"Aussie" wrote in message
...
On 18-May-17 6:39 PM, Jim Wilkins wrote:
"Aussie" wrote in message
...
On 18-May-17 10:47 AM, Jim Wilkins wrote:
"Steve W." wrote in message
news Aussie wrote:
I'm playing around potting some electronics for immersion in
water
up
to 10 metres.

A cable with a polyethylene sheath and PVC insulated wires
connected
to a circuit board are embedded in some rigid setting epoxy.

I know I'm not going to get the epoxy to adhere to the
polyethylene.

What can I do to enhance the bond between the epoxy and the PVC
wire
insulation to keep the water from wicking along the wire to
the
circuit board?

I've read that brushing with PVC pipe glue primer (MEK) can
help,
as
well as flame treating.

Flame treating is impractical as the job is too small & tight
to
get
into the area where the PVC insulation is.

Any suggestions?

Is the MEK priming likely to help?

Perhaps painting on some sort of low viscosity RTV silicone
that
might form an intermediary seal?


Use a heated plate to melt a rolled lip on the end of the poly
tubing. The lip will give the epoxy a mechanical bond. To ensure
it
doesn't leak you could put a common O ring that fits the tubing
above the lip. The epoxy then encapsulates the ring and the lip.

--
Steve W.

I haven't had much luck sealing tube feedthrus with O rings
unless
there was a threaded packing nut to compress them.




I did read that to seal some potted sonar assemblies a couple of
tight large section o-rings are placed on the cable inside the
potted volume.

The o-ring material is selected to bond well to the potting
material.


The theory is the o-ring tension provides a seal against the cable
sheath and the potting compound to o-ring bond seals the ingress
path around the outside.

It sounds viable but I'm a little skeptical about long term
sealing.

I needed to contain internal pressure that expands the O ring while
you have external pressure compressing it. It sounds like you found
a
proven solution.

10 meters deep is the same pressure differential as a vacuum, which
these are thick enough to resist:
http://www.apiezon.com/



Am I understanding correctly?
You think a smidge of thick vacuum grease on the embedded wires
might block the water??
Thats an interesting idea.

Or maybe chain lube. Perhaps you could adapt pipe fittings to compress
on one end of a wire sample, apply likely diluted or melted greases to
the other and see if they block bubbling when pressurized.
-jsw