Electronics under pressure (marine electronics)

[cprobertson1]

I've been discussing an idea for a simple hydrophone with a friend (an ultrasonics engineer).

What I was wondering about were the implications of having electronics in the hydrophone probe - specifically an amplifier to send the signal back up the line.

What got me thinking was first how to waterproof the electronics (electronics + salt water = dead electronics + salt water ) - and suggested simply potting it in epoxy.

However, even potted, if the epoxy flexes even slightly it will experience an increase in pressure - and it got me wondering: how do electronics respond under pressure?

Apart from the obvious heat dissipation changes between a lower and higher pressure - what sort of effects would an increased pressure (in our case, we would aim for a depth of 10m to start with - which is ~2atm - it increases by ~1atm for every 10m below the surface).

Obviously my first fear would be that any force applied to the board could crack solder joints - though hopefully with a rigid enough epoxy the pressure will be fairly even (though I do wonder about the effects of voids in the epoxy - particularly under components.)

I was more interested in the effects on the characteristics of the components themselves though - for instance, does compressing a capacitor increase the capacity (by virtue of forcing the electrode plates closer together?) - does increased pressure mimic some of the effects of increased temperature?

I assume crystals would experience a similar effect - though one would hope that increased pressure would only serve to attenuate their oscillation rather than changing it (if they oscillate in a denser medium, it will take more energy to move them the same distance so I assume the end effect would be that the amplitude of their oscillation will change (but not the frequency unless the morphology of the crystal is affected - which I doubt it would be under these sorts of pressures! Hopefully...!)

Anyway, would love to hear all your thoughts on the matter!

Ps - we aren't planning on actually making a hydrophone - just an idea we're toying around with

PPs - we'll probably place the electronics in a small pressure vessel so that the internal pressure doesn't change too much - though I'm still interested in the effect of pressure changes since I can't rely on it working!

[Kalvin]

Just my uneducated 2c: How about combination of epoxy and silicone. Or potting into plastics using chemical plastics? Metallic outer container which can handle the pressure so the pressure inside the container will not change, and feedthroughs for cabling which can take the pressure difference without leaking?

[dmills]

Polyureathane potting and over moulding, job done (Especially at only 10m, I have used this at 2km depth)!

The key is getting all the surfaces to wet out properly, and there can sometimes be some chemical magic involved (Teflon does not wet without chemical and mechanical treatment).

One lovely gotcha is that some PVC cables turn out to suffer from osmosis, and this can cause interesting problems in a long term install.

Another is the interesting realisation that a multicore cable can if slightly damaged provide a ingress route down the inside of the cable between the cores and can even provide a route between the strands, one helpful trick to to place a small amount of epoxy on the board covering each wire to board soldered connection, this provides both some strain relied and stops any water that gets inside the wire making it onto the board, then pot the whole thing in Pur.

Top tip: FR4 of appropriate thickness happens to sit at the geometric mean of the acoustic impedance of sea water and common piezo ceramics, while you still need to over mould (FR4 is slightly hydroscopic), this makes it a useful material for narrow band acoustic transformers. 

Regards, Dan.

[cprobertson1]

That was more-or-less the plan I've been looking at cable transits (just cone-shaped compression fittings) that are suitable up to 100 bar - I'm pretty sure one could also use an hydraulic fitting to the same effect provided the right diameter of cable is used with it.

I'm more curious as to the effects of pressure on the (electrical) characteristics of components though - more out of curiosity than anything else!

[cprobertson1]

Top tip: FR4 of appropriate thickness happens to sit at the geometric mean of the acoustic impedance of sea water and common piezo ceramics, while you still need to over mould (FR4 is slightly hydroscopic), this makes it a useful material for narrow band acoustic transformers. 

Regards, Dan.

Nice one - will need to remember that!

[dmills]

Component wise, anything with significant trapped gas is a bad idea,  but then again, 10m, meh.

Electrolytics are probably not good (apart from anything else, the potting stops them venting), and inductors usually want vacuum impregnation to avoid oddities when you wind up the pressure, just use a desiccator with a roughing pump (Open the gas ballast to avoid issues with water vapour in the pump, pull down, let the worst of the bubbles out, bring back up to atmosphere before the varnish dries). 

I had mini-circuits do a custom transformer variant with a slot in the bottom of the case to admit the potting compound.
A nice trap is MLCCs being microphonic, look out for this, it can cut both ways.

