Ocean STEMulation: Saturation Diving
|Aquanaut DJ Roller in training for saturation diving at Aquarius. Photo © Tim Grollimund.|
Focusing on science, technology, engineering, and math (STEM), as they pertain to the ocean.
This is part of our ongoing coverage of Mission Aquarius, what may be the last mission to the world’s only remaining undersea research base. For the full story, visit our Mission Aquarius expedition page.
Aquarius dramatically extends the amount of time divers can spend in the water because it enables saturation diving, which has one huge difference from typical scuba diving – you don’t waste time coming up to the surface, and decompressing, after each dive.
Here’s a breakdown of why compression is such an issue for divers, and how saturation diving deals with it.
In the underwater world, pressure changes much more rapidly than it does on land, because water is much more dense than air. This causes complications with divers’ physiology that must be handled properly or serious problems and even death can occur.
The weight of our atmosphere, the roughly 62 miles of air above us all, is referred to as atmospheric pressure. At sea level this is measured at roughly one atmosphere (atm). Our bodies function well breathing air at this pressure – it’s what we’re made for.
Another atm of pressure is added with every 10meters (33 ft.) a diver descends underwater, squeezing the diver’s body. Air spaces like the lungs, sinuses, and inner ears can get painfully compressed. Divers don’t feel as much pressure elsewhere, since the body is made mostly of liquid, which doesn’t compress. Under pressure, air gets squeezed too; its volume goes down by half with every additional 1 atm, and it becomes denser.
Scuba divers breathe air that is part nitrogen and part oxygen, just like we do on the surface. A diver’s body doesn’t use the nitrogen, but under pressure, the gas will dissolve into a diver’s blood. The longer one stays underwater, and the deeper one goes, the more nitrogen dissolves into the blood.
The pressure and dissolved gas aren’t harmful on their own. Divers relieve pressure by equalizing; air from the lungs fills the sinuses and ears to equal the water pressure outside. Scuba divers automatically equalize by breathing in more air from their tanks; the air inside a diver’s lungs will be the same pressure as the surrounding water. But the deeper a diver goes, and the more pressure there is outside the body, the more air is required to fill the same volume in the lungs, so the faster their tanks will become depleted. This faster consumption of tank air and increased nitrogen in the bloodstream is why deeper dives must be shorter dives.
Freedivers, who hold their breath when they dive, also equalize the sinuses and ears with air from the lungs, but since they can’t inhale, the water pressure causes the lungs to shrink – not normally a problem since the lungs are flexible and return to normal size as they ascend to the surface.
|Because filmmaker Marc Ostrick scuba dove to Aquarius and did not saturate, he will only be able to stay an hour before decompressing and returning to the surface. Photo © Brian Lam.|
Dissolved nitrogen in a scuba diver’s blood is not normally harmful in shallow water but can cause temporary mental impairment that some have likened to being drunk, through nitrogen narcosis when diving below about 30m (100 ft.). Divers going deeper use a mix of breathing gases with helium, reducing the nitrogen narcosis.
Once a diver returns toward the surface and lower pressure, things get more complicated. If the lungs are full of expanding air and the diver doesn’t exhale, the lungs could over-expand, a condition called arterial gas embolism. This is easy for divers to avoid simply by breathing normally and ascending slowly. They key is to never hold your breath when scuba diving!
A more difficult issue is the nitrogen dissolved in the blood stream, which comes back out of solution as the diver nears the surface. If a diver ascends slowly enough, the nitrogen gradually leaves the body through exhalation, but if a diver comes to the surface too quickly, it’s like opening a bottle of soda – the nitrogen forms bubbles in the blood stream and body tissues, causing decompression sickness. This can lead to dangerous health problems and death, depending on the severity and where exactly the bubbles form, including some symptoms such as severe joint pain known as “the bends.”
Scuba divers must budget their time diving so they don’t spend too much time absorbing nitrogen, and make sure they have adequate air to ascend slowly, releasing their nitrogen. Dives, 60 feet or less in depth, often last about an hour, and the number of dives one can do in a day is limited because of nitrogen absorption, so for scientists conducting research underwater, scuba diving from the surface can be inefficient.
How do you extend dive time safely? This is where Aquarius and saturation diving come in!
Aquanaut DJ Roller in training for saturation at Aquarius Reef Base. Photo © Brian Lam.
Saturation diving was first researched in the late 1950s. It involves diving to a certain depth and staying there, absorbing the breathing gases until no more gas can be absorbed. At this point, the body is saturated with nitrogen (and helium, if using a trimix gas mixture to dive deeper – but the mixture at Aquarius uses just nitrogen and oxygen because it’s fairly shallow).
Once saturated, the aquanauts can stay at depth indefinitely, though no one has stayed beyond 58 days. Underwater bases, or habitats, provide a dry place to eat and sleep (and even do lab work, in the case of Aquarius), while maintaining the pressure the divers were diving under. Because they don’t have to decompress during each dive, divers can spend all day in the water doing work, as opposed to a few hours.
|Aquanaut Mark Patterson in training for saturation diving at Aquarius Reef Base. Photo © Tim Grollimund.|
In Aquarius, the divers conduct one to two-week missions. Before returning to the surface, the aquanauts have to conduct just one gradual decompression. The main compartment of the base is sealed off so that it doesn’t flood when the interior pressure drops, and it is slowly depressurized over 17 hours, while the people inside continue their activities. Once safely decompressed, the habitat is briefly re-pressurized, then unsealed, so that the aquanauts can exit the habitat using scuba, and swim back to the surface.
Saturation diving is often used for deep ocean industrial work, including oil rigs and pipelines. As with research, industrial work is now increasingly being conducted by robotic vehicles to reduce risk and cost.
But many scientists feel robots cannot replace a human being on location, doing research. Aquarius is the last underwater habitat, and if it is closed, there will be nowhere left for humans to saturate and explore the ocean. Countless discoveries over the last 50 years would never have been made without saturation diving technology.