Questions re: sealing DIY dive lights

[naimis]

I'm at the point where I'm ready to get the housing for my dive light and put it all together, but I have a few concerns...

Like most folks, I'm building a cannister light. But a cannister light has a lot of opportunities for leakage, with multiple penetrations with the cable, the lens and possibly the back of the light.

The penetrations into the head I'm only a little worried about, the coefficient of thermal expansion should be similar enough between the 6061 aluminum and the brass the gland is made out of, but PVC has a very different COE. PVC doesn't make a good threaded seal on a good day (I've built water rocket launchers and used threaded PVC and it never sealed very well, even with teflon tape).

Also, I've been concerned about the possibility of galvanic corrosion between the gland and the head, and contact welding as well. These can probably be addressed reasonably well by throwing some teflon tape on the threads. Has anyone out there addressed these issues yet, and if so, how?

I spoke with an engineer at the office about these issue and he didn't think that 6061 was a good alloy to be using for this application because it's not very corrosion resistant. He subsequently referred me to Online Materials Information Resource - MatWeb (http://www.matweb.com) and McMaster-Carr (http://www.mcmaster.com) (the latter I had already visited from following other related threads) to get information about various alloys.

I've also seen at least one person anodize his machined light head, but the same engineer indicated that we didn't really have any good shops here in Austin to do that.

So, in summary, what are people doing to address these issues?
1) contact welding / galvanic corrosion
2) differences in coefficients of thermal expansion
3) corrosion due to exposure to (salt) water

[PhilEllis]

I'm at the point where I'm ready to get the housing for my dive light and put it all together, but I have a few concerns...

Like most folks, I'm building a cannister light. But a cannister light has a lot of opportunities for leakage, with multiple penetrations with the cable, the lens and possibly the back of the light.

The penetrations into the head I'm only a little worried about, the coefficient of thermal expansion should be similar enough between the 6061 aluminum and the brass the gland is made out of, but PVC has a very different COE. PVC doesn't make a good threaded seal on a good day (I've built water rocket launchers and used threaded PVC and it never sealed very well, even with teflon tape).

Also, I've been concerned about the possibility of galvanic corrosion between the gland and the head, and contact welding as well. These can probably be addressed reasonably well by throwing some teflon tape on the threads. Has anyone out there addressed these issues yet, and if so, how?

I spoke with an engineer at the office about these issue and he didn't think that 6061 was a good alloy to be using for this application because it's not very corrosion resistant. He subsequently referred me to Online Materials Information Resource - MatWeb (http://www.matweb.com) and McMaster-Carr (http://www.mcmaster.com) (the latter I had already visited from following other related threads) to get information about various alloys.

I've also seen at least one person anodize his machined light head, but the same engineer indicated that we didn't really have any good shops here in Austin to do that.

So, in summary, what are people doing to address these issues?
1) contact welding / galvanic corrosion
2) differences in coefficients of thermal expansion
3) corrosion due to exposure to (salt) water

As reasonable cost metals go, aluminum, in general, is pretty corrosion resistant. In my view, 6061, 6063, and 6351 are all pretty good for "weather" or normal atmospheric corrosion resistance. However, I have no metals educational background, so you might be advised to listen to other here on the board on that issue.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment (http://www.divesports.com)

[fppf]

If you use aluminum in saltwater you need to have it anodized. Surface corrosion is not bad with 6061, however crevice corrosion can be huge. Anyone doing visual tank inspections can attest to this. The material under the valve is always chewed up far more than the tank.

I don't think you will have many galvanic issues. Your parts are small and localized. I don't think eddy currents will have a path to setup. But I would still look up the materials and see if there is a way for currents to setup. One thing to watch for on threaded parts is galling. Aluminum and stainless gall bad. Lubricate the threads.

Thermal expansion issues will depend on your o ring designs. But I highly doubt you will have issues with this. If your using a face seal I almost would not even look at it. If your using an axial seal then find out what your worse case clearance will be and check the o ring guides for extrusion limits.

[lucca brassi]

I don't think eddy currents will have a path to setup.


