introduction to dive physics. we will cover air pressure boyle’s law vision sound
TRANSCRIPT
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Introduction to Dive Physics
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We will cover• air• pressure• Boyle’s law• vision• sound
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Air
21%
79%
OxygenNitrogen
• Oxygen (O2)
• Nitrogen (N2)
• Carbon dioxide (CO2)• others in trace amounts
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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
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Pressure terms• Atmospheric pressure• Underwater pressure• Absolute pressure
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Atmospheric pressure• Pressure exerted by air at sea
level• Acts on divers both above and
below the surface
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Atmospheric pressure• 100 km x 1 cm2 column of air
weighs 1 kg• Resulting pressure: 1 bar
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Underwater pressure• Water is much heavier than air• Pressure changes underwater are much
greater• Each 10 m depth = 1 bar pressure
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Absolute pressure• Total pressure experienced by diver• atmospheric + underwater• Absolute pressure at 10 m = 2 bar– 1 bar (atmospheric) + 1 bar (underwater)
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Pressure and depth
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Pressure and depth
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Boyle’s Law• describes the effect of pressure on gas volume• the single most important gas law for divers• explains most diving injuries
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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
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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Boyle’s Law
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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
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Equipment issues• BCD• Mask• Suit
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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
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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
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Never hold your breath on SCUBA• Never hold your breath on SCUBA– Never hold your breath on SCUBA
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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
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Refraction
Light Light
Air
AirWater
Water
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• Refraction changes our perception of objects– 33% larger– 25% closer
Vision underwater
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Vision underwater
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• Refraction changes our perception of objects– 33% larger– 25% closer
• Divers adjust with experience
Vision underwater
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Colour• Water absorbs light• Rays are absorbed in order of frequency– Low-frequency light is absorbed first
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Colour
RED
5
YELLOW
11
GREEN
19
BLUE
23
VIOLET
28
ORANGE
8
GREY
31
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Colour• At depth everything appears grey• Torches bring colour back
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Sound• Speed of sound on land
330 ms-1
• Speed of sound underwater1345 ms-1
• Four times faster underwater
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Sound
330 ms-1
1345 ms-1
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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
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SummaryWe discussed• air• pressure• Boyle’s Law• vision• sound
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Questions
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Questions1. The composition of air is approximately
(b) 21% Oxygen / 79% Nitrogen
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Questions2. The pressure at sea level is
(a) 1 bar
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Questions3. The effect of refraction on vision underwater
causes objects to appear(b) larger and closer
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Questions4. The pressure at a depth of 20 metres in sea
water is(c) 3 bar
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Questions5. The pressure at a depth of 40 metres in sea
water is(c) 5 bar
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Questions6. 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
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Questions7. 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
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Questions8. Effects governed by Boyle’s Law are
(b) ear clearing and sinus squeeze