Open Water 20 - Physiology

Transcription

1 Open Water 20 - Physiology

2 PHYSIOLOGY Open Water 20 - Physiology INTRODUCTION... 2 SECTION OBJECTIVES... 2 RESPIRATION AND THE CIRCULATORY SYSTEM... 3 BREATHING STIMULUS... 3 TERMINOLOGY... 4 HYPERCAPNIA... 4 SHALLOW WATER BLACKOUT... 5 OXYGEN... 5 WHAT IS HYPEROXIA?... 6 OVEREXERTION... 7 OVERHEATING AND HYPOTHERMIA... 7 PRESSURE RELATED PROBLEMS BAROTRAUMA... 8 NITROGEN... 8 DECOMPRESSION SICKNESS (DCS)... 9 ARTERIAL GAS EMBOLISM EAR AND SINUS SPACES EQUALISATION DIVE COMPUTER RULES DIVING AFTER MEDICAL TREATMENT Section 5 - Page 1

3 RAID OPEN WATER 20 INTRODUCTION Physiology is defined as the biological study of the functions of living organisms and their parts. As a qualified diver, you need to understand how being underwater affects your body. If you have any questions regarding any points that you feel may affect you, then seek professional medical advice from a registered Diving Doctor. Eat healthy The Physics section covers buoyancy, decompression sickness, and most importantly, the impact of increasing water pressure on gasses. You will read about the effects these physics phenomena can have on your body. SECTION OBJECTIVES Describe the respiratory and circulatory systems in our body Describe the effects of various gases we breathe and exhale Define the effects of temperature change on our body Describe effects of hypothermia and overheating Explain pressure related effects on the diver Explain the terms nitrogen narcosis and decompression sickness What Does Diving Physiology Entail? The knowledge of how water pressure impacts your body, what can happen, why it can happen, how to recognise any adverse symptoms, and what to do in case of an accident. It also includes things that can happen before you go underwater. Health for Diving Even though diving is said to be 60% mental and 40% physical, there may be times when the diver needs to exert high physical exercise; for example, to swim against a current or back to the beach after a dive. So a certain amount of physical strength and a good cardiovascular health is required. This does not mean that you have to be an Olympic athlete, but at the same time being overweight and short of breath from a short walk, will not be good for diving. So start keeping yourself healthy and work on your cardio, if you are not doing so already. Section 5 - Page 2

4 PHYSIOLOGY RESPIRATION AND THE CIRCULATORY SYSTEM The cardiovascular / circulatory system and respiratory system are two systems in the human body that are extremely vital: they continuously provide the body with a flow of nutrients and oxygen and take care of the removal of waste products. Any break down to these systems result in permanent damage to your body within minutes. These two body systems are the most influenced by the underwater environment. Every part of the human body constantly needs oxygen. Oxygen is needed for what is called the oxidative metabolism: the oxidation of nutrients in order to supply energy required for living. All body cells need oxygen. The respiratory system takes care of bringing oxygen into the body and removing carbon dioxide from the body via the lungs. The cardiovascular system takes care of the circulation of blood and transport of oxygen and carbon dioxide. It mainly consists of the heart, arteries, veins and capillaries. BREATHING STIMULUS When the reflex respiratory centres in the brain detect high carbon dioxide levels in the blood, they stimulate breathing. So carbon dioxide is one of the main stimuli for breathing. Section 5 - Page 3

