The body's tissues absorb inert gas until they reach a point of equilibrium with the surrounding pressure. Once that point is reached — saturation — no more gas goes in, whether three days pass or three weeks. That's why decompression time depends on depth, not on how long the stay lasts.
This is the question that best sums up why saturation diving exists: if decompressing from 120 metres takes days, how can a diver work down there for a whole month and only decompress once at the end?
The answer lies in a physiological principle that's simple to state. And understanding it properly also explains the whole economic and operational model behind the industry.
What happens to the body under pressure
When you breathe a gas mix under pressure, the gases dissolve into the blood and pass from there into the tissues. Oxygen gets used up in metabolism, but the inert gas — nitrogen in air, helium in saturation mixes — isn't consumed: it accumulates.
How much dissolves depends on that gas's partial pressure and the tissue's affinity for it. Each type of tissue loads at a different rate: blood and highly vascularised tissue load in minutes, while fat, bone or cartilage take hours.
The equilibrium point
That process isn't infinite. Each tissue loads more and more slowly as it approaches equilibrium with the surrounding pressure, following a curve that flattens out. There comes a point where the pressure of the gas dissolved in the tissue equals the pressure of the gas being breathed, and net exchange stops.
That's called saturation: the tissue is full for that pressure. From that point on, continuing to breathe the same mix at the same pressure adds nothing more. The diver can spend three days or three weeks there and the gas load in their body will be exactly the same.
How long it takes
The slowest tissues in the body take on the order of a day to reach equilibrium. That's why, in operational practice, a diver is considered saturated after roughly 24 hours at constant pressure. From that point on, the decompression obligation stops growing.
The operational consequence
This is the key to the whole model. In surface-supplied diving, every dive carries its own decompression: you go down, work for a limited spell, come up decompressing, and repeat the next day. Productive time on the bottom is a small fraction of the total time.
In saturation, decompression is paid for only once. The team is pressurised at the start, kept at pressure for the whole campaign, and decompressed at the end. All the time in between is time available for work.
Multiplied across weeks of campaign and several teams rotating in continuous shifts, the productivity difference against surface-supplied diving is overwhelming. It's what justifies the cost of a saturation system and a DSV. The full differences between both regimes are covered in the article on saturation versus surface-supplied diving.
What the final decompression looks like
Saturation decompression doesn't look like the stops in conventional diving. There are no marked stages or waiting times at set depths: it's a continuous, very slow pressure ascent inside the system's chambers.
The rate is expressed in metres per hour or per day and is remarkably slow. As an order of magnitude, decompression from typical saturation depths is measured in days, not hours: several days for the working depths typical of the industry.
What matters is that this time depends on the system's storage depth, not on how long has been spent at it. It's exactly the same length of time whether the campaign lasted ten days or twenty-eight.
What happens during those days
The divers stay inside the system, in the living chambers, not working. They sleep, eat, read, watch films. It's paid time and counts as part of the campaign, but it's dead time from a productivity standpoint, which is why operators don't extend it unnecessarily.
The atmosphere is monitored continuously throughout the whole process: gas composition, oxygen partial pressure, carbon dioxide, temperature and humidity. Life support technicians watch those parameters around the clock.
Why so slow
The reason is bubble formation. When ambient pressure drops, the gas dissolved in the tissues no longer fits in solution and tends to come out. If that pressure drop is slow, the gas is carried by the blood to the lungs and eliminated on breathing out. If it's fast, it forms bubbles inside the tissues and the bloodstream.
Those bubbles are decompression sickness. They can lodge in joints, in the spinal cord, in the inner ear or in the lung, and the consequences range from joint pain to permanent neurological damage.
Helium and how it behaves
Saturation mixes use helium because nitrogen at those partial pressures produces narcosis, and because helium, being far less dense, reduces the work of breathing. But helium behaves differently: it diffuses faster than nitrogen, which means saturation is reached sooner, and also that desaturation is different.
A particular phenomenon with helium in saturation is dysbaric osteonecrosis, a bone injury associated with prolonged hyperbaric exposure and inadequate decompression. It's one of the reasons saturation decompression tables are deliberately conservative, and why medical monitoring of saturation divers is so demanding.
Excursion limits
There's an important detail a lot of people miss. Being saturated at a given depth doesn't mean the diver can move freely through the water column.
The body is in equilibrium with the storage pressure. If the diver ascends above that depth, even while still within the saturation regime, they experience a pressure drop and can form bubbles. If they descend below it, they absorb extra gas that will later have to be eliminated.
That's why excursion tables exist, setting how many metres above and below the storage depth the diver can move. When the job calls for a different depth, the excursion isn't stretched: the whole system's storage depth is changed instead, in a controlled way and with its own corresponding time. How the transfer from the system down to the seabed works is covered in the article on the closed bell.
What happens if an evacuation is needed
This is the point that makes saturation something planned with such care. A saturated diver can't leave the system. If they suffer appendicitis, a fracture or any medical emergency, they can't simply be taken out and rushed to hospital: decompressing them takes days.
The industry's answer to that problem has two legs. The first is prior selection and medical monitoring, aimed at minimising the chance of a preventable emergency arising during the campaign. It's one of the reasons behind the strictness of the commercial diving medical exam.
The second is the hyperbaric rescue chamber. Saturation systems carry a detachable chamber, transferable under pressure, that allows the system's occupants to be evacuated while keeping them pressurised and moving them to another hyperbaric facility. It's mandatory equipment, and its use is drilled periodically.
For minor medical issues, the vessel's crew usually includes training in hyperbaric medicine, and specialist medical advice is available remotely.
Who decides the tables
Saturation decompression tables and procedures aren't improvised by each operator. They come from decades of physiological research and the industry's accumulated experience, and are set out in internationally recognised guidance and in each contractor's own procedures, audited by their clients.
That body of knowledge is one of the reasons why saturation diving, despite being a risky activity, has a reasonable safety record when done properly. It's also why the industry is so intolerant of shortcuts: the margins built into the tables are there for a reason.
If you want the full picture of the saturation regime, start with the guide to what a saturation diver is, and to understand how a campaign is structured, with how long a saturation campaign lasts.
Frequently asked questions
How long does it take a diver to become saturated?
The slowest tissues in the body reach equilibrium with the surrounding pressure in roughly 24 hours. From that point the diver is considered saturated, and their decompression obligation stops increasing no matter how much longer the stay is extended.
How long does decompression at the end of the campaign last?
It's measured in days, not hours, and depends on the system's storage depth. It's a continuous, very slow pressure ascent inside the chambers, not a series of stops like in conventional diving.
Does it make any difference spending ten days versus a month in saturation?
From a decompression standpoint, no, it doesn't. Once the tissues are saturated, the body's gas load doesn't increase and the decompression time is the same. What does change is the physical and psychological toll on the person.
Why is helium breathed in saturation?
Because nitrogen at those partial pressures produces narcosis, and because helium, being far less dense, reduces the work of breathing. In exchange, helium has high thermal conductivity, which means the system's temperature has to be controlled very strictly.
What happens if a saturated diver needs urgent medical care?
They can't be taken out of the system, because decompressing them takes days. The systems carry a detachable hyperbaric rescue chamber that allows the occupants to be evacuated while kept pressurised and moved to another hyperbaric facility.
Can a saturated diver move freely through depth?
No. Excursion tables exist that limit how many metres the diver can move above and below the storage depth. If the job requires a different depth, the whole system's storage depth is changed in a controlled way instead.
Published 20 August 2026 · By Arthur More, professional saturation diver
Photo: SFU - Communications & Marketing · CC BY via Openverse