas boundary...

Anybody know?

Thanks, Rich

Well, it would be 'atmosphere'. It's not even certain that Jupiter has a distinguishable surface; the atmosphere may just get denser and denser and more viscous.

It seems, at present, that the atmosphere has a multi-layer structure, much more complicated than Earth's, and it's possible that there are layers in which carbon-based life could exist. But it wouldn't be much like our land-based life-forms, because of the gravity. More like jellyfish, perhaps; neutral buoyancy.

As is not at all unusual, Arthur C Clarke has been there already; in

ISTR that the mass/density of Jupiter didn't work unless it had a rocky core.

Why would Jupiter have refined its core when the rest of the solar system didn't? The argument is that the lighter elements were blown outward in the solar system by the solar wind, leaving the inner planets rocky and the outer gaseous. Jupiter may then be somewhere inbetween (or maybe all the giants have a rocky core).

Not necessarily. Above the tripple point, there is no difference between gaz and liquid.

Rene

Avoid can electrolytics - they can squash under pressure. Oil is even worse for them - they will absorb oil like a sponge, swell up and pull themselves off the board.

Other devices in cans (like crystals) squash under pressure too.

I've never seen can electrolytics absorb oil. What kind of oil were you using?

Yes. The crystal is the extreme form of the part in metal can case.