Beyond the no-deco limit E-Book

Luca Cicali

• Herausgeber: Luca Cicali
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2024

Beschreibung

Beyond the no-deco limit - A journey through technique, psychology and emotions in the world of diving

Have you ever looked for a book on diving that combines technique and passion, science and true stories, training and introspection?

Luca Cicali, manager and diver, wrote ‘Beyond the no-deco limit” because, among the many books available on the market, he couldn't find one that explained everything a diver really wants to know.
It is not just a manual. It is a passionate and accessible story, designed for those who:

• Are new to technical or recreational diving
• Want to learn more about decompression diving
• Are curious to understand what really happens to the human body underwater
• Are looking for a book that guides them through emotions, theory, equipment and safety

What you will find in this book:

• Practical examples, real-life episodes and personal stories experienced underwater
• Explanation of the effects of breathing compressed air at depth
• A journey through decompression models, tables and dive computers
• An entire chapter dedicated to Nitrox, useful and educational
• Insights into equipment and its evolution
• A practical guide to dive planning, the most important skill for any technical diver

Beyond the no-deco limit is a book for everyone, written in simple but scientifically rigorous language, which takes the reader on a journey where every dive is a complete experience: physical, mental and emotional.
The book combines technical training and personal narrative, answering questions that every diver asks themselves and explaining what many other texts take for granted.

Leseprobe

Luca Cicali

Beyond the no-deco limit

A journey into the world of scuba diving

UUID: c6f5d736-5cca-4887-a959-411a09946d11

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1. Beyond no decompression limit. Why?

1.1 A book, not a manual….

1.2 Interview with the author

1.3 No-deco limit, (NDL)

2. Land mammals and underwater environment

2.1 Deep Blue

2.2 Underwater

2.3 Pressure

2.4 Weightless

2.5 Eyes wide open

2.6 Cold abyss

2.7 The dynamics of the false fish

2.8 I think, so I breath

2.9 Automation

2.10 Up, down, right and left

3. Scuba physics

3.1 A mixed fried of basic concepts

3.2 A few bricks to build the universe

3.3 Force, the cause of everything

3.4 Weight, probably an unnecessary chapter

3.5 Density, (how much mass fits into a space...)

3.6 Specific weight, (weight versus volume)

3.7 Energy, a spendable capital

3.8 The St. Vito dance of atoms and molecules

3.9 Heat, energy in transfer

3.10 The pressure, how much I lean to push

3.11 Some sink and some get by...

3.12 Gas grappling with temperature, pressure and volume

3.13 Impeccable ideal gas

3.14 Each gas takes his portion…. of pressure

4. Gas dissolved in tissues

4.1 A bit of nitrogen in a cup of water … and shake well

4.2 The gas makes its way by pushing

4.3 Tension of a dissolved gas

4.4 Exponential law

4.5 Our circulatory system

4.6 Our respiratory system

4.7 Tension, compartments, saturation, desaturation…

4.8 Perfusion or diffusion?

5. Decompression theories, history and topicality

5.1 Mysteries of decompression

5.2 “You only pay when you leave…”

5.3 Paul and John’s great ideas

5.4 Brief history of US Navy decompression tables

5.5 Other models, other tables

5.6 Haldanean models, (the tradition)

5.7 Non-Haldanean models, (innovation)

5.8 Probabilistic models (unlikely to be found…)

5.9 Model verification

6. From Haldane to dive computers

6.1 Starting blocks

6.2 Workman and “M” values

6.3 Bühlmann’s work

6.4 A dive in the tension-depth reference system

6.5 Safety margins

7. Bubble models

7.1 What works …works!

7.2 Bühlmann: almost the end of the line for “liquid models”

7.3 Bubbles for all tastes

7.4 The “push and pull” of bubbles…

7.5 VPM - Varying Permeability Model

7.6 The “Reduced Gradient Bubble Model” - (RGBM)

7.7 Deep stops... Why not ?

7.8 So, which is the ideal decompression profile?

8. Breathing compressed gas

8.1 Deep air, an insalubrious mix

8.2 Be careful not to drink too much .... nitrogen

8.3 Nervous system, command and control centre

8.4 Nitrogen narcosis

8.5 Nitrogen, psyche and other nuisances

8.6 Oxygen, the gas of life

8.7 Pulmonary oxigen toxicity

8.8 CNS Oxygen toxicity

8.9 The pitfall of carbon dioxide

8.10 Stress and narcosis, two devious enemies

8.11 The displacement of the lungs …

9. Nitrox, "umbalanced" air

9.1 EAN 21

9.2 Why Nitrox?

9.3 Extension of no-deco limit

9.4 A handful of simple formulas

9.5 Equivalent Air Depth, “EAD”

9.6 Summary tables and approximations

9.7 Best mix at all depths

9.8 Deco with nitrox: take it or halve it?

9.9 A puff of oxygen … opening the window

9.10 Nitrox equipment

10. Scubaland

10.1 The newly certified diver

10.2 The “over-equipped diver”

10.3 The “over-cautious” diver

10.4 The “over-performing” diver

10.5 The “unaware diver”

10.6 The … “normal diver”

10.7 From OWD to instructor … it is only a short step

10.8 Diving centres

10.9 Occasional diving buddies

10.10 Diving instructors

11. Tools of the trade

11.1 Technical or recreational equipment?

11.2 Two-stage regulators

11.3 Testing regulators: strength in numbers

11.4 Diving tanks

11.5 The buoyancy compensator device, BCD

11.6 Diving weighting system

11.7 Mouthpiece

11.8 Mask and fins

11.9 Dive Computer

11.10 Diving suit

11.11 Must-have accessories

11.12 Useful accessories

11.13 Unnecessary or harmful accessories

11.14 Scuba equipment configuration

11.15 Baseline configuration

12. Dive planning

12.1 Experience is the name we give to our mistakes

12.2 Within or beyond the no-deco limit

12.3 Main steps of dive planning

12.4 Which dives are we talking about?

12.5 Configuration parameters

12.6 The fuel for the trip

12.7 Dive planning

12.8 Method 1: planning with decompression tables

12.9 Dive planning example 1

12.10 Dive planning example 2

12.11 Method 2: use of a dive planner

12.12 Dive planning example 3

12.13 Method 3: Dive simulator paired with dive computer

12.14 Summary of planning methods

13. What if

13.1 What if….

13.2 Calling off a dive

13.3 Buoyancy control problems

13.4 Availability of bottom gas

13.5 Availability of deco gas

13.6 Non-compliance with the dive plan

13.7 Other mishaps of various kinds

13.8 Problems related to physical condition

14. Dos & don’ts and poetic licences…

14.1 Don’ts …

14.2 Dos …

14.3 Underwater legends

15. Appendices

15.1 Appendix 1 - Hydrostatic pressure

15.2 Appendix 2 – The Babel of pressure measurement units

Appendix 3 – Eureka!

