1. Introduction to Dive Physics
Can you hear me?
Question policy

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

3. Air Oxygen (O2) Nitrogen (N2) Carbon dioxide (CO2)
others in trace amounts
For convenience we ignore trace amounts of CO2 and other gasses when discussing diving

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 = Pascal

5. Pressure terms Atmospheric pressure Underwater pressure
Absolute pressure
3 different kinds of 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)
To go from 1 bar to 0 bar you have to travel ~100km

10. Pressure and depth
So, how does this work? What is the pressure at 0 m? (1 bar)
At 10 m (10 bar)?

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
Important terms are: volume / varies / pressure
(constant mass and temperature also important --- later lecture covers this)
In layman’s terms

14. Boyle’s Law

15. Boyle’s Law

16. Boyle’s Law

17. Boyle’s Law

18. Boyle’s Law

19. Boyle’s Law
next slide: relative pressure to surface

20. Boyle’s Law
next slide: relative volume

21. Boyle’s Law
It’s really as simple as that. If the mass and the temperature stay the same, pressure is inversely proportional to volume
Invite audience to think about what happens in reverse --- if you fill a balloon with air at 30 m, what happens when you ascend?

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
What we need to consider when we are diving

23. Equipment issues
BCD
Mask
Suit

24. Equipment issues BCD Mask Suit Equalise with inflate/deflate valves
Equalise by breathing out through nose
Suit
Neoprene compresses at depth
Drysuit divers add/remove air from suit
Discussed in more detail in Pressure lecture
Mass of air in BCD changes volume like the balloon --- we use valves to add/remove air
Air in mask compresses too
Suits compress; neoprene provides less insulation

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
Valsalva manoeuvre is what you do to equalise in airplanes --- hold nose and blow out
This is why nose cover of mask is rubber
Remind Ss of balloon example. Lungs burst like balloons if pressure increases
Burst lungs are very dodgy and can kill in minutes
Almost the first rule of SCUBA: don’t hold your breath.

26. Never hold your breath on SCUBA
I’m going to stress this tediously
If you have to panic, scream out loud --- it could save your life

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
As we learned in equipment lecture last week...

28. Refraction Light Light Air Air Water Water
Different materials have different refractive indices, but the main thing is
that light bends between the water and your eyes

29. Vision underwater Refraction changes our perception of objects
33% larger
25% closer
What is the effect of refraction on us as divers?

30. Vision underwater Diver is looking at the smaller fish
The light rays are bent during their passage from water to glass to air
The fish appears to be closer and bigger

31. Vision underwater Refraction changes our perception of objects
33% larger
25% closer
Divers adjust with experience
When you start diving, it’s hard to grab hold of things --- you gradually learn with experience to adjust to it

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

33. Colour 5 8 11 19 23 28 31 RED ORANGE YELLOW GREEN BLUE VIOLET GREY
At 5 m, red disappears (turns to grey)
At 8 m, orange...
... and so on all the way through the spectrum

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
Four times faster underwater

36. Sound
330 ms-1
1345 ms-1
Look, nice picture!

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
So why is this an issue for us?
Rule of thumb: if you can hear a boat, you’re in danger. Low viz means you may not be able to see it; important not to surface

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