O-ring and lens questions/advice

[gcbryan]

My specific questions are referring to this light:
http://www.dealextreme.com/details.dx/sku.26498

I realize this isn't a light that most of you are all that fond of nevertheless I could still use some general advice.

Picking out appropriate o-rings and relative strength of various lenses...how do you know what durometer is appropriate. In this particular case there are two soft silicone o-rings in the removable tail cap area. Soft means they will seal easily under lower pressure but it also means they could "bottom out" under higher pressures and leak. How do you know which duro would be an improvement without being so stiff that they would leak under lower pressure?

The 50mm x 2.5mm "coated glass" lens is not something that can be replaced with a thicker lens due to the limited thread free area in the head but is there a stronger lens material that could be used...Plexiglas or something still being limited to a thickness of 2.5mm?

There haven't been a whole lot of floods in these lights so I'm really just trying to learn a little more on this subject. If I can make easy fixes great but if not it's still good info to have.

Any Mechanical Engineers out there or anyone else with some info?

Thanks.

 

[wquiles]

From various searches here in this forum and over the Internet, the most common answer is to use duro 70 O-rings, either EDPM or Nitrile.

Here is a good link on the duro values:
O-ring USA ...

and here are some older post from Packhorse, who is very experienced with light builds:
O-rings for Mag conversion ...

Packhorse also uses duro 70 o-rings in his W200 conversions:
Ultrafire W200 conversion


The question on lens thickness I don't know much. The only data point I can offer is that the dive-rated lights from Barbolight are tested to 200 meters, and they use a 2" dia polycarbonate 5 mm thick lens, and that at 200mm there is less than 0.5mm deflection on the lens. Glass is stronger (although not impact resistant as the poly lens), so you should be able to make it thinner than 5mm - how much, I don't know. This is also the type of information that I am also looking to learn more about.

Will

 

[gcbryan]

So I guess that might explain why so far no one has had their 2.5mm glass lens from the above flashlight fail at up to 145 fsw (just going by dive reports).

But if I'm understanding you correctly if you were to bump into the glass lens or drop the flashlight glass would be more like to crack or shatter than a polycarbonate so glass might be expected to handle greater depths (at the same thickness) in a static sense but dynamically maybe polycarbonate is best?

As far as leaking it also seems from personal experience and reading these forums that water pressure tends to seal the front lens and most leaks seem to be from twisting the head or from switches. Is that a fairly accurate conclusion?

From various searches here in this forum and over the Internet, the most common answer is to use duro 70 O-rings, either EDPM or Nitrile.

Here is a good link on the duro values:
O-ring USA ...

and here are some older post from Packhorse, who is very experienced with light builds:
O-rings for Mag conversion ...

Packhorse also uses duro 70 o-rings in his W200 conversions:
Ultrafire W200 conversion


The question on lens thickness I don't know much. The only data point I can offer is that the dive-rated lights from Barbolight are tested to 200 meters, and they use a 2" dia polycarbonate 5 mm thick lens, and that at 200mm there is less than 0.5mm deflection on the lens. Glass is stronger (although not impact resistant as the poly lens), so you should be able to make it thinner than 5mm - how much, I don't know. This is also the type of information that I am also looking to learn more about.

Will

 

[wquiles]

So I guess that might explain why so far no one has had their 2.5mm glass lens from the above flashlight fail at up to 145 fsw (just going by dive reports).

But if I'm understanding you correctly if you were to bump into the glass lens or drop the flashlight glass would be more like to crack or shatter than a polycarbonate so glass might be expected to handle greater depths (at the same thickness) in a static sense but dynamically maybe polycarbonate is best?

As far as leaking it also seems from personal experience and reading these forums that water pressure tends to seal the front lens and most leaks seem to be from twisting the head or from switches. Is that a fairly accurate conclusion?

Yes, if you drop the light on a hard surface, the glass lens is far more likely to shatter/chip/break than the policarbonate lens.

As to front sealing better with increased pressure, it depends on the design. For a Mag, the standard setup of the lens in the bezel has the o-ring on the outside, to at increased pressure the original setup gets worst from a theoretical point of view. If however, you look at the designs in this link (look at diagrams in post #7 and #18) you will see that in those cases at higher pressures the seal against the inside/inward o-ring is better:
how to design a press fit lens ...

Javier (designer/founder of Barbo light) has said more than once that you only need ONE o-ring in the tailcap to do the job properly, but he likes the extra assurance of using two o-rings in his dive-rated lights, so I guess you can say they are over-built/over-designed from that point of view.

A fantastic resource on o-rings is the Parker handbook:
Parker O-ring handbook ...

Will