Open Water 20 - Physics

Open Water 20 - Physics

www.diveraid.com PHYSICS Open Water 20 - Physics INTRODUCTION... 2 SECTION OBJECTIVES... 2 BUOYANCY... 2 PRESSURE, VOLUME AND DENSITY RELATIONSHIPS... 5 GAS MATTERS... 12 PARTIAL PRESSURE OF A GAS... 13 DALTON S LAW... 15 CONTAMINATED AIR... 16 VISION... 16 COLOUR... 17 SOUND... 18 HEAT LOSS IN WATER... 18 Section 4 - Page 1

RAID OPEN WATER 20 www.diveraid.com INTRODUCTION The physical world can have a very significant impact on the way a diver moves, sees, hears and feels underwater. Additionally, there are physiological effects that will be discussed further here. Understanding the physics that causes these impacts is critical to the safety and enjoyment of diving. Some examples: Water density causes drag and effectively decreases your ability to move freely. Colours appear different because some of them are absorbed by water. Water is a much better conductor of heat than air. As a result, we lose heat much more quickly in water. Most importantly, at sea level, water weighs about 800 times more than air. Therefore, as you descend underwater, you are quickly subjected to pressure. This greatly affects the gasses inside the human body. You do not need to understand every part of every law or equation, but you must understand what it all means to you and your body when you are underwater. RAID Note: This section is a little involved and we encourage you to talk to a dive professional at your RAID Dive Centre if you need help. SECTION OBJECTIVES Explain some of the physical effects on divers underwater Describe the states of buoyancy Explain pressure, volume and density relationships Describe the effects on light and sound underwater Explain how to calculate surface air consumption rates (SAC) BUOYANCY The dictionary describes buoyancy, as far as it applies to being in or underwater, as the tendency or capacity to remain afloat in a liquid. Section 4 - Page 2

www.diveraid.com PHYSICS That phenomenon is based on something first discovered by the ancient Greek scientist Archimedes. He found that an object which is partly or fully immersed in water is pushed up by a force equal to the weight of the fluid displaced. That, obviously, is of significant importance to divers. Divers break this down into three types of buoyancy: Positive Buoyancy The object displaces more water than it weighs. Generally this would mean the object is floating on the surface. If the object were held underwater and if it were to be released, it would head straight for the surface. Divers use this positive buoyancy by inflating their buoyancy compensation device or BCD while on the surface to keep them floating, saving energy by not fighting to keep afloat. Neutral Buoyancy The diver s goal while underwater is to neither sink nor float. The amount of water displaced is equal to the body weight and equipment used. Neutral buoyancy is important for swimming over a reef without touching or damaging corals and to be able to hold a depth at 5m / 15ft while completing a safety stop. This feeling of weightlessness is a fun part of the dive experience. This skill takes some time to master. Negative Buoyancy This is when the object sinks. The weight of the object is more than the water displaced by it. The diver can achieve this by deflating the BCD and sinking. Positive Buoyancy Neutral Buoyancy Negative Buoyancy Section 4 - Page 3

