Potting for under water

Get electronic potting compound instead. There is normally a choice of epoxy, polyeurethane, and silicone elastomer. The problem with normal resins, is that unless you investigate the chemistry carefully, they may contain acids or alkalis, which can themselves cause corrosion (Silicone RTV's, often for example use acetic acid). Even though you use a potting compound, add a grommet around any wire entrances (the tendency for water to penetrate 'along' a surface at a joint is amazing, and you also want strain relief where the wire enters a rigid material).

Best Wishes

You should look in the catalogue of a specialist potting compound manufacturer (robnor, Dow Corning) as there are a wide variety of types for different applications.

There is not much of a problem in potting things so they are immersible.

The fun starts when you want to have wires going into it.

Water will get in in between the strands of a stranded cable.

It will also get in between the encapsulant and the wire insulation - unless you ensure there is a chemical bond. Usually this bond doesn't occur; it's very hard to make anything stick to soft bendy plastic. Many years ago I achieved this in production by using irradiated vinyl insulation (high temperature rated) which adheres OK to epoxy resins.

You also have to watch shrinkage during curing - this can crush many components. Some epoxy resins are particularly bad, especially the thermally conductive ones.

Peter.

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Does it *have* to be completely potted?

What about a thick layer of conformal coating? The ingress along wire entrances issue still applies, however.

Chris.

Hello Robert,

One of the industries you could check out (publications etc.) would be undersea robots. These guys really know since their stuff is exposed not just to water, but usually to salt water under extreme pressures.

Anyway, you also have to consider what kind of water your board will have to survive in. Salt? Chlorine? ph value? Is it chemically stable? How many years? Then there are some living underwater beasts that can eat into almost anything.

We have a pool and over the years I have learned some hard lessons. even the low doses of chlorine in a pool will eat away at stuff. Then, after installing a water filter, big surprise: The tap water around here when measured before the filter had the same chlorine level as our pool.

Regards, Joerg

I have no experience at all in this field, but a gut feeling says: take an enclosure, fill it with a liquid which doesn't harm the circuit and make sure the inside pressure can be the same as the outside pressure. Any comments?

Number of ways of doing this: Potting ======= If your depth isn't too great (couple 100 meters), you shouldn't have to worry about parts crushing. Over 500 meters, you need to use parts with no air cavity. Crystals and hermetically sealed packages have an air cavity. Aluminum electrolytics don't handle pressure. To reduce the chance of wicking, use a large solid bare wire with an o-ring over the wire.

1. Polyurethane is commonly used in the undersea world. PR-1570 is commonly used by the industry.
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2. Various industrial expoies can be used.

1. Silastic J-RTV (Dow Corning). Has quite a bit of shrinkage.

Housings ========

1. Housing filled with inert fluid like castor oil or fluorinert. Parts are subjected to water pressure. Useful mainly for very deep housings where weight is a concern.

1. Standard pressure housing. Gets expensive with connectors and tight-tolerance machining. See Parker o-ring manual for guidelines on how to make the sealing areas. Use the commercial practices, not the military practices in the Parker manual.

Mark

Hello Nico,

Then the problem is reversed: How do you keep that pressure up over months or years? Even a sealed container leaks some, unless the liquid hardens in which case it basically becomes a potting compound.

Regards, Joerg

You don't need to keep the pressure up. A flat box will provide enough flexibility to level the pressure between inside and outside. Or a flexible membrane may be used. In my idea, equalizing pressures will prevent liquid from leaking in or out.

The problem is not leakage, which can be made negligible fairly easily, but differential thermal expansion rates, which can not be made negligible. A solution which has been used in liquid silicone filled instruments is the metal bellows, which can allow for expansion and contraction while maintaining internal pressure very close to external pressure.

Regards, Glen