15.4 Appendix 4 - The defects of non-ideal gas

15.5 Appendix 5 - Haldane compartments

15.6 Appendix 6 - Up and down… but in no hurry

15.7 Appendix 7 - What's in the alveoli?

15.8 Appendix 8 – The uncertainty of an estimate…

15.9 Appendix 9 – Critical supersaturation ratio as a function of M

15.10 Appendix 10 – From Workman to Bühlmann and back

15.11 Appendix 11 - The tug of war at the surface of a bubble

15.12 Appendix 12 - The critical radius of a bubble

15.13 Appendix 13 - From VPM to RGBM

15.14 Appendix 14 - Propagation of the nervous impulse

15.15 Appendix 15 - Even oxygen sometimes exaggerates

15.16 Appendix 16 - Deflating the bubbles by opening the window

15.17 Appendix 17 - Balanced first-stage tricks

15.18 Appendix 18 – Acronyms

Beyond the no-deco limit ... is a journey

into the world of scuba diving

facebook: diving web

Issue 1.1 -14032024

This book is dedicated to a great man.

my Father.

FOREWORD

"Going beyond" ... the "no-decompression limit" opens a fascinating world for us: better control of situations, awareness of our capabilities and limits, the ability to plan, assess and optimise operational parameters to allow safe decompression diving. This makes every diving experience like the accomplishment of a small exploratory mission, even if only experienced at a depth of a few metres, without hyper-technological equipment, alongside our faithful buddy and with only one fundamental goal to achieve, the most peculiar of the human species: having fun.

"Going beyond" is synonymous with curiosity, the desire to explore, to broaden our horizons, to test ourselves by winning a challenge ...but it is accompanied by the fear of not succeeding, of not being up to the mark. 'Going beyond' is a categorical imperative of the human species, always dominated and driven by a creeping dissatisfaction, the real driving force behind us to exceed our limits. This applies not only to memorable feats or records, but also to the many ordinary challenges of life, in which we are called upon to test our certainties and capabilities. Challenges that, even when not overcome, have enabled us to become what we now are. Overcoming ourselves is always a source of great satisfaction and pride, even when achievements seem insignificant.

Let us embark on a brief journey into the world of diving, to imagine how such fascinating experiences can be enriched once we cross the reassuring boundary of the "no-deco limit," a barrier with strong psychological as well as purely technical significance. It is a very personal attempt to answer the questions and curiosities that this sporting sphere inevitably generates; the desire to discover what lies behind the phenomena, rules and operating customs. Much of this effort is probably an end in itself: the many fascinating topics concerning the physics, physiology and psychology of scuba diving are far from being intimately understood and harnessed, and research in the various fields is as fruitful and ongoing as ever. Many topics are barely touched upon, and it could not be otherwise; this serves to fuel curiosity more than to provide complete and exhaustive answers.

STOP! I WOULD LIKE YOU TO READ THIS FIRST…

As we well know, there are no diving tables, computers, algorithms, procedures or diving techniques capable of completely eliminating the risk of decompression sickness or other accidents of any kind, which remain an unavoidable eventuality in scuba diving. In the most serious and unfortunate cases, such accidents can have permanent consequences, even very serious, or cause the death of the injured person. The only way to eliminate these risks completely is

NOT TO SCUBA DIVE.

This book is not a manual, and in no way replaces the necessary training to be carried out through the appropriate courses of international educational agencies, which require sports diving to be performed within the “No-Decompression Limit” and within a maximum depth of 40 metres. The suggestions, procedures and examples presented aim at informing on possible ways of planning decompression dives but involve the risk, when used in scuba diving, of incurring decompression sicknesses or other accidents, the consequences of which may be of extreme gravity and can cause death. Therefore, the author states that the profiles, tables and examples presented are not directly usable for scuba diving. It cannot therefore be held liable in any way for any damage, particularly to health, that may be reported by anyone who inappropriately decides to use the examples and plans presented for scuba diving.

1. Beyond no decompression limit. Why?

“When there is a great will, there can be no great difficulty.”

Niccolò Machiavelli, Italian Renaissance politician and philosopher

1.1 A book, not a manual….

It was a late summer evening when I read a strange sign in a diving centre on an enchanting island in the Tuscan archipelago. At first glance it seemed like a serious thing, then it turned out to be a humorous joke but, after reading it again carefully, it resembled a pearl of wisdom. More or less, he said so:

When you know the theory well you know everything, but nothing works. When you have a lot of experience everything works but you don't understand why. When you know the theory well and have a lot of experience, nothing works and you don't understand why.

The last paradoxical statement reveals a fundamental truth. In all things we never stop learning, and scuba diving is the clearest confirmation of this. The idea for this book was born shortly after I had completed a course for decompression diving to a maximum depth of 54 metres, at the end of which I realised I was not at all up to the task. I felt that I had received a generic smattering of theoretical notions and an even more approximate practical training, and this generated a very annoying feeling of discomfort. I didn't feel at all capable of tackling a decompression dive on my own without the assistance of an instructor. The will to react to what I perceived as a flaw in the preparation, able to open the way to possible risk situations, was the spring that pushed me to deepen with passion and determination many of the theoretical and practical aspects of the world of decompression diving, which I tried to include in this work.

Of course, however, a simple course, no matter how complete and accurate it is, cannot on its own form an expert diver, otherwise it would have to include a long training during which all possible critical operating conditions are simulated and assimilated. More than normal, therefore, that basic training be complemented by experience, an indispensable factor in safely carrying out any activity in life. But at the end of a course that enables the diver to reach depths close to the operational limit of air and to carry out decompression, divers should feel able to dive within the limits of their licence on their own, even if they are still short of experience. This is often not the case, for one simple reason: market requirements. Recruiting a number of customers sufficient to generate a fair volume of business is only possible by using a few keywords: ease, speed, results within everyone's reach. But we know that in life nothing is within everyone's reach, and nothing is simple and immediate if we want to achieve good results. There are several declared enemies of the student diver: the need to hurry, (few lessons and few dives, before the plane leaves for the return from the tropics), the need for many diving schools to attract as many customers-enthusiasts as possible, and in too many cases the absence of a serious final verification of the skills actually achieved by the student, in his own interest. This verification should be carried out by an independent evaluator who can, if necessary, deny the licence, asking the student for a further training session or definitely advising him not to go ahead, (I have seen some newly certified divers jump into the water forgetting their fins or re-surfacing at rocket speed because they started the descent breathing from the snorkel instead of the regulator).

