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We are building a marine instrument that will operate at up to 30m depth in sea water. Part of the design requires that the middle part of a USB type cable is exposed to seawater.

Most electronics cables that I know of are sheathed in black or grey PVC material, would this type of sheath be waterproof? Waterproof at 30m? Note that the cable should in not likely to be bio-fouled.

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It's likely to be ok for low pressure immersion in fresh water for short periods, but not for the highly corrosive salt water and high pressure environment that you are planning to operate in for long term. If you can't find an appropriate marine grade cable, you should use conduit which is similarly specified and sealed.

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    \$\begingroup\$ Salt water is corrosive to PVC? By what mechanism? How does high pressure increase the effect? Thanks for the answer, but I need more detail as currently I am not convinced. Note the cable cannot be sealed or rigid as it is connected to a moving part. It also must be the typical diameter of a USB cable - approx 3mm. \$\endgroup\$ Nov 19, 2011 at 9:32
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If you can replace it should it fail, it may be simpler to try the experiment than to pre-emptively source something more specialized.

Unless there is a manufacturing flaw, the problem may be more sealing to the cable jacket than the cable itself leaking. You may have similar problems if you have anywhere that cables exit the hull, (especially if the bare wires are ultimately wetted by design, though things like wetted motors are probably reserved for fresh water)

You could consider pressurizing the housing on the surface end of the cable as well as the hull to greater than the pressure at depth (though beware the explosion hazard if you use materials not rated for this)

It's not entirely clear what you are doing though - I don't think USB is rated for 30 meters (ie, to the surface), so presumably both ends of the cable are on the submerged device. In that case, can you cut the connector off one end, cover the cable with some hose, and solder the wires back onto a compatible connector or directly to the PCB?

Not quite sure what you mean by the bio issue, but if you are concerned about contaminating the non-exchanged volume of water in something like a giant aquarium tank, covering the cable in hose made of a known-suitable material would seem wise. Or perhaps you just mean that there aren't any barnacles to grow on it?

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  • \$\begingroup\$ The cable is used to connect one part of the instrument to another and both ends are sealed and soldered directly to the boards. I actually don't need help with that aspect of it, I'm just interested whether water will penetrate the PVC sheath of typical cables. If it does I will have to source another type. Thanks for the reply. \$\endgroup\$ Nov 22, 2011 at 8:55

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