Comments on: TV Tonight: Toolbox http://toolmonger.com/2007/01/27/tv-tonight-toolbox/ All tools. All the time. Your source for news, information, and reviews of hand tools, power tools, and tools of all kinds. Tue, 02 Dec 2008 02:54:55 +0000 http://wordpress.org/?v=2.6.1 By: Daniel Rutter http://toolmonger.com/2007/01/27/tv-tonight-toolbox/#comment-16647 Daniel Rutter Sun, 28 Jan 2007 12:35:37 +0000 http://toolmonger.com/2007/01/27/tv-tonight-toolbox/#comment-16647 I don't think an external pressure increase from any escapable sunken-car depth will make any difference at all to the breakability of the window. The compressive stress on the surface of toughened glass is in the few-hundred-atmosphere range, which is a LOT. The pressure at the bottom of the 13 foot deep pool the Mythbusters used was less than half an atmosphere higher than the pressure at the surface. To get a pressure of a hundred atmospheres, which may still be less than a quarter of the surface compressive pressure of the glass but might still influence its behaviour, you'd need to be slightly more than a KILOMETRE underwater. (In which case it doesn't really matter if you open the window or not.) Of course, lots of things that don't seem plausible on the surface are actually true. But I'd be willing to bet you a good-sized wad of cash that anything that'll break "safety" glass at zero atmospheres relative pressure will do it just as well with 100 atmospheres pushing one way or the other. Heck, there was even a Mythbusters episode a while back in which they demonstrated that pushing by hand on the inside of a car window made no perceptible difference to its breakability. I don’t think an external pressure increase from any escapable sunken-car depth will make any difference at all to the breakability of the window. The compressive stress on the surface of toughened glass is in the few-hundred-atmosphere range, which is a LOT. The pressure at the bottom of the 13 foot deep pool the Mythbusters used was less than half an atmosphere higher than the pressure at the surface.

To get a pressure of a hundred atmospheres, which may still be less than a quarter of the surface compressive pressure of the glass but might still influence its behaviour, you’d need to be slightly more than a KILOMETRE underwater.

(In which case it doesn’t really matter if you open the window or not.)

Of course, lots of things that don’t seem plausible on the surface are actually true. But I’d be willing to bet you a good-sized wad of cash that anything that’ll break “safety” glass at zero atmospheres relative pressure will do it just as well with 100 atmospheres pushing one way or the other.

Heck, there was even a Mythbusters episode a while back in which they demonstrated that pushing by hand on the inside of a car window made no perceptible difference to its breakability.

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