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Thread: Cable blocking, aka "Why your cables are so damned expensive"

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    Cable blocking, aka "Why your cables are so damned expensive"

    I had a break in a wire on the cable running from my AV1 to the head of my Inspo last weekend.

    Alexey was most helpful, as always: "Send it all back and I'll replace it or I'll send you a cable and you can DIY". About AUD100 delivered.

    "Sod that", says I: "I'm not waiting a month for the turnaround with shipping, I'll try fix it myself first".

    Assuming it was my own wiring, I started in the head and cut everything back. Nope. It ended up being a break under the gland at the handset. That was an easy fix, but the head end had some weird setup which I'd hacked through and now needed to replace.

    A little more research explained what was going on and I thought I'd share how I sorted it (mostly a clone of what AV-UWT did) and the pain involved. All up I lost a day's billable hours stuffing around with this, which is much more than the cost of just buying the cable - but I learned stuff so I'm happy.

  2. #2
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Let's start with the basics. The handset is sealed, and expects to be at 1atm internally with no moisture. A fair enough expectation.

    So the cable needs to block, at the other end (preferably both but I can understand, given the restraints of space in the handset, that a sheared cable is going to cause problems and that's part of the risk), pressurised and 100% humid air being pushed up the cable as we descend.

    AP divers with either molex or older coax connectors will appreciate the black death that can creep up the cell cables as humid air gets pushed up there and condenses, same kinda thing but on a larger scale.

    Diagram shows potential ingress points on a cable end.


    #1 - around the sheath and between cores, is pretty obvious. What's not so obvious is #2 (along and through the sheath between copper fibres) and #3 (same, between the strands of copper in each core).

    So we need to block it all.

  3. #3
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    One way to do this is using what's known as a "penetrator". Essentially a solid block with a bonded, solid rod for each core which is sealed onto a bulkhead. In a through cable we can create something similar by:
    1. Stripping the sheath, and binding the shield into another conductor.
    2. Stripping back a short (5mm?) length of each core at a short distance from where the sheath stops.
    3. Tinning (which makes multistrand into solid) each of those short segments and the shield at the same distance from the sheath.
    4. Potting the whole thing.

    You end up with a plug at the end which blocks all water/pressure ingress points.

  4. #4
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Of course, it's fiddly with a 12 core cable that has an OD of 7mm (just over 1/4"). But the stripping and tinning is very doable.

    Potting it gets tricky because you need a very low viscosity potting compound to get amidst all the conductors without leaving air gaps. And low viscosity potting compound likes to run everywhere and make a mess.

    So a jig/mould is needed. That was my next step.

    The idea was basically to drill a 9mm hole in a block of HDPE, block off the bottom and fill it around the wires and cable sheath. This has some practical dramas, though:
    1. All those bare bits of wire need to be kept parallel and spaced apart to prevent shorts.
    2. Needs to be a decent seal to stop the PU leaking out.

    My solution was to make a plug through which I could thread each of the cores, that would hold them in position and slide into the bottom of the hole. A quick CAD job gave me this.


    A couple of hours later on the CNC and I had this.


    Took a couple of goes to get feeds and speeds of the cutting tools right to prevent burring etc, and get the diameter of those holes to the sweet spot where I could (just) feed the wires through but they wouldn't let any pesky liquid through.

  5. #5
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Today's mission was to get it all loaded and potted.

    Start off by loading the plug. This got finicky but I got there in the end - zero tolerance for any bumps at the ends of the wires etc.


    I actually had a test run yesterday and my old nemesis of foaming PU struck again, which is why there's a bit of gunk on the wires in that pic.

    Today's effort was a little more serious.

    I vacuum de-gassed the PU after mixing it to get rid of any bubbles, then dripped it through the wires to get it good and soaked, then slid it into the 9mm hole in the block and finished filling it up.

    Hands full and working to a time so no photos of that process, sorry, but here it is sitting in the mould and curing:


    And now I wait until tomorrow before pulling it out and seeing how much of a mess I've made :)

    Anyway, a bit of show and tell but also it maybe goes a little way to explaining why those fischer cables and the like are so damned expensive. Even forgetting that the cable itself with a PU jacket, abrasion and tensile strength ratings and reduced creep factors would be cheaper if it were made of solid diamond, the labour and stuffaround to get the ends right is significant. You're applying subsea oil and gas tech (with the prices that go with it) to a niche market with zero cost advantages of mass production - because it ain't mass production - and that's all going to add up to $$$.

    Photo of the next steps hopefully tomorrow!

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    RBW Member turtle dude is an unknown quantity at this point turtle dude's Avatar
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Ooooops
    If it goes well si I might need to borrow this idea , bet you can't guess what I done when installing my Av1 lol.
    Baz

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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Yeah.

    That block is kinda important :)

    You might get away with making the tinned sections, splaying the leads, loading it up with hot snot and smooshing it all back together in an oversized blob, laying heatshrink over the top and then heating & shrinking until the hot snot oozes out the ends.

    But I had a CNC machine sitting in the shed unloved and this was as good an excuse as any to nerd it up

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    RBW Member Mike_Eitel is an unknown quantity at this point Mike_Eitel's Avatar
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Good job Captain
    Yes, I've been throug all that mess when i did my first O2 metters years ago. Had more than one overpressure "explosion" of my housings when coming up again.

    I finally used double sided pcb, soldered long golden soket pins from both sides and burried that in epoxy. Probably not realy longterm pressure resistant, but god enough. I found that also fisher is not perfect, but much better than pure cables.
    Have not yet done it, but i'm thinking sometimes that next cable i will treat with pressure chamber and -1 to 15 bar cycles, covered by oil. Just have not yet the idea for the best oil. Should be not to viscous, not longtime agressing plastic, nore longtime hardening. Maybe oil for wappons, that we call balistol??
    Mike

  9. #9
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    Re: Cable blocking, aka "Why your cables are so damned expensive"

    Cheers Mike

    The pins soldered through sound like a pretty workable solution too.

    Oil? I've used paraffin oil before (as have Uwatec and others) for filling computers so they're not squeezy under pressure but I think, provided the end exposed to pressure and wet in the head is properly blocked, that's pretty much all you need - the rest of the cable can squeeze its airspaces and then release again but there's no bulk movement and no worries.

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    RBW Member Franck13 is an unknown quantity at this point Franck13's Avatar
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    kiss poseidon lard ....

    Thumbs up Re: Cable blocking, aka "Why your cables are so damned expensive"

    congratulations captain i know more of one than would have quit (démissionné in french).
    in front of the work to be made.


    franck

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