Rebreathers are becoming more and more popular in Sport diving.  New manufacturers are springing up all the time. With a range of choices, how do we know which rebreather to buy? Kevin Gurr looks at some of the issues.

Testing

Look at your SCUBA regulator first stage. You will find a label that says EN250. This is a European design standard that specifies how a regulator should perform. Several years ago the industry backed the introduction of this directive to ensure a minimum safety standard for regulators was maintained. SCUBA regulators like rebreathers are life-support, hence the importance of a standard.

A similar standard exists for rebreathers, it is called EN14143. Unlike SCUBA regulators, most rebreather manufacturers do not yet adhere to this standard. One reason for this is that in order to design a product to the standard is quite difficult and often means that the designer’s ideal of a compact, light unit cannot be met. That said, as with SCUBA regulators, the need for such a standard is vital in order to ensure as safe a product as possible and it is one of the first things to check when buying a rebreather.

Many manufacturers quote ‘in-house’ testing and ‘field trials’ as suitable test methods, this as in other industries such as car safety has been shown to be inadequate.

Light

For many divers the weight of their equipment is an issue. That said as divers we are always fighting Archimedes Principle while diving. The more volume of gas we have in our equipment, the more lead we need. So while a rebreather may be light on the surface, if it has a large volume it will take lead to sink it! Counterlung sizes can range from 4.5l to 14l. Every litre means 1kg in weight. Put simply, (ignoring suits and other equipment) if a rebreather weighs 10kg but has an internal volume of 14l then you will need another 4kg of lead to get down. So while a unit may appear light on paper, check it’s relative volume if you have a bad back!

Should a problem occur with a unit with large counterlungs and the unit floods, you will then need the corresponding volume of lift in a BCD to compensate. Large counterlungs also generate other issues like unnecessary venting and shifts in oxygen partial pressure tracking. Humans generally only need 4.5l of counterlung volume (within certain pressure limits) to function.

Where a light ‘dry weight’ is of advantage when transporting the unit, most airlines now have a good sporting goods allowance.

Small

Small can be good but small means several things will have been compromised. These might include;

1. The amount of CO2 absorbent material the unit carries

Small amounts of CO2 absorbent mean short duration. If you plan to dive to a certain depth and in certain conditions, ask the manufacturer how long the absorbent will last. EN14143 insists on testing under a range of conditions within a worse case environment being 4 degree C water and high ventilation and CO2 generation rates. While this is extreme for most divers, it provides a safe baseline. Many manufacturers quote a depth independent absorbent life time. This is misleading because for a variety of reasons the duration of absorbent is massively affected by depth. If a manufacturer can’t tell you how long it lasts they probably don’t know!

2. The internal size of the breathing circuit

Small hoses and mouthpieces look sexy but may mean a higher Work of Breathing (WOB) or breathing feel. WOB is a result of the position of the counterlungs and all the internal restrictions of the breathing circuit. A unit that feels like you are breathing through an old sock at the surface may well become unbreathable in deep water especially if you are breathing hard. EN14143’s WOB limits are set to allow for these often experienced diver maximums.

3. The comfort and fit of the harness

Comfort is a function of how well the unit breathes, how well it swims (hydrodynamic drag), how comfortable the harness and counterlungs feel. Some people prefer counterlungs on the front and some on the back, each type has it’s own ‘feel’. This is a personal preference but you should try several types before making a decision.

Maintenance and set-up

Units that need specialist tools and detailed set-up procedures are probably not for the average diver. Contact a good instructor or dealer and have them talk you through the basics before you make a decision.

Cost-consumables

Unlike open circuit, rebreathers can have expensive consumables such as batteries, oxygen, absorbent material and oxygen sensors. Use the internet forums to find out how each units handles and uses it’s consumables. Some will be expensive, with oxygen sensors being replaced every year or less as one example.

So in summary; use the internet to get user feedback, find someone knowledgeable to run you through the basics on several units, take your time to make a decision, this is after all by definition, life-support equipment. Finally and most importantly, choose a good experienced Instructor should you intend to buy.