Just a little deeper…..

PADI, and other training organizations, set their limit for recreational deep dives at 40 meters/130 ft. Going a little deeper than this does not necessarily mean you will immediately find yourself in difficulties or requiring a decompression stop, although you will have an increasingly limited amount of remaining no decompression time left as you go deeper, so if we can go deeper why set the limit at 40 meters/130 ft?

There a several factors contributing to the necessity of this limit, arguably gas availability and narcosis among the primary concerns.


As the surrounding ambient pressure increases, the pressure of the gas entering divers lungs increases. This results in a higher density of inert gases entering our blood stream.

gas density

Some theories suggest inert gases dissolving in the lipids of cell membranes cause narcosis. The mix of gases that makes up what we breath, such as O2, Nitrogen, or helium (for technical divers), have differing solubility characteristics making them more (nitrogen) or less (helium) effective as a narcotic agent than others.

O2 narcosis

Some theories also suggest an amount of unmetabolized oxygen that remains present in the body also has the potential to cause narcosis. This is a contentious issue and although advocated by some others remain unconvinced there is any real evidence that O2 is narcotic. PADI suggest in their training materials, for the purposes of dive planning, to er towards caution and expect the same level of narcosis for high O2 Nitrox blends as you would for air.

CO2 narcosis

Carbon dioxide plays a role in this, in addition to having a high narcotic potential it can also increase the narcotic abilities of other gases. Carbon dioxide may accumulate due to exertion or erratic breathing making it essential to keep your work rate as low as possible and plan deeper dives in calm conditions.

Signs and symptoms of narcosis

It’s difficult to provide a reliable example of the effects of narcosis at a given depth, it hits us all slightly differently from one dive to the next. It is however commonly agreed that there are increasing risks of significant effects below 4 bar or 30 meters/100 feet, the risks increasing further at depths of 40 to 50 meters 130/160 feet. Technical divers can offset this with the use of helium in their breathing gas, which has a low narcotic potential, allowing deeper dives with reduced levels of narcosis.

Signs and symptoms of narcosis vary, but in the early stages you may expect feelings of mild euphoria, slowness or numbness of thinking and delayed responses. More significant signs and symptoms may include anxiety, extreme euphoria, a false sense of confidence, uncontrolled laughter, terror or extreme fatigue which can lead to blackouts.

Nitrogen narcosis
Lippmann, John; Mitchell, Simon J (2005). “Nitrogen narcosis”. Deeper into Diving (2nd ed.). Victoria, Australia: J.L. Publications. p. 103. ISBN 0-9752290-1-X. OCLC 66524750.

You may have heard divers claiming, occasionally even bragging, that they do not get “narked”, what I understand when I hear such claims is the diver is simply not aware enough to notice. With experience some divers learn to manage levels of narcosis, but no one is born with or develops immunity to it. Good diving practices such as slow descents can reduce the affects, some lifestyles can make divers more or less susceptible, for example a diver with a generally high alcohol intake, poor hydration and lack of rest may find themselves suffering narcosis at relatively shallow depths.

Gas consumption

How much does your average diver really know about their gas consumption? Aside from anecdotal information such as “i’m an air hog” or “I hardly breath at all” how much useful data, in terms of dive planning, can your a typical diver provide. Before embarking on a deep dive ask yourself at 40 meters how much gas would you need to get to the surface while maintaining at a safe ascent speed.

Technical divers spend a great deal of time in training trying to analyze this, during a decompression dive direct access to the surface is no longer an option so they have to be certain they have enough gas with them to make the dive. Recreational divers need only get themselves back to the surface, but at 40 meters while ascending at a safe speed, that’s quite a long way off when low or out of gas.

Surface air consumption SAC

Try this exercise from the PADI Tec 40 course on your next dive. Swim at a fixed depth for a fixed period of time and monitor how much bar you use. As a suggestion try swimming at 10 meters for 10 minutes, check your gauge just as you start and immediately as you finish, make a note of the SPG readings on a slate. When you get back use the formula below to calculate your surface air consumption (SAC).