NOT :no:

Eddy current - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Eddy_current)

BUT :coffee:

Galvanic series - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Galvanic_series)

:)

[fppf]

Great another wiki know it all. :shakehead:

It takes CURRENT to actually preform electrolysis.
Galvanic corrosion - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Galvanic_corrosion)
Please note the section on the bottom "Factors that influence galvanic corrosion"
Please note the term "current" is used in many points.
Please note that the area of parts effects corrosion rates.
Please not that Aluminum is one of those metals that forms a hard patina. That is what anodizing is.

Also, most water tight bulkheads that they are using for cable pass through are stainless.

[adm3745]

Divegearexpress.com has some good priced cable glands. Some more advanced DIYers may have a better source. Those are 316 Stainless and you shouldn't have an issue. If I use aluminum It is anodized. I believe that Delrin is the most popular material for light heads and cannisters. It is a hard plastic but it can be more expensive. That is your best bet.

If you don't want to use PVC for a cannister because of threading issues, and you don't have access to a machine shop...
I am experimenting with old paintball 12oz CO2 tanks for cannisters. A band saw can cut the neck off, then you just need to make a head for it. Those are anodized Aluminum and can be found for around $10.00.

If that's to much there is always JB weld on the PVC seals.

[adm3745]

"well priced" , "canister".. I'll be alright

[fppf]

Divegearexpress.com has some good priced cable glands. Some more advanced DIYers may have a better source. Those are 316 Stainless and you shouldn't have an issue. If I use aluminum It is anodized. I believe that Delrin is the most popular material for light heads and cannisters. It is a hard plastic but it can be more expensive. That is your best bet.

If you don't want to use PVC for a cannister because of threading issues, and you don't have access to a machine shop...
I am experimenting with old paintball 12oz CO2 tanks for cannisters. A band saw can cut the neck off, then you just need to make a head for it. Those are anodized Aluminum and can be found for around $10.00.

If that's to much there is always JB weld on the PVC seals.

I like the old paint ball cylinder idea. You are correct, most mfgrs are using Delrin for can lights. It has good machining and strength characteristics.

[adm3745]

http://www.scubaboard.com/gallery/showphoto.php/photo/141518/ppuser/127224
http://www.scubaboard.com/gallery/showphoto.php/photo/141519/ppuser/127224
http://www.scubaboard.com/gallery/showphoto.php/photo/141520/ppuser/127224

Sorry for the bluriness.. I just thought of this this morning and made a head cap on the lathe. I'm going to order a couple latches for it and it should work great. Comfortably holds 8 Li-Ion 18650 for my P7 LED. It could definately fit some of the smaller NimH Bat packs...

It should definately work out better than your pvc.

[adm3745]

ScubaBoard Gallery - PA300053 (http://www.scubaboard.com/gallery/showphoto.php/photo/141520/ppuser/127224)

[350xfire]

What in the world is that cable gland???

[lucca brassi]

Great another wiki know it all.

It takes CURRENT to actually preform electrolysis.
Galvanic corrosion - Wikipedia, the free encyclopedia
Please note the section on the bottom "Factors that influence galvanic corrosion"
Please note the term "current" is used in many points.
Please note that the area of parts effects corrosion rates.
Please not that Aluminum is one of those metals that forms a hard patina. That is what anodizing is.

Also, most water tight bulkheads that they are using for cable pass through are stainless.

Real sorry from me but you don't understand electrochemical basics at all!

- current it is not just another current you have in your mind !

- EDDY CURRENT begins in the metal parts under influence of electromagnet field
which must be alternating type(not DC as by galvanic series) . This current is normal
in High friquency fields and makes so called ''Skin effect''
( by my job as electroengineer in the lab I have studied sensors special eddy ;))

- first you have two parts of metal with different galvanic numbers (condition)
- parts causes electrochemical tension (volatage) between them
- than you must have resistance in metall ( Al2O3 which made anodization Al
known as eloxal process makes Alu hard but also made resistance from low -
very high and the electro tension can not go through)

- voltage and resistance causes CURRENT - not anyone but DC. Parts become
battery which melts material on higher galvanic ledder and brings it to the metal
with lower galvanic part. THAT IT IS CORROSION :shocked2:

tor example : if you squeeze SS backplate with iron vice then will SS become rusted !

Regards

[adm3745]

Haha, we're not gonna talk about the cable gland. I'm not even sure we can call it that.