5 RAID OPEN WATER 20 RAID Note: Did You Know? For every litre of oxygen metabolised, about 90% is converted into carbon dioxide At rest, the average breathing rate is breaths per minute. Due to a demand of oxygen by the body as a result of exercise and/or anxiety, the levels of carbon dioxide will increase and the breathing rate will increase in relation. As the carbon dioxide levels reduce, (at rest) the breathing rate will return to normal. If oxygen levels are too low but the respiratory centres do not detect overabundance of carbon dioxide, it is possible that breathing is not stimulated. This is what happens in case of shallow water black out, (mostly breath-hold diving). When diving, the high oxygen partial pressure level is more than enough for the body to function, but as the partial pressure of oxygen drops on ascent, possible symptoms of feeling light headed, dizzy, nauseous, and/or headache might appear. In extreme cases there may be loss of consciousness. The simple solution to this problem is to take three or four depth full breaths and flush before ascent. TERMINOLOGY Hypercapnia - too much carbon dioxide Hyperoxia - too much oxygen Hyperthermia - too hot Hypocapnia - too little carbon dioxide Hypoxia - too little oxygen Hypothermia - too cold Hyper means too much Hypo means too little HYPERCAPNIA This potentially fatal condition for divers can be caused by an excessive build-up of carbon dioxide in the respiratory and circulatory system. Elevated levels of carbon dioxide lead to headache, confusion and eventually to loss of consciousness. Section 5 - Page 4

6 PHYSIOLOGY Hypercapnia is sometimes associated with full-face masks, rebreathers and skip-breathing (open circuit). It is not very common in recreational open circuit diving, although it may still occur, especially when skip-breathing or hyperventilating at depth. Therefore we recommend taking two or three slow, deep breaths before ascending, and keeping a close watch on your buddy. SHALLOW WATER BLACKOUT This refers to a diver who is not using scuba equipment- that is free diving or snorkelling. It is the opposite effect of hypercapnia. Excessive hyperventilation (more than 5 times of deep inhaling and exhaling) before breath hold diving may lead to shallow water blackout. Three to four deep full breaths is enough for most people. When a diver hyperventilates excessively before a breath hold dive, carbon dioxide levels will become extremely low. Since it is the carbon dioxide level which stimulates breathing (and not low oxygen levels), this stimulus will not be present, even when oxygen levels become too low. At depth, the increased oxygen partial pressure in the lungs of the diver allows the respiratory system to receive oxygen. However, when the diver makes an ascent (or goes to shallow depths), the partial oxygen pressure in his lungs will drop. As a result, the lungs no longer get oxygen from the air, leading to blacking out and possibly drowning. If you feel light headed, nausea or even a headache you should stop and rest. Get your breathing back to normal before deciding to continue. 8 O OXYGEN Oxygen is required to sustain life and the body requires a balance of the gas to operate within. With oxygen exposure, too much, too little or more than average over a long duration can cause serious problems. You learnt in the Physics Section about partial pressure. Here is a reminder: Air is made up of 21% oxygen and 79% nitrogen at sea level. So the partial pressure of oxygen at sea level is As we descend underwater the pressure increases, and so do the partial pressures. Section 5 - Page 5

7 RAID OPEN WATER 20 The respiratory and circulatory systems will operate within an oxygen partial pressure limit of 1.6 bar/ata (maximum) and 0.16 bar/ata (minimum). Going above and or below these parameters is dangerous. WHAT IS HYPEROXIA? Hyperoxia is when the oxygen partial pressure exceeds 1.6 (Recreational Diving Limit) and/or when you are exposed to elevated partial pressures of oxygen over an extended time. This exposure may lead to any number of the following symptoms: Convulsions Visual disturbance Ringing in the ears Muscle twitching Dizziness The symptoms may be sudden or gradual. Generally open circuit air divers will be unlikely to experience this, our depth limits and our NDL will not allow us to build up a partial pressure of oxygen that could be harmful. For example with 21% oxygen in our normal air mix we would need to descend to around 8 atmospheres (ATA) of pressure to take us over the partial pressure of oxygen limit. 21% = 0.21 ATA partial pressure of oxygen at the surface. -- Examples: 40m / 130ft dive we are now at 5 ATA of pressure. 5 x 0.21 = 1.05 ATA -- 60m / 200ft dive we are now at 7 ATA of pressure. 7 x 0.21 = 1.47 ATA These examples are way over our limit of 20m / 66ft for this program but illustrate how the oxygen pressure increases with depth and how we can stay safe by not exceeding these limits. RAID Note: Your maximum depth for training will be 18m / 60ft and maximum recommended depth once certified RAID Open Water 20 Program is 20m / 66ft, the maximum recreational depth limit is 40m / 130ft once you receive the appropriate training and experience It is however possible with divers who use higher concentrations of oxygen in their breathing gas known as Nitrox diving to come closer to oxygen toxicity. Specialised training is required for Nitrox diving. Section 5 - Page 6

