COMPOSITION OF THE AIR AT SURFACE LEVEL

The air that we breathe is composed of a mixture of gases, for simplicity let’s say that is made up mainly of 20% of oxygen and 80% of nitrogen.

Dalton’s Law States “ The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume” At sea level with a pressure of 1bar, the partial pressure of the oxygen will be 0.2 bar, & the nitrogen will have a partial pressure of 0.8 bar. IT is the oxygen that is metabolised by the human body for energy, transformed into carbon dioxide or expelled; while the nitrogen is inert and circulates as a dissolved gas in the blood stream. It has a partial pressure of 0.8 Bar.

COMPOSITION OF THE AIR DURING DIVING

Whilst underwater a diver breathes compressed air or, more precisely, air that is supplied at ambient pressure. At a depth of 10 metres, the pressure inside the lungs will be the equivalent of the ambient pressure, 2 bar. In these conditions as the maximum volume of the lungs cannot change the density of the air varies according to the ambient pressure. We have therefore, at this depth, doubled the quantity of air supplied. Likewise the partial pressures of each of the gases shall increase.
At this point another law of physics comes into play. Henry’s Law. This
states, “The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas that is in contact with the liquid and the solubility coefficient of the gas in the particular liquid”.
This means that the deeper the diver descends the greater the quantity of oxygen and nitrogen passes into the blood stream and the tissues. The increase in the partial pressure of the oxygen causes no problems at the recommended depths for recreational diving. However, the quantity of nitrogen absorbed (which is 4 times greater) can cause nitrogen narcosis during the dive or decompression sickness after the dive.

NITROGEN NARCOSIS

From the middle of last century, it was observed that men exposed to hyperbaric air, which is higher pressure than normal, behaved as if they were drunk. For this reason these symptoms were called nitrogen narcosis.
As soon as a diver leaves the surface and descends he is exposed to an increase in the partial pressure of nitrogen. At the same time the first symptoms of narcosis become apparent. Near the surface the symptoms are mild and the effects increase as the diver descends, affecting his awareness and behaviour. The danger is that as the Nitrogen Narcosis affects reason, a diver may not realise that that he is affected.
Nitrogen narcosis adversely affects the thought processes causing difficulties in carrying out normal activities such as reading instruments, communicating with diving buddies, recognising the right direction to the surface, any of which can be potentially dangerous. Nitrogen narcosis effects different divers differently depending on the physical conditions and the ambient. It is difficult to establish a depth at which these symptoms appear. It can be said that from studies carried out at random, that dives carried out above 18 metres depth have fewer accidents. If effected by Nitrogen Narcosis the treatment to reduce or to eliminate the symptoms completely is simply to ascend a few metres and breath normally.

DECOMPRESSION SICKNESS

As we have seen the increase in pressure during descent, corresponds to a rise in the partial pressure of the gas breathed. According to Henry’s Law the pressure in the tissues increases at the same rate. With increasing the depth these percentages vary in the same proportion, the longer spent at a given depth, and therefore pressure, the more gas absorbed. On ascent, maintaining a rate of 9 metres a minute, the opposite happens. The excess nitrogen is released from the blood through the lungs, at a partially higher pressure. If you ascend too quickly the nitrogen is cannot be released quickly enough thus contributing to the formation of bubbles in the divers body. This can lead to Decompression Sickness. The symptoms are different according to the tissue affected and can appear between 15 minutes and 2 hours after surfacing. The symptoms of DCS may appear as late as 48 hours after surfacing. Flying after diving may bring on symptoms if attempted too soon.

SKIN DECOMPRESSION SICKNESS

This can range from a mild rash to an angry measles like rash or a bluish marble like mottling of the skin. The mottling is associated with severe DCS and is caused by the bubble blocking the blood vessels of the skin. Mild cases may disappear even if untreated. However medical help should be sought if ANY symptoms are seen, the skin symptoms may be masking more serious neurological problems.

