December 14th, 2009

Clarity on ExtendAir cartridge duration for rebreathers


The O2ptima rebreather uses the ExtendAir cartridge for a number of reasons.  Some like to argue that granular is cheaper and easier to get.  I won’t argue with that because if that’s your belief then my arguments won’t matter. However, convenience is key for me and that’s what the ExtendAir rebreather cartridge provides. Duration sometimes comes into question. Let’s look at that.

The cartridge is designed to scrub 240 liters of CO2 and since you metabolize 1 liter of oxygen for every .92 liters of CO2 its basically liter for liter. So it’s easy to look at your oxygen gauge and calculate duration based on oxygen use. We did a rebreather dive two weeks ago at Peacock Springs, swimming from Orange Grove sink to Peacock Springs and back. The swim took 150 minutes and the team average was 30 bar of oxygen to do it.  The quick math for usage is 240 liters (cartridge rating) divided by the bottle size ( in liters).  I was using a steel 27 LP (4 liter), so 240 divided by 4 equals 60 bar. Using this math I only used half of the cartridge duration, so I actually had a 5 hour cartridge duration for that particular dive.

This is very important because canister duration is really about workload and the amount of oxygen you use (or CO2 you produce). Using the cartridge gives me peace of mind for scrubber packing and repeatability for doing the calculations for canister duration. My point is any canister time quoted by someone is based on a set rate of CO2 production and oxygen usage that may not be matched with how you dive. The cartridge testing is done based on 1.35 liters per minute, I know CE is 1.6 liters per minute, but I challenge anybody in reasonably good physical shape to maintain that rate. I know I can’t and people tell me I swim fast.  More advantages later.

** tests based on MicroPore testing at 40m at a 1.35 liter CO2 production.