Diving Physics
We will cover • Pressure • Behaviour of Gases • Vision • Sound • Buoyancy
1 Bar Atmospheric Pressure Results from the weight of this air Normal Atmospheric Pressure = 1 Bar = 1 AT = 1 Kg/cm2 Pressure A column of air rising above one sq. cm. of the Earth’s surface
Atmospheric and Water Pressure Absolute Pressure is the sum of 1 Bar 2 Bar 10 Metres 1 Bar Pressure
Pressure 0 Metres 1 Bar 10 Metres 2 Bar 20 Metres 3 Bar 30 Metres 4 Bar 40 Metres 5 Bar
Definitions Mass or Weight Quantity of matter in a body Measured in grammes (g) or kilogrammes (Kg) Pressure Force per unit area acting on a surface Measured in Kgs/cm2, Bar, Atms
Definitions Ambient Pressure Local or surrounding pressure Absolute Pressure The total pressure including atmospheric pressure acting on a body, as distinct from gauge pressure which ignores atmospheric pressure.
Definitions Volume A measure of space occupied Measured in litres (Lts) or cubic centimetres (cc or cm3) Density Mass or weight per unit volume Measured in Kgs/m3, g/cm3
Definitions Relative Density Ratio of density of a substance to that of water at 4°C Water = 1, Sea Water = 1.027 Temperature Measured in degrees Kelvin (°K) 273°K = 0°C, 373°K = 100°C 0°K = Absolute Zero
Boyle’s Law “At constant temperature, the volume of a given mass of gas varies inversely with the ABSOLUTE pressure”
0 Metres 1 Bar Absolute Pressure x 2 Volume x 1/2 10 Metres 2 Bar Absolute Pressure x 3 Volume x 1/3 20 Metres 3 Bar Absolute Pressure x 4 Volume x 1/4 30 Metres 4 Bar Absolute Boyle’s Law
Dalton’s Law In a mixture of gases, each constituent gas exerts a “Partial Pressure” in proportion to it’s percentage of the mixture
Dalton’s Law(Example) • Pure air at 1 Bar pressure • Air consists 79% nitrogen and 21% oxygen • Partial pressure of nitrogen =.79 Bar P.P. • Partial pressure of oxygen =.21 Bar P.P.
Dalton’s Law of Partial Pressure 0 1.0 .21 .79 10 2.0 .42 1.58 20 3.0 .63 2.37 30 4.0 .84 3.16 40 5.0 1.05 3.95 50 6.0 1.26 4.74 Depth Absolute Metres Pressure ppO2 ppN2
Henry’s Law “The amount of gas that dissolves in a liquid is proportional to the partial pressure of the gas in contact with the liquid” (Providing temperature is not changed)
Charles’Law “At constant pressure the volume of a gas varies directly as to the absolute temperature” or “At constant volume the pressure of a gas varies directly as to the absolute temperature”
P V P V 1 1 2 2 = 200 12 P 12 T T x x = 2 1 2 283 313 313 x 200 x 12 P = 2 283 x 12 Universal Gas Law Examples = 221 Bar
Universal Gas Law Examples Total contents in litres of cylinder:200(P1) x 0.4(V1) = 80 litres At 30 metres the effective contents will be:80/4 = 20 litres Giving 11/4 fills at this depth
Universal Gas Law Examples A diver breathes at a rate of 28L/min. How long will a 10L/232 Bar cylinder last at 20M ? What is the volume of air available at 20M ?
P x V 232 x 10 = = 1 1 V 2 P 3 2 Universal Gas Law Examples P1 = 232 Bar V1 = 10L P2 = 3 Bar V2 = ? = 773.3L available at 20M
773 . 3 Time = 28 Universal Gas Law Examples Breathing at 28L/min. How long will 773.3L last = 27.6 minutes
Vision Underwater Light Light Air Air Water Water
Vision Underwater Fish appears Larger and Closer
RED ORANGE YELLOW GREEN BLUE VIOLET GREY 5 8 11 19 23 28 31 Colour Absorption DEPTH (metres)
Sound 4 times faster
Archimedes Principle “Any object wholly or partially immersed in a liquid experiences an up-thrust (apparent loss of weight) equal to the weight of liquid displaced”
Archimedes Principle Compare:- Density of object with density of displaced liquid, to determine state of Buoyancy. Fresh Water Density = 1000kgs/m3 Sea Water Density = 1027kgs/m3
Buoyancy - Archimedes Principle Wood Steel WOODSTEEL Volume occupied by block 1000 cm3 1000 cm3 Weight of displaced water 1.0 kg 1.0 kg Weight of object 0.7 kg 5.0 kg Weight of object in water -0.3 kg +4.0 kg
Summary • Pressure • Behaviour of Gases • Vision • Sound • Buoyancy
Questions?
