oxygen rebreather

In late 2022 I purchased an oxygen diving rebreather. The specific unit is a salamander.

A rebreather is a device that allows users to breathe underwater. Unlike open circuit systems, where gas exhaled by the user is lost as bubbles into the water, a rebreather recycles that gas by removing the CO2 and replenishing the oxygen consumed by the user and then allowing the user to “re” breathe it again. Because gas is recycled they typically have a much longer duration for a given gas volume than open circuit systems; in practice, this means you can use fewer, much smaller cylinders to achieve the same dive (ignoring bailout). The inhaled gas is also warm and humid, since it was just inside the diver's body, unlike open circuit where the gas is typically cold and dry.

An oxygen rebreather is a special case of rebreather where the gas composition breathed by the user is nominally 100% oxygen. This is in contrast to a “mixed gas” rebreather which delivers a gas mixture such as nitrox (oxygen & nitrogen) or trimix (oxygen, nitrogen & helium). Those units require the use of a diluent gas in addition to an oxygen supply - the diluent and oxygen are mixed by the unit to produce the desired gas. This makes them very capable units, but much more complicated, large, and prone to failure. An oxygen rebreather is comparatively simple as it only has one job to do - replace the consumed oxygen and remove the CO2.

Here is mine:

why?

This question is most often asked by other divers. The funny thing is that the answer is usually obvious to non divers - it lets you breathe underwater! But when divers ask it, they mean something different. What they mean is “why would you buy such a device in light of its limitations?”

The thing about oxygen is that it's a highly reactive gas. This property is critical to human chemistry. However, in excess concentrations, oxygen becomes toxic. Thus you have to keep the oxygen concentration low. In diving, we do this by limiting the partial pressure of oxygen. The “maximum” generally accepted to be “safe” is 1.6 ATA. Oxygen partial pressure increases with depth; 1.6 ppO2 corresponds to 20ft of freshwater, so you cannot safely dive an oxygen rebreather below 20ft. This is quite limiting. Most dive sites are deeper than 20ft.

So why would you want one of these things? Well, for starters, not *everything* worth diving is deeper than 20ft. There are some sites, like Blue Heron Bridge, that are considered world class and have plenty to see in an o2rb's depth range. Also, not every dive has to have an objective. You can have a very enjoyable dive and never go below 20ft! For dives like that, the oxygen rebreather is awesome, better than open circuit. Because exhaled gas is captured within the system instead of being exhaled as bubbles, bubble noise is eliminated and the unit is almost completely silent (you can still hear the sound of oxygen pushing through the breathing hoses, but it's very quiet). Consequently aquatic life is much less scared of you.

The unit is also very lightweight. Fully assembled and ready to dive it weighs 6kg (~13lb) including an oxygen bottle (and not counting ballast). Compared to a typical OC scuba system, where the most basic equipment configuration exceeds 40 pounds (not counting ballast), it is much easier to transport. It's also much smaller, both in storage and in the water. In the water it's as close as you can get to wearing nothing at all. This makes it very enjoyable to dive.

It's also cool. Oxygen rebreathers are very uncommon in sport diving (meaning non-military). Every time I've dived it people are curious about it because they've never seen one before.