8 PHYSIOLOGY IMPORTANT: The oxygen partial pressure limit of 1.6 is reached or even passed if uncontrolled descents are not stopped. OVEREXERTION You can experience overexertion by swimming against a current, long distances or carrying excessive weight. You may feel tired, short of breath, weak, and have a tendency to cramp and feel panicky. What do you do? Stop, breathe and think about the situation. Work out a rational approach and implement it. For example: over exertion at the surface requires you to establish buoyancy, rest, catch your breath, signal for help and/or move slowly to the boat or shore. If over exertion occurs underwater Stop, breathe and think. OVERHEATING AND HYPOTHERMIA In an ideal world it would be wonderful to be neither hot nor cold under water. To achieve this wear exposure protection suitable to the conditions you ll be diving in. If you don t wear appropriate exposure protection, you could suffer from overheating (Hyperthermia) and/ or cold (Hypothermia). Just 2 C above or below the standard body temperature will lead to either of these conditions. Water will conduct heat away from your body 20 times faster than air. So what feels like a mild temperature out of the water can be an effective cold temperature in the water. Further, even with exposure protection, the length of submersion will eventually cool you down to the extent it becomes uncomfortable. As soon as this happens get out the water and warm up! Section 5 - Page 7

9 RAID OPEN WATER 20 Over-heating can also be a problem before diving. Your exposure suit should be put on after getting your dive gear ready, just before the actual dive, and taken straight off after exiting the water, because a suit limits the normal emission of heat in the air. Remember, if you are not feeling comfortable for the environmental conditions that you are in, advise your buddy/divemaster/instructor and take appropriate action to either cool down or in the case of cold, warm up. RAID Note: Exposure protection is one of the most important considerations in diving. Make sure you get good advice from your RAID dive centre and choose an exposure suit that is best for the environment you are diving in. PRESSURE RELATED PROBLEMS BAROTRAUMA Body air spaces like the lungs, ears and sinuses can suffer barotrauma (pressure injury) on descending or ascending. During descending or ascending the hydrostatic pressure changes. Trouble occurs if the diver fails to equalise the air space. On descending a barotrauma is called a squeeze. On ascending a barotrauma is called a reverse squeeze, reverse block or expansion injury. NITROGEN Nitrogen represents almost 79% of standard breathing air and has specific properties that can lead to two important dive maladies- Nitrogen Narcosis and Decompression Sickness. Nitrogen Narcosis Many experienced divers have felt the effects of what has been called Rapture of the Deep or getting the narks at around 30m / 100ft. Nitrogen narcosis is a fairly harmless phenomenon, if you are aware of its existence. The narcosis develops with an increasing partial pressure of Nitrogen. Signs of nitrogen narcosis can include a feeling of false security, poor judgment, becoming uncoordinated, having euphoric feelings, fixations, and anxiety; all similar symptoms to being drunk on alcohol. The onset of nitrogen narcosis varies from diver to diver. Inadequate hydration and the use of some drugs, amongst other things, can worsen symptoms. Section 5 - Page 8

