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. This is accomplished by routing exhaled gas into a bag (called a “counterlung”). Along the way the gas is forced through a chemical filter to remove CO2 and the oxygen consumed by the body is replaced from a supply tank. The user then inhales from the bag on their next breath.

Because gas is recycled rebreathers typically have a much longer duration for a given gas volume than open circuit systems. Roughly speaking this means you can use much smaller cylinders to achieve the same dive, ignoring backup gas carried in case of a rebreather malfunction (“bailout”). The inhaled gas is also warm and humid since it was just inside the diver's body, in contrast to open circuit where the gas is supplied directly from a pressurized cylinder and is consequently 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 two jobs 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 a device with such limited utility?” I'll explain what they mean.

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. Therefore in a breathing gas you have to keep the oxygen concentration below a certain level in order to avoid poisoning the user. In diving, we do this by limiting the partial pressure of oxygen (ppO2). The “maximum” ppO2 generally accepted to be “safe” is 1.6 ATA. Oxygen partial pressure increases with depth. For pure oxygen, at the surface (where pressure is 1 ATA) your breathing gas is already at 1.0 ppO2. Every 33ft of freshwater adds another atmosphere of pressure, so for pure oxygen a ppO2 of 1.6 is reached at about 20ft. In short, you cannot safely dive an oxygen rebreather below 20ft. In some respects this is quite limiting; most dive sites are deeper than that.

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 oxygen rebreather'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.