JOINT DECOMPRESSION SICKNESS

This usually starts as a feeling of tenderness or numbness at or near a joint, soon becoming a dull ache.
The affected joint may swell and become red. The pain will increase over the next 12 or 24 hours and will not be relieved by heat or pressure – as a bruise may be. It is most common in the shoulder.
Commercial divers are more prone to this type of DCS as it is more commonly associated with exertion and long duration’s of diving. It is
common among caisson workers. Medical help must be sought, as more serious neurological DCS may also be present.

CENTRAL NERVOUS SYSTEM DECOMPRESSION SICKNESS

The brain and spinal cord both have very large blood supplies and are both very susceptible to any bubbles in the blood stream. The symptoms for a CNS Decompression Sickness are varied and diverse. Some of the more common are; extreme fatigue, a strong feeling of malaise, pins and needles and numbness. Partial or total paralysis, loss of bladder function, blurred vision, confusion and even death can be the result of a CNS DCS. This is the most common type of DCS among sport divers and is usually associated with insufficient decompression.

FIRST AID

Contact the nearest Recompression Facility. 100% Oxygen must be given to the casualty as soon as possible after the incident & during the journey to the hyperbaric chamber. The application of pure oxygen guarantees greater oxygenation of the tissues and helps to lessen the risk of damage. In parallel with the administration of oxygen the casualty should be encouraged to drink fluid at a rate of about 1 litre per hour.
Avoid diuretic fluids such as coffee or any alcoholic drinks. Still water or Isotonic sports drinks are best.
Maintaining a good level of hydration helps the blood volume and reduces the risk of more bubbles forming.

Prevention of accidents

Diving should be considered as an enjoyable and relaxing activity. For this reason diving should be undertaken by divers with good levels of fitness and training at depths of not exceeding 20 metres. This will minimise the risks of decompression sickness. The rules for the prevention of decompression sickness can be divided as follows:

MEDICAL

A visit to the doctor should discover the presence of any pathological problem (for example epilepsy, diabetes etc.)

BEFORE DIVING

Avoid alcoholic or caffeine drinks (coffee, coke etc.) as they dehydrate you and cause diuretic problems Always drink plenty of still liquids before diving, especially in summer Avoid stress and excess physical exertion.
Avoid certain medicines or drugs; if in doubt consult your doctor.

DURING DIVING

Follow the tables and keep to the correct rates for descent and ascent If you are tired end the dive & surface, tiredness causes an increase in the absorption of nitrogen Avoid “see-saw” or “saw tooth” dive profiles Be cautious if it is cold, this causes vascular constriction, dehydration, and stress, which increases the circulation activity

DURING ASCENT

Keep to an ascent speed of 10 metres a minute, and obey any ascent alarm given off by instruments. Always carry out a safety stop at 3 metres for 3 minutes.

AFTER DIVING

Avoid heavy physical activity
No snorkelling after scuba diving
Don’t fly immediately after diving
Re-establish normal body temperature

Dive tables

download: Navy_air_no_deco_tables

In order to dive safely, the nitrogen absorbed during the dive must be allowed to escape from the body without producing bubbles of a size or in a quantity, that may cause decompression sickness. The rules are; not exceed the ascent rate of 9 metres a minute, to allow the tissues to get rid of the nitrogen by breathing normally on ascent. As an extra precaution a safety stop of 3 minutes at 3 metres must be carried out. On the surface the rest of excess nitrogen is released through the lungs until the partial pressure drops to 0.8 bar.
During this course all dives will be performed within the limits of non-decompression. Over the years several dive tables have been put together, some of which are particularly suitable: Buehlmann, BSAC 88, & French navy, tables. The most commonly used dive tables in use today are the US-Navy tables. For many years these tables were the only method used by divers to calculate decompression requirements. Even today, despite the rapid evolution and diffusion of diving computers dive tables are still probably the most common system in use, especially amongst those beginning the sport who prefer to spend their money on other equipment.

TERMINOLOGY

Descent rate
The speed at which the diver descends from the surface. A maximum speed of 20 metres a minute is advised.