The book is mainly dedicated to those who are curious and want to know more, but do not have enough preparation, patience, or time available to digest books, documents and monographs full of complicated formulas and dedicated to super-specialists. The author's ambition is to try to bridge, at least in part, the sidereal distance between the articles we can find in scuba magazines, which often offer a little more than ten lines of text underneath the usual giant photographs of spectacular dives in exotic places, and the learned monothematic publications of super-specialists, on average incomprehensible to those who are not a university professor of physics or mathematics. But it is also a book dedicated to habitual trespassers, i.e., all of us "very recreational" divers who often violate the depth limits provided by our licences and overtake by a few minutes the no-deco limit because we now feel like experts. Gradually, we realize that beyond this limit there is no gloomy precipice, that we no longer feel particular anxiety or stress, that having to face a few minutes of "deco" to be recovered with a slightly slower ascent does not seem to be a big gamble, and that's it. This is what happens to everyone sooner or later, since there is no underwater policeman to issue fines for exceeding the depth or duration of the dive. As experience and confidence increase, we gradually get used to extending these exceptions, making them a habit, but without having acquired a knowledge of the possible major risks we face, and without having practiced some simple contingency procedure, necessary for dealing with any inconvenience. These brief outings in decompressive territory are conducted using equipment that is often profoundly inadequate, and this is perhaps the most neglected detail. It is better therefore to stop for a moment and reflect, concluding that it is probably useful to dust off and consolidate some notions learned during the basic courses. Reflection will also help us to understand whether we really just want to strengthen our theoretical and practical foundations as responsible divers, or if we want to make decompression diving our new goal as sports divers. In the latter case, of course, we will still have to go through a good course, appropriate to our expectations.

This work is the result of the integration of various heterogeneous contributions, matured through personal observations, the study of specialized texts, diving experiences, chats among divers, old notes of youthful studies, integrated and harmonized according to a logical thread I hope sufficiently coherent. Basically, it addresses the general problems of longer bottom times and increased maximum depths compared to recreational diving, which are generally dealt with in so-called 'Decompression' or 'nitrox advanced' courses (or others) offered by most international diving schools. These courses provide for maximum depths generally varying between 40 and 56 metres, and include decompression, possibly carried out breathing nitrox mix, to reduce its duration. We will therefore try to explore a few technical and operational issues related to this range of depths. We will also try to unravel some mystery about the dynamics of uptake and release of the inert gas in diving, addressing the fascinating topic of the various decompression models designed to try to ensure safety in diving, and which are the subject of continuous research and updating, providing also some historical information. Some slightly more technical details will be dealt with in dedicated appendices, for those who wish to delve deeper. Some attention has been paid to the dive planning, something which is perhaps a little too much neglected even in course dives. Unfortunately, this limitation also applies to deco dives, which are too often framed and managed as normal dives to which the "variant" of decompression is added. The ultimate goal of this reading, which we naturally hope will be both easy and enjoyable, is to arouse greater curiosity about all the topics addressed. Even (and perhaps above all) for those who do not dive beyond the 18 metres of the first level licence and have no intention of changing these healthy habits.

1.2 Interview with the author

The au thor of this book is not an instructor but a simple diving enthusiast. He does not have a three-digit number of dives behind him, he has never reached depths greater than 60 metres, he has not done any specialised studies on hyperbaric physiology, he is also a bit overweight... In short, he does not seem to have the specific qualifications to write a text on this subject. He practises this sport when he can find some time off from work and other commitments, but is always irresistibly attracted to the world of silence and underwater flying. He shares this passion with the reader, the great feeling of freedom and fulfilment that we feel every time we abandon ourselves in the descent of a dive, the light diminishes and takes on a uniform blue colour, the silence wraps us, broken only by the hissing of the regulator and the gurgling of the bubbles, and the body frees itself from weight and gently glides. Furthermore, and perhaps because he is no longer very young, he has become a little more sensitive to diving safety and is convinced, even more so than at the beginning of his diving career, that during a dive, concentration and common sense are more important than any equipment. "Like so many others, I started scuba diving almost by chance, and then, one licence after another, (more or less useful or significant), I slowly progressed through the stages leading to greater depths, up to the occasional trespass beyond the no-deco limit, but not too far. And despite the experience and prudence gradually growing over time, I have always had to and still have to take the time to reassure my mother and wife every time I start packing my bag of equipment and checking my regulators. Then, the step towards the "decompression diver" licence, which allowed me to adventure beyond the no-deco limit and reach a depth of 54 metres, carrying out the decompression with nitrox mix. But here too, as immediately after the previous courses, I didn't feel that I had achieved the operational autonomy and confidence necessary to feel at ease. The desire for greater safety and familiarity with the new procedures has sparked curiosity and a desire to delve deeper into the many technical and operational issues involved in decompression diving". The insights and reflections that have come out of this, along with the most significant observations based on the experiences made, have become this book.”

1.3 No-deco limit, (NDL)

Here we are at the "no-decompression limit", very well known to all divers. It is better to make it clear at once that NDL does not represent a rigid boundary between safe and unsafe dives, as if the dives within the limit were absolutely safe, (which is completely false) and those beyond the limit unsafe, (and this is also obviously false). The safety in diving, (but also in any other context), is instead linked to behaviour, i.e., to the correct application of operational rules capable of guaranteeing, whatever the context, to be within a calculated and considered acceptable risk. The no-deco limit can be represented as a broken line in a depth-time reference system; it shows the relationship between two parameters: the maximum depth reached during a dive, and the time spent underwater until the start of the ascent, called bottom time. Fig. 1.3‑1 shows the no-deco limit for depths greater than 15 metres. The graph is based on the no-decompression times set out in the "U.S. Navy Diving Manual", rev.7; published 1 December 2016.

