Physics Lecture

Water vs. Air

Density

Pressure

Buoyancy

Sound

Light

Absorption

Refraction

Heat Loss

Air

Ingredients

Boyle's Law -> Pressure

Charles' Law -> Temperature

Dalton's Law -> Partial Pressures

Henry's Law -> Absorbable

Water vs. Air

Density

It is the fact that water is a lot more dense than air that the other properties exist.

At Sea Level

Water Density does not change because it is incompressible

 Piston Example

Air is Compressible.

One can squeeze air into a smaller volume increasing its density.

 Water is Incompressible.

Putting force on water will not change its volume or density.

Therefore water's density is the same at any depth.

Pressure

The weight of an entire column of air yields the pressure. The weight of a square inch of air at sea level is 14.7 lbs.

A large force presses down on a lunch box, however that external force is counteracted by the air pressure inside the lunch box. If the inside of the box was vacuumed out, a

6 x 8 x 14.7 = over 700 lbs

force would crush the lunch box.

For incompressible water, pressure increases linearly with depth. One additional atm of pressure for every 33 ft of depth.

Gauge pressure is the pressure difference the depth gauge indicates, the pressure change due to the depth of water.

Absolute Pressure is the combined pressure of water and air.

Absolute Pressure
 1 atm  14.7 psi
 2 atm  29.4 psi
 3 atm  44.1 psi
 4 atm  58.8 psi
 Gauge Pressure
0 atm 0 psi
 1 atm  14.7 psi
 2 atm  29.4 psi
 3 atm  44.1 psi

Buoyancy

Displacement

Archimedes Principle: Any object wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.

The force of buoyancy equal to the weight of the displaced water.

 

Buoyancy Example

A 150 lbs diver where a 10 lbs weight belt to become neutrally buoyant in fresh water. How much more weight will they need in salt water ?

Diver Total Weight
= 150 lbs + 10 lbs = 160 lbs
Since the diver is neutral the force of buoyancy matches the diver's weight
= 160 lbs
The amount of displaced fresh water required to generate that buoyancy
= 160 lbs / (62.4 lbs/cu ft) = 2.564 cu ft
That volume will create a larger buoyancy force in salt water
= 2.564 cu ft x 64 lbs/cu ft = 164.1 lbs
This increase buoyancy force can be compensated by adding more weight to the weight belt
= 164.1 lbs = 160 lbs = 4.1 lbs added

 

Sound

Sounds travels farther in water, due to higher density allowing for more collisions of particles.

Sound travels 4x faster in water than in air, this makes it difficult to distinguish which direction the sound came from.

Can hear many things: ships miles away, shrimp eating, helicopters overhead. An underwater bell can be heard 15 miles away.

Light

Absorption

Absorption of light occurs in the water, it filters out the colors progressing through the color spectrum from red to violet. As one dives deeper the natural light will not provide as much color information.

A dive light will allow a closer light source provide the diver with more colors. An underwater flash will help with underwater photography.

Red is the first color to be filtered out. If you bleed underwater it will appear black until illuminated by a nearby dive light when the red color will then be seen.

Refraction

Things appear larger and closer underwater.

Light bends as it transfers media from water to air.

Heat Loss

Heat Loss is 20 times faster in water than in air, due to higher density of water promoting more conduction.

A wet uit allows water inside. The body heats up the water which remains trapped between the wetsuit and skin. The wetsuit's warming effectiveness is decreased as the water is circulated.

Air

Ingredients

78.084% Nitrogen
20.946% Oxygen
0.934% Argon
0.033% Carbon Dioxide
0.003% Trace Gasses

Simplified model

Boyle's Law -> Pressure

For any gas at a constant temperature, the volume will vary inversely with the absolute pressure while the density will vary with the absolute pressure.

 Volume
 Pressure
 Density

Therefore if one was to hold their breath, descend, take a breath of compressed SCUBA air and ascend to the surface, the following would occur.

This occurs with BCs, air squeezes, air embolism, buoyancy, etc.

Charles' Law -> Temperature

For any gas at a constant volume, the pressure of the gas will vary directly with the absolute temperature.

Tanks heat up as they are pressurized.

Tanks laying out in the sun will heat up, increasing the pressure may blow out the burst disk.

 Temperature
 Pressure

Dalton's Law -> Partial Pressures

The total pressure exerted by a mixture of gasses is equal to the sum of the pressures exerted by each of the gasses if it alone were present and occupied the volume.

20% Oxygen at 5 atm is like 100% Oxygen at 1 atm.

Henry's Law -> Absorption of Gas

The amount of gas that will absorb into a liquid at a given temperature is almost directly proportional to the partial pressure of that gas.

Partial Pressure
Absorption Rate