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

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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.

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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.

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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.

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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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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.

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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.

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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

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Components
Scuba equipment comprises a
• Cylinder
• Buoyancy compensator
• Regulator
The cylinder
The cylinder holds the diver’s supply of breathing gas.
Features
• Contains breathing gas at high pressure
Dive cylinders are usually lled to either 232 bar or 300
bar pressure, from a suitable compressor. The cross ow or pillar valve in the
cylinder neck is used to prevent or allow gas ow into or out of the cylinder.
• Steel or aluminium
Typically, cylinders are made of steel or aluminium. The base of the cylinder is
often protected by a rubber or plastic boot.
• BS/EN standard; CE mark
Diving cylinders contain large amounts of stored energy. They are made to a
very robust design, manufactured to strict safety standards. These standards
are marked on the shoulder of the cylinder. It is vital that there are appropriate
standards and that cylinders are used and maintained to ensure the safety of
both the divers who use them and the people who ll them. In the EU, a cylinder
used with diving apparatus cannot be put on the market unless it conforms to
the Pressure Equipment Directive (PED), which is implemented in UK legislation
by the Pressure Equipment Regulations 1999, as amended.
All diving cylinders manufactured to the PED will bear the CE mark. Cylinders
made before the application of the PED will not bear the CE mark but may
continue to be used as long as they have been manufactured in accordance
with an appropriate standard and are maintained in serviceable condition.
Applicable standards include BS EN ISO 0909-2:2010 for steel cylinders and
BS EN ISO 7866:2012 for aluminium cylinders. Older cylinders with older
specications are still valid subject to Periodic Inspection and Testing.
Outside the EU, instructors should describe the corresponding local
requirements.

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Testing
Diving cylinders should be subject to a suitable inspection and test regime to
ensure they are safe.
• Hydrostatic test every 5 years
Scuba cylinders require a hydrostatic test every ve years. On successful
completion, the date (year/month) of the test is stamped on the cylinder
shoulder.
• Visual inspection every 2.5 years
Scuba cylinders also require a visual inspection test every two and a half years
in line with the ve-year cycle of hydrostatic tests. The date a hydrostatic test
is due is not affected by the date of the last visual inspection. If a cylinder is
unused for a period of time such that it has a visual inspection four years after
its hydrostatic test (rather than at the two-and-a-half--year mark) the cylinder
will only then be ‘in test’ for a further one year before the next hydrostatic test is
due.
Cylinder testing regulations and the required periods between test are changed
periodically, so please check the BSAC web site for the latest information and
advice.
Care
• Rinse in fresh water
Cylinders should be rinsed in fresh water after each dive and the boot (if tted)
removed periodically to clean the base of the cylinder.
• Dry before storing
Cylinders should be stored dry. If possible store upright, but ensure cylinder
cannot fall over.
• Do not store empty
Storing cylinders completely empty should be avoided; rather store them with a
small amount of residual gas pressure to prevent ingress of contaminants.
Cylinder markings
In addition to the manufacturing standard there is other
important information marked on the cylinder and some
labels that divers should understand.
Breathing gases
Gases used by Ocean Divers are air and some nitrox mixes.

