Gas Density and SCUBA

What prompted this blog post?

Diving On The Edge (DOTE) conference was held at DDRC in October 2022. With a variety of underwater topics in the field of exploration, medicine, and technology, it was enjoyed by all present. One of the presentations was by Gavin Anthony about gas density in diving. This is an expanding topic of interest in diving, Gavin has expertise in this area, having published a paper with Professor Simon Mitchell ‘Respiratory Physiology of Rebreather Diving’. Page 70 https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf#page=70 The presentation at DOTE went into depth about this topic, as well as covering the background data that lead to subsequent recommendations for SCUBA breathing mixture density.

Although the publication is based around rebreather diving, the principles are transferable to open circuit diving. Divers often customise their gas mix to achieve a desired max partial pressure of oxygen (PO₂) or to minimise nitrogen narcosis. There is now an increasing understanding of the importance of maintaining an appropriate gas density during SCUBA diving, as demonstrated by several organisations, such as the British Sub Acqua Club, publishing their own recommendations.

www.bsac.com/advice-and-support/technical-diving/gas-density-tables/bsac-recommended-diving-gas-mixtures-for-open-circuit-diving

Tell me more.

The principal concern surrounds the effect of breathing a very dense gas on the body’s ability to control Carbon Dioxide (CO₂) levels in the blood. CO₂ is a by-product of normal cellular energy generation, if it accumulates it renders the blood acidic and has lots of undesirable effects.

• Our breathing (speed and depth) is controlled by sensors in the blood vessels and brain, these detect subtle changes in CO₂ With rising levels, such as during exertion, there is increased depth/speed of breathing.

 Ok, so we breath to eliminate CO₂, how does this relate to diving?

If a process impairs the ability of the body to increase breathing speed/depth, the CO₂ in our tissues will rise. This leads to a range of unpleasant symptoms, starting with breathlessness/anxiety and culminating in unconsciousness and death.  It is also important to recognise that high levels of CO₂  will also affect other factors during the dive. These include potentiation of nitrogen narcosis with worsening effects at shallower depths and central nervous system oxygen toxicity at potentially lower partial pressure of oxygen.

So why is CO₂ control linked to gas density?

Boyles law states that with constant temperature, there is a direct inverse relationship between pressure and volume. Following this, a fixed mass of gas compressed has is a linear increase in density as well as pressure. SCUBA equipment delivers gas at ambient pressure, so for example, a breathing mixture will be twice as dense when used at 10 m compared to the surface.

There is experimental evidence from ‘dry’ (chamber) diving that the maximum volume of air a person can breathe per minute  at 30m is half the volume that can be breathed at the surface.

Gas with a high density is more likely to develop turbulent, inefficient flow, within the airways.  The high resistance of a breathing mixture can result in participants creating high pressure in the chest as they breathe out (expiration.). If the pressure in the chest is greater than in the smaller breathing tubes leading into and out of the lungs (the small airways) this can result in these small airways collapsing at points in the breathing cycle. This is known as ‘dynamic airway collapse’ and is an additional factor limiting the maximum volume a person can breathe in or out. When breathing is less efficient, and the level of CO₂ can rise. As mentioned earlier, the body will respond to this by changing increasing the breathing speed and depth. However, this response can worsen this situation. As increasing the speed and depth for breathing, can increase the exertion of the breathing muscles. This increase in exertion can increase the production of C0₂, leading to a cycle.

How do we know what gas density is safe for SCUBA diving

There has been no specific investigation into what gas density is safe for SCUBA diving. However, the paper mentioned previously reports on a series of hundreds of dives with a range of exposures from 4-80 m. The investigation was not designed to investigate gas density, rather the completion/failure of a dive, with the rational for dive failure being documented. Critically, there was a variety of maximum breathing mixture densities, as this had not been standardised in the study design. The graph below shows the incidence of dive failure due to high CO₂ levels in rebreather divers:

It is evident that there was a sharp increase in failed dives due to CO₂ retention with breathing mixture density>6 g/L. This contributed to the following recommendations:

• recommended maximum breathing gas density 5.2 g/L
• maximum advised breathing gas density 6.2 g/L

Why is it important to consider this in the dive plan?

It is vital to consider gas density in your dive plan. Divers should not rely on being able to detect symptoms of C0₂ excess. These can be hard to detect, and some divers may not experience early signs or symptoms. These individuals who do not have a normal response to elevated CO₂ levels when diving are labelled CO₂ retainers. They can tolerate slightly higher blood CO₂ during a dive. However, with no action, they will succumb to elevated CO₂ eventually and may have less warning signs such as breathlessness before incapacitation. This has been demonstrated experimentally by SCUBA divers exercising underwater suddenly falling unconscious from CO₂ narcosis, despite indicating they felt fine moments before.

Secondly conditions which precipitate a CO₂ excess can be inescapable. Remember the effect of exertion, leading the greater CO₂ production, and an obligation to greater breathing effort. This could be during a situation such as an underwater entanglement or rescuing a buddy.

Finally , the effect of CO₂ excess can precipitate nitrogen narcosis or oxygen toxicity. These can rapidly incapacitate a diver.

OK, fine, you have convinced me. Is there anything else that can have effects underwater?

As mentioned, CO₂ as a by-product of exercise. Minimising underwater exertion and being physically fit will help minimise excess CO₂ production and optimise the body’s ability to cope.

Equipment considerations

• A poorly maintained open circuit regulator can increase resistance relative to breathing at rest.
• Rebreather divers must generate all gas flow with their own breathing effort. Considering components such as a tightly packed scrubber it is easy to see how extra resistance is generated.

Relative pressure

• Open circuit equipment delivers gas at ambient pressure. This is relative to the regulator and not the airways. If the diver is in the upright position, the regulator and mouth will be at lower ambient pressure than the water surrounding the chest. This will lead to greater resistance to inhalation, limiting maximum breathing effort.
• For CCR divers the position of counterlung has similar effects, with hydrostatic imbalance depending on counterlung/diver positioning.

Are there any resources to access to reference safe gas density?

There are ‘ideal gas’ tables produced by various agencies. Gavin Anthony has produced these example gas tables for DDRC to share. It is important to note that these are recommendations only.  Divers should always use equipment/plan dives that they are trained for and have experience in.

The following table is for open circuit divers, using a maximum PO₂ of 1.4 bar. The ‘equivalent narcotic depth’ (END) assumes that nitrogen is the only narcotic gas in the breathing mixture.

35 m is the first depth, all usable breathing gas mixtures 30 m or shallower will have a suitable density. Once 35 m is reached, a nitrox mixture or air will have a density more than 5.7 g/l and are therefore not recommended in the following tables.

The table below is for CCR divers and applies the same principle to ideal loop density, but with partial pressure of oxygen in the loop 1.3 bar and diluent ≤ 1.1 bar.