RAID OPEN WATER 20 www.diveraid.com How Do You Control Your Buoyancy? Use lead weight to compensate for the effects of the exposure protection worn in water. The thicker the wetsuit, the greater the positive buoyancy. Next, while swimming underwater you can displace more water or less water by inflating or deflating the air in your BCD. Thirdly, when using open circuit scuba, the diver s lung volume will affect buoyancy. This allows you to achieve positive buoyancy at the surface and neutral buoyancy while underwater. To correctly weight yourself, you need to adjust your weights before you go for a dive, or any time you change part of the equipment you dive with, especially thermal protection, and when you move between fresh water and salt water dive sites. Generally, adjust the weights that you carry, so that with an empty BCD and your equipment, you float with the top of your head breaking surface, while holding a full breath of air. Make sure your cylinders have only 30 bar/500psi of pressure when adjusting your weight. This simulates a near empty cylinder much like at the end of a dive. This is important because your cylinder has less negative buoyancy at the end of the dive, as the air has been used, so you need to adjust weight for this scenario. Otherwise you will float to the surface while trying to execute a safety stop at the end of the dive. When you have established the correct weighting, simply dump the air from the BCD and breathe out through your open circuit regulator. Spend some time finding your correct weighting and the rewards will be noticeable. Over weighting yourself can be dangerous and make it hard to control your buoyancy correctly. Your RAID Advanced 35 or Explorer 30 program builds on this skill, by fine tuning your buoyancy. Buoyancy will be affected by diving in salt or fresh water as they have a different density and thus different buoyancy characteristics. The Dead Sea is so dense that people can float easily without needing to swim, while fresh water is less dense and to stay afloat you need to swim. Section 4 - Page 4

www.diveraid.com PHYSICS Seawater is heavier than fresh water by 2.5 3% because of the greater amount of salts dissolved in it. When moving from fresh water to salt water a diver has to ADD weight. The amount is not a precise calculation as it depends on factors other than just the buoyancy effects of salt water. The one factor which is difficult to judge is lung capacity due to the anxiety a diver may have going into a new environment for the first time. REDUCE weight when moving from salt to fresh water. Fresh water is less dense and therefore has less upward force and less buoyancy, resulting in less weight to be carried by the diver. It is recommended to complete a buoyancy check before diving in a new environment - don t waste a dive because you didn t sort out your weighting! As you gain more experience through further training and diving, you will find it much easier to redistribute your weight to achieve optimum buoyancy and trim. This three dimensional sensation is all part of the excitement of diving, being able to move up and down in any direction, or just float, in itself is relaxing and a good stress relief. PRESSURE, VOLUME AND DENSITY RELATIONSHIPS Pressure is the first and most important factor that must be overcome for divers to be able to breathe underwater. The pressure on you underwater consists of two forces which act together: the weight of the water and the weight of the atmosphere over the surface of the water. You don t feel much pressure underwater because you are made up mostly of liquids that do not compress. However, they are affected where pressure is exerting a force on any gas cavities. Section 4 - Page 5

RAID OPEN WATER 20 www.diveraid.com Pressure can be expressed in pounds per square inch, or Newton s per square metre. Pressure of the atmosphere at sea level can hold up 760 mm of mercury in a vacuum tube. So at sea level the pressure is 1 Atmosphere (ata) which is equal to 1 bar or 14.7 psi or 760 mm Hg. You are used to living at 1 atmosphere (ata) of pressure so you rarely take notice of it. You do notice changes of pressure in your ears when swimming down, flying, driving up in the mountains, going in a lift etc. This is because your ears/sinuses have air spaces in them and therefore they are compressible. The gas will compress proportionately to the amount of pressure exerted on it. According to Boyle s Law, gases can be compressed into a smaller volume. This is what happens when your cylinders are filled, and the air is squeezed together and is taking up less space which increases the pressure inside the cylinder. The problem comes when we look at your lungs, they do not like to expand and contract very much, so the volume of the lung remains much the same. When you take a full breath of compressed air at 10m / 33ft, or 2 bar you will breathe twice as much air as you do at the surface, due to the increased pressure, but your lungs will remain the same size/volume as normal. As absolute pressure increases on a gas (with descent), the volume of gas in your lungs will decrease proportionately. For example, descend to 10m / 33ft the pressure is twice as that on the surface, so the volume of gas would be half. AIR AIR Your lungs cannot work with this, so we need to breathe more air per breath to maintain the volume. Left: A breath hold divers lungs on descent will compress and then expand again on ascent. Right: A scuba diver breathes more air on descent to keep the lungs full but must exhale on ascent to let the expanding gas escape. Section 4 - Page 6

www.diveraid.com PHYSICS Table 1 The table below shows the balance between Gas volume and Gas density. As the pressure increases with depth so the volume will decrease and the density will increase. This is what happens with any gas space in your body and equipment, for example your ears, sinuses, mask and lungs. Depth Pressure Gas Volume Gas Density Example Sea Level 1 bar/ata/14.7 psi 1 x 1 10m / 33ft 2 bar/ata 1/2 x 2 20m / 66ft 3 bar/ata 1/3 x 3 30m / 99ft 4 bar/ata 1/4 x 4 Table 2 The table below shows that even though the pressure increases, the lung volume remains the same. To do this, the amount of litres of gas you inhale has to increase substantially due to the increased density of inhaled gas. Depth Pressure Lung Volume Litres of Gas inhaled to fill lungs Sea Level 1 bar/ata 4L 4L 10m / 33ft 2 bar/ata 4L 8L 20m / 66ft 3 bar/ata 4L 12L 30m / 99ft 4 bar/ata 4L 16L As absolute pressure decreases on a gas with ASCENT, the volume will increase proportionately. So the volume of gas will expand. Section 4 - Page 7

RAID OPEN WATER 20 www.diveraid.com If a diver should hold their breath at 10m / 33ft and ascend with 8 litres of compressed gas inside the lungs (Table 1), which can only hold 4 litres, the expanding gas would cause serious damage to the lungs.! NEVER HOLD YOUR BREATH WHEN SCUBA DIVING Have you noticed anything? The biggest pressure and gas volume change is between 10m / 33ft and the surface. The volume of gas doubles on ascent or halves on descent. In only a couple of metres/a few feet - the pressure differential is large. Because of this rapid change in just 10m / 33ft, this is the area where you have to be the most cautious, especially with regard to barotraumas (holding your breath). RAID Note: Remember, move up and down slowly, and do your safety stop on ascent. Practical pressure change examples for scuba diving: Mask Squeeze - as we descend the volume of gas inside our mask is decreasing due to increasing pressure. Add gas to the mask to make the volume consistent by blowing out enough gas to equalise through your nose. Descending - as we descend we add gas to the BCD to control our descent. Our exposure suit has gas in it either in the material (neoprene - in the form of bubbles for a wetsuit) or in the suit itself for a drysuit. The gas in the exposure suit will compress making you more negative. You will master how to deal with this and achieve neutral buoyancy using your BCD in your confined water session. Ascending - the opposite happens due to the decreasing pressure on our ascent. Your exposure suit and BCD will both have expanding gas and your will need to vent it from the BCD to control your ascent. It is important that you should never ascend faster then 9m / 30ft per min. Why slow ascent rates are important will be explained in detail in the next chapter. Section 4 - Page 8

www.diveraid.com PHYSICS Ear equalisation is affected on your descent. Volume of gas spaces in the middle ear will reduce and this needs to be equalised just like when you fly in an aircraft and feel the pressure in your ears. This will be explained in the Physiology section. Our lungs - as previously explained these need to be equalised by continuous breathing or we can possibly cause injury. Remember to never hold your breath on SCUBA and continuously breath. Key points on pressure changes: As pressure increases, volume decreases and visa versa. This is why a diver adds air to his BCD, mask and ears as he descends and lets air out of his BCD when he ascends. As pressure increases, gas density increases. This is why we breathe more at depth and we need to inhale more air into our lungs as we descend to keep their size consistent. Remember: At depth the gas density increases and you will consume more gas. Monitor your instruments frequently. Divers will have longer dive times the shallower they are. Definition of Absolute Pressure (bar/ata): The entire or total pressure. For example the atmospheric pressure (bar/atm) plus the water pressure. For recreational divers bar and atm are equal. Definition of Gauge Pressure (g): Is a measurement that ignores the atmospheric pressure (atm, pressure of the air pushing down). At sea level with no added pressure, gauge pressure is zero, and underwater your submersible pressure gauge will show 10m / 33ft at a depth of 10m / 33ft. What is the gauge and absolute pressures at a depth of 12m / 39ft in salt water? Metric Gauge pressure (g) 1.20 atmospheres gauge and it will read 12 m Absolute pressure: 1.20 bar + 1 atm = 2.20 bar/ata. Section 4 - Page 9

RAID OPEN WATER 20 www.diveraid.com Imperial Gauge pressure (g) 1.20 atmospheres gauge and it will read 39 feet Absolute pressure: 1.20 bar + 1 atm = 2.20 bar/ata To calculate absolute pressure at depth in Salt water, use the following formula: Metric Pressure (ata) = Depth (msw) + 1 atm 10 So the pressure at 15 msw is: Ata = 15 + 1 atm 10 P = 2.5 bar/ata - Simple! Imperial Pressure (ata) = Depth (fsw) + 1 atm 33 So the pressure at 49 fsw is: Ata = 49 + 1 atm 33 Ata= 2.5 bar/ata Msw = metres of sea water & Fsw = feet of sea water Table 3 Salt Water Depth Gauge Pressure 6m / 20ft 15m / 49ft 20m / 66ft 25m / 82ft 32m / 105ft Pressure Absolute 1.6 bar/ata 2.5 bar/ata 3.0 bar/ata 3.5 bar/ata 4.2 bar/ata Note: If you did not have to add the atmospheric pressure then the pressure at 15m / 49ft would be 1.5 bar/ata. By adding the atmospheric pressure of 1 bar/atm you get 2.5 bar/ata. Section 4 - Page 10

www.diveraid.com PHYSICS Now you know that salt water is more dense (weighs more) than fresh water so to get the same pressure in fresh water as salt water you need to descend just a little more. This means you also need a little extra weight in salt water compared to fresh water. This will be discussed in more detail in the next level of training. While freediving, (with no scuba unit) on the surface, you take a breath at 1 bar/ata and hold it. While descending the increased water pressure compresses the air in your lungs. When you ascend, the air expands to the same volume you originally had, 1 bar/ata. You can hold your breath while freediving with no scuba unit. You are not breathing compressed air at depth. Scuba diving is very different. Underwater you breathe air at a pressure equal to the surrounding water pressure. Therefore your lungs will always be at their normal volume at any depth. Refer to Table 1. If you hold your breath on ascent, your lungs would over expand and likely rupture. Lung overexpansion can force air into the bloodstream and chest cavity which can lead to serious injuries, paralysis and even death. Lung injuries are very difficult to treat but very easy to avoid! So the most important rule in scuba diving is to never hold your breath and breathe continuously. How long can an Open Circuit Diver stay underwater with a cylinder? That depends on your breathing rate per minute (RMV - Respiratory Minute Volume) at depth. (We always revert this back to surface air consumption or (SAC). When diving open circuit, your regulator supplies your air equal to the surrounding pressure. The deeper you go the more air you consume, per breath. For example: at 20m / 66ft / 3 bar you breathe 3 times as much to fill your lungs to normal volume. Therefore, your air supply only lasts one third at 20m / 66ft / 3 bar as at the surface. Simply put, if a cylinder lasts 60 minutes on the surface it will only last 20 minutes at 20m / 66ft. Section 4 - Page 11

