Introduction to Dive Physics Can you hear me? Question policy

We will cover air pressure Boyle’s law vision sound

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

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

Pressure terms Atmospheric pressure Underwater pressure Absolute pressure 3 different kinds of pressure.

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

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

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

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

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

Pressure and depth

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

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

Boyle’s Law

Boyle’s Law

Boyle’s Law

Boyle’s Law

Boyle’s Law

Boyle’s Law next slide: relative pressure to surface

Boyle’s Law next slide: relative volume

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?

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

Equipment issues BCD Mask Suit

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

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.

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

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...

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

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

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

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

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

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

Colour At depth everything appears grey Torches bring colour back

Sound Speed of sound on land 330 ms-1 Speed of sound underwater Four times faster underwater

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

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

Summary We discussed air pressure Boyle’s Law vision sound

Questions

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

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

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

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

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

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

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

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