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• Air (21% oxygen)
Standard air from the environment is compressed and ltered into scuba
cylinders. Air has an oxygen content of 21% and contains 79% nitrogen.
• Nitrox (32% or 36% oxygen)
A nitrox mix has extra oxygen in it in place of some of the nitrogen. Nitrox 32
has 32% oxygen and 68% nitrogen whereas nitrox 36 has 36% oxygen and
64% nitrogen. The reasons that these nitrox mixes might be chosen are covered
later in the course.
Key marks and labels
Several of the additional markings contain essential information about the cylinder.
• “Breathing Air” or “Nitrox” labels
Scuba cylinders should be labelled to indicate whether the contents are air or
nitrox. This could be a large sticker around the whole cylinder.
• Cylinder size
Cylinders can have different capacities. Capacity is measured in litres, or put
another way, how much water the cylinder could hold if you took the valve off
and poured water in. Hence, the term water capacity (WC)
• Working and test pressures
The working pressure, labelled WP, indicates the maximum operating pressure
of the cylinder. Usually this is 232 bar or 300 bar. Some cylinders may have this
value marked as CP, charging pressure.
There is another mark, TP, which stands for test pressure. This is a value higher
than the WP and is used during the hydrostatic test.
• Test dates
The manufacturer tests the cylinder and stamps it with the date of this test.
Subsequent test dates are also stamped on the cylinder and these are looked
at by compressor operators to ensure the cylinder is ‘in test’ before being lled.
If a cylinder is not ‘in test’, it should not be lled. Test dates may be of the
format YYYY/MM or since 2013 just YY/MM. Where the date refers to a visual
inspection then there will be a ‘V’ stamped next to the date.
Cylinders used for nitrox need periodic cleaning
Cylinders used for nitrox mixes that are produced using partial pressure mixing
techniques need to be in oxygen service. which means they need to have
oxygen compatible parts in the valves and need to be oxygen clean to ensure no
hydrocarbons are present in them. Nitrox mixes below 40 per cent produced by
continuous blending or by using premixed gas do not need such cleaning. Cleaning
is recommended every 15 months.

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Pillar/cross-ow valves
A-clamp
The A-clamp or international tting cylinder valve has an
O-ring set into the valve outlet and the regulator clamps
around it. Sometimes this is referred to as a yoke tting.
• 232 bar cylinders (max)
The A-clamp type of cylinder valve can only be used on cylinders with a working
pressure up to 232 bar.
DIN
The DIN cylinder valve has an internal 5/8-inch BSP thread and no sealing O-ring
as this is on the diving regulator instead.
• 232 -300 bar cylinders
DIN valves can be tted on cylinders up to 300 bar. A 232 bar DIN cylinder
valve has ve thread rotations within the valve whereas a 300 bar DIN valve
has seven thread rotations. This means that it is not possible to attach 232-bar-
rated equipment to a 300 bar cylinder supply. It is possible to screw an A-clamp
insert into some 232 bar DIN valves to convert them to allow use of regulators
equipped with A-clamp ttings.
M26
• Nitrox cylinders
In 2008, an EU directive came into force requiring an M26 tting to be used on
equipment where the gas being used has an oxygen content above 22 per cent.
This tting appears similar to a DIN tting but is a 26mm tting rather than a
5/8”. However, this valve type has not been generally adopted in the UK.
Buoyancy compensator
Supports the cylinder
The buoyancy compensator or BC carries the cylinder. It
is a jacket or harness system. They are manufactured in
various styles but have many important common features.

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Buoyancy adjustment
Gas is added to or dumped from the BC to assist the diver with their buoyancy
control. Students will learn in the pool/sheltered water how the BC is used to adjust
a diver’s buoyancy.
Features
• Gas bladder
An integrated gas bladder gives support to the diver and equipment on the
surface when inated. It also allows the diver to adjust buoyancy underwater.
Note: Point out that breathing in from the bladder of a BC should be avoided
due to risk of lung/chest infection.
• Direct feed
A direct feed uses gas from the cylinder to inate the BC. An intermediate
pressure hose connects the direct feed to the rst stage of the regulator. A
simple ‘push button’ is used to inate the BC.
• Oral inator
BCs also have a back-up oral-ination mouthpiece. The oral inator also has
a push dump. Some have an alternative method of ination, such as a small
cylinder.
Note: Point out that lling of emergency BC cylinders with nitrox should be
avoided unless oxygen clean.
• Dump valve
A manually operated “dump” mechanism allows easy venting from the bladder
so the diver can descend. This is a pull dump.
• Over-pressure relief valve
This prevents the bladder from bursting if gas is continually added.
Care
• Rinse in fresh water (inside and out)
The BC should be rinsed with fresh water. Mild disinfectant solutions can
also be used inside the bladder. Do not breathe the gas from the BC bladder.
The bladder can build up bacteria and breathing from it may cause serious
respiratory problems.
• Dry before storing slightly inated
The BC should be dry before storing and should be stored slightly inated.
• Service to manufacturer recommendations
The BC should be serviced according to the manufacturer’s recommendations.

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Regulator
Delivers breathing gas
The gas in a diving cylinder is at high pressure, anything
up to 300 bar. It would be impossible to breathe directly
from the cylinder so the regulator has been designed to
reduce the pressure of the gas leaving the cylinder and deliver it to the diver at
ambient pressure so that it is easy to breathe whenever the diver wants to. The
regulator is also referred to as a demand valve, or DV, for this reason.
Features
• First and second stages
The regulator rst stage ts to the cylinder valve with either with an A-clamp or
DIN tting (5/8” BSP or possibly M26).
When the cylinder valve is opened the rst stage reduces the pressure in the
cylinder to an intermediate pressure around 10 bar above the local absolute (or
ambient as it is also known) pressure.
The gas at this inter-stage pressure is passed down an intermediate pressure
hose to the regulator second stage.
The second stage further reduces the gas pressure from inter-stage pressure to
equal ambient pressure and delivers gas to the diver when they breathe in.
When the diver breathes out the exhalation is expelled via the exhaust valve
into the water.
If water enters the second stage it can be easily cleared by either breathing out
or by pressing the purge button located on the front face of the regulator.
The fail-safe design of regulators usually means that they will ‘free-ow’,
continuously delivering gas if a mechanical fault occurs. Training includes how
to breathe from a free-owing second stage.
• Hoses
As well as the inter-stage, intermediate pressure hose, a regulator set will
probably have further intermediate pressure hoses for connection to a buoyancy
compensator direct feed and a dry suit direct feed.
• Alternative supply
The alternative supply is an additional second-stage regulator connected to the
rst stage via its own intermediate pressure hose to provide a backup system.
This additional second stage is commonly known as an octopus rig and, as a
backup system should be easily identiable. Yellow marked hoses and second

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stages are a common method of identifying this backup system. Regulator
rst stages manufactured after 2014 will be stamped with an “A” if they are
compatible to be tted with an alternative supply.
As a backup system, an alternative supply second stage should be of equal
standard to the primary second stage. This alternative supply is for use in an
emergency, usually by the diver’s buddy.
Note: The latest version of the regulator standard EN250 (2014) advises that an
octopus rig is not a preferred option if the depth is greater than 30m or the water
temperature is less than 10°C, instead an alternative fully independent gas
supply is advised.
• Contents/pressure gauge
Divers need to know how much gas is in their cylinder at all times throughout
their dive, so a contents gauge is connected via a hose to the rst stage. This
has to be a high-pressure hose as it delivers high-pressure gas for the gauge
to “read”. This contents gauge is sometimes mounted in a console that also
includes a depth gauge and possibly a compass.
Types
• Cold water (less than 10°C)
Many entry-level regulators are designed for water at a temperature above
10°C. EN250 (2014) compliant regulator rst stages which are not designed for
cold-water performance are marked with “>10°C”. Regulator rst stages may
be marked with a lower working temperature if specied by the manufacturer.
• Nitrox
Regulators used with nitrox should be in oxygen service. A regulator is in
oxygen service when all its internal components are compatible with high
oxygen levels and have been cleansed of hydrocarbons.
Care
• Rinse in fresh water
Regulators should be rinsed in fresh water after use. It is good practice to rinse
regulators while still attached to a cylinder and pressurised.
Warn about not pressing the purge button while rinsing a regulator that is not
under pressure, as water can enter the hoses and rst stage. Also the rst stage
should not be submerged in water unless sealed with an adequate cover; some
rst stage covers are only dustproof and not watertight.
• Dry before storing
Regulators should be dry before storing and should be stored without placing
strain on the hoses.

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• Service to manufacturer recommendations
Regulators should be professionally serviced at least once a year according to
the manufacturer’s recommendations. Remember they are a diver’s ‘life line’.
Problems with the gas supply, or a lack of it, are not conducive to happy diving.
Weights
The weights that divers need to take with them to
maintain control of buoyancy can be of several types/
congurations.
Types
• Weight belts
A simple belt with quick release buckle onto which cast weights are threaded.
Weight blocks come in various sizes and can be plastic coated. Straightforward
to release and drop/dump in an emergency
• Weight harness
Where a diver has a problem with a simple weight belt; perhaps due to
it slipping from the waist past the hips or where the weight on the hips is
uncomfortable a harness system may be used instead. This harness is usually
donned rst so is harder to jettison in an emergency unless the weights are
secured in pockets/sleeves that can be removed from the harness.
• Integrated weights
Some BCs have weight pockets integrated into the harness/sides of the device.
These can be removed for emergency jettison. Integrated-weight BCs often do
not have sufcient capacity to take the amount of lead needed for drysuit diving.
Quick-release mechanisms
All weight systems must have a way to easily jettison weights in an emergency.
However, it is also important that weight is not lost unintentionally as this would
create a dangerous positive-buoyancy situation. Release mechanisms vary
between a simple fold-over buckle, through Velcro sealed pockets to release clips.
Trim
Weight can be placed at different places on a diver to help improve their trim in the
water. For example, weights may be stowed in small pockets towards the top of a
BC or threaded on the cam-band attaching the BC to the cylinder.

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Care
• Wash in fresh water
Weight belts, harnesses and pockets should be washed in fresh water after the
dive.
• Dry before storing
As with all equipment, they should be allowed to dry before being stored.
Using scuba equipment
As scuba equipment delivers breathing gas at ambient
pressure, when divers ascend, regardless of depth,
they must never hold their breath. To do so would mean
that the breathing gas volume increases in the lungs
and, unless they breathe out, could cause serious lung
damage. It is very important that divers breathe normally
on ascent to equalise pressure in the lungs with that of the surrounding water. If
snorkellers were to dive down and take breathing gas from a diver, they would
increase their lung volume and would need to breathe out on ascent to avoid lung
damage.
Warning
• Scuba delivers breathing gas at ambient pressure
• Breathe normally at all times
• Or gas in lungs will expand on ascent resulting in damage
• Breathe normally on ascent
Buoyancy
It was a Greek philosopher, Archimedes, who worked out
the basic principle that allows things to oat or sink.
Water displacement
• Occurs when an object is placed in water
When an object is placed in water, its weight pushes down moving the water out
of the way.

Instructor Manual | Ocean Diver | Adapting to the underwater world
Copyright © BSAC 2017 20
• Creates an upwards force equal to the weight of water displaced
The water pushes back in an upward force (upthrust) equal to the weight
of water displaced by the object. Any object immersed in water therefore
apparently loses weight. Lifting someone out of a swimming pool is easy while
most of their body is still in the water; the more they are lifted out of the water
the heavier they become.
• Density is relevant
• Objects sink when they are heavier than water displaced
An object will sink if it weighs more than the upthrust - it is heavier than the
water it displaces
• Objects oat when they are lighter than water displaced
An object will oat if it weighs less than the upthrust – it is lighter than the water
it displaces.
As an example: A solid ball of modelling clay will sink in a bowl of water – it is
heavier than the water it displaces. (The water displacement makes the level of
water rise.)
If the ball is reshaped into a boat, the volume of the object has increased.
The boat and the air that it contains displace a greater volume of water
so the upthrust has been increased and the boat oats. (With the greater
displacement, the level of water in the bowl is higher than with the submerged
ball.)
Steel ships weighing thousands of tonnes oat because of their large
‘displacement’ and they contain a large volume of air.
Buoyancy and divers
The only times divers want to be able to oat on the
surface are at the beginning and at the end of the dive. It
is underwater where we need to control our buoyancy.
Neutral buoyancy
This is a state when a diver is in equilibrium with the surrounding water.
• Weightless state
It is a weightless state, which reduces physical effort. A diver’s buoyancy needs
adjustment throughout the dive to maintain neutral buoyancy – to be able to
hover above, below or to the side of interesting marine features.
• Reduces physical effort
Happiness is neutral buoyancy.