Decompression Theory

Part 2 - information about decompression theory

Decompression Theory – Part II

During my last blog I wrote about the very beginning of decompression theory until the first diving bells or Caissons started to be used regularly. During this blog I want to focus on the scientists who changed how we understand decompression theory, and its subsequent development with their ground breaking research.

Paul Bert Bert was born in Auxerre in 1833. He always wanted to become an engineer but he changed his mind and studied law and finally he took up physiology. After graduating as a Doctor of medicine in 1863 and science in 1866, he was appointed professor of physiology successively at Bordeaux and the Sorbonne.

After the revolution of 1870 he started to be involved in politics and four years later he was elected to the Assembly, where he sat on the extreme left. He was elected to the chamber of deputies and served as a minister of education and worship.

Early in 1886 he was sent to Indochina and appointed resident-general in Annan and Tokin. Five months later in November 1886 Bert suddenly died of dysentery in Hanoi. He was 53 years old.

Worldwide, Bert is known more as a man of science than as a politician or administrator.

He is remembered well for his work ‘La Pression Barometrique’ in 1878, laying the foundations that led to our knowledge of physiological effects of air-pressure, both above and below atmospheric pressure.

Bert became interested in the problems that low air pressure caused for mountain climbers and balloonists. This led him to study the problems that divers experienced with increased pressures. He reviewed current research in this area. He was struck in particular by the experiences that Dr. Alphonse Gal had while diving in Greece.

Dr. Gal was the first Doctor to actually to dive in order to study how the body reacted underwater. Bert studied Gal’s diving experiences and reports on divers who were injured or killed.

The research and experiments Bert completed led him to his conclusion that pressure does not affect as physically, but rather chemically by changing the proportions of oxygen in blood. Too little creates oxygen deprivation and too much creates oxygen poisoning.

He showed that pure oxygen under high pressure can be deadly. To this day CENTRAL NERVOUS SYSTEM (CNS) Oxygen toxicity is known as the ‘Paul Bert effect’.

Bert’s most important discovery was the effect of nitrogen under high pressure. This for the first time explained decompression. In investigating the causes of decompression illness Bert exposed 24 dogs to a pressure of 9.75 ATM (equivalent to a depth of 87.5 meters or 290 ft) and decompressed the dogs rapidly, between 1-4 minutes. 21 of the dogs died, while only one showed no symptoms.

After hundreds of experiments, looking at methods of treating compressed air illness once the symptoms had appeared, Bert’s experiments show that once bent, the symptoms could be relieved by returning into the compressed air environment of the Caisson or Tunnel, and then decompressing the patient slowly. This heralded the start of recompression therapy.

Bert attempted to explain why oxygen worked when he was treating patients with the bends with different gases.

He said “I thought if the subject were to breathe a gas containing no nitrogen, pure oxygen for example, the diffusion would take place much more rapidly, and perhaps even rapid enough to cause all the gas to disappear from the blood.”

John Scott Haldane Scott Haldane was born in Edinburgh into a notable family. The Haldanes had been Lords of Gleneagles since the 13th century and John Haldane’s brother Richard, Lord Haldane of Cloan, was secretary of War from 1905-1912, Lord Chancellor and also the founder of the Territorial Army.

Haldane is considered to be father of modern decompression theory. He was the first person to approach predicting decompression and his methods form the basis of the majority of modern decompression theory.

After his graduation he went to Queens College, Dundee and after he was transferred to Oxford University. At Oxford he lectured on medicine and conducted medical research.

In 1906, in collaboration with J.G. Priestley, he discovered that respiratory reflex is triggered by an excess of carbon dioxide in the blood rather than a lack of oxygen.

Haldane became an authority on the effects of pulmonary diseases on industrial workers. He was Director of the Mining Research Laboratory in Doncaster and was known to the miners in Yorkshire as ‘The Doctor’.

Haldane and his wife lived in the same house in Oxford for fifty years. The Haldane residence was an amazing house and would later become part of Wolfson College. Haldane often worked at home. He had a study in the attic as well as a laboratory complete with a pressure chamber so he could expose his test subjects to the effects of gases under pressure.

Haldane also founded the Journal of Hygiene and it was here that the first set of diving decompression tables were published. is most widely remembered for his work on decompression, especially amongst divers. In 1905 he was elected by The Royal Navy’s Deep Diving Committee to carry out research on a number of aspects of their diving operations.

His most important research was looking at ways to avoid ‘the bends’, or ‘caisson disease’ as it was them worldwide known.

It had been observed that workers in pressurized construction areas known as caissons, would sometimes complain of pain in their joins. As the working depth increased, the symptoms worsened.

Many suffered total paralysis and also there were often deaths. After research suggested that gases, breathed under pressure, were diffusing into the body’s tissues and when these gases came out, bubbles will form in the body, the subject became known as decompression illness (DCI).

Divers were told to minimize this risk by ascending slowly to begin with, and then rising faster as they got close to the surface. Thanks to Haldane’s work we know now that was completely wrong and very dangerous.

Haldane began experimenting with goats because they are of a similar size of humans. He found that the body could tolerate a certain amount of excess gas with no apparent ill effects. Caisson workers pressurized at 2ATM (10m) experienced no problems at all, no matter how long they worked. Similarly, goats saturated to 6ATM (50m) did not develop DCS if decompressed to half ambient pressure.

