So we’ve been a little quiet on the writing front lately owing to seasonal commitments in the community. Many who know us know that we have a solid track record of community involvement, especially around Christmas and the holidays when it’s pantomime time. This year is no different and the work involved can be significant. It promises to be a good one though again this year.
Individually I have also been involved with the development of the upcoming training and operations manual for when we do eventually get to the point of bringing a chamber online. This document represents a considerable time investment and promises to be a comprehensive resource for operators, supervisors, physicians, patients and just about anyone with an interest in HBOT. The goal is to produce a manual that can introduce an individual to hyperbaric chambers and HBOT for the first time and develop their knowledge to supervisor level. It will hopefully receive favorable review and become something many can benefit from. It is written in accordance with prevailing guidelines currently underpinning the industry in the UK and includes modules on legislation, health and safety, physics, physiology, concepts of pressure, design and layout and more.
For now we are happy to share brief extracts from the manual, not necessarily full sections and in running order, as we do hope for it to become an income generator in time by offering FIFO, (fly in, fly out), training services.
Accordingly today we share the introduction to the physics module followed by the section on Henry’s Law, one of the main gas laws.
It is important to note that this isn’t a DIY manual. It won’t ever be made available as such. It will involve classroom contact time when it is complete. One cannot simply read the book. Knowledge does not equal competence and proper training is absolutely necessary to operate hyperbaric chambers.
Module 3 Diving Physics
As with most things, some form of physics always applies. This is no different and even more so the case in hyperbaric applications. The physics involved are in fact diving physics which date back hundreds of years. The basis for the physiological processes discussed under physiology is a set of laws which relate to gasses and the behaviour of gasses under pressure. Being a diving tool essentially, now a medical tool as well, a hyperbaric chamber and its occupants and processes are subject to these laws of physics. Diving has a well-established history dating back centuries. Over the years, many have contributed to research and development including navies, medical institutions, research companies, commercial pioneers and so on. What all this research has in common is the origins of the basic laws governing gas and its behaviour in a pressurised environment. Diving physics is taught from day one on even the most rudimentary diving courses all the way up the most advanced saturation procedures and it’s all the same physics.
The same physics also applies to aerospace engineering and space exploration. Astronauts are subject to diving physics as well. Although any self-respecting astronaut would have it the other way around and insist that all divers are subject to space physics. Meeting in the middle we could perhaps agree that both disciplines share this commonality as they do with atmospheric and gas science.
The phrase “all divers are equal in the water” means just that. The physics applies universally no matter what the level of qualification one holds. All divers must have a working knowledge at least of the physics behind diving and what makes it safe and possible. No diver is expected to be a physicist or scientist though. The physics is irrefutable. It is not subject to opinion or post truth interpretation and it is empirical. It can be observed directly in experiments.
It begins for a diver, when they submerge under the water and increase the surrounding or ambient pressure. This brings about changes in the body and the body’s response to the changing gasses it breathes and its air spaces. For chamber occupants this is when the chamber is pressurised.
Physical changes in the way gasses behave in relation to pressure and physiology affect us in immediate and very real terms in the chamber just as they do in the water with only a few exceptions such as cold and physiological hydrostatic pressure responses which are not evident in a dry environment inside a chamber.
For the purposes of this module, a chamber treatment or session shall be referred to as a dive. The legal definition of a dive includes pressurisation in a fluid as well as a chamber where the pressure increases more than 0,1 atmospheres absolute above normal atmospheric pressure for longer than a minute. In physics, gases are indeed fluids and fluidic physics apply. Air is a fluid. Indeed, chamber compression is legally a dive and subject to the same rules and processes.
Henry’s law is one of the gas laws, formulated by the British chemist, William Henry, in 1803. It states that: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Discounting temperature we can now discuss the law which explains how a gas dissolves into a liquid under pressure. This law works hand in hand with Dalton’s Law which we will discuss next.
Another way to phrase it is: In chemistry, Henry’s law is a gas law that states that the amount of dissolved gas is proportional to its partial pressure in the gas phase. The proportionality factor is called the Henry’s law constant.
Remember that we are under pressure even when not diving or in a pressure chamber. The earth’s atmosphere exerts pressure. It could be referred to as the ultimate (hyper)baric chamber. Although we refer to atmospheric pressure as “Baric” or “Normobaric” air. In other words, barometric, or atmospheric pressure that is measurable with a barometer. Ergo the previous description on concepts of pressure. This is measured at sea level as, 760mm of mercury (mmHg), 1000hPa or 100kPa or 14,7 pounds per square inch (PSI) and indeed 1Kg per centimetre squared.
