Zen and the Art of Underwater Explosions...

A cautionary tale: last weekend I submerged into a well-known quarry in middle England as it begins to warm up (5 Celsius, 278 Kelvin or about 40 Fahrenheit). Later that day I changed to my nice, big 9" dome which, I believe, came via Nauticam but originally from Ryan and Zen. The air in my housing was still pretty chilly when I started to shoot in the dive centre's pool at about 27 Celsius, 300 Kelvin or 80 Fahrenheit. Two hours later there was an unexpected "glugg!", an even more unexpected bubble, and, as I lifted my housing up, my partner caught a large lump of exquisitely crafted glass and two o-rings going down!

Back-of-the-envelope engineering suggests that the forces on the dome are rather large: the internal area of the dome is, roughly, 70 square in or 400 square cm, a temperature differential of 20 K is equivalent to a pressure of about 6.5kPa or 1 psi: the expanding air exerts a force of 70 lbf or 30kg on the dome!!

Air pressure caused the explosive disassembly of my port!

I can't claim that this is my idea as I heard that Hong Kong and Florida have already discussed my plight. I am pleased to say that the initial prognosis is highly positive and I can't thank Edward and Ryan enough, not to mention the intecession of Alex Tattesall, the first recipient of my anguished email on Saturday afternoon!

Tim

Wow - very scary. Lucky it happened in the pool.

Your description, Dr P, is rather vague on the details (despite the excellent prose) - just to be clear - the glass popped off the dome port back plate?

Alex

Scary situation indeed.

Zen Underwater produces a competing product, but does not manufacture that aluminum mounting plate.

That is quite a temperature differential... We've done in house explosive testing on almost every dome made, and most would tolerate that, though.

Scary situation indeed.

 

Zen Underwater produces a competing product, but does not manufacture that aluminum mounting plate.

 

That is quite a temperature differential... We've done in house explosive testing on almost every dome made, and most would tolerate that, though.

Apologies for spreading false information...

... I'm impressed by the Nauticam dome in use...

... and from outside to inside in the UK at the moment, the temperature differential is indeed over 20 Celsius (dry suit & thermals to shorts & T-shirt!).

Outside:

Inside:

Tim

Your description, Dr P, is rather vague on the details (despite the excellent prose) - just to be clear - the glass popped off the dome port back plate?

Yup!

Dr T is now on the case Dr M. Dr P is looking at a glass of cold sauvignon blanc...

Tim

No need for apologies.

... I'm impressed by the Nauticam dome in use...

It is a nice piece of glass, isn't it!

Back-of-the-envelope engineering suggests that the forces on the dome are rather large: the internal area of the dome is, roughly, 70 square in or 400 square cm, a temperature differential of 20 K is equivalent to a pressure of about 6.5kPa or 1 psi: the expanding air exerts a force of 70 lbf or 30kg on the dome!!

Would like to see your math here?

Force to separate the dome from the mount is in the axial (not radial) direction. Projected cross-sectional area will overestimate this force - you need to take the integral of the load in spherical coordinates, which (quick calculations) seems to only result in ~33 lbf (15 kg) separation force on the dome.

Anyway, this is insignificant relative to the pressure on the dome of the water at depth, which (as we learn in basic training), increases at ~1atm/10m. At 1m, the external pressure of the water on the dome is 16.2psi (or a delta of +1.5psi assuming 14.7psi ambient air conditions), already outweighing what would have been an internal 1psi increase.

It seems like something failed, but as Ryan suggests, I wouldn't attribute it purely to the temperature change - all domes should tolerate these conditions.

Hmm. A salutatory tale about positive pressure inside an enclosure that's designed to see pressure the other way round. Sorry to hear about your camera Tim.

However, the way I see it is little to do with the cold quarry water. The air mass was sealed into the housing before you got into pool, so it's more to do with the difference between the ambient air temp when you closed the housing and the final temperature that the air mass got to in the pool.

Another Tim - but not an esteemed Dr, just a humble CEng.

There you go: never trust a biologist's back-of-the-envelope approximations, as a physicist or engineer will laugh at you (for myself, if I'm within an order of magnitude, I'm doing well)!

I did not want to suggest that there was a defect with the dome, as it has done well by me. I was speculating what was different about the day's diving, or what I may have done, to cause the failure of the dome. My respect for engineers suggests that it's usually an "RTFM" problem rather than a manufacturing one: that is, user error!

I would like to say that I am still impressed by my Nauticam equipment, and hope to rekindle my enthusiasm (I have to admit that diving and photography are not much fun at the moment) when I can replace my camera. This is not a gripe against the manufacturer, but an attempt to warn other photographers not to repeat my (potential) mistakes...

Tim

Tim and Matt:

- the temperature differential between air and pool was about 20 Celsius, the water depth about 10-20cm.

The combination of a marked temperature change, a long time over which the air inside could warm up and low external pressure seem significant contributory factors to me.

Tim

Ruling out pressure as a huge contributor - my guess would be thermal shock fractured a bondline (adhesive layer). What's the CTE mismatch between the dome glass and the aluminum plate?