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Introduction to Dive Physics

Introduction to Dive Physics. We will cover. a ir p ressure Boyle’s law vision sound. Air. Oxygen (O 2 ) Nitrogen (N 2 ) Carbon dioxide (CO 2 ) others in trace amounts. Pressure. Every medium exerts force on objects within it Force is exerted equally from all directions

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Introduction to Dive Physics

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  1. Introduction to Dive Physics

  2. We will cover • air • pressure • Boyle’s law • vision • sound

  3. Air • Oxygen (O2) • Nitrogen (N2) • Carbon dioxide (CO2) • others in trace amounts

  4. Pressure • Every medium exerts force on objects within it • Force is exerted equally from all directions • Divers are subject to pressure from atmosphere and water • Divers measure pressure in bar • 1 bar = 100000 Pascal

  5. Pressure terms • Atmospheric pressure • Underwater pressure • Absolute pressure

  6. Atmospheric pressure • Pressure exerted by air at sea level • Acts on divers both above and below the surface

  7. Atmospheric pressure • 100 km x 1 cm2 column of air weighs 1 kg • Resulting pressure: 1 bar

  8. Underwater pressure • Water is much heavier than air • Pressure changes underwater are much greater • Each 10 m depth = 1 bar pressure

  9. Absolute pressure • Total pressure experienced by diver • atmospheric + underwater • Absolute pressure at 10 m = 2 bar • 1 bar (atmospheric) + 1 bar (underwater)

  10. Pressure and depth

  11. Pressure and depth

  12. Boyle’s Law • describes the effect of pressure on gas volume • the single most important gas law for divers • explains most diving injuries

  13. Boyle’s Law “At a constant temperature, the volume of a given mass of gas varies inversely with the absolute pressure.” • If you double the pressure, you halve the volume

  14. Boyle’s Law

  15. Boyle’s Law

  16. Boyle’s Law

  17. Boyle’s Law

  18. Boyle’s Law

  19. Boyle’s Law

  20. Boyle’s Law

  21. Boyle’s Law

  22. Boyle’s Law for divers • Any compressible air space will change in volume on descent and ascent • Equipment air spaces • Body air spaces • We add and remove air from these spaces to equalise the pressure

  23. Equipment issues • BCD • Mask • Suit

  24. Equipment issues • BCD • Equalise with inflate/deflate valves • Mask • Equalise by breathing out through nose • Suit • Neoprene compresses at depth • Drysuit divers add/remove air from suit

  25. Physiological issues • Ears and sinuses need equalising • Equalise with Valsalva manoeuvre, etc. • Lungs can burst on ascent if you hold your breath • This can kill you • Never hold your breath on SCUBA

  26. Never hold your breath on SCUBA • Never hold your breath on SCUBA • Never hold your breath on SCUBA

  27. Vision • Human eyes can’t focus underwater • Masks trap a layer of air between our eyes and water • Light rays bend as they move from one medium to another • They appear to be coming from elsewhere

  28. Refraction Light Light Air Air Water Water

  29. Vision underwater • Refraction changes our perception of objects • 33% larger • 25% closer

  30. Vision underwater

  31. Vision underwater • Refraction changes our perception of objects • 33% larger • 25% closer • Divers adjust with experience

  32. Colour • Water absorbs light • Rays are absorbed in order of frequency • Low-frequency light is absorbed first

  33. RED ORANGE YELLOW GREEN BLUE VIOLET GREY 5 8 11 19 23 28 31 Colour

  34. Colour • At depth everything appears grey • Torches bring colour back

  35. Sound • Speed of sound on land 330 ms-1 • Speed of sound underwater 1345 ms-1 • Four times faster underwater

  36. Sound 330 ms-1 1345 ms-1

  37. Sound • Humans use timing cues to localise sounds • Determining direction of sound is almost impossible underwater • Changes in loudness can tell you if something is approaching • When in doubt, assume the boat is above you

  38. Summary We discussed • air • pressure • Boyle’s Law • vision • sound

  39. Questions

  40. Questions 1. The composition of air is approximately (b) 21% Oxygen / 79% Nitrogen

  41. Questions 2. The pressure at sea level is (a) 1 bar

  42. Questions 3. The effect of refraction on vision underwater causes objects to appear (b) larger and closer

  43. Questions 4. The pressure at a depth of 20 metres in sea water is (c) 3 bar

  44. Questions 5. The pressure at a depth of 40 metres in sea water is (c) 5 bar

  45. Questions 6. A flexible container full of air at atmospheric pressure is brought underwater. At a depth of 10 metres its volume will be (c) half its original size

  46. Questions 7. A snorkel diver takes a deep breath and fills his lungs with air on the surface and then descends to a depth of 20 metres. His lungs will be (c) a third of their original size

  47. Questions 8. Effects governed by Boyle’s Law are (b) ear clearing and sinus squeeze

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