Tutorial 2: Pressure, Buoyancy and Bernoulli Questions Question 1: Diving Deep I
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Tutorial 2: Pressure, Buoyancy and Bernoulli Questions Question 1: Diving Deep In diving many different breathing gases are used besides pressurized air. One reason for this is that oxygen can actually become toxic if its partial pressure is too high. Partial pressure of oxygen is the product of total pressure and oxygen content in percent. For example, air (which contains 20% oxygen) at atmospheric pressure (101kPa) contains oxygen with a partial pressure of 20% . 101kPa 20kPa. The NOAA recommendation is to avoid being exposed to oxygen with a partial pressure of 160k Pa for longer that 45min in order to avoid oxygen poisoning. a) How big is the total pressure at a depth of d meters? Assume you're diving in seawater, pw = 1.025kg/l. If you were to go diving with pure oxygen instead of pressurized air, how deep could you go while still b) respecting the NOAA limit for a 45min dive? How deep could you go while respecting this limit if you took pressurized air instead? c) For very deep dives other gas mixtures are used. One example is Trimix 10/70/20 (10% oxygen, 70% helium, 20% nitrogen). How deep could you go using Trimix and still be within the limits for oxygen poisoning?Explanation / Answer
a). The pressure on an object submerged in water will pe given as
P = rho * g * h , where rho = 1025 kg/m3 for sea water
P = 1025 * 9.8 * d = 10045*d.
However, if the fluid body is open to atmosphere as in case of ocean, there is atmospheric pressure also. Therefore, Total pressure is some of atmospheric pressure and pressure of fluid. At sea level, atmospheric pressure is around 1.01 * 105 Pa.
b). As in the case of pure oxygen (100% oxygen), the partial pressure will be 100 % of 101 kPa which equals 101 kpa which is extremely high. Uusually, pure oxygen becomes toxic below the height of 10 meters. The ,more we go down, more is the chances to get sudden seizures.
c). As in the case of trimix, it has 10% of oxygen with partial pressure of 10.1 kPa with respect to atmosphere. It is obvious that we will be able to dive much deeper. We can dive as deep as 90 meters or around 300 feet with trimix
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