RMV formula

* if you are not sure what your cylinder capacity it ask your Divemaster to point out the tank markings that will provide this information. 

** The sum (depth in meters + 10 / 10) allows us to calculate our ATA for any given depth. ATA or atmospheres absolute can be used as a means of measuring the combined pressure of air and water. Presumably you’re in one atmosphere of air as you read this, as water is heavier than air every 10 meters of sea water is equivalent in weight to 1 atmosphere, at 10 meters you are at 2 ATA, one of air and one of water.

An example of a diver who swam at 10 meters for 10 minutes and used 30 bar from an 11 liter tank

RMV example

From this we learn that this person on a dive would breath 16.5 liters of gas per minute at the surface (if all other factors remaining unchanged).

With that understood we can use this information to calculate what they would breath at any other depth during a dive.

For example were I to plan a dive to 40 meters for 6 minutes and wanted to know the volume of gas I would need for the bottom part I would multiply my SAC rate by the ATA (see ** above) and then by the time:

Gas requirementNow consider that your typical 11 liter scuba tank filled to 200 bar holds 2200 liters of gas (11 X 200 = 2200), that’s a big chunk of that gone already and we have not even factored in the gas needed for the ascent, emergency reserve or that in an emergency you will probably find yourself breathing 4X as much as you normally would, that is to say the diver above would have a SAC rate of 66 liters per minute vs the 16.5 he planned for.

As incredible as it sounds anyone who has found themselves in a scary situation such as a motor accident, or even someone startling you, will remember the rush of adrenaline, increased heart rate and increase in respiration’s. This has massive implications when breathing from a limited gas supply at depth.

Gas requirements

An example of gas requirements of a dive to 40 meters, for a diver with a common SAC rate of 18 LPM, for 8 minutes, using an ascent speed of 10 meters per minute.

 Depth Time SAC ATA Gas volume
40 meter 8 18 5 720
Ascent* 4 18 3.2 230
5 meter safety stop 5 18 1.5 135

* Depth averaging used for ascent depth. Bottom depth – safety stop/2 + safety stop (40-5/2+5=22M or 3.2 ATA)

With the gas requirements listed here added to the 50 bar emergency reserve  that totals 148 bar, leaving 52 bar from the 200 you started with.

Faced with a free flowing regulator, hose blowout late into the dive, or what would happen to your breathing rate when faced with such an emergency 52 bar or 550 liters of gas doesn’t sound so much does it?

Partial Pressure of oxygen PPO2

For Nitrox divers this will be a familiar topic, as you descend the ambient pressure increases, which in turn increases the density of your breathing gas.

As there are a mixture of gases in whatever you breath (Air is approximately 79.% nitrogen and 21% oxygen), the partial pressure of a gas in a mixture of other gases is equal to the pressure it would exert if it occupied the same volume alone.

To calculate a partial pressure of O2 multiply the ATA by the fraction of O2 in your breathing gas, for example breathing air at 50 meters gives a PPO2 of 1.26 ( 6 ATA x .21=1.26)

As we descend on a dive the partial pressures of all gases we breath increases, our bodies have a limited tolerance for high PPO2. The National Oceanic and Atmospheric Administration (NOAA) recommend a maximum of 45 minutes at 1.6 bar absolute or 150 minutes at 1.4 bar absolute for single exposure.

Commonly we use 1.4 as our maximum PPO2 on a dive, 1.6 being used as a fallback or contingency by recreational divers and for decompression stops by technical divers.

PPO2 increase comparison table
This table shows a comparison of how PPO2 increases with depth. The vertical column shows partial pressure, the horizontal depth in meters and ATA.

 These PPO2s provide us with our maximum depths for the breathing gas we are using, for example with air we would reach 1.4 at 56 meters.

