Introduction  

We (humans) didn’t evolve to breathe underwater. However, the natural curiosity we all have and the high appeal for underwater exploration has led us to develop various systems that enable us to visit the underwater environment. Each form of diving invented caters to different underwater activities, depth and time requirements, individual preferences, or sea conditions. In this article, we briefly discuss the key working principles behind free diving, SCUBA diving, hookah diving, and few others, and we look into basic requirements of each apparatus and its limitations. 

1. Free Diving, aka Breath-hold Diving, or Apnea 

(Working principle: “hold your breath, don’t breathe underwater”) 

The oldest and most rudimental approach to diving simply involves holding your breath for period of time while you’re submerged. The humble beginnings of freediving date back to ancient times. Today, it’s a fun activity that many water-lovers practice. The unparalleled freedom of diving without any sophisticated apparatus is very appealing. Grab a mask, fins and perhaps a weight belt (and wetsuit in colder waters), and you’re ready to go! 

Hold your breath, and as you descend, the water pressure shrinks your lungs, according to Boyle’s law, to one half of their original volume at 10m, one third at 20m, etc. Ascend towards the surface and your lungs regain their original volume. This is a fundamental difference compared to diving with any of the breathing apparatuses mentioned later. 

The downside of freediving is that your time underwater is very limited, hardly enough to explore the environment or interact with marine life. Average people can hold their breath for about 30 seconds to 1.5 minute and dive comfortably to depths somewhere between 3 and 15 meters. Of course, champion freedivers can do upwards of 7 minutes and dive over 150 metres deep, but that’s another story. 

While shrinking the lungs during freediving is not known to cause major damage to the lung tissue (unless we’re talking about “lung squeeze” which may occur in competitive apnea), it doesn’t mean that freediving is without risks. Freedivers can suffer from oxygen starvation leading to Shallow Water Blackout (SWB) and potentially drown. They also risk Barotrauma (of eyes, ears, sinuses, or lungs), Nitrogen Narcosis, Decompression Sickness (DCS), entrapment and drowning, etc. 

In summary, as magical as freediving may be, unless humans grow gills, we’ll always need some kind of breathing apparatus to stay underwater for extended period of times. The desire to stay longer underwater motivated the inventions of various underwater breathing apparatuses mentioned below. 

Pros and cons of free-diving  

Pros	Cons
Simple, accessible, independent	Very limited dive time and depth
Freedom of movement underwater	No room for error, no bailout systems
Minimal gear, lightweight, portable	Lack of education on health risks
Low initial cost, no ongoing cost

 

2. SCUBA Diving 

(Working principle: “breathe bottled air that you take with you underwater”) 

Currently, the most wide-spread form of diving amongst recreational divers is SCUBA (Self Contained Underwater Breathing Apparatus) that rapidly developed since Cousteau’s and Gagnan’s invention of the modern demand regulator in 1943 and the commercial success of their company Aqua Lung shortly after. 

The basic working principle of an open-circuit SCUBA system is to “pack” large volume of air in a relatively small cylinder, that is then slowly released on demand as the diver breathes through a 2-stage diving regulator. Exhaled air is released in water. For example, the most popular SCUBA tank, AL-80, with 11 litres internal capacity holds about 2300 litres of air compressed to 210 bar. Compressing air to such enormous pressure requires a huge amount of energy and a large, powerful compressor. Imagine, the energy stored inside a SCUBA tank is equivalent to kinetic energy of a 3-ton truck travelling on highway at 110 kmh.

However, human breathing underwater only requires air pressure slightly higher than the ambient water pressure at the diver’s depth, for example 3 bar (absolute) pressure at 20m. Hence, the vast majority, over 98% of the pressure and energy required to fill a SCUBA tank is for “air storing” rather than “diving” purposes. SCUBA is very energy inefficient form of diving. This is where the key disadvantage of SCUBA in terms of complexity, size and weight of the gear stems from. 