10 PHYSIOLOGY What Do You Do When Nitrogen Narcosis Strikes? Unlike the effects of alcohol, nitrogen narcosis goes away as you ascend. So simply ascend until the feeling subsides, whilst keeping an eye on your buddy to make sure they are okay. As with getting drunk there is a funny side Nitrogen Narcosis should be taken seriously. Everyone reacts differently and it is important you are on the lookout for symptoms in yourself and others and always ascend if significant symptoms present. DECOMPRESSION SICKNESS (DCS) Additional gas will dissolve in body tissues under pressure. Oxygen is metabolised (used by the body) as you descend, nitrogen however gets absorbed into your body tissues as the surrounding pressure increases. When the tissues have absorbed gas equal to the surrounding water pressure it is called saturated. This state must be avoided. Tissues high in fat absorb a lot, other tissues less, and tissues that have a large blood flow will absorb and release gas more quickly than tissues with less blood flow, such as tendons, cartilage, or fat. While under pressure the increased gas circulating in the blood stream is contained but as you ascend the gas will need to be eliminated. If the pressure is released slowly, the nitrogen outgasses slowly and will safely travel to the lungs where you breathe it out. However, if the pressure drops too quickly, (rapid ascent) nitrogen forms larger bubbles in the blood and that can be harmful. While tiny bubbles, generated by slow, controlled ascent, are harmless, larger bubbles can get stuck and create blockages in your circulatory system. They may block blood circulation, and/or compress nerves causing pain. Signs and symptoms would vary from pain, to irritation, soreness and swelling. You will hear divers talk about the bends or having been bent. That s when nitrogen are bubbles blocking parts of the circulatory system, (small veins of joints, elbows, knees, shoulders) causing pain that a diver may seek to relieve by bending those joints. Even very experienced divers can get bent, so do take this very seriously. There is a wide range of symptoms from skin rash to articular pain and neurological problems, and they tend to come on gradually. Section 5 - Page 9

11 RAID OPEN WATER 20 The biggest problem is denial by the patient and delayed response. Regardless of the severity of the symptoms, consider all cases of decompression sickness serious. Treatment is simple: discontinue diving, administer 100% oxygen and seek professional medical attention as soon as possible. It is important to have the correct contact information for the closest diving emergency facility/re-compression chamber as treatment by such a facility will most likely be necessary. What is Decompression Illness? Decompression illness refers to both decompression sickness (DCS) and lung over-expansion injuries (AGE - arterial gas embolism). Although the term decompression illness is sometimes used interchangeable with decompression sickness there is a difference. DCS refers to the condition caused by inert gas coming out of solution within the body, and AGE is a pressure related injury/ problem. The reason for the singularity term is that in an emergency the first aid provider will follow the same procedural response Can DCS Be Avoided? Yes, in most cases, by following safe, proper diving procedures. While it is very difficult to exactly predict how much nitrogen is absorbed by the wide range of human bodies, there are models and algorithms that give fairly accurate approximations. Open circuit dive tables let divers determine how much nitrogen is absorbed, how long they can safely remain at a certain depth, and how quickly they may dive again. Dive computers do the same thing, only more quickly and more accurately and are recommended for RAID training. You will be trained to make a safety stop on every dive as a precaution to help avoid decompression sickness. A few precautions you can take to stay as safe as possible: Make sure you ascend slowly 9m / 30ft per minute or less Stay well hydrated before and after the dive Do not exceed you No Decompression Limits (NDL) Be more conservative with cold or strenuous dives Do not dive if you are unwell Section 5 - Page 10

12 PHYSIOLOGY ARTERIAL GAS EMBOLISM Alveoli Capillary O 2 O 2 CO 2 CO 2 As air in the diver s lungs expands on ascent (increasing volume) to keep the pressure equal to the surrounding pressure (Boyle s law), the expanded gas must be able to escape. This is achieved by continuous breathing or breathing out during an ascent. If the maximum volume of the lungs is reached by holding your breath on ascent, the lungs get over pressurised, and may start to tear/ rupture. Arterial Gas Embolism, abbreviated as AGE, occurs if air escapes through ruptures in lung alveoli into the bloodstream (pulmonary capillaries). Air can enter tissue or the bloodstream which may lead to serious injury or death. Symptoms are: Confusion Dizziness Disorientation Shock Paralysis Loss of consciousness Although these symptoms are comparable to those of decompression sickness, AGE symptoms are rapid and dramatic, whereas DCS symptoms are somewhat delayed. Symptoms occur during and immediately after surfacing. Over pressurising can easily happen: even ascending 1 to 1.5m / 1 to 3-5ft with full lungs can give rise to lung injuries. Especially in shallow water, the pressure drop (respiratory volume increase) per ascended 1m / 3ft is greatest as you get closer to the surface. RAID Note: NEVER HOLD YOUR BREATH while scuba diving and always blow out little bubbles when you are not breathing from the mouthpiece and/or regulator The early management of air embolism and decompression sickness is the same. Although a diver with severe DCS or an air embolism requires urgent recompression for definitive treatment, it is essential that they be stabilised at the nearest medical facility before transportation to a chamber. Section 5 - Page 11

