Q. A reader asked for recommendations regarding a typical weekend of diving. If he dives two hours on Saturday, is it fine to dive the same canister on Sunday for two more hours?
A. The answer is not exactly black and white. If you look at the PRISM2 Assembly and Operational Checklists, they clearly state the tested duration of the scrubber. It reads:
“Maximum Scrubber Duration: 240 min (0.5% SEV CO2) using 6-12 @ 40 °F/4.4 °C, 1.35 slpm CO2, 40 lpm RMV, 60 fsw/28 msw” Operational Checklist
If you dive deeper, colder or in more strenuous conditions, the duration of the scrubber will be reduced. Hollis additionally suggests that once a scrubber is packed, then it should be used within 24 hours.
Based on easy, shallow diving, the math adds up. However, there are a few more things to consider.
Imagine that late Sunday afternoon, beyond the third hour of scrubber time, you run into a problem that forces you to exert a significant workload, such as executing a rescue. Is there a situation where you might be required to extend your dive time such as freeing an anchor, executing a search or descending to retrieve a lost piece of dive gear? Those unforeseen circumstances could take you well beyond the life of the scrubber. In the case of extreme workload, symptoms of high CO2 can go unnoticed since you are already breathing hard.
Caution is prudent. If you want to leave yourself some solid contingency room, it is always best to start with fresh sorb on the second day. If you are diving from a boat that does not permit repacking a canister on the vessel, then buy an additional inexpensive scrubber basket that you can take pre-packed and sealed in a dry bag.
Note: The Prism2 rebreather has completed 3rd party testing to the EU standard 14143 and met all acceptance criteria. These tests include unmanned testing of scrubber capacity in 40 m and 100 m depth.