SCUBA gear is relatively heavy. The air tank alone can weigh around 16 to 20kg when full, depending on its size and material. If you include the BCD, regs, weight belt, wetsuit, fins, mask, etc., SCUBA divers easily carry some 25-30+ kg of gear. Typical recreational dive takes about 45-60 minutes reaching depths of up to 30m with advanced training and 18m with open water training. The maximum depth limit for recreational diving is 40m to keep the risk of Decompression Sickness (DCS, aka “Bends”), Nitrogen Narcosis or Oxygen Toxicity manageable. Recreational divers must not exceed the No Decompression Limit (NDL), which is a nitrogen loading threshold beyond which safety stops become mandatory to prevent DCS. Diving beyond 40m is called Technical Diving, which requires additional skills, equipment with greater redundancy, and often different gas mixtures. 

All-in-all, SCUBA is a widely popular form of diving among recreational divers, especially for guided dive tours thanks to its versatility. Immense energy required for “storing” the air and complexity of the air filling make this form of recreational diving practically dependable on external infrastructure (dive centres/ tour operators/ resorts), which in turn limits the spontaneity and increases ongoing (variable) costs for the divers. 

Pros and cons of SCUBA diving  

Pros	Cons
Versatile, developed, widespread	Heavy, bulky, cumbersome
Sufficient dive time and depth for most	Dependency on dive centres, opening hours
Allows diving in various sea conditions	Ongoing costs and logistics
Can be used in obstructed environments	Time-consuming pre and post dive

 

3. Hookah Diving, aka SSBA – Surface Supplied Breathing Apparatus 

(Working principle: “breathe air from a surface compressor through a hose”)  

Unlike SCUBA where you “bottle” the air at a facility to later take underwater with you, SSBA (colloquially called “hookah”) continuously compresses air from the surface and delivers it to a diver on demand, through a long hose and regulator. 

Surface supplied diving has been existing for over 200 years, which makes it older than SCUBA. You remember it from the old pictures of divers in brass helmets. The recreational version of surface supplied (aka “hookah”) diving spans over 6 decades since the introduction of gas-powered units to the wider public, such as the Evinrude Aquanaut or Johnsons Air Buoy in the in the 70’s. Nowadays, thanks to modern materials and technologies, new kind of small, lightweight, battery-powered systems emerged, and the recreational hookah diving enjoys renaissance. Battery-powered compressors significantly reduce the inherent risks of its petrol predecessors, such as CO poisoning from own exhaust fumes, engine stalling due to water in petrol, flooding, etc. 

Whereas the roots of hookah diving are predominantly in commercial diving and aquaculture where gas-powered units remain relevant until today due to their extended runtimes (3-5+ hours), the compact and light-weight battery powered floating units are becoming increasingly popular amongst recreational divers for various self-guided shallow water diving activities. Especially in scenarios where SCUBA gear is too impractical, cumbersome or costly and diving depth greater than some 10-12 metres is not required, such as: 

• recreational diving at shallow reefs or lakes 
• spontaneous diving on travels, e.g. by campervan or sailboat 
• boat maintenance, hull cleaning, fouled propeller untanglement or retrieval of stuck anchor 
• underwater photography of marine life, or fashion pictures in pools 
• recreational lobstering, collecting abalone, scallops or sea urchins 
• underwater treasure hunting with metal detector, or fossil collection 
• coral restoration projects, management of crown-of-thorns outbreaks 
• maintenance of swimming pools, mould cleaning, grout repairs, drain or lights installations 
• etc. 