Haldane suggested that we consider the body as a group of tissues which absorbed and released gases at different rates, in order to explain these observations, he proposed 4 basic principles.

Gas absorption and elimination in a tissue occurs exponentially.

Different tissues absorb and release gas at different rates.

Decompression is achieved by decreasing ambient pressure.

Gas tension is a tissue must not exceed approximately twice ambient pressure.

This meant the tissues were all exposed simultaneously to the breathing gases at ambient pressure but each tissue reacted to gas pressure in a different way. Then he went on to suggest a mathematical model, describing how each of the tissues absorbs and releases gases, each tissue having a limit of the amount of nitrogen that the tissues can tolerate.

Haldane introduced the concept of half times to the model, measuring the absorption and release of nitrogen in the blood, a concept all scuba diving professionals learn during their Divemaster course and IDC, often one of the more challenging aspects of scuba diving theory to fully understand but it’s pretty simple.

The half time is the time required for a particular body tissue to become half saturated with a gas. Haldane suggested 5 tissues compartments with half times of 5, 10, 20, 40 and 75 minutes.

He also demonstrated that the most significant period was when a diver was closest to the surface. One the most relevant of Haldane’s works, and one that is still relevant today, is that ‘it is the relative pressure differences that are important rather than the absolute depth changes’.

Haldane also developed practical dive tables based on his research that include slower ascent rates as the diver get closer to the surface. Haldane’s dive tables were published in 1908 in the Journal of Hygiene.

After the report of the Admiralty’s Deep Diving Committee it was decided to publish the Committee’s conclusion in the form of a blue book available to the public. The conclusions were universally accepted and it became the foundation of all diving operations, both in the UK and abroad.

Haldane was sent in 1915 to investigate the poison gas being used by the Germans in France. During his research into these gases Haldane ended up breathing samples of these gases while testing new designs of gas mask. As a result of his work Haldane developed one of the first effective gas masks saving thousands of lives.

Haldane died of pneumonia in 1936 he was 75 years old. His lungs had never recovered from his experiment during the First World War.


Before 1912, US Navy divers rarely dived below 18 metres. In that year a Chief Gunner George D. Stillson proposed the US Navy study the work of Haldane in order to allow divers to safely dive below 18metres so he set up a program to test Haldane’s diving tables and methods of stage decompression.

Another goal of the program was to develop improvements in Navy diving equipment and procedures. The first test was made in tanks ashore, then in open water dives followed in long Island Sound from the USS Walke. The divers began to dive deeper and deeper, eventually reaching 83m/274 fsw.

Stillson also tested the affects using pure oxygen during decompression, the result was that the US Navy published their own dive tables in 1915, known as the C&R as they were published by the Bureau of Construction and Repair. the beginning of 1917 Stillson skills where put to dramatic use when a US submarine sank near Honolulu, Hawaii. 22 men lost their lives in the accident and it was the first time ever a submarine had been lost.

US Navy divers dived the submarine, recovering the bodies of the crew.

This extreme effort lead to the development of new skills and techniques, but what was most remarkable was that a US Navy diver completed a dive at the extreme depth of 92m/304fsw, using air as a breathing gas.

These dives remain the record holders for the use of the standard deep-sea diving dress equipment in both maximum depth and longest dives.

Because of the depth and the ensuing decompression, each diver could only remain on the bottom for ten minutes. Even for that short period of time, the men found it hard to focus on the job at hand.

They were of course experiencing the effects of ‘nitrogen narcosis‘.

In 1935 J.A. Hawkins concluded that a surfacing ratio of 2:1 was too conservative for faster tissues so rather than a single surfacing ratio for all the tissues compartments each compartment should have its own individual surfacing ratio.

Hawkins found that the 5 and 10 minute compartments could tolerate such an over pressurization ratio that they could be ignored so he reduced this in order to make the tables more conservative. The new tables were published with just the 20, 40, and 75 compartments. After 20 years of US Navy experiments and tests they restored the 5 and 10 minute compartment and also added a much slower 120-minute compartment.

Robert Workman

Workman revised Haldane’s model to take into account the fact that each of the various compartment can tolerate a different amount of over-pressurization and that this level changes with depth. He introduced the term of M-Value to describe the amount of over-pressurization each compartment could tolerate at any depth.

Workman also added three further slow tissue compartments 160, 200 and 240. He also made the observation that a linear projection of M-Values is useful for computer programming as well.

Professor Albert Buhlmann

Buhlmann began his studies into decompression in 1959 in the University Hospital in Zurich at the Laboratory of Hyperbaric Physiology. His research over thirty years made a number of important contributions to the dive science.

For the majority of his life his main interest was professional deep diving. In 1959 he supervised experimental dives to a depth of 120m in Lake Zurich using TRIMIX gas mixtures and changes of mixture during decompression. In the next two years he demonstrated the practical results of his research with simulated dives to 300 metres.

In the following years Buhlmann worked with the US Navy who funded a series of experimental extended dives from a range of 150 to 300 meters. Buhlmann also worked with Shell Oil who were interested in the practical implications of his research so they could be applied to commercial dives involved with undersea oil fields.

Much of his research was intended to determine the longest half-time compartments of nitrogen and helium. As a result of his work the number of half-time compartments was extended to 16.

Author: Guillermo Sanchez (PADI IDCS #288160)

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