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Old 8th Nov 2017, 7:28 pm   #21
G6Tanuki
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Default Re: Best oil-proof sealant?

The issue with 'sealing' coax is that once any kind of liquid gets either into the gap between the inner insulation the braid and the outer jacket, or the core and the inner dielectric, you get spectacular 'wicking' because of surface-tension effects.

These can drag liquids over tens/hundreds of metres in impressively-short times. The smaller the gap the greater the effect!

[Trust me on this - I had to replace the wiring-loom on my Defender because poorly-specified sealing on the wiring-harness caused oil to wick-back all the way from the engine to the ECU under the seat.]
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Old 9th Nov 2017, 9:24 am   #22
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Default Re: Best oil-proof sealant?

Dow Corning make sealants for just about every application. For EHT units a vacuum chamber is used to draw out trapped air as the sealant cures.
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Old 9th Nov 2017, 9:50 am   #23
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Default Re: Best oil-proof sealant?

Quote:
Originally Posted by G6Tanuki View Post
The issue with 'sealing' coax is that once any kind of liquid gets either into the gap between the inner insulation the braid and the outer jacket, or the core and the inner dielectric, ....
These can drag liquids over tens/hundreds of metres in impressively-short times. The smaller the gap the greater the effect!
Woah, that is going to be an 'interesting' one to crack. I guess I could use multiple cable ties next to eachother to really squeeze the coax, but it may degrade the outer. I can well believe the capillarity you describe, I won't test your claim, I accept it!

Thanks for pointing that out!
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Old 9th Nov 2017, 12:09 pm   #24
Geoff 555
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Default Re: Best oil-proof sealant?

Hi, Wellseal works for me, used it on all kinds of castings sealing against oil,anti-freeze grease etc.
Does not set and a joint can be separated years later and its still sealing and sticky.
All the best.
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Old 10th Nov 2017, 4:29 pm   #25
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Default Re: Best oil-proof sealant?

I use Dow Corning 3145 RTV (to MIL-A-46146 if that means anything to anyone) which is a military-grade non-corrosive sealant. It is fairly viscous (even when fresh), so tends to stay put and not drip or run, and is recommended for use with electrical equipment. I got mine from Farnell some years ago, and although its use-by date of Feb 2014 has long since passed, it is still good to use. Not cheap. Use Meths or IPA for cleaning.

Reputable quality type N and BNC connectors used with a bulkhead socket or feed through should provide effective sealing. They incorporate synthetic rubber seals at both the mating surfaces and cable clamps to prevent ingress of moisture. I believe that the "N" designation comes from it having been designed for Naval applications, although this is only one of a number of theories that I have come across. My late brother-in-law's sea-going yacht used type N feedthroughs and exterior connectors for his external wireless antenna and they never gave any trouble. The connector catalogues I have only refer to sea water performance and make no explicit mention of oils.

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Old 15th Nov 2017, 7:14 pm   #26
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Default Re: Best oil-proof sealant?

Quote:
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Does not set and a joint can be separated years later and its still sealing and sticky.

Ah, I wish I'd read that properly earlier! I've just done the first trial joint with it. But from what you're saying, it will only do one of the jobs it needs to - it needs not only to seal the two surfaces watertight (oiltight) but to also hold two pieces of lexan together permanently. From what you're saying, it won't cure in a way that will give structural rigidity to the two pieces?

It sure is messy old stuff! I'm keeping my fingers crossed that I won't have to strip it off and try another product if it fails to keep the pieces together enough. I'll see what happens overnight - I've just put it outside so it can release all those VOC's in the fresh air!

Not blaming your suggestion at all, Geoff, just cursing myself a bit for not reading your post properly the first time!

I'll report back tomorrow.

Afterthought: it's not a tidy solution, but if the product forms an effective seal but doesn't bind the pieces together tightly, I suppose at a pinch I could always just tape around them.

I mean, using teflon/PTFE plumbing tape. I wouldn't be asking the tape to be watertight, just to keep things from sliding around. I cringe when I think of this 'botch,' though!
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Old 15th Nov 2017, 9:17 pm   #27
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Default Re: Best oil-proof sealant?

Post 22!?
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Old 15th Nov 2017, 11:12 pm   #28
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Default Re: Best oil-proof sealant?

In seeking help on this, it could be useful to post a drawing of your design.
Are you intending to use some sort of catch tray in case of a leak?

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Old 16th Nov 2017, 11:58 am   #29
D Cassidy
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Default Re: Best oil-proof sealant?

Hello.
May I ask why you are using oil rather than a potting compound? Any small amount of heat generated by bleed resistors would be dispersed in the compound. You then would have no issues with your coax feeder or the possibilty of oil leaking.
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Old 16th Nov 2017, 12:31 pm   #30
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Default Re: Best oil-proof sealant?

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May I ask why you are using oil rather than a potting compound?
Hi there, it's a good question.

Oil has a lower specific heat capacity, so the resistors can do their job without overheating. The oil will heat much faster.

I'm also not convinced that a flashover would still not occur inside the potting compound, along the body of the resistors, since we're dealing with RF currents at a very high power. I can visualise this travelling down the body of the resistor even inside the potting compound. That can't happen with oil as it will to some tiny but sufficient extent penetrate the body material of the resistor.

The difficulty with the coax issue may have been resolved now. I am only now dealing with a bare copper lead-out wire. The coax will now be external to the unit.

I'm trying a combination of epoxy resin around the inside of the lead-out wire and Wellseal on the inside and outside.
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Old 17th Nov 2017, 9:18 am   #31
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Default Re: Best oil-proof sealant?

Hello.
I do remember it was Ferguson I believe had line output transformers that used Dow Corning jelly type potting compound, they were small and could get warm and have 8kv floating around.
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Old 17th Nov 2017, 9:42 am   #32
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Default Re: Best oil-proof sealant?

Snot pots

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Old 17th Nov 2017, 10:10 am   #33
D Cassidy
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Default Re: Best oil-proof sealant?

Hello.
I'd make doubly sure Wellseal doesn't contain any conductive material. Also a small antichamber to allow for oil expansion as well. Use top quality transformer oil and heat it to 100c, let it cool and then bottle it in a dry atmosphere, when ready to do the filling ensure no dampness can encroach.
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Old 17th Nov 2017, 12:54 pm   #34
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Default Re: Best oil-proof sealant?

I have not had much practical experience with really high voltage stuff but from what I have read, flashover rarely happens inside solid bodies but at the solid/air interface. Failure of insulation in high voltage cables is usually initiated at voids within the insulation. The oil used in high voltage transformers and some high power high voltage cables not only helps dissipate heat, but also eliminates voids at which flashover might occur. So if you can ensure that there are no voids at the resistor/potting interface, you should be OK.

At Plesseys, the curing of the potting compound of high reliability stuff was often carried out in a vacuum chamber to facilitate removal of any small air bubbles that were created during mixing. The low pressure caused small bubbles that would otherwise have stayed put in suspension, to expand and rise to the surface. I used to heat up the Araldite I was using in order to shorten its (2 hrs at room temp) cure time, but I did always notice that, in its highly fluid warm state, any entrapped bubble of air rose to the surface and burst, resulting in void-free potting.
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