Dive physics

[gcbryan]

If one wanted to be somewhat analytical regarding front lens glass strength for a dive light certified to 300 fsw is this a reasonable way to look at the problem...?

Pressure at 300 fsw is 10.09 ata. One ata is in the light so the force on the front lens would be 9.09 ata or 9.09 * 14.4 psi = 131.8 psi

So does this roughly mean that static pressure at 300 fsw on either the "ugly light" or the W200 is more or less equivalent to placing an object weighing 132 pounds where it would be directly supported by the glass lens?

It seems that for lights with a diameter in the 40 mm - 50 mm range that perhaps 6 mm would meet the standard, for glass thickness anyway, for a 300 fsw spec?

I'd just like to start to get a clear idea of the ideal spec's for a dive light in this size range and for a 300 fsw spec. I realize there is o-ring design consideration along with other factors but as far as the glass lens considerations is this in the ballpark?

 

[350xfire]

If one wanted to be somewhat analytical regarding front lens glass strength for a dive light certified to 300 fsw is this a reasonable way to look at the problem...?

Pressure at 300 fsw is 10.09 ata. One ata is in the light so the force on the front lens would be 9.09 ata or 9.09 * 14.4 psi = 131.8 psi

So does this roughly mean that static pressure at 300 fsw on either the "ugly light" or the W200 is more or less equivalent to placing an object weighing 132 pounds where it would be directly supported by the glass lens?

It seems that for lights with a diameter in the 40 mm - 50 mm range that perhaps 6 mm would meet the standard, for glass thickness anyway, for a 300 fsw spec?

I'd just like to start to get a clear idea of the ideal spec's for a dive light in this size range and for a 300 fsw spec. I realize there is o-ring design consideration along with other factors but as far as the glass lens considerations is this in the ballpark?

I've had 3/16" boro glass to 265 with no problems...
A way to test, would be to use a 1 sq inch bar and stack 132lbs on it and see what happens. I have a pressure pot so I don't need to do that, but it could work if you can balance it or use some type of press with a scale as an indicator.

Of course, if the lens is more than 1 square inch in surface area then the pressure in lbs would increase... A 2" lens is 3.14sq in (A=pi r^2) inches so 132*3.14=414lbs of force spread over the face... Now that lens will probably be supported for quite a bit of its area so the force will only be spread out over a smaller area.

I am now using 1/4" glass anyway, just for a safety margin.. You really need to figure out the tensile/shear strength of the material to be able to determine specifics here.

 

[Packhorse]

Ouch. My head hurts.
Why do you guys have to be so difficult and use imperial?

1 bar = 1 kg/cm2= 1 ata


At 100 meters the pressure on the outside is 11 bar, the pressure inside is 1 bar. So the pressure differential is 10 bar.
So for every square cm of surface area on a flat plane there is 1 kg for every bar of pressure.
a 50mm diameter lens ( like a mag) has a surface area of 19.63cm or 19.63kg's per 10 meters of depth. or 196.3kgs at 100 meters. This force is spread evenly over the whole lens. So yes this is the same as a 196.3 kg weight pushing against the lens.

What happens if you use a concave lens? Is there more pressure working against the larger surface area?

Yes and no.

Yes there is more pressure acting against the lens. But the pressure is pushing at right angles to the curve of the lens. The inwards pressure remains the same regardless of the external shape of the lens ( cone, hemisphere or flat) as long as the diameter is the same.
Yes this is the same as a 196.3 kg weight pushing against the lens.

I realise I am saying the same as what 350 said. I just thought I would do it in metric since TROTW uses it. :twothumbs

 

[Packhorse]

The question of knowing what lens to use is a hard one.
I dont know and have not found any data to work with.
I can only tell you what I have experienced.

I used to use a stock Maglite polycarbonate lens in front of a MR16 bulb (the type with a glass lens). At 42 meters the lens pushed hard against the MR16 and it smashed. The stock mag lens did not fail and the light remained dry.
I have has 2 of the DX 50mm 1.6mm thick lenses in a mag in a pressure pot down to about 60 meters with out fail.
I currently use 4mm acrylic lenses and dive have several dives between 50-57 meters with no issues.

 

[gcbryan]

The question of knowing what lens to use is a hard one.
I dont know and have not found any data to work with.
I can only tell you what I have experienced.

I used to use a stock Maglite polycarbonate lens in front of a MR16 bulb (the type with a glass lens). At 42 meters the lens pushed hard against the MR16 and it smashed. The stock mag lens did not fail and the light remained dry.
I have has 2 of the DX 50mm 1.6mm thick lenses in a mag in a pressure pot down to about 60 meters with out fail.
I currently use 4mm acrylic lenses and dive have several dives between 50-57 meters with no issues.

It is interesting that no one that I know (or have read about) has had a glass lens fail.

The "ugly" light uses a 2.5 mm glass lens and no one has had a lens fail.