That means that for every 1 cm2 of surface area on the earth, the atmosphere exerts 1kg. That 1 cm2 column of air would extend to outermost reaches of the measurable atmosphere in terms of pressure, and it weighs 1Kg. The weight of two large bricks of butter. As previously mentioned, these are all near measures of pressure and are accepted as equal even if they are mathematically slightly unequal. In diving calculations, the slight difference is inconsequential.
A fun calculation is to consider that in a square meter of surface area there are 10 000 cm2. That totals 10 000 kg per meter squared. The surface area of the earth is more conveniently measured in square kilometres though which is 10 000 000 000 cm2. That’s 10 billion cm2 with a pressure of 10 billion kilograms per square kilometre of surface area. The earth has a surface area of 510,1 million km2. So, the total weight of the atmosphere can be calculated as equal to 510 000 000 X 10 000 000 000 kg (Rounded). The atmosphere therefore weighs 5.1e +18kg or roughly 5’100’000’000’000’000 tons. Amazing isn’t it. Five thousand one hundred trillion tons or 51 quadrillion tons.
In a different vein, imagine a liquid, such as water, exposed to an increased pressure in a pure gas environment, for example carbon dioxide (CO2). With either a high enough pressure gradient, as discussed under “Gas Exchange”, or a long enough time, the water would become saturated with that gas. It would seek to achieve equilibrium according to Henry’s Law. This is why a bottle of pop goes Pttsshhh! It holds carbon dioxide gas in solution under a maintained pressure.
This is how soda water is made, as whoever owned a Soda Stream machine will know. The bottle is sealed in place and pressurised CO2 is exposed to the water surface causing it to dissolve into the liquid and go into solution. The gas is bubbled through the liquid to increase surface area which facilitates quicker saturation. As long as the pressure is maintained it will stay in solution. When the pressure is relieved, by opening the cap, or releasing the lever in the case of a SodaStream, it will come out of solution forming bubbles, which will also be discussed later when we discuss decompression sickness and pressure gradients.
Water exposed to any gas will have that gas dissolve into it. Nitrogen for example will also make soda water. Nitrogen makes finer bubbles and is not quite as soluble in water as CO2 though, and they don’t taste the same. It would take longer to make bubbly water with nitrogen. Which is why drinks companies use carbon dioxide instead. We will also discuss why oxygen does not, and is not known to come out of solution in this way under cellular metabolism. The oceans on earth accordingly serve as a massive carbon dioxide sink into which excess atmospheric carbon dioxide will dissolve. [Henry’s Law dictates so] As concentration increases so does solubility in the ocean according to Henry’s Law. There is also no limit really. The higher the fraction of gas the higher its partial pressure and the more of it will dissolve into solution. The problem with this is eventually the ocean will become carbonic acid. It would take a huge number of tons of CO2 to achieve this though somewhere along similar lines to the atmospheric calculation above.
The relevance of this law is that when breathing a gas mix containing a metabolically inert gas such as nitrogen or helium, that gas dissolves into the blood and tissues. This necessitates the need for proper decompression practice and the use of decompression tables and schedules. Failure to adhere to established decompression schedules can lead to decompression illness. The greater the pressure, and by extension the partial pressure, the greater the inert gas uptake and the greater the need for longer and deeper decompression pauses or stops. Higher fractions of gas also increase partial pressure and render the gas subject to increased solubility into solution.
A 50-meter air dive in a chamber can incur rather hefty decompression penalties in a short time as opposed to a 10-meter chamber dive which incurs far less decompression debt in a much longer time. This is discussed in more detail when learning about dive tables. Oxygen behaves differently as it is metabolically active and while it goes into solution, it is then used up and doesn’t need to come out of solution again. That oxygen, which is inclined to outgas, doesn’t form a bubble meniscus and therefore oxygen is highly unlikely to become a gaseous embolus such as inert gas is inclined to. In fact, that is the reason HBOT works. More oxygen Is used than normal, and the incurrence of no decompression penalty is evidence of that. It is why recompression therapy for divers suffering from decompression illness works. It squashes any bubbles present and re-dissolves them into the tissues while not incurring further inert gas penalty by breathing oxygen at treatment depth.
Above is the extract, or much of it, is as written on Henry’s Law in the manual. The underlying scientific and empirical law that dictates that a gas will dissolve into a liquid as its partial pressure increases. It’s the basis on which blood plasma saturates to an exponential degree with oxygen, which surpasses, by a long way, the ability of hemoglobin to carry oxygen as the sole carrier and transporter. This law can be demonstrated with a simple soda bottle experiment in which a valve is fitted and the pressure increased inside the bottle. Liquid in the bottle, or a blood analogue if one wishes, absorbs gas and strives to achieve equilibrium. When the pressure is released, that gas comes out of solution again as shown in the graphic.
As the manual develops we will endeavour to share snippets of interesting information and hope it proves useful and interesting. We hope to make this available as an income generator in the new year.