RAID OPEN WATER 20 www.diveraid.com The management manual explains this in detail and shows you how to work out the duration of your scuba cylinder for a given dive profile. GAS MATTERS When it comes to diving, the behaviour of gasses at the pressures encountered underwater becomes a very serious matter. If not properly understood and respected, they can become a matter of life and death. The major issue here is that gasses have a very low density compared to water. For all practical purposes, water does not compress enough to matter for diving, even at the bottom of the ocean. We assume it is not compressible for all calculations. Gas, on the other hand, can easily be compressed, and things happen when it does. That is why it is essential to have a basic understanding of the laws that govern the behaviour of gas. What Are We Concerned About? Is it just air in general, or the different components of air? Both: What we call air actually consists of a number of gasses, but two make up almost all of it: oxygen and nitrogen. The First Gas is Oxygen 8 O 15.99 Scientists know it as a chemical element in the periodic table that lists all chemical elements known to man. It is known by the symbol O and carries the atomic number 8. Oxygen is the second most common element on earth and makes up about 21% of the air we breathe. Oxygen easily bonds with other elements and thus carries nutrients around our body and enables life. Oxygen is tremendously useful element. Of course, it s also the reason why things rust, and too much oxygen can be poisonous for your body. We ll get to that later. Section 4 - Page 12

www.diveraid.com PHYSICS The Second Gas is Nitrogen N 14.01 7 Another element in the periodic table, carrying the letter N and the atomic number 7. Nitrogen is by far the largest component of your air, 78%. In the human body it doesn t really do anything. Above water, whatever nitrogen you breathe in, you also breathe out. Some nitrogen gets absorbed by our bodies. The remaining 1%of gas in the air consists of a variety of elements or compounds such as Argon, Neon, Helium, Methane, Krypton, Hydrogen, Carbon dioxide and others. With the exception of Carbon dioxide, these trace gasses do not matter when you re diving. Oxygen and nitrogen, however, do matter. Oxygen is needed to live and Nitrogen, because it can harm your body, must be accounted for. The effects of oxygen and nitrogen are, in fact, the primary reason why divers must know about the physics of gasses. PARTIAL PRESSURE OF A GAS What is Partial Pressure (Pp)? Pressure is made up of the sum/total of partial pressures. For example, the pressure of air/gas at sea level is 1 bar/ata and it is made up of 0.21 oxygen plus 0.79 nitrogen (78% N 2 +1% inert gas). So the partial pressure of oxygen is 0.21 Why is Partial Pressure Important? Open circuit works with a fixed fraction of oxygen and the partial pressure changes with depth. For example we have 21% oxygen in our air which is affected by pressure at depth. Partial pressure is important to know when diving a rebreather or open circuit Nitrox which are both covered in further RAID programs. The air we breathe is made up of a mixture of gasses, mainly oxygen and nitrogen. Each gas s partial pressure in a mixture adds up to the total pressure of the gas. The fraction of oxygen at sea level is 0.21 and Nitrogen is 0.79. Section 4 - Page 13

RAID OPEN WATER 20 www.diveraid.com Open Circuit SCUBA Table 4 Depth Pressure Oxygen % Oxygen Pp Nitrogen % Nitrogen Pp Total Pressure Sea Level 1 bar/ata 21 % 0.21 79 % 0.79 1 bar/ata 10m / 33ft 2 bar/ata 21 % 0.42 79 % 1.58 2 bar/ata 20m / 66ft 3 bar/ata 21 % 0.63 79 % 2.37 3 bar/ata 30m / 99ft 4 bar/ata 21 % 0.84 79 % 3.16 4 bar/ata 40m / 132ft 5 bar/ata 21 % 1.05 79 % 3.95 5 bar/ata The table below shows that with a fixed gas percentage (%), the partial pressures (Pp) will change in open circuit diving. This calculation is very simple all you need is a calculator and to refer to Table 4. What is the Total Pp at sea level? Answer: 0.21 + 0.79 = 1 bar What is the Pp of oxygen at 20m / 66ft / 3 bar? The fraction of oxygen = 0.63 How do you get 0.63? Answer: All you need to do is a quick calculation which is 0.21 oxygen (at 1 bar/sea level) x 3 (3 bar) = 0.63 Questions (using normal air): a. What is the Pp of oxygen at 30m / 99ft? b. What is the Pp of nitrogen at 40m / 132ft? c. What is the total PpO 2 and PpN at 20m / 66ft? Answers: a. 0.21 x 4 = 0.84 PpO 2 b. 0.79 x 5 = 3.95 PpN c. 0.63 + 2.37 = 3 Bar Table 4 above is with reference to Open Circuit SCUBA, air is a fixed fraction of gas in the cylinder which cannot change while underwater. You will notice the percentage of gas (fraction) does not change. If the percentage of gas remains the same, then the partial pressure will change. This has some advantages, although there are more disadvantages and you can read more about this under the Physiology section. Section 4 - Page 14

www.diveraid.com PHYSICS The Total Pressure exerted by a mixture of gases is the Sum of the pressures that would be exerted by each gas if it were alone, present and occupying the entire space P (total) = P1+P2+P3.Pn DALTON S LAW As we said before when using open circuit scuba the fraction of gas remains the same and the partial pressure of gas changes with depth. So what about the nitrogen? Well as we descend and ascend diving open circuit nitrogen pressure change is proportional to the oxygen pressure change. But what does this mean to the diver? In the Physiology section you will learn that the less nitrogen we are exposed to, the less sub-clinical decompression stress to the body. Maximum Open Circuit Depths MAXIMUM DEPTH OF 20M / 66FT IS MANDATED CERTIFICATION FOR THIS PROGRAM The maximum depth advanced non decompression divers will go to is 40m / 130ft. Remember you need to do RAID Deep 40 and RAID Advanced 35 for these depths. At 40m / 130ft there is little time to do anything as our nitrogen loading is building up fast. Have a look at your computer and see what the planned time for 40m / 130ft is. Time here is so minimal at these depths that some divers go on to technical or decompression diving to allow greater times. There are certain benefits to decompression diving but this is a subject of further high level training. Your instructor can explain the pathway if you desire to go down this track. For now we are concentrating on your first program and the exciting things you can see when diving less than 20m / 66ft. The great benefit is that we have much more time to relax and enjoy what we are seeing. Section 4 - Page 15

RAID OPEN WATER 20 www.diveraid.com CONTAMINATED AIR Scuba cylinders are filled by compressors that use special filters to prevent contamination; for example, carbon monoxide and oil vapour. This is important because as you descend, pressure proportionately increases the effects of the gas you breathe, so traces of contamination can be toxic underwater. This usually tastes and smells bad, but can also be tasteless and odourless. A diver affected by this may experience headaches, nausea, and dizziness. Excess carbon monoxide will produce signs such as cherry-red lips and fingernail beds. In severe cases, seek medical attention. This is very rare, but make sure you purchase your air fills from reputable and professional dive stores. VISION Why Do You Need A Mask? Your eyes are designed for an air environment and focus depending on how the light enters the eye. When underwater, if you didn t have the air gap that the mask provides, the light would enter your eyes differently, and as a result, things would be out of focus. Section 4 - Page 16 You see differently underwater, and the reason, once again, is physics. The diving mask itself, with its flat glass lens or lenses, bends light (refracts) so that objects underwater are magnified by about a third. We say about because the magnification factor depends both on how far the lenses are from your eyes, how far away an object is and the turbidity of the water. For some reason, inexperienced divers tend to underestimate the distance of close objects, and overestimate the distance to objects further away. Take the example of a pencil in a glass of water and see how the pencil is distorted.

