BSA OT1 User manual

Copyright © BSAC 2017 01
Adapting to the
underwater world
Module objectives
This module provides a basic understanding of air and water pressure and the
effects of the underwater environment on the diver. It also covers the purpose
and function of diving equipment, and the other specialist kit that divers need, and
introduces the concept of buoyancy. Heat loss from the body and how this can be
reduced through the proper choice of protective clothing is considered.
Achievement targets
At the end of this module students should:
• Have a basic understanding of air and water pressure and the physical effects
on the diver of the underwater environment
• Understand the purpose and function of basic equipment
• Understand the purpose and function of scuba equipment
• Understand buoyancy
• Understand heat loss, body temperature control and thermal protection options
Additional visual aids needed
• Mask, ns, snorkel, and weight systems
• Scuba equipment – cylinder, regulator with alternative supply, and buoyancy
compensator
• Wetsuits and drysuits
Module OT1
Ocean Diver

Instructor Manual | Ocean Diver | Adapting to the underwater world
Copyright © BSAC 2017 02
Module content
This module considers the effects of depth and pressure
underwater, basic equipment, scuba equipment,
buoyancy and thermal protection.
Understanding air and water pressure and the
physical eects on the diver in the underwater environment
Explain that before discussing diving equipment, students need to have some
understanding of the diving environment and its effects on divers and how
equipment, both snorkelling and scuba, is designed with this in mind. There will
also be an introduction to buoyancy control and heat loss. Finally explain how
divers can reduce heat loss through thermal protection options.
The module covers the following topics:
• The effects of depth and pressure
Divers need to have a basic understanding of the effects of depth and pressure
on their bodies and equipment while diving. This knowledge will be the
foundation of many of the topics introduced in future stages of their training and
is required to ensure safety.
• Snorkelling equipment
The sport of diving all starts with snorkelling equipment which allows us to see
underwater, swim efciently and breathe air while swimming on the surface
face-down.
• Scuba equipment
Self-contained underwater breathing apparatus (Scuba) has allowed millions
of people to enjoy the underwater world. Ocean Divers need to know about
the key features and functions of this equipment to be able to dive in comfort
and safety.
• Buoyancy
A key skill for Ocean Divers to master is that of buoyancy control. A basic
knowledge of the principles which impact a diver’s buoyancy are important in
developing these skills.
• Thermal protection
Keeping warm is a key consideration when diving. Even in warm tropical waters
divers need to consider the impact of water temperature on their bodies.

Instructor Manual | Ocean Diver | Adapting to the underwater world
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Air pressure
It is important that students have a basic understanding of
air and water pressure and the effects that this can have
on our bodies. The bodies of land-dwelling animals are
acclimatised to life on the Earth’s surface and the need
to breathe air to stay alive. When we venture underwater,
the new environment will subject our bodies to unfamiliar
stresses.
Air
• Compressible gas
Air is a compressible gas. Students may have experienced this for themselves
when inating the tyres on a bicycle with a simple hand or foot pump, which
compresses the air to make the tyres rm.
• Surrounds the Earth
A layer of air approximately 10 to 12 kilometres deep surrounds the Earth
and makes up the atmosphere in which we live. This layer of gas, under the
inuence of the Earth’s gravity, exerts pressure.
• Exerts force in all directions
This air pressure acts in all directions, but generates an overall downward force
on the surface of the Earth called atmospheric pressure.
Atmospheric pressure
• What is atmospheric pressure?
The weight of the air in a column with a cross section of one square centimetre,
stretching from the Earth’s surface to the edge of the atmosphere is one
kilogramme at sea level and this is atmospheric pressure. Students can
visualise this as a column of air about the size of a ngernail stretching up about
10-12km and weighing about the same as a bag of sugar. The air gets thinner
and weighs less the higher up in the column you go.
The reason we do not sense the weight of the air above and around us is that
the body, consisting of lots of water and some air, is in balance or ‘equilibrium’
with the surrounding air pressure. We know the air gets thinner and weighs
less the higher up the column and beyond, that’s why astronauts have to wear
pressurised suits to keep their body in equilibrium to survive.
• 1kg/cm2= 1 bar
A pressure of 1kg per square cm is known as one atmosphere or one bar
(barometric pressure). Although the actual air pressure at sea level varies a little
due to weather conditions, we use one bar as a measurement of air pressure at
sea level for diving purposes.