Fig. 1.3‑1 – NDL for depth over 15 m, (U.S. Navy Diving Manual, rev.7, 2016)

The broken line divides the plane into two areas: the upper area contains no-decompression dives (no-deco zone); the lower area contains decompression dives (deco zone). Each point of the graph indicates the maximum time that can be spent at a certain depth without having to make decompression stops, and for this reason it is called the "non-decompression limit", (NDL). For dives with maximum depth and bottom time represented by a point above the broken line, the diver is not required to make decompression stops to return to the surface, subject to compliance with a certain maximum ascent speed. For points below the broken line, on the other hand, the diver is instead obliged to make decompression stops whose number, depth and duration depend on the maximum depth and bottom time reached. Exceeding the no-deco limit requires a completely new and different operational and psychological approach to diving. In the case of a no-decompression dive, it is only necessary to establish in advance a simple criterion for starting the ascent, typically several minutes before the no-decompression limit expires, or when the cylinder pressure reaches a minimum value. In the case of a deco dive, the decompression obligation absolutely prevails, so it is imperative to ensure, through proper planning, that the available gas is sufficient to complete this fundamental sector of the dive. Moreover, an emergency ascent in a decompression dive can no longer be performed as in the case of a no-deco dive. Therefore, since we certainly cannot know in advance when problems may occur, we must try to develop the ability to prevent and solve them during diving.

Fig. 1.3‑1 show the NDL graph for depths greater than 15 metres. The graph is based on the no-decompression times provided by the "U.S. Navy Diving Manual", rev.6; published on April 14, 2008. Once a depth value is chosen on the vertical axis, a horizontal segment is drawn until it meets the broken line. The time in minutes shown on the vertical at this point is the allowable dive time without decompression stops for the chosen depth. It is evident that, as we move down the vertical axis of the depths, this time is reduced. For example, NDL is reduced from 92 to 10 minutes for depths between 15 and 40 metres and reaches 5 minutes at greater depths. However, these variations occur in a non-linear way: the NDL variation as a function of depth is much more pronounced in shallow water, while at greater depths it is much smaller. The NDL chart represented in Fig. 1.3‑1 is based on a descent speed of no more than 23 metres/min, and an ascent speed of 9 metres/min. Finally, it can be seen that, taking just over two minutes to reach a depth of 45 metres, only 3 minutes remain deco free, as the NDL at such depth is only 5 minutes. Therefore, dives to 45 metres depth and beyond are almost automatically deco dives.

2. Land mammals and underwater environment

"...But what drives you to put all that stuff, to go so far… but then why? Do you catch the fish? ... No, so what? I don't understand you. Do you put lead weights on too? My God!"

My mother-in-law's recurring grumbles.

2.1 Deep Blue

I turn slowly in my bed, trying to separate reality from the last fragments of sleep. Then I pull myself up decidedly, by now I have understood that I'm awake. Without turning on the light, I throw a few handfuls of water on my face and chest, shiver a little and dry off quickly. I swallow two spoonfuls of sugar and check the duffle bag prepared the night before; the mental check list is now running faster and finds two bugs. I retrieve the missing material, make one last check and load everything on the car. It's cold outside, but not for long. The sky has that indefinite colour that precedes sunrise, but the day is clear and windless. Still, everything moves in slow motion, silently, at low temperature, it seems like a human way of life. I get into the car and turn the key. The engine starts, then idles, quiet and regular. I do everything calmly and silently, so as not to wake up the world. I put on my dark glasses, fasten my seatbelt, now I'm comfortable. I press the accelerator and the car starts slowly; I face the silent road. My mind is clear, everything seems to work naturally. I see the sunrise on the way, I am serene and feel free. I reach the port when the journey begins to become monotonous. I quickly transfer all the equipment to the boat, climb up to the upper deck and lie down against the right-hand wall, with the towel rolled up under my head. My eyes look up to the blue morning sky, I stand lazily between the sky and the sea that flows slowly below, pierced by the bow of the boat as it makes its way between two wings of foam. We get to our destination quickly, less than I wanted. The sailors manoeuvre frantically with the lines to moor the boat, they repeat the operations many times and are never happy with the way the boat is set. Finally, the engine stops, an unreal silence returns, broken only by the screams of the seagulls. I take a look at the blue sea that stretches as far as the eye can see, merging with the sky, with a thousand reflections that force me to narrow my eyes. There is north-westerly wind, a bit of undertow and the colourful, quivering patches of the reef under the boat. I stand up, turn my gaze to check my surroundings. I gather all the equipment around me and decide to go. I slip my legs into the black neoprene armour which starts to constrict from the knees, and gradually upwards, stretching out the artificial skin that reluctantly adapts to the shape of my body. At the end it no longer oppresses me in a clearly defined place, but it seems to want to suffocate me as a whole. I let out the air and close the zip firmly, surrendering to the constriction. I pick up the weight belt, grey in its sinister weight. Leaning forward, I place the belt on my kidneys, contract my stomach and fasten the buckle at the front, tightening it to the limit. Then I try to breathe, spit on the glass and rinse. I press the purge button of the regulator and turn the bottle valve. A hiss that sounds like a groan responds to the manoeuvre and the hoses stiffen. I rest my back on the steel tank, tighten the shoulder straps, close the Velcro, and snap fastex and carabiners. Now I sit down, slip on my fins, pull the straps on both sides, slam my feet between them to settle down. I stand up again, with a certain effort; make a quick check of the pressure gauge and the regulator; the computer keeps quiet. I am a technological product, clumsy and intubated, staggering and squeezed by neoprene and rubber, encumbered by lead and steel, stuck in the BC. I bite the mouthpiece and approach the edge, while the sweat begins to run down and the glass fades. One last mental check, I open my arms and hold them to my body gathering everything that sways, keep my gaze straight and I take a step in the void. I float easily and feel the sweet relief of the fresh water that slowly wets my back and arms; I unzip the wetsuit and ease the water in. Below me almost a thousand metres of salt blue water, then black. A few fish go up quickly to check what's new and watch me from a safe distance. I move a little to adapt well, the wetsuit relaxes and fits the body, it becomes immaterial. The weight is gone; I'm still a little clumsy and swollen from the wetsuit and equipment but now the muscles can relax. My strength immediately regenerates, every movement becomes easy and the backwash gently rocks me. I lazily wait for my buddy. Now there are two of us, close together, we exchange a glance of understanding, then we go down. Mouthpiece tightened between my teeth, I let the air out and empty the BC. Gentle and safe, gravity defeats Archimedes and we sink into the blue, while I push air against my eardrums. A long and modulated hiss in my ears, then everything is smooth and quiet. The view changes, we are in flight. The music starts, as the bubbles, the breathing and everything else disappear, only the silent gliding next to the magic of the blue remains. The colour of the light changes, the blue surrounds us, everything is mysterious and reassuring at the same time. We fly, weightless. All we have to do is move a muscle to turn, flip, move left and right. A few fin shots and we pass the tip of the reef, we go down a few more metres; the current, weak at first and then stronger, takes us over. The curtain of paradise opens, the scenes prepared by Allah flow slowly in front of us, in a glitter of colours and bizarre shapes, of sea monsters and tales of magic. Here we are on the flying carpet, lazily watching the images unravel and flow gently before our eyes as in a dream. I choose a fish, I follow it with my gaze as far as I can, hypnotized by the brushstrokes of saturated colour humbly displayed. A tuft of soft red blood coral crosses my gaze as other actors enter the scene, with the singularity of the forms and the bizarreness of the appearance. Shoals of Dascyllus around gorgonians and madrepores have a snack of plankton, leave their shelter among the coral branches for a moment, grab a bite, derail as drunks pushed by the current and immediately go back to their hiding place in reverse, to take another turn immediately afterwards. The glass fish shoal is a strange animated species that changes shape under the attacks of groupers. They move, sway, grow and shrink, without peace. Sumptuous and lazy, the barracudas move forward, circling tightly together, similar to an equatorial typhoon. These impostors seem aware of their supremacy, reluctantly tolerating the violation of their world by pouting and changing colours. We fly weightlessly over crevasses and escarpments that open onto the abyss, where blue turns black. We are far from the gloomy seabed, resembling death and oblivion, but we measure its limit and keep it at a distance. We sail mid-height next to the coral skyscraper, preceded by the carousel of barracudas and followed by snappers. We scan the tenants of the skyscraper who mostly ignore us, only a few is curious or anxious. Everything seems to be swaying in slow motion, in a lazy, gentle, quiet and reassuring atmosphere. But the struggle for life is evident in every nook and cranny. Inside every hole of the cliff the harmony of colours goes hand in hand with death, the struggle is relentless and unsparing. Every violent colour is a weapon, every shape is a strategy, every appearance is a deception, every behaviour is an offence, a defence or a deterrence. There seems to be opportunities only for the contemplation of paradise, where everything is sweet, serene, and consoling, and the drama seems not to be happening. Instead, it is the comedy of the survivors. Behind the scenes of the stage there is a smaller scale theatre, which mimics the main scene. I stop to observe the details, and in that fragment of cliff I glimpse the richness of life pulsating as in the whole picture of blue ocean that the eye can cover. Now we have arrived beyond the cape, the current strengthens, it joins the one on the opposite side and pushes us forward like wagons of a ramshackle train.