13 RAID OPEN WATER 20 Early oxygen first aid is important and may reduce symptoms substantially, but this should not change the treatment plan. Symptoms of air embolism and serious decompression sickness often clear after initial oxygen breathing, but they may reappear later. Because of this, always contact Divers Alert Network (DAN), your countries diving emergency services or a dive physician in cases of suspected DCS or Air Embolism - even if the symptoms and signs appear to have resolved. EAR AND SINUS SPACES There are air pockets in your ears and sinuses. As you descend, water pressure squeezes the air in these spaces. Therefore you must equalise the pressure by the various methods taught by your instructor. Never dive with a cold or sinus infection as this will impair your ability to equalise and it may result in damage to your ears or sinuses. You can have blockages caused by mucous build up that can damage ears or lung alveoli. You also don t want to spread the cold to other divers. You may get a squeeze as you descend. A squeeze is a pressure imbalance in which pressure outside an air space exceeds pressure inside an air space, resulting in discomfort and pain. To avoid this you need to equalise. Although very rare, you may have airspaces in filled teeth where a filling has eroded. If this is a problem seek medical advice as this space cannot be equalised. Frontal sinus Sphenoid sinus Maxillary sinus Ethmoid sinus Nasal passage EQUALISATION It is very important to maintain equal pressure in your air spaces to the surrounding pressure. This is known as equalisation and involves adding air to these spaces as you descend. You will most likely already have experienced these changes in pressure at some point in your life. Whether going over a mountain, in a plane or the bottom of a swimming pool. Small changes in pressure may be managed by simply swallowing or wriggling the jaw. You will typically hear a pop or crackling sensation in your ears when the pressure equalises. Section 5 - Page 12

14 PHYSIOLOGY During a scuba dive pressure changes rapidly. The fastest change is in the first few feet/meters. Therefore it is very important to equalise early and often and approximately every 1m / 3ft on the way down. Mouth Hand Nasal Cavity Tongue The Valsalva Manoeuvre Opening to eustacian tubes Soft palate Glottis Trachea The easiest and best way to equalise for most people is called the Valsalva manoeuvre. This simply involves holding the nose closed and blowing gently. This process forces air into the Eustachian tubes and from there to equalise the space behind the eardrum this procedure simultaneously forces air into the sinus cavity to equalise these spaces. It is always a good idea to equalise before you enter the water. This helps open up the spaces and helps to ensure you can in fact equalise. Do not force or blow too hard as it may cause damage. If you feel discomfort or pain on descent, ascend about a 1m / 3ft and try equalising again. Swallowing and wriggling the jaw can also help. Do not over do it - if you continue to feel pain or discomfort it is better to abort the dive. Make sure you let your buddy / dive guide / instructor know you are having issues by use of the correct hand signal. If you need to abort the dive ensure you follow correct ascent procedures- do not rush but follow a slow controlled ascent. If pain persist or you have a full feeling in your ears or sinuses, trouble hearing or dizziness, or blood in your mask after a dive seek immediate medical advice. While most ear or sinus problems will be relatively minor and resolve themselves in a few weeks or so seeking medical advice is very important to prevent further complications. A less common occurrence is what is known as reverse block. This occurs when an air space closes off during a dive perhaps as a result of inflammation or mucus forming. As you ascend the expanding air will cause discomfort if it cannot escape. If this happens try descending, equalising and starting to ascend very slowly. Repeat if needed. The expanding air will typically find an escape route if you ascend slowly enough. If pain persists after the dive seek immediate medical advice. Section 5 - Page 13