Hookah diving is not without risks. Surface-supplied diving does involve breathing compressed air, so the underlying dive theory and basic safety principles taught in SCUBA equally apply to hookah diving. Pulmonary Barotrauma remains a major risk, just as with any other form of diving with compressed air (or gas mixture). Divers must never hold their breath on ascent. The risk of Decompression Sickness is present, but rather unlikely due to the limited time and shallow depth of recreational hookah divers using the battery-powered floating units. For example, the NDL (No Decompression Limit) of a 10-metre dive is 3:39 hours or 2:27 hours for a 12-metre dive according to PADI recreational dive planner. Practically speaking, as the NDL exceeds the typical 45-60min runtime of the battery powered floating hookahs, you would need to swap multiple batteries and dive with minimal surface intervals to those 10-12m depths to reach the NDL. 

Pros and cons of hookah diving  

Pros	Cons
Easy, lightweight, portable and stowable	Shallow water diving only
Independent, spontaneous, grab-and-go	No diving in obstructed environments
One device can support multiple divers	Limited use in rough seas, or boat traffic
Diver(s) location visible from the surface	Not suitable for large groups of divers
Very low operating costs

 

4. SNUBA Diving 

(Working principle: “breathe bottled air from surface through a hose”) 

SNUBA is a hybrid system between SCUBA and hookah. It uses a SCUBA cylinder floating on a raft, supplying air to the diver through a long hose. It is used by some dive tour operators (mostly in Hawaii, Mexico and Caribbean) to introduce untrained people (even children) to diving, following a 15-20 minute safety briefing. There are several divers breathing from one SNUBA system, accompanied by an experienced guide, who, unlike the rest of the group, is using SCUBA gear for greater flexibility. The depth is typically limited to about 6m. 

Outside of this specific scenario, the use of SNUBA is rather limited. The weight and size of SNUBA is equivalent or exceeds full SCUBA gear. The system is using a SCUBA cylinder, hence it’s burdened by the same logistical issues and costs as much as SCUBA diving. Cylinders can only be hired by certified divers, who in most scenarios prefer taking the tank with them underwater. A major drawback of SNUBA system is that the diver has no indication of the remaining air pressure inside the cylinder. Running an additional, long high-pressure (i.e. very stiff) hose from the tank on the surface down to the diver is impractical and wireless transmitters only work for short distances underwater. Acoustic (sonar) transmitters haven’t made much headway so far, as they face unique set of challenges. 

In terms of safety, the risk profile of SNUBA is similar to recreational hookah diving. The common risks stemming from breathing air underwater are present, albeit some of them diminish (or practically become irrelevant) due to the shallow dive profile. 

Pros and cons of SNUBA diving 

Pros	Cons
First underwater experience without training	Largey, bulky, difficult to transport and carry
Less intimidating to novices than SCUBA	Dependency on 3rd parties for air-filling
Relatively free movement underwater	Diver(s) don’t see tank pressure information
No need to wear and operate BCD

 

5. Diving with a Rebreather 

(Working principle: “recycle the air (or gas) by removing CO2 and replenishing O2”) 

Oxygen is only about 21% of air. Our body metabolises about 1/4 of the oxygen (O2) from the air we breathe (at rest about 250ml per minute). Thus, we only use a small fraction of the air volume we breathe. The vast majority of the gas volume, we just ventilate in and out of our lungs. Why waste the air and discharge it into water if it could be recycled (in a closed loop circuit)? This realisation sparked the idea behind rebreathers.   

Rebreathers are relatively complex systems containing counter-lungs (i.e. strong “bags”) that temporarily hold the exhaled air for recycling, CO2 scrubber which is a chemical granulate (typically soda lime) that absorbs CO2 from the exhaled air, various sensors and computer(s) to analyse the quality of the air (or gas mixture), cylinder with oxygen and a cylinder with diluent gas (air, nitrox, heliox or trimix), various manual (mCCR) or automatic (eCCR) valves for oxygen injection, specialised dive surface valve and regulator, breathing hoses, batteries, etc.  

Rebreathers allow diving longer (2-10+ hours) and deeper (40-200+ metres) depending on the type of the rebreather, size of the cylinders, the gas mixture used, diver’s training, etc. They have additional, interesting benefits, such as silent operation without bubbles, reduced heat loss and higher air (gas) humidity leading to slower dehydration of the diver. Rebreathers are popular for technical diving, cave diving, military use or amongst underwater documentarists who film shy marine species. 