www.diveraid.com PHYSICS COLOUR Colours look different underwater as well. Colours are really nothing more than different light wavelengths reflected by an object. Underwater, waves travel differently, and some wavelengths are filtered out by water sooner than others. Lower energy waves are absorbed first, so red disappears first, then orange, then yellow. Green stays longer and blues the longest, which is why things look blue the deeper you go - as long as the water is clear, that is. Typical Conditions 5m / 17ft 8m / 24ft 15m / 50ft 25m / 85ft 35m / 116ft Colours at Depth Perfect Conditions 10m / 33ft 20m / 66ft 35m / 110ft 45m / 150ft 100m / 330ft In murky turbid water there is less light penetration and things tend to look greenish-yellow. If a diver cuts himself, the blood will seem green or brown. Even under the best conditions colours will fade the deeper you dive. The way colours are absorbed has an impact on underwater photography: a good light source is needed for vibrant images. Photographers often use a red filter for deep natural light images which helps remove some of the predominant blue. But wait, there s more. Or rather, less. The water surface is reflective and can act a little like a mirror. At high noon, with the sun being right overhead, almost all of the sun s light enters the water. However, early and late in the day when the sun s rays hit the water at shallow angles, almost all light is reflected away and doesn t make it into the water. So be aware that it can get dark sooner than you think. Poor light source Good light source Section 4 - Page 17

RAID OPEN WATER 20 www.diveraid.com SOUND Sound travels much faster in water than in the air, about four times as fast. Sound also travels further underwater. Whales use this to communicate with each other over great distances, and some shrimps use sound to hunt. This means that your hearing, which is designed to interpret sound in the air, can easily get confused underwater. On land, your brain detects the source, and to some extent the distance, of a sound by the difference between its arrival in the right and left ear. When underwater sound is so much faster, your brain cannot process direction, so you think the sound is directly above you! It is the best surround sound you ll ever hear. The continuous sound of the exhalation of gas of an open circuit diver is loud and it reduces the diver s ability to hear other sounds underwater. HEAT LOSS IN WATER Have you ever noticed that you feel perfectly comfortable walking around in 20 C/68 F, but water of the same temperature feels quite cold? It is because water is a much better conductor of heat than air. When diving, the water that comes in contact with your body warms up, expands, and quickly carries the heat away from you. As a result, you feel cold in no time. In fact, water removes heat from your body about 20 times faster than air. Another reason for greater heat loss when diving is breathing on open circuit. Breathing on open circuit generally accounts for about a quarter of your body s heat loss in the form of cold air inhaled, and after warming it up, the warm air is exhaled. The inhaled gas from a regulator can be as much as 15 C to 20 C/59 F to 68 F colder than the surrounding water temperature. So a diver cools from the inside out on every open circuit exhalation. Temperature is important to consider when immersed. Even in warm water, a small drop in body temperature can be serious. Divers wear exposure suits to reduce heat loss. Diving suits are either sealed dry suits that do not allow any water in, or wet suits where some water gets in. Section 4 - Page 18

www.diveraid.com PHYSICS Unfortunately, underwater physics throws divers another problem: the increasing pressure compresses the insulating gas bubbles inside the exposure suit, making it less effective the deeper you go. That s why there are recommendations as to what type and thickness of wetsuit you should wear in different water temperatures. Consult the Dive Centre in the area you are planning to dive regarding what type of protection that is best for you. Section 4 - Page 19

RAID OPEN WATER 20 www.diveraid.com LICENSE AGREEMENT End User License Agreement (Non-transferable) and Limited Warranty. Read carefully before using the License Agreement and Limited Warranty. Your use of this product must be within strict accordance of the License Agreement. This means you may not copy this program for any purposes other than to maintain a backup copy for your own personal use. It is against the License Agreement to sell, give, or lend this program, or a copy of it, to a third party. Copyright 2007 Revised 2016 This document is the property of Rebreather Association of International Divers AB, Sweden. All rights reserved. ACKNOWLEDGEMENTS Primary Author: Barry Coleman, Celia Coleman Contributing Editors: Paul Toomer, Barry Coleman, Mark McCrum, Jim Holliday, Karen White, Steve Bates, Alex Woodhams, Gary Hawkes, Chris Haslam, Lisa Toomer, Watson Devore, James Rogers Images: Stephen Frink, Mark McCrum, Jacques De Vos, Barry Coleman, Paul Toomer, Kristen Goldsworthy, David Molina Grande Graphics: Kristen Goldsworthy Version 2.04 Section 4 - Page 20