Instructor Manual | Ocean Diver | Adapting to the underwater world
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Water pressure
To understand the effects of the underwater environment
on our bodies and our diving equipment, students should
understand some basic properties of water.
Water
• Non-compressible liquid
Water, unlike air, is a very dense medium and is not compressible.
• Exerts force in directions
Like air, water exerts a force on things immersed in it.
• Weight of water exerts a pressure of one bar for every 10m of depth
Water on its own, in a column with a cross section of one square centimetre,
exerts a downward pressure of one bar for every 10m of depth.
Divers use depth gauges – they measure the water pressure but the readout on
the dial face is given in metres, as divers need to know how deep they are.
Absolute pressure
Understanding the term ‘absolute’ pressure is important. We have to combine the
air pressure and the water pressure at any depth to give us the total or absolute
pressure we will experience at that depth underwater.
• Water (or gauge) pressure + atmospheric pressure
Absolute pressure = water pressure (gauge) + atmospheric pressure
At 10m, we will experience one bar water pressure plus one bar atmospheric
pressure; therefore, two bar absolute pressure.
If we descend a further 10m to a depth of 20m, the water pressure increases to
two bar, adding on the one bar for atmospheric pressure gives us an absolute
pressure of three bar.
Depth and pressure
Eect of pressure on an air space
• As pressure increases, volume decreases
If pressure is exerted on a body of air, remembering
that air is compressible, it will be squeezed and
the volume reduced. So if a body of air is taken
underwater, say in an upturned, open, rigid container, the water pressure will
‘squeeze’ the air and reduce the volume the deeper the container is taken.

Instructor Manual | Ocean Diver | Adapting to the underwater world
Copyright © BSAC 2017 05
The compression of the air volume for every 10m water depth is easy to
remember.
At 10m, at an absolute pressure of two bar, the volume of air reduces to half of
its original surface volume.
At 20m, three bar, it reduces to one-third of its original volume.
At 30m, four bar, it reduces to one-quarter of its original volume, and so on.
• As pressure decreases, volume increases
If the surrounding pressure decreases, such as on an ascent, then the volume
of a body of air will increase.
Impact on divers
Fortunately, as human bodies have a high-water content of 70-85 per cent, our
bodies can readily adapt to the increase in water pressure that recreational divers
will normally experience. However, any air spaces within the body cannot adapt so
readily. The most important of these are the lungs.
• Lungs
The lungs are not a rigid air container like a bucket, but are a exible air space
more like two balloons. As we breathe in and out, the elasticity of the lungs
allows expansion and contraction. If a swimmer takes a breath and dives down,
immediately the water pressure squeezes the air volume in the lungs and they
reduce in size. We do not feel this reduction unless we dive down deep.
• Diving equipment
To survive underwater we need equipment that enables us to adapt to the
underwater environment. This equipment is also subjected to the effects of
depth and pressure. The following sections covering snorkelling and scuba
equipment will explain the effects in more detail.

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Quiz 1
Instructors should routinely check for transfer of
knowledge to the students.
A diver is at a depth of 15m; what is the water
pressure?
• 1.5 bar
What is the absolute pressure?
• 2.5 bar
The diver now descends to 25m; what is the absolute pressure?
• 3.5 bar
Snorkelling equipment – mask
To move easily through the water and see life below us
from the surface, or to dive down to have a closer look,
we need basic diving equipment; mask, ns, and snorkel.
Allows underwater vision
Divers need a mask to see underwater. The eye is designed to work in air not
water – open your eyes underwater and everything is blurred. Putting an air space
in front of your eyes allows you to see as normal.
Features
Features to look for in a mask are:
• Rigid frame
A mask frame should be rigid to hold the glass.
• Tempered glass
For safety reasons only use masks with tempered glass. Rather like older car
windscreens, if it breaks it will form “pebbles” rather than “shards”.
• Prescription lenses, if needed
For divers who wear glasses, masks with prescription lenses or special frames
that clip into mask are available.