1.4 (PPO2)  / .21 (% of O2 in the gas) = 6.6 ATA or 56 meters

Using higher blends of O2 found in enriched air we reach these PPO2 limits in much shallower depths, for example using EANx32 we reach 1.4 at 33 meters.

1.4 PPO2 / .32% = 4.3 ATA or 33 meters

CO2 comes back into play again here also, acting as an exciter for O2 toxicity, on deeper dives approaching PPO2 limits it is quite likely we will find ourselves working a little harder, possibly even laboring against some of the effects of narcosis, CO2 accumulation is a real concern over extended periods.

Divers exceeding the depth limits by going beyond the maximum PPO2 or their respective time limits are at risk of oxygen toxicity, leading to an underwater convulsion, the typical result of which is drowning.

Decompression sickness DCS

Ask an average diver if he understands decompression sickness there is a better chance than not he will answer in the affirmative, ask an expert he will probably say something along the lines of not entirely, or simply no.

With this in mind there is no table, computer or technique that can guarantee you will be safe from DCS. You can reduce your risks somewhat by diving well within No Decompression Limits (or as a Tec diver decompressing conservatively), staying well hydrated, sleeping well the night before a dive, avoiding rapid temperature changes after a dive and being mindful of good overall health and fitness with particular attention required for anything affecting your circulatory and repertory systems.

Ascent rate

Staying within the no decompression limits is only part of the game, the rate of ascent is also crucial.

Many divers are aware of the maximum ascent rate of 18 meters per minute but fail to notice the word “maximum”.

This should be reserved for emergency ascents and treated very cautiously. Many divers nowadays are advocating ascent speeds of 5 to 10 meters per minute, slower still on the final ascent from safety stop to surface.

As long as you respect the no decompression limits of a dive there is no such thing as ascending too slowly, ascend to quickly however and you may be visiting your local recompression chamber.

Equipment failure

Ever experienced a low pressure hose exploding on a dive, a free flowing first or second stage or an O ring giving up the fight at the deepest point of a dive? In most cases the answer is probably no and that’s great, but it does happen and more often than most divers would wish to believe.

Deep dives do not only put the diver under more strain, their equipment is working harder as well. With increased depth comes increased risk, not just of malfunction but of the likelihood of exhausting the gas supply before a diver can safely reach the surface without exceeding a safe rate of ascent.

Maintaining your gear is a good step towards preventing this but it certainly does not remove the risk. At depths of 5 ATA and on some of your gear may be being pushed to, or past, its limits.

Redundant life support systems such as bail out or pony tanks are a sensible measure to take. In excess of 40 meters/130 feet consider technical equipment where everything, including your regulators has a back up as mandatory.

 The error chain

In most cases it is probably safe to say that no one factor can be identified as the cause of an accident, it is more likely attributed to a combination of smaller problems. Ignoring the finer details or missing a small and seemingly insignificant problem combines with other such factors. A very short chain of errors can manifest into something very dangerous and very quickly.

In reading this far you may have come to understand deep dives present some very real dangers if not properly planned and executed. The limits of recreational diving whilst seemingly arbitrary, and ultra conservative in some cases, are there for good reasons. These limits have been shaped, amongst other factors, by previous incidents and the accumulated knowledge drawn from them.

Diving beyond 40 meters carries with it the risks of exceeding the increasingly small amounts of no decompression time available, omitted procedures or errors due to narcosis and insufficient volumes of breathing gas, especially in the case of equipment failure or emergency decompression requirements.

Your own personal limits need to be equally well considered, going beyond your comfort or experience level should be approached with caution. Set your own boundaries and get properly trained before venturing further.

If you do feel the challenge of deep dives calling you get the training required to get there and back safely. look at the courses on offer and find the right instructor, equip yourself with the appropriate level of knowledge, skill and experience needed to do it properly and use equipment designed for the task.

 For more information on Technical diver training click here.





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