15 RAID OPEN WATER 20 WARNING: Do not dive if you are unwell or congested. Please consult a qualified diving physician before taking any treatment for sinus or ear problems related to diving. Please inform your dive professional about any medical advice or treatment you may be receiving. Mask Squeeze Section 5 - Page 14 If a diver does not equalise early or often enough, the pressure differential can force the soft tissues together, closing the ends of the tubes. Forcing air against these soft tissues just locks them shut. No air gets to the middle ears, which do not equalise, so a barotrauma results. Even worse, blowing too hard during a Valsalva manoeuvre can rupture the round and oval windows of the inner ear. Most of the time the air inside the mask will equalise from small amounts of air coming from your nose during respiration. It s a good idea to consciously breathe out through your nose every 1m / 3ft during descent. If you feel any pressure on your face breathe out through your nose and if this does not work ascend a 1m / 3ft and try again. Rarely mask squeeze can cause blood vessels in the eyes or face to burst - this looks much worse than it really is but a trip to the doctor is recommended as a safety precaution. DIVE COMPUTER RULES We mentioned earlier that a great way to more accurately monitor your safe dive times is to use a dive computer. Here are a few rules and safety tips relating to dive computers: 1. Follow established safe depth limits and dive times must remain within No-decompression limits. 2. Ascend slowly to allow plenty of time for the nitrogen to out-gas: recommended 9m / 30ft a minute. 3. Follow the dive computer s required safety stops. 4. Ensure the correct gas mix is loaded into the dive computer, even if it is normal air. 5. Make a 3-5 minute safety stop at 5m / 15ft or follow the computer required stops.

16 PHYSIOLOGY 6. Limit repetitive dives to 30 metres/100 feet or shallower. 7. Ensure you read and understand the manufacturer s guidelines on the use of the computer or have an experienced operator show you how to use it. 8. Maintain good cardiovascular fitness. 9. Never dive dehydrated (drink sufficient water). 10. Allow extra surface interval time between dives if possible. 11. Allow 12 to 24 hours before you fly in a commercial airplane after diving or follow the no fly dive time displayed on your dive computer. 12. RAID strongly recommends that divers limit their depths to their certification level or the depth calculated by the computer or whichever is more conservative. 13. Divers within the sport programs should ascend before any required decompression stops are required. Decompression diving requires specialised training and procedures. 14. Perform pre-checks. Always activate and check the dive computer before diving in order to ensure that it is working and (LCD) segments are completely displayed, that the device has not run out of battery power, and that any oxygen, altitude, personal, Safety/Deep Stop adjustments are correct. 15. THE DIVE COMPUTER SHOULD NEVER BE SHARED OR TRADED BETWEEN USERS WHILE IT IS IN OPERATION. An individual s physiological make-up can vary from day to day. A dive computer or dive tables cannot account for these variations. You are strongly advised to remain well within the exposure limits provided by the instrument to minimise the risk of DCI. As an added precaution, you should consult a physician regarding your fitness before diving. Its information will not apply to someone who has not been wearing it throughout a dive, or sequence of repetitive dives, even if the other diver was on the same dives. If it is left on the surface during any dive, the dive computer will give inaccurate information if used on subsequent dives. No dive computer can take into account dives made without the computer. Thus, any diving activity up to four days prior to initial use of the computer may cause misleading information and must be avoided. 16. DIVING WITH NITROX: make sure the analysed value of the gas in the cylinder is entered into the dive computer. You should also obtain the correct training for nitrox diving by attending a RAID Nitrox program. Section 5 - Page 15

17 RAID OPEN WATER Select the correct altitude adjustment setting. When diving at altitudes greater than 300m / 1000ft the Altitude Adjustment feature must be correctly selected in order for the computer to calculate the decompression status. Most dive computers are not intended for use at altitudes greater than 3000m / 10,000ft. Failure to select the correct Altitude Adjustment setting, or diving above the maximum altitude limit, will result in erroneous dive and planning data. 18. SELECT THE CORRECT PERSONAL OR CONSERVANCY ADJUSTMENT SETTING. Whenever it is believed that factors that tend to increase the possibility of DCI exist, it is recommended that you use this option to make the calculations more conservative. Failure to select the correct Personal Adjustment setting may result in erroneous dive and planning data. RAID Note: Read the manufacturer s computer user manual thoroughly before diving with your computer. RAID Note: There is always a risk of decompression illness (DCI) for any dive profile, even if you follow the dive plan prescribed by dive tables or a dive computer. No procedure, dive computer, or dive table can prevent the possibility of DCI or oxygen toxicity. Emergency Ascents In the unlikely event that the dive computer malfunctions during a dive, follow the emergency procedures provided by your certified dive training agency or alternatively, follow these steps: a. Assess the situation calmly and then ascend at a rate of 9m / 30ft per minute and to a depth between 3m and 6m / 10ft and 20ft. b. Stay there for as long as your air supply will safely allow and conditions permit. After reaching the surface, do not dive for at least 24 hours. Section 5 - Page 16

18 PHYSIOLOGY Dive Computer Limitations While the dive computer s calculations are based on current decompression research and technology, it is important to realise that the computer cannot monitor the actual physiological functions of an individual diver. All decompression schedules currently known to the authors, including the U.S. Navy Tables, are based on theoretical mathematical models, which are intended to serve as a guide to reduce the probability of decompression illness. Diving with nitrox may provide you with an opportunity to increase bottom times or possibly reduce the risk of decompression illness by reducing the nitrogen content in the gas mix. This requires specialised training which is not covered in this program. Freediving in combination with scuba diving, may present risks that have not been researched and are not commonly known. It is recommended to wait a minimum of 24 hours after scuba diving before freediving. RAID Note: Always plan your diving conservatively to give yourself extra safety margins. Definition of NDL The no decompression limit (NDL) is the maximum allowable dive time that you can remain at a specific depth and ascend directly to the surface without accumulating excess nitrogen in your system and requiring decompression stops. Remember the higher the partial pressure of nitrogen (PpN2) the shorter the dive time (NDL). You can use the RAID air dive tables to determine your NDL but the best way is to consult your computer and plan the depth you will be diving. It is important to know how to manually calculate NDL in the case of the computer not working or having a flat battery before the dive. Section 5 - Page 17

19 RAID OPEN WATER 20 Table 1 - open circuit The table below shows the sliding Open Circuit partial pressure of both oxygen and nitrogen as applied to Air. Depth Sea Level Pressure Oxygen % 1 bar / ata / 14.7 psi Oxygen Pp Nitrogen % Nitrogen Pp Total Pressure 21 % % bar/ata 10m / 33ft 2 bar/ata 21 % % bar/ata 20m / 66ft 3 bar/ata 21 % % bar/ata 30m / 99ft 4 bar/ata 21 % % bar/ata 40m / 132ft 5 bar/ata 21 % % bar/ata What Should You Do If You Accidentally Exceed a No Decompression Limit? Follow the recommend stops displayed by the computer. If required decompression stops were omitted then exit the water, do not do any more diving, breathe 100% oxygen, monitor for signs and symptoms of DCS. Seek medical attention, by contacting DAN/emergency services and/ or the closest medical facility, as soon as possible. NEVER DELAY MEDICAL TREATMENT DIVING AFTER MEDICAL TREATMENT If during training or anytime thereafter, you received any type of medical treatment, medical clearance from a registered diving doctor must be obtained before continuing to dive. If you fail do to this, you will not only put yourself at risk, but also the lives of other divers, Divemasters and Instructors. It is your obligation to seek professional advice after receiving medical treatment, before further diving is undertaken. RAID Note: TWO GOLDEN RULES - Never hold your breath while scuba diving & know how much gas you have at all times Section 5 - Page 18

20 PHYSIOLOGY 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, Tim Curran, Karen White, Steve Bates, Alex Woodhams, Lisa Toomer, Watson Devore, James Rogers Images: Barry Coleman, Paul Toomer, Martin Strmiska, John Rigby, Kristen Goldsworthy Graphics: Kristen Goldsworthy Version 2.04 Section 5 - Page 19