I would say that salt water is actually easer then some other places but still not easy.
Think of a oil or gas well. Some things easer here some harder.
Some wells are greater then 200c in temp, real fun to try to make things work at high temps. Things don't work or grow in size to work at high temp's. For a well you have size and weight limits. In Salt water Temp could go cold or hot.
When trying to build something for high temp does have it's fun moments, As you go higher in temp.
Forget to use the high temp solder and a lot of things fall apart. Some plastic parts turn in to a working plastic blob while a second part of the same batch turns in to a dead plastic blob.
In simple terms for wells you don't try to seal your cable connection from the well, you really can not do it. What you do is seal your cable with something that you can get a good seal with and use that something for the seal barrier to the well. Most times I have seen grease used here. You might have a conductor through glass for example. You still need to create a good seal/isolation between the conductors and to the cable even in this case. Is the something good for a long time or short time, or is the time based more on to and from pressure?
Going to two cables adds a lot of problems. It's easy to say but very hard to do and get it to work. In the water you really need them tied together or you will have tangles. On a winch spool if you winch two cables you could crush one cable. Use two winches and you have to add and remove the ties between cables.
Then you have the pressure effects on the cable. Want to guess what happens when you put a lot of pressure on the outside of a cable. Is that nice shield brad of coax strong enough to prevent the collapse of that foam dielectric around the center wire? For that many twisted pair cable in the video, I would guess that the 4000 foot end will end up being smaller and the plastic around the wires will get deformed so that there are no air gaps any longer. And if the pressure is high enough the plastic will bond together. The side effect of this is that the cable's electrical character will be changed.
If I was not using cable built for this use, I would start with a burial version of cable that has it's air spaces filled but this adds costs also.
Yes fiber sounds nice for this, but try finding some that still has power conductors. Keep in mind that no power conductors = batteries which = weight which = stronger cable while also adding limited run time.
The longer the distance the more power is needed just for the communications. The higher freq used on copper also = more power.
A long copper cable is a great filter. Pump in a nice big square wave in one end and you may get lucky and be able to detect a sign wave with a great radio type receiver.
For salt water and to keep that expensive cable usable for a longer time you need to seal everything. Plastic sheath to plastic cover of wires and plastic cover of wires to the actual copper, And from this to the ROV.
You also need to remember that all cable has weight. Here salt water may provide some support but your max limit is less than the length of the cable being able to lift it's self. How much of the strength do you want to leave for pulling on the ROV?
While it may sound easy to do, in practice it can be very hard to do. Expensive equipment at each end to allow cheaper cable. More expensive cable to make each end cheaper.
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