Does Pre-breathing your rebreather matter?

July 16, 2015 4:07 pm / Category: Uncategorized

Training agencies and rebreather manufacturers have advocated pre-breathing the rebreather prior to diving. The rationale has been that CO2 bypass and scrubber faults can be detected prior to jumping into the water. As one instructor told me: “Better to pass out on the boat deck than underwater”. This practice appears to have common sense but does it actually pan out?

A randomized single-blind study from March 2015 on the subject drew the following conclusions:

  • While pre-breathes are useful to evaluate other primary functions;
  • The five-minute pre-breathe is insensitive for CO2 scrubber faults in a rebreather. Partly failed conditions are dangