Seiko & Citizen Watch Forum Message Archive

Consider that pressure at sea level (or the weight of one atmosphere) is equal to about 15 lbs/square inch (i.e. a vacuum will raise a 1 square inch column of water about 33 ft. at sea level, and this column weighs about 15 lbs.). Furthermore, this weight increases linearly as we descend down into the sea., i.e. pressure at 66 feet down is equal to twice that of sea level, or two atmospheres. Furthermore, consider that Bernoulli's law tells us that total pressure at any liquid depth is equal to static pressure + dynamic pressure. Given that we already know what the static pressure is at any depth, we then need only concern ourselves with dynamic pressure "" that is, how much more would we ADD, by anything we might do or encounter, above what we already expect? Therefore, if a watch is rated for say, 20 atmospheres, what might be the maximum pressure that that watch might ever encounter, due to any condition or circumstance, at any lesser depth? And therefore, what might this be as a maximum? As a generous estimate (interpolating from various on-line sources), I would say that no person could generate a velocity of greater than 50 feet/second (or 34 mph), either INTO or UNDER water, and survive. By reference, this is approximately equal to the terminal velocity of a free dive into a pool from 150 feet (for reference, a sky diver generates a terminal velocity of only about 170 feet/second, or about 120 mph). Few people have ever survived a free dive from higher heights. Therefore, nothing you yourself could do, and no condition you could ever survive, would ever generate a momentary or continuous water velocity greater than 50 feet/second. Now, 50 ft/sec = 600 inches/second. Thus, any movement into or under water at any velocity up to 600 inches/second, would load 600 cubic inches of water on every square inch of surface area, or .3472 cubic feet (i.e. 600 / (12*12*12)). At 62.427 lbs./cubic foot of water, this yields a pressure of 21.67 lbs./square inch, or about 1.44 atmospheres (21.67 / 15) of maximum momentary dynamic pressure. Note that this is a VERY generous maximum, and I stand by my prior assertion that no one is likely to ever encounter or generate an INCREMENTAL pressure of greater than 1 ADDITIONAL atmosphere above what the watch wearer is already experiencing in any given condition. Therefore, unless the watch is improperly assembled or specified, a true 200 meter watch would NEVER experience a momentary pressure greater than 20+1.44 = 21.44 atmospheres at any depth UP TO 200 atmospheres. Therefore, take your 200 meter watches no deeper than 20-1.44=18.56 atmospheres, or 185.6 meters (550+ feet deep)and you should be fine!