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• Flexible seal or ‘skirt’
The mask should have a exible seal or ‘skirt’ that moulds easily to the face.
Most are made of silicone rubber.
• Nose pocket
The mask skirt must enclose the nose. Remembering that a volume of air will
compress even in shallow water, being able to breathe into the mask will enable
a diver to equalise the pressure inside with that of the surrounding water. If
the pressure is not equalised, the mask will be squeezed onto the face and
become uncomfortable. The seal inside the mask skirt under the nose pocket is
designed so that if water enters the mask (generally seeping in because stray
hairs have broken the seal) breathing out through the nose will displace the
water from the mask.
• Adjustable strap
To secure the mask comfortably, it should have an adjustable strap.
Fit
As masks come in a variety of sizes and designs, it is important that it ts the face
comfortably. To test a mask for t offer it up to your face, without using the strap,
then inhale through your nose. A good-tting mask should remain on the face until
you exhale. It should even resist gentle pulling.
Care
• Rinse in fresh water
A mask should be rinsed in fresh water after each dive.
• Dry before storing
Avoid drying in direct sunlight as this could affect the exibility of the silicone
rubber and therefore the t of the mask.
Snorkelling equipment – ns
Give underwater propulsion
Water provides considerable resistance to body
movement. Using a n to extend the diver’s foot creates
a higher surface area, which increases the propulsion the
diver can generate with a minimal increase in effort.

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Copyright © BSAC 2017 08
Features
Fins come in a variety of shapes and sizes. Before buying a set of ns you need to
consider whether the style of n suits the type of diving being undertaken.
• Two basic styles
There are two traditional, basic styles: the shoe n and the strap n. The shoe
n has a foot pocket with an enclosed heel and is generally used in warm water
or in the pool, where extra foot protection is not worn. The strap n has a foot
pocket designed to t over boots and an adjustable strap or spring tting around
the heel to hold the n in place.
• Flexible blade
The blade should allow some exibility as the legs move up and down with the
nning action.
• Stiffening ridges
The basic design of a n blade should include stiffening ridges to maintain the
shape of the blade, the blade itself should decrease in stiffness towards the tip.
Too rigid, too exible or overlong ns will increase strain on the legs.
• Shaped to maximise efciency
Manufacturers have rened their ns over the years to increase nning
efciency. Most ns include slots, grooves or have shaped blades that assist the
nning action.
Fit
• Comfortable foot pocket
The most important consideration is that the foot pocket is the correct size and
a comfortable t. If too big or too small, it will generally result in cramp and
discomfort. The foot pocket should be foot size for shoe ns and boot size for
strap ns.
Care
• Rinse in fresh water
Fins should be rinsed in fresh water after each dive.
• Dry before storing
Dry them standing up on the foot pocket end as standing ns up on their blades
can distort their shape over a period of time.

Instructor Manual | Ocean Diver | Adapting to the underwater world
Copyright © BSAC 2017 09
Snorkelling equipment – snorkel
Allows surface breathing
The snorkel is a simple breathing tube to allow a diver
to breathe while face down on the surface observing the
underwater scenery or nning along. Snorkels can be
worn under the mask strap or attached to the strap with a small clip.
Features
• Tube
A rigid or semi-rigid open topped tube. May have a exible corrugated section
towards the mouthpiece end.
• 40-45cm long
The tube usually forms a ‘J’ shape to t close to the side of the head. Anything
longer would require too much effort to breathe.
• 20mm diameter
Anything narrower will require too much effort to breathe. Anything wider and it
will be more difcult to blow clear of water following a surface dive.
• Self-drain valves
Some snorkels have a self-drain valve at the lowest point to assist with clearing
water out of the snorkel following a surface dive. Some designs also include
splash protection at the top of the tube.
Fit
• Mouthpiece
Mouthpieces come in different sizes, they should t comfortably in the mouth
gripped lightly by the teeth.
Care
• Rinse in fresh water
Snorkels should be rinsed in fresh water after each dive.
• Dry before storing
Snorkels should be allowed to dry before storing.

Instructor Manual | Ocean Diver | Adapting to the underwater world
Copyright © BSAC 2017 10
Breathing underwater
Snorkelling equipment
• Diver breathes gas at atmospheric pressure
Using basic equipment, the diver breathes gas at
atmospheric pressure on the surface and then breath
holds during a surface dive.
• Lungs are compressed on descent
The lungs react as a sealed balloon does; the air volume compresses but we do
not feel this. The reason for returning to the surface is the urge to breathe.
Scuba equipment
To spend more time underwater, divers take their breathing supply with them.
• Diver breathes gas at surrounding pressure
If the lungs are compressed the action of breathing becomes difcult. Divers
need a gas supply that will maintain, as near as possible, their normal lung
volume. Scuba equipment does this by delivering breathing gas at the same
pressure as the surrounding water. A higher pressure of gas is needed
the deeper a diver goes and this is incorporated into the design of scuba
equipment.
• Lungs are not compressed on descent
When diving with scuba equipment the lungs are not compressed on descent.
Scuba equipment
Scuba
Scuba gear is also known as the aqualung.
• Self
• Contained
• Underwater
• Breathing
• Apparatus