Glass Lenses For Dive Lights
JRE
has a color temperature of 5700K. A canister light has a battery pack in
a separate case from the head, which contains the bulb, and the two are
joined by a hose within which runs the wires. This has been a really
fun project, in part because I've done nothing like it for at least 20
years. All the parts are 6061-T6 Al, all seals are O-rings.
With my old (belt-driven!) lathe I've solved all the problems now except
the last, which is finding a reasonably economical way to make the
lenses live.
I started with acrylic because it was cheap and easy to find. Bad idea.
After about 30 minutes, the entire side of the lens begins to melt
where it touches the bulb.
Polycarbonate lasts nearly an hour before showing a 10-12mm melted spot
in the middle. If all else fails, it's cheap enough to replace the
lenses as necessary and go diving but I'd really like something more
permanent.
Untempered 1/4" glass fractures from the temperature differential within
30 minutes (all tests run in a bucket of cold water). The pressure
differential (14.7 PSI for every 33' of salt water) would doubtless make
this happen faster underwater.
Borosilicate glass is hard to locate in .250" thickness now that Pyrex
isn't made of it any more. It sounds good but I've not found a place to
buy any yet. Anyone know where to find it? And someone who stocks it
in small quantity for a reasonably affordable price? And, of course,
whether it's likely to live in the application?
Fused silica sounds wonderful for both strength and temperature
resistance, but I've been quoted $100 per lens, a "bit more" than I had
in mind.
Nobody who cuts small tempered glass circles seems to want to guarantee
the dimensions within reason, and from what I read it can't be ground to
fit afterward (.040" of the edge and it will shatter, less off and it's
weaker). A tolerance of +/- 1/16" would be OK, but +/- 1/8" is not.
Another thought is bonded layers of thinner (untempered) glass. Since
each layer will have a smaller temperature differential across its
thickness, it seems to me that it should develop less stress (strain?)
from inside to outside and stand up to the temperature differential
better than a single thicker piece while having approximately the same
mechanical strength. Am I on the right track?
Also, are there plastic materials I've missed that would have better
temperature resistance?
Last, some additional info:
- The lens is recessed in the face of the head and rests against an
O-ring. A stepped ring fits against the lens from the outside and is
secured by four machine screws to preload the O-ring.
- I need the lens to stand up to about 150PSI pressure differential
(plus a good safety factor) across a 2 3/8" diameter.
- I don't know the actual temperature of the inside of the lens but it's
got to be in the 275-300F range to melt the polycarbonate. It can't be
much more than that because of the small area affected for a
polycarbonate lens. The minimum temperature on the outside of the lens
is salt water's freezing point, 28F.
- I have no way to pressure test the assembly with the bulb on other
than by taking it under water, unfortunately. (I did pressure test the
heads with compressed air but of course the load was in the opposite
direction. Since the O-ring glands are symmetrical and the normal
direction of load improves the seal I'm sure they will seal under
water.) I could machine a test rig just for this but that seems like
overkill for what I thought would be easier than this. ("Experience is
what you get when you don't get what you want," right? ;-)
- This is not a homework project! I'm a diver. I have a lathe. I
wanted lights...
Any pointers or information much appreciated.
--
John Eells
JRE
Problem solved, I think.