If doing O ring grooves, make sure you don't get a hair across the seal (or similar), and LIGHTLY grease the ring, you do NOT need to slobber it on.

If the assembly has significant trapped air then it is a really good idea to fit a pressure relief valve (Ally-Bronze bolt with a side hole and O ring, opening this before undoing the access hatch bolts makes it MUCH less exciting when the thing has been deep and suffered a leak (It does not take much residual pressure to make opening a square foot of access hatch a memorable experience - it hit the ceiling)!

Generally you want to be vary careful about metallic components in contact with sea water, especially if they are connected to the cable in any way, both dissimilar metals and electrolysis due to the cable voltage drop will rot things surprisingly quickly, 316L is not horrible (The L means low pitting), but the professionals use ally-bronze.

Oh yea, Morgan electroceramics have a very good set of notes on materials, well worth tracking down.

Regards, Dan (Who used to do sonar transducer electronics for a living).

[mac.6]

A good presentation about pressure tolerant electronic:

[cprobertson1]

Component wise, anything with significant trapped gas is a bad idea,  but then again, 10m, meh.

Electrolytics are probably not good (apart from anything else, the potting stops them venting), and inductors usually want vacuum impregnation to avoid oddities when you wind up the pressure, just use a desiccator with a roughing pump (Open the gas ballast to avoid issues with water vapour in the pump, pull down, let the worst of the bubbles out, bring back up to atmosphere before the varnish dries). 

I had mini-circuits do a custom transformer variant with a slot in the bottom of the case to admit the potting compound.
A nice trap is MLCCs being microphonic, look out for this, it can cut both ways.

If doing O ring grooves, make sure you don't get a hair across the seal (or similar), and LIGHTLY grease the ring, you do NOT need to slobber it on.

If the assembly has significant trapped air then it is a really good idea to fit a pressure relief valve (Ally-Bronze bolt with a side hole and O ring, opening this before undoing the access hatch bolts makes it MUCH less exciting when the thing has been deep and suffered a leak (It does not take much residual pressure to make opening a square foot of access hatch a memorable experience - it hit the ceiling)!

Generally you want to be vary careful about metallic components in contact with sea water, especially if they are connected to the cable in any way, both dissimilar metals and electrolysis due to the cable voltage drop will rot things surprisingly quickly, 316L is not horrible (The L means low pitting), but the professionals use ally-bronze.

Oh yea, Morgan electroceramics have a very good set of notes on materials, well worth tracking down.

Regards, Dan (Who used to do sonar transducer electronics for a living).

Excellent stuff there! Thank you very much!

That's a good shout on the pressure release valve - well, I mean, everything you said was a good shout but you know what I mean!

I recall a diving story of one of my Uncles involving a deep-sea diving bell that experienced a leak during a dry run (oh, the irony!). When they got it up to the surface, the window (a 6" deep cylinder of glass) was blown clear of its fittings, followed by a blast of water. I seem to recall it was blown a good distance clear of the deck of the ship, but I can't recall the details I'm afraid!

As for metallic components in water, I seem to recall that marinas have issues relating to ships plugged into the mains undergoing galvanic corrosion thanks to their convenient path to earth! That's actually a good call as well - especially since I was considering encasing the electronics in a piece of metal pipework; I'll need to look into isolation and stuff (or run it from a boat, or just records the audio onto an SD card or something so I don't need to run wiring to the surface... I wonder what sort of battery tech would work best underwater actually - I'd be wary about compressing a LiPo cell too much for instance!

Then again, I don't imagine recording would require too much in the way of power.

Now here's a question for you - I assume this idea won't work (otherwise there's a decent chance it would actually be used somewhere).

Looking at the principle of a stethoscope - that is the sound is transmitted via a working medium through a tube. Where I to replicate this using flexible hydraulic line filled with mineral oil with a diaphragm at the subsea side, and a fairly simply hydrophone at the surface side (thus keeping all the electronics at the surface)... would it work? I assume there is something (probably several somethings) preventing this being a viable idea (weight of the cabling, pressure changes would require a compensation tank at the top, offgassing of the oil when it is pulled up from depth, poor efficiency, etc)

That's more of a brainfart btw - just curious as to if it would work, and if not, why not

A good presentation about pressure tolerant electronic:

That's tonight's entertainment sorted! Many thanks