Goofs from 47 Meters Down
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- The amount of time a diver can breathe on a tank of compressed air decreases with depth. At 47 meters down, a diver would have limited time as the air they breathe is equalized to the pressure at that depth. Every 10m the volume is divided (10m = 1/2, 20m = 1/3, 30m = 1/4, etc.). So 10m below the surface a diver breathes 2x the volume of air. At 50m each breathe is 6x as dense as the air they breathe at the surface. If the diver's surface air consumption (SAC rate) is 30psi per minute that means they would breathe 150 psi per minute at 50m... which is approximately 20 minutes of air. The most common type/size cylinder in scuba diving is an AL80 / 11L at 3,000PSI / 207 bar). Factors that contribute to gas consumption include physiology, fitness level, and stress/panic (such as that caused by being circled by Great White sharks). Thus one hour is highly exaggerated. US Navy dive tables establish the no decompression limit ("bottom time") for 47 meters (>150 ft) to 5 minutes (not including descent or ascent time). A diver has 20 minutes of bottom time at 100 feet using air (21% oxygen). When using an open circuit breathing apparatus, as in the film, a diver breathes compressed air. Air is 21% oxygen and 79% nitrogen. 100% oxygen can potentially result in oxygen toxicity at depths past 19.8ft / 6m depth. It's also quite possible a diver would experience nitrogen narcosis which is similar to being drunk, and simple tasks can be difficult to achieve. Technical diving generally requires Air / Helium (aka Trimix) when diving below 50 meters / 165 feet in order to reduce nitrogen narcosis.
- At 47m, without direct light, blood looks green, not red, as spectrums of light are filtered out by the water. However, there are powerful lights attached to the cage and divers have flashlights, which would negate the effect of depth and explain why the red color is visible (both for blood and for the red patches on dive suits).
- The coast guard divers would have been wearing shark shields knowing there were great whites in the area. None of the divers appeared to be wearing one.
- At some point, the instructor on the boat tells the divers that he delayed sending more air tanks because that would increase their risk of narcosis. 47 meters have limited effect on divers (a common saying among divers is "40 meters one drink, 50m two drinks") and the risk of "hallucinations" is largely exaggerated in the case of two young women in good health. Besides, delaying the delivery of additional air tanks drastically increases the risk for the divers to die from lack of air, obviously a far worse scenario than narcosis (you should also consider the risk that the fall might have damaged their equipment and caused abnormal air consumption). A normal reaction should therefore have been to send additional tanks as soon as possible, ideally attached to a cable that can lead the divers back to the boat (instead of just dropping the tanks in the water with a flashing light).
- The gauge they use to see their air consumption is electronic. Most divers use analog gauges that use a dial to display air pressure, which led some to believe this was a goof. But there are digital pressure gauges that display the pressure in the manner indicated.
- One of the characters advises their air is running out because the shark punctured their BCD. Air does not leak from a BCD.
- The girls were told to do a 5 minute decompression stop at 20 meters. At the depth they were and with the amount of time they were underwater, this would not be sufficient and an additional safety stop would be needed at 5 meters. Instead they were told to then swim directly to the surface. This would likely have killed them due to decompression sickness.
- The speed at which the girls fell in the cage would probably have burst their eardrums. This is because they did not appear to equalize once and it would have been very difficult for them to do so effectively wearing a full face mask.
- While they are submerged in the ocean, they communicate to each other by some sort of radio device on the diving mask despite the fact that their ears are in permanent contact with the water, so they would not be able to hear anything.
- When the cylinder is on the seabed it is an int fitting. When she attaches her regulator it is a din fitting as she screws it in.
- One of the divers in the cage attempts to show the rescue diver where they are by banging on the cage. Our ears determine direction by measuring the difference in the time it takes for sound to reach both ears. Due to the increased density of water vs air, sound travels at a different speed and our human ears cannot determine direction underwater. Divers are trained to know this, but it is possible that the divers were panicked and not thinking correctly.
- One diver successfully hides from a shark by hiding behind a rock. She's wearing an open-circuit SCUBA system that ejects bubbles, which is a very noisy process. The shark would have been able to hear the bubbles, as well as look at them rising from behind the rock. She would not have been able to hide from the shark.
- A certified diver sufficiently skilled to clear a full face mask would know that the only way to survive this scenario would be to immediately slowly ascend to the surface. The scenario in the movie is literally not survivable.
- The Divemaster on the boat states that it might be necessary to equalize on a dive to 5 meters. A 5 meters dive involves a 50% increase in pressure and will always require equalization.
- Switching tanks underwater require not only a new tank but a new regulator. Switching tanks as depicted in the movie would compromise the functionality of the regulator.
- The bends is not a universally fatal disease. Severe cases can be treated with hyperbaric chambers. A common complication is a nitrogen bubble in the bloodstream lodging in the hip and causing aseptic necrosis, or dead bone. It's treated with an artificial hip.
- At 41:58, a character is supposedly swimming up but bubbles are apparently going down below her. Which means this was shot as she was swimming downwards, not upwards.
- The cage has its lights disconnected from any power supply but still working.