Despite various interesting advantages, the adoption of rebreathers amongst recreational divers is low. Rebreathers appeal to smaller, specialised groups of divers who can fully utilise their potential in tech or cave diving. The equipment is quite heavy (ranging around 20-40 kilograms), expensive when considering the initial purchase price of the gear and ongoing costs. Divers require equipment-specific training, and the availability of the consumables (CO2 scrubber, gas fills) is limited, especially in remote areas and less developed countries. Only a few dive charters allow the use of rebreathers for various reasons, such as lack of appropriately trained staff, insufficient support in emergencies, short time limit due to boat schedule, etc. Rebreathers are complex with additional failure points and safety concerns compared to open circuit systems. Furthermore, set-up, pack-up and cleaning of rebreathers takes more time and requires discipline and attention to detail, which reduces the spontaneity and convenience. 

Pros and cons of rebreathers 

Pros	Cons
Extended dive time and depth	Complexity, more failure-points, lower safety
Silent, bubble-less operation	Expensive investment and operation costs
Warmer, more humid air (gas)	Require gear-specific training, task loading
Efficient, multiple dives on one refill	Limited availability of the consumables

 

 

6. Other/ Experimental Methods of Diving 

There are additional, mostly theoretical or experimental approaches to diving. 

One theoretical approach involves extraction of air dissolved in water. In theory it’s possible, but the energy and equipment (or rather facility) required to extract sufficient volume of air for one diver would be enormous, thus impractical. 

Another approach that you may be familiar with from the movie The Abyss involves breathing perfluorohexane, i.e. oxygenated fluorocarbon fluid. While it has been laboratory tested on mice with partial success, it is bound with problems related to overcoming the basic human instinct that prevents us from breathing liquids (voluntarily “drowning yourself”), finding an effective way to ventilate the fluid to remove CO2 (to prevent respiratory acidosis), and finally removing the fluid from the lung alveoli after diving to prevent subsequent pneumonia. 

An interesting device under the name Hydroid Aquabreather turned heads at DEMA show couple years ago. It’s an all-in-one rebreather helmet that uses small replaceable canisters with potassium hydroxide (KO2) and sodium superoxide (NaO2), a technology known from over 50-year-old Russian military rebreather IDA-71. However, the chemical compound has an aggressive and dangerous reaction with water, which raises serious safety concerns, especially in configuration worn on diver’s head. 

Another interesting device, under the name DiveMan appeared in late 80’s and just few years ago resurfaced in media under the name ExoLung. It uses a plastic bell (external lung) attached to diver’s chest with a flexible diaphragm that is manually extended out by straps attached to diver’s legs to suck air from the surface through a hose. In essence, a simple, manual hookah. While it promises “unlimited” dive time and simple, inexpensive gear for shallow dives (up to 5m), the key concerns relate to coordination of diver’s swimming motion, breathing and buoyancy. Further questions relate to the air delivery performance, especially at increased demands, and finally the daunting consideration of what happens if the diver stops moving legs. 

Which Form of Diving is Right for Me? 

There is no ultimate answer. We all have different preferences, desires, and constraints. A good starting point is to understand what fascinates you about diving and in what scenarios and for what purpose you’d be typically diving. Then determine your requirements in terms of depth, time, underwater activity, environmental conditions, safety, frequency, budget, portability, etc. which in turn will inform the selection of suitable gear. As they say, “horses for courses”. 

Having elaborated on the range of diving methods, we know that each of them carries its unique appeal and challenges. Whether you opt for the simplicity of free diving, the versatility of SCUBA, the convenience of hookah diving, or the ultimate performance of rebreathers, your choice shapes your diving experience. Acknowledge the disadvantages equally as the advantages and understand the risks and limitations to make sure you have not only a pleasurable, but safe diving experience.