Maximum depth
This indicates the maximum depth reached during a dive, even if the time spent at this depth represents only a small part of the total.

Dive time
It is the time spent from the beginning of the descent to the moment when the ascent begins.

Ascent rate
This is the speed that the diver must not exceed when he moves towards the surface, either for final ascent or to reach another depth level during the course of the dive.

Dive letter group
This indicates the saturation level of the residual nitrogen in the tissue at the end of the dive and after a time on the surface. It is used to calculate how much any following immersion might be penalised. It is calculated in letters from A to O.

Surface interval
The time elapsed from leaving the water at the end of one dive to the beginning of the following one.

No decompression dive
It is a dive performed without exceeding the limits indicated by the tables, always maintaining the ascent rate and the safety stop.

Repetitive dive
It is a dive performed more than 10 minutes but less than 12 hours after the first. This means that the divers body tissues have not yet become completely desaturated of nitrogen. Any new dive within 10 minutes of the last must be considered part of the previous one.

Decompression dive
It is a dive for which the tables indicate the need for decompression stops. This subject will be dealt with in a later course.

Use of the tables

DIVE TABLES

On dive tables the time and depth of the dive are correlated. Along the top, the maximum depth is indicated with intervals from 1.5 to 3 metres. When calculating one must always take the higher figure, for example with 11 metres, 12 is used as the previous number is 10,5 metres. The column along the side indicates dive time. Here too the higher time must be considered. Following the two indications the letter group is indicated.
On the extreme right are indicated the times of the surface interval. From this point moving down the column the new letter group is found. The meeting point between these and the depth of the next planned dive indicates the minute of penalisation or residual nitrogen time (RNT).

EXAMPLE OF CALCULATION USING THE TABLES

When you plan a dive is a good rule to follow the scheme below, in which the depth one wants to reach and the relative time are indicated.

In this way an exact dive profile is created. The first thing is to establish the maximum depth to be reached, for examples 13 metres, naturally considering 15 metres on the tables following down the column the limit of permanence at this depth is shown at 100 minutes. Planning to dive for 35 minutes, 40 minutes must be calculated.

At this point on the right the corresponding dive letter group, in this case F is found.

Now, wanting to plan a second dive, after a break on the surface of 2 hours and 30 minutes, moving to the right to find the box corresponding to this time, we find the interval from 2 hours 29 minutes to 3 hours 57 minutes. Below the new dive letter group is indicated: in this case C.

The change of dive letter group shows that the tissues have got rid of a part of the nitrogen during the interval on the surface.
Now wanting to plan a second dive where the maximum depth must never exceed the previous one, in this case 13 metres for 25 minutes, the first thing is to find the minutes which penalise the dive and add them to the real dive time. To do this one follows down under letter C to the point corresponding to the maximum depth programmed, always calculating the lower figure. This cross check between letter C and the depth of 12 metres gives 25 minutes. This means that a dive of 25 minutes must be calculated at a total time of 50 minutes.

The tables can be used also to calculate the surface interval that indicates when a second dive can be carried out following the relative data of the two planned dives.

Dive computer

Progress in technology has made it possible to make diving computers easy to use and reliable in operation.
They are becoming a common part of diving equipment.
Nonetheless it should be pointed out that any computer, however sophisticated and precise could never be a substitute for the human brain. Careful preparation and common sense is better than any instrument.

HOW TO USE THE COMPUTER

A personal dive computer cannot calculate the exact saturation state of our tissue but works with theoretical calculations, being unable to consider all the personal characteristics such as obesity, age, tiredness, smoking etc….

Let ‘s try to sum up the five main points in the use of the computer:

In water

BUDDY BREATHING ASCENT

ALTERNATIVE AIR SOURCE ASCENT

EMERGENCY ASCENT USING THE FINS AND B.C.D.

 

EMERGENCY ASCENT WITHOUT BELT

IN WATER WEIGHT BELT DRESSING
ROTATION

BACK PASSAGE