I took a tiny Romisen non-diving light down to 30 mts and it probably has a 1.5 mm glass lens.

I guess flooding and not glass breakage is and should be our biggest concern with dive lights.

 

[BVH]

Just thinking out loud...I would think you'd also need to be sure the final lens size is small enough so it allows for the reduction in size of the bezel/body of the light as pressure increases?

 

[Packhorse]

If your body deforms that much its not the right one for the job.

 

[350xfire]

If your body deforms that much its not the right one for the job.

Right!

 

[znomit]

For deformation...
http://www.misumi.co.th/products/technical/mold/pdf/pl1113_1114.pdf

Note deflection goes down as a function of thickness^3

 

[beekeeper5]

I'm planning to use a Borofloat 52.1mm x 2.00 mm on a Maglite. Do you think this is thick enough to withstand a max depth of 100'? That's about 30.48 meters (for Packhorse )

 

[gcbryan]

I'm sure it would work. The "Uglylight" has a similar diameter and I've had it deeper than that (it's 2.5 mm glass). Is Borofloat glass or polycarbonate?

I wouldn't want to bang the glass against anything at that thickness/depth however.

 

[beekeeper5]

I'm sure it would work. The "Uglylight" has a similar diameter and I've had it deeper than that (it's 2.5 mm glass). Is Borofloat glass or polycarbonate?

I wouldn't want to bang the glass against anything at that thickness/depth however.

It's a Borafloat 52.1 mm x 2.00mm that I bought from Flashlightlens. I'm waiting for 350xfire to give me a quote on the shipping charges for one of his lens. I may just go ahead with the 2mm for now and get the 5mm from 350xfire at a later date. I'm itching to get my maglite completed.

 

[Codiak]

Rushing may cost you in the end, experience talking here ;-)

 

[beekeeper5]

Codiak, Haha.. Yes, I know. Patience is a virtue.

Oh.. to answer your question, the Borofloat is glass. Does that matter at all? The link is located here http://www.flashlightlens.com/str/index.php?app=ccp0&ns=prodshow&ref=boro_lens

 

[SmokedCPU]

Hi there,

With 44mm aspheric Im using stock mag plastic lens, with a oring between external rim shim, and lens go down to the aspheric.
Tested to 130psi, tree at time with complete canister assembled. Didnt push test furter as i dont want my small compressor to die.

With reflector i use 4-5mm std glass, and it work.

Cheap DX 52mm x1mm glass implode around 75' and its pretty cool too. sound like a shotgun shoot. Led still lit all the 45 min dive in salt water, but needed a new power cord.

I wish it was not too mush theorical

 

[DIWdiver]

Copied and heavily modified from a post I made here: http://www.scubaboard.com/forums/do-yourself-diy/284363-acrylic-thickness.html

From Marks' Standard Handbook for Mechanical Engineers, the max stress in a disk supported uniformly around its edge is S=1.24wr^2/t^2, where:
S = max stress, PSI
w = pressure against the plate, PSI
r = diameter, inches
t = thickness, inches

Re-arranging the equation,
t = sqrt(1.24wr^2/s)

I got the flexural strength of borosilicate glass as 69 MPa from:
http://www.makeitfrom.com/compare/?left=Borosilicate_Glass&right=MACOR&prop=Flexural_Strength
Regular (soda lime) glass, also 69 MPa:
http://www2.bakersfieldcollege.edu/mrozell/documents/Engr B45/tensile flexure.pdf

From Pocket Ref, 1 Pa = 0.0209 lbs/ft^2
69 * 10^6 * 0.0209 / 144 = 10,000 psi

Seawater gives about 0.5 psi/ft, so 300 ft gives w=150psi.

Thus t = sqrt(1.24 * 150 * 1^2 / 10,000) = 0.136 inches (3.45mm).

Since the strength goes up (stress goes down) as the square of the thickness, for a safety factor of 4 you'd want 2 times the thickness. Or, 5mm would give a safety factor of (5/3.45)^2 = 2.

As a sanity check, let's analyze SmokedCPU's data:
75 feet => 37.5 psi, and 1mm = 0.040", so
s = 1.24 * 37.5 * 1^2 / 0.040^2 = 29,000 psi
That's almost 3 times the strength I would have expected. SmokedCPU, are you sure that wasn't 1.5mm glass? Or is the o-ring or some other supporting structure well in from the outer edge of the lens? Or maybe both?

As far as testing, there's a big difference between putting a weight on the lens and putting it under pressure. When under pressure, the force is evenly distributed across the lens. Because most of the lens is near the edge, most of the force is also near the edge. When you put a weight on it, more of the force probably ends up being near the center. Also, if you have hard edges on the block, that is a stress concentrator. Both of these things tend to make the lens fail earlier, so this is a harder test to pass. Unless your block ends up placing most of the force very near the edge, in which case it might be an easier test to pass. In other words, pressure testing is really the more reliable way to test.

D