The movie speeds up, now we have to catch flashes from the underwater world. I look up and the light; I turn my body and I lie down on the water that pushes me like a river in flood. Between me and the dazzling glow of the surface, here are the large dark figures of the jack fish cutting through the light, swimming at half height. Serious, disciplined, an orderly and resolute platoon. The skyscraper curves, the light gradually changes its inclination and is now grazing the wall. Huge fans open their arms on the edge of the ridge, enveloping as much water as possible. The currents strengthen and meet right here, pressing on the gorgonians and upsetting the Alcyonarians. The silhouettes of the fish resemble black cardboard collages, inlaid in the flickering glow of the background. We approach the reef, resist the pull. We take shelter behind a ridge where we find a sandy plateau, here the water is calm. We are in Allah's garden, the anemones greet us swaying, forcing us to visit them. The attraction of the purple tips swaying lazily is irresistible, and we indulge in twirling in this hanging garden like big plastic bees on the flowers of a meadow. Violated and offended in his silent calm, the clown fish comes out of his temple as resolute as a samurai, ready to sacrifice himself for the salvation of the dwelling. He stands halfway between me and his turgid anemone, opens his opercula and his pectoral fins to make himself look bigger. I approach him slowly; it almost breaks out and looks like a desperate goalkeeper trying to anticipate the opponent's centre forward. Its white stripes against the orange background look like white lead laid on top of wax, repainted in pencil. It is a mask. Desperate, he throws himself at me, banging on the glass and nibbling at the edge of the mask. I walk away from the brave clown as he returns home proud of having put me on the run. Napoleon, on the other hand, arrives from afar, almost touching my shoulder before I see him, then my surprise turns to emotion. He has a huge face of a gentle giant, the big eyes that rotate and two huge lips. I get closer, he turns to his side and looks at me questioning and puzzled... I convince myself that he loves me, but I make the usual mistake. I feel a touch on my ankle. I turn my gaze, get two signals from my friend, glance at the instruments, forgotten along the side to do the dirty work. Yes, it's time to go. We are alive, we must stay alive. We have to reason, to check, to verify, to do what needs to be done. We must wake up from the dream, check the air, the plan, the decompression, the ascent, the procedures. The enchantment is broken, we were out of the world and we cannot stay there forever. A blow to the BC that discharges two puffs of bubbles, we ascend towards the light, the air. All that comes next is clearing the table after eating.

2.2 Underwater

Below the surface of the sea everything is against us, land mammals who realize how much our organism is adapted to the air and not to the water as soon as we peep below the surface. Our senses, perfectly functioning when we are surrounded by air, are deceived and put to a severe test by replacing air with water. A few metres down and the pressure increases rapidly, forcing us to frequently compensate for the imbalance on our eardrums. The weight is cancelled, but the up and down become difficult to distinguish; if we do not have references, we must observe the direction of the rising bubbles to establish where the surface is. The density and viscosity of the water oppose advancing, forcing us to make slow, measured movements; nothing like the freedom and dynamism we are capable of in our natural environment. We begin to lose heat very quickly; at around 15 degrees Celsius the air feels cool, the water icy. Our vision becomes blurred and inaccurate, so much so that we are forced to remove the water from our eyes with a mask, which also defends us from the irritating action of the salt. The light is reduced and the colours distorted, each variation in depth further reduces the amount of light and alters its colour; quickly a dark blue surrounds us. Visibility is extremely limited, even the clearest waters limit the view that to no more than a few dozen metres. The sounds are more intense and partly distorted, but above all it is impossible to identify their direction. Obviously, we must understand each other by gestures. Breathing becomes a much less automatic and natural act. We need a supply of air and a mechanism to provide it at the pressure of the surrounding environment, otherwise there is no way to expand the lungs. Breathing becomes slower and deeper, its delicate and complex mechanism is altered, oxygen saturates haemoglobin and dissolves into plasma, carbon dioxide is eliminated with reduced efficiency, dissolved nitrogen in tissues increases, and to meekly return it to the gaseous state and eliminate it with breath we are forced to comply with decompression procedures. The efficiency and precision of our nervous system in exchanging signals alters, our reflexes slow down and we suffer a subtle drunkenness, while our friend oxygen, the gas of life, becomes a threatening ordnance to be kept under strict control. To cope with this environment, we must have air supplies contained in heavy steel cylinders slung over our shoulders, breathe through the proper functioning of the first and second stages of a regulator, and obey the orders of the "wrist brain" that keeps the dive under control and guides us during ascent and decompression. In addition, we have to endure the discomfort of slipping into a neoprene wetsuit that clutches us from head to toe like a glove, and tightening a heavy weight lead belt at the hips. The air comes through a tube directly into the mouth, dry and cold, while the nose is almost unusable, confined inside a mask that marks the face and limits the view. In a way, the underwater environment can be compared to the space environment, with the fundamental difference of being more within reach. The diver is the little astronaut of the depths. A fun dive is only proof of how adaptable our body is to adverse environmental conditions, which in this case are decidedly unnatural for a terrestrial mammal. But also, of how indispensable technical equipment, the result of human ingenuity, is for dealing with the underwater environment.

2.3 Pressure

There are many effects of the pressure on the diver. One of the most significant is the inability to breathing air at atmospheric pressure even just below the surface. To expand the lungs, we have to overcome external environmental pressure, and this is impossible if the air we breathe does not have the same pressure as the water around us. We only have to try to breathe with a 1-metre snorkel to realise this limitation. Hence the need for regulators. Breathing compressed air, however, is not natural. It causes mechanical problems, such as increased resistance to breathing due to the higher density and thus viscosity of the air, and the increase in partial pressures of gases it contains. As we all know, this effect is the primary cause of the limitation of the maximum depth at which we can breathe air, due to the nitrogen narcosis and the oxygen toxicity. From the very first meters, we have to compensate for the pressure imbalance between the outer and inner ear due to the non-instantaneous adjustment of the pressure in the inner ear to the airway pressure. Finally, pressure reduces the volume of anything that is not rigid and contains gas, so especially the jacket, but also the neoprene of the wetsuit, whether wet suit or dry suit, undergoes compression that decreases its protective effect against low temperatures. The squeezing effect makes us less and less buoyant and causes us to sink faster when we descend. It seems impossible, but the effect of pressure exerted directly on the surface of our body is not directly perceptible. And this is despite the fact that even at a depth of twenty meters the pressure is three times that of the atmosphere.

2.4 Weightless

In diving we are not in the absence of gravity, we are in the absence of weight. This unique condition due to Archimedes thrust provides one of the most beautiful and rewarding sensations of underwater flight, which can only be experienced in another slightly less convenient condition: orbiting in space around the earth. In fact, to simulate weightless conditions, astronauts train in large swimming pools. Weight is the force with which our feet press against the floor, attracted towards the centre of the earth by gravity. Underwater this attraction remains unaltered; however, it is counterbalanced by another opposite force, Archimedes’ buoyancy. And the weight is precisely the result of the sum of these two equals and opposing forces, so that they are cancelled out. This particular condition is due to the high density of water, which gives us a strong upward pull compared to when we are immersed in air, which has a density almost a thousand times lower. The underwater descent is a controlled flight, during which the jacket can be used to vary the Archimedes’ buoyancy at will, until the weight is perfectly counterbalanced and we can remain neutral in mid-water. Once the weight is off, we can experience the fascination of weightlessness, as if we were in the orbiting space station. Although this free buoyancy thrust prevents us from exerting a strong muscular effort to keep ourselves at the desired depth, the high density of water compared to the air hinders our progress. And so, we have to work hard, with fin strokes, to get where we want to go.

2.5 Eyes wide open

Colour is light. Metre by metre, water acts as a filter against the available light, selectively reducing the amount of light by wavelength, i.e., by colour. From the sun we get intense almost white light, which penetrates through the surface of the water with all the colours of the rainbow. However, each of them is "absorbed", i.e., attenuated, according to a precise absorption coefficient. This causes a corresponding gradual fading of the colours, as we go deeper into the water. As we move towards the bottom, first red, then orange, yellow and green are absorbed and tend to disappear from objects, fauna and the natural environment, but also the total amount of light available decreases. At just over 40 metres a soft and weak blue light surrounds us. Around 60 metres, especially on cloudy days or with not very clear water, we are not far from the conditions of a night dive. It is a small additional complication to handle in deep dives, requiring torches of adequate power to light the environment. The first underwater experiences with the phenomenon of red absorption even at shallow depths, could lead to the belief that there are species of completely black starfish. Such gross misunderstandings are quickly removed as soon as we use a powerful underwater torch, thanks to which the star shines bright red again. If we rely on a particular colour of the suit, hood or equipment to recognize our buddy, we must pay careful attention to colour changing. It is better to rely on shapes (fins, mask, etc.), rather than on the garish red of the wetsuit insert, which will appear black even after a few metres. We also need to get used to poor visibility situations, which are more frequent than we think even in tropical waters. It only takes a moment to lift the very light suspension resting on a muddy seabed and see our fins disappear, especially in caves or wrecks. Even in very wide visibility conditions, the total lack of landmarks can cause the same problems as poor visibility. Instruments help us but we must be able to interpret their indications. Looking through the glass of a mask does not give us a complete and free horizon. Sometimes we have to turn around twice to catch sight of our buddy who is .... only half a metre higher.

2.6 Cold abyss

Body heat loss is much faster in water than in air, and this is because water is a better heat conductor. Staying 5 minutes underwater at low temperatures is the same as being in the air at the same temperatures for almost two and a half hours, as far as heat loss is concerned. Therefore, thermal protection in diving is absolutely essential. Unfortunately, the wetsuit restricts movement, makes breathing less natural, must be of appropriate thickness for maximum depth temperature, and is somewhat laborious to put on. The dry suit protects us even better in cold water, but it is a bit more complicated to use, so we have to learn to use it gradually. Insufficient thermal protection is only bearable if the dive has a very short bottom time. But as soon as the bottom time increases, the simple discomfort becomes a serious handicap and a risk that should not be taken at all. Cold causes greater absorption of inert gas in the tissues, aggravates the effects of oxygen toxicity and accentuates the symptoms of nitrogen narcosis. Especially in temperate seas, the temperature tends to decrease in a nonlinear way as the depth increases, and with large differences between summer and winter. In winter the temperature is lower but more homogeneous, varying less with the depth, and requires the use of a drysuit with appropriate internal protection to deal with temperatures that can fall below ten degrees Celsius. In late summer, on the other hand, the temperature in the surface layers, which are mostly warmed by the sun, differs considerably from that in the deeper layers. This situation of temperatures stratification tends to be maintained and consolidated because the warmer surface waters have a lower specific weight and therefore remain above the colder ones, preventing mixing. This create a significant temperature discontinuity between the surface layers and deeper layers, called thermocline, which stabilizes closer to the surface at the beginning of summer and deeper in late summer. The discontinuity is sometimes so marked that in a few metres the temperature can vary by as much as 4 or 5 degrees. In practice, a diver standing vertically at the thermocline will find his head warm and his feet cold. In tropical seas the temperature is much less variable, and its decrease with the increase in depth, although very weak, tends to approach linearity, allowing for greater seasonal uniformity and less dependence on depth. This greatly reduces the need for thermal protection at all times of the year. Apart from local or particular exceptions, tropical seas almost always allow the use of wetsuits, often only three millimetres thick. It is therefore better to pay attention to the thermal protection, choosing it well suited to our degree of coldness. Being very cold in diving is not only a problem of discomfort, but it is a real risk we should avoid running. It is always easy to find the superman that goes without a hood even in winter, next to those who use a drysuit even in the Red Sea. We have to know ourselves and find our ideal configuration in order not to chatter our teeth in the cold but also not to sweat. And that's not always easy.

2.7 The dynamics of the false fish

What water gives us as an aid to buoyancy, it takes away as freedom of movement. Nearly a thousand times denser than air, water offers considerable resistance to advancing; this is why the fastest fish have an extremely hydrodynamic body shape. This form is inimitable for humans, who must therefore try to avoid an overly water-resistant equipment configuration and assume an appropriate trim. Moving underwater means advancing by the thrust of the fins while maintaining a horizontal trim. Arms and hands are of little use, perhaps if the suits had pockets suitable for this purpose, it would be better to put our hands there. It is necessary to make controlled movements, at a regular pace, and avoid sudden and intense efforts. Although all this is taught from the very first dives of every course, the acquisition of a regularity of movement without wasting energy is neither immediate nor obvious. With time and experience, movements become rare; a few fin strokes are sufficient for the necessary movements. Buoyancy control must also be automated and reduced to a minimum, and skilfully integrated with the contribution of breathing. Our autopilot must be fine-tuned little by little. Deep divers act calmly and with concentration, making the movements that are needed, and only those. Instead, the diver who flaunts great experience seems practically motionless, as if he were a frozen squid. We will meet them again in a chapter devoted to some stereotypes of divers that we can find in all the seas of the world. Good physical fitness allows one to achieve and maintain good diving performance and to sustain occasional, intense exertion if necessary without immediately falling prey to breathlessness. However, there is one type of diving that allows us to overcome great distances at reasonable speed, without any effort at all, while quietly enjoying the flow of the underwater landscape before our eyes. This is drift diving. But like everything beautiful and easy, it has some drawbacks: these are dives to be performed in environments perfectly known, or led by people who are experts in the area, and with a crew assisting us in the boat who are perfectly aware of what we are doing, so that they can be found at the exact spot where we will surface. Being swept along by the current is as nice as sledging, but one mistake is enough to be caught in the wrong slipstream and splash away like a table tennis ball.

2.8 I think, so I breath

Breathing underwater is anything but sitting in our armchair while reading a book. Out of the water we just breathe, according to an automatic process that depends on the physical activity in progress, and that, in resting conditions, takes place at a regular pace and with minimal and constant ventilation. In practice, in normal life we breathe unconsciously, even though this activity is subject to our control. In diving, on the other hand, we realize that we are breathing, at least because of the hissing of the regulator and the rhythmic gurgling of the bubbles emitted, which also invade part of the field of view chasing towards the surface. In this situation the process is somewhat reversed: we have to adapt the effort we make to the proper respiratory rhythm, rather than the other way around, in order to maintain a slow pace and deep breathing. Slow, deep and continuous breathing goes hand in hand with the ability to move in a careful and controlled manner, and the two must be well coordinated to keep gas consumption to a minimum and keep breathlessness at bay at all times. This is precisely one of every diver's worst enemy, especially when going down deeper than 100 feet, in which case the consequences of breathlessness are magnified. Therefore, when we dive we must focus on breathing, committing to doing it the right way, paying special attention to exhalation. It seems a paradoxical statement, but it is necessary to make a seemingly unnatural way of breathing become natural. Appropriate breathing in diving is the key to preventing the carbon dioxide build-up, and all the dangers it entails.

2.9 Automation

Natural behaviour in water is acquired gradually; before reaching this comfortable condition, it is necessary to endure for a while with a certain discomfort and stress, mainly due to not feeling fully up to the tasks to be carried out in an unfamiliar or even hostile environment. But then the automatisms are gradually acquired, we become able to look at an instrument out of the corner of our eye, all the equipment becomes familiar and at hand, we sense the presence of our buddy even without seeing him, we find a slow and regular breathing rhythm and can finally enjoy the dive. But no matter how experienced and prepared we are, the underwater environment still requires self-control and caution, especially in decompression diving. These are the situations in which things cannot be left to chance, but must be planned. In a way, dealing with this type of diving takes us a little step backwards from the autonomy and feeling of safety we had before. In fact, equipment increases in weight and complexity, a decompression tank must be managed and carried throughout the dive, we must be able to react to an unexpected event promptly and, if necessary, anticipate the ascent while maintaining our decompression obligations. Gas consumption must also be kept under strict control, as the total volume of breathing gas must be sufficient for the entire decompression including a certain safety margin. At depths of more than 40 metres, the no-deco limit is exceeded after a few minutes, and unless it is an insignificant (and also risky) dive to the bottom and a quick escape, decompression stops must be carried out and careful planning is required. All this makes decompression diving a little less natural than no deco diving. But we have no alternative. After all, we are not fish and the abysses are not our natural environment... We are therefore obliged to maintain constant self-control and always ensure an adequate margin between this state of normal vigilance and the anxiety that can be generated by any reason for discomfort. It can be an unexpected event, a minor malfunction of the equipment, and so on. Without being able to master and easily solve small and predictable incidents we run the risk of becoming prey to dangerous enemies: anxiety, stress and panic.

2.10 Up, down, right and left

Nice to know where we're going when diving. But since at sea we have no roads, no crossroads, no signs, and even less a satellite navigator, the right way is to be found with the help of a few reference points. The ability to orientate in diving is perhaps one of the most important things for people in the depths, and the least easy to acquire. Critical situations for orientation arise all the time: poor light, lack of reference points when passing through the blue, areas of poor visibility, etc. But orientation is not easy even in the presence of submerged structures, natural or artificial. Here is a promontory, a shoal, a strange rock or a landslide to keep in mind to find our way back. Or the hatchway or a turret of the wreck we are visiting. But then the doubt arises that the rock taken as a reference is the very one we saw earlier, when it seemed so close to a posidonia bush that we no longer find; or that we really noticed that wreck porthole, or that we mistook it for something else... Shall we try to help ourselves with the compass? Many tricks and gimmicks can be deployed, but of course nothing replaces a good knowledge of the place... And here's a classic that is never out of fashion: not being able to find the boat. It’s a contingency to be solved by throwing the SMB. In this case we will probably surface far from the boat, and this can only be a big nuisance having to fin with difficulty until we cancel the distance. But in case of strong adverse current or rough sea the scenario is quite different, and it can quickly evolve towards a really critical and unpleasant situation. Getting noticed by the people on board before sliding quickly at an excessive distance becomes a goal to be achieved without delay.

3. Scuba physics

"You do not really understand something unless you can explain it to your grandmother" Albert Einstein

3.1 A mixed fried of basic concepts

This chapter can be read in its entirety, only in part, or used only for reference, when necessary. It contains a somewhat haphazard mix of basics useful to an experienced or at least curious diver. In theory we should know almost everything about these topics since we have taken courses and obtained patents… but you never know. The concepts presented may seem of limited importance or less relevant than the practical experience of diving, and immediately after the first courses we can’t wait to forget what has been absent-mindedly taken in. But conscious and responsible divers must have a fairly solid knowledge of some of the basic physical laws they have to deal with in a diving environment. The practical experience of the physical phenomena we will briefly describe is indeed recurrent in scuba diving. We don't need a degree in physics to understand what happens to us in diving, but some simple thoughts on some basic concepts, which require only a minimum of concentration to be understood. This is all it takes to create a greater confidence with the underwater environment, and the key to progressing seriously in our sport. Although it may seem like an academic clarification, it is useful to remember that every time we talk about physical quantities, we must indicate a reference system in which they are measured. In the following we will use the International System of Units, (SI), the metric system that allows us to switch from one unit to another simply by multiplying or dividing by 10 or multiples of 10. It is always possible to switch from metric to Imperial with appropriate equivalences.

3.2 A few bricks to build the universe

Everything that occupies a space and has mass is matter. Therefore, every portion of matter has a volume, which measures how much space it occupies, and a mass, which measures the quantity of matter by which it is formed. Matter is made up of a number of basic elements, which have their own physical and chemical characteristics. Elements are, for example, nitrogen, aluminium, chlorine; but not water or steel, which are compounds formed by the union of several elements. If we have a certain amount of an element, for example one kilogram of aluminium, and we break it up into smaller and smaller parts, all these parts are nothing but smaller and smaller pieces of aluminium, and therefore retain all the characteristics of this element. How far can we go in subdivision, but still deal with aluminium? Up to the elementary unit of an element, the atom. This is the smallest portion of an element that retains its physical and chemical characteristics. Therefore, each different existing chemical element corresponds to a specific atom. In nature we have 92 different elements, consisting of atoms that all have the same basic structure: a nucleus of particles called protons and neutrons, and a cloud of electrons that orbit at a great distance from the nucleus like small satellites, in number equal to that of protons. Protons have a positive electric charge and electrons have an equal and opposite (negative) charge. Neutrons (hence the name) are neutral, i.e., they have no electric charge, therefore, being the number of protons equal to that of electrons, the whole atom is electrically neutral. The peculiar thing is that the atoms of an element differ from those of another element only for the number of protons, called atomic number. Therefore, great differences between the characteristics of different elements are due only to the difference, we might almost say merely "quantitative", in the atomic number, i.e., how many protons (and electrons) are contained in the atom. Take oxygen, for example, which is an element of atomic number 8, shown schematically in Fig. 3.2‑1.

Fig. 3.2‑1 – Atomic structure of oxygen

The nucleus contains 8 protons, and 8 electrons orbit around it. The diameter of an atom is about 10,000 times larger than that of the nucleus, so the atom is practically an empty structure. The electrons revolving around the nucleus are arranged in shells called orbitals, which have a specific shape, corresponding to a certain level of energy of each electron. By means of a so-called chemical bond, the atoms of different elements can join together permanently to form compounds. The basic unit of compounds is called a molecule. Like the atom for elements, it is the smallest portion of a compound that has its own physical and chemical characteristics. Just as the few letters of the alphabet are sufficient to construct thousands of words, the 92 elements existing in nature are sufficient, when combined, to form the more than 4 million known compounds. For example, carbon dioxide, which has a chemical formula CO 2, is a compound whose molecule is formed by an atom of carbon and two of oxygen, as the chemical formula suggests. Similarly, for water, which has an H 2O formula, the molecule consists of two hydrogen atoms and one oxygen atom. Elements and compounds form all the substances that we find in nature. If we now take more compounds or elements and mix them together, we can get two things: mixtures or solutions. The former is a mixture of spatially randomly distributed, more or less homogeneous substances. The latter, on the other hand, are a homogeneous mixture of substances, with identical composition at each point, and in which h the individual components are not recognizable with the naked eye or by optical systems. Air is an example of a solution. Sand, on the other hand, is an example of a mixture. If we put a teaspoon of sugar in the coffee we create a mixture, but as soon as we stir the coffee and dissolve the sugar, we turn the mixture into a solution. And indeed, the sugar disappears from view, but not from taste.... Let's recap:

Atoms: the smallest portion of matter that retains the properties of an element.Elements: simple substances consisting of identical atoms (same atomic number).Compounds: substances formed by several elements, consisting of molecules all identical.Substances: the sum of compounds and elements.Solutions: perfectly homogeneous mixtures.Mixtures: a combination of several substances with a composition varying from one point to another

A diver is immersed in water (a compound), breathes air (a solution), swims above the sandy bottom, (a mixture) and can decompress with oxygen, (an element). (Such a diver attends imaginary pristine environments, in which water is a compound. In the real case of sea water, it is actually a mixture of hundreds of different elements and compounds....)

3.3 Force, the cause of everything

In everyday speech, force is an intuitive and simple concept. However, when it comes to physical laws, this is not the case. Force is an interaction between two bodies: it acts on one body and is originated by another body. Its action has the effect of varying both the velocity of the body to which it is applied and the body that causes it (it can be a hammer blow, the attraction of a magnet, the wind blowing up a sail, a falling object...). Variation in speed means acceleration, deceleration or change of direction. Moving from a state of rest to a state of motion and vice versa are always the results of applying a force. Pressing the accelerator or brake of a car is equivalent to applying forces to the vehicle. The great Isaac Newton first described, with his famous laws of dynamics, the effects of applying forces to bodies.

Fig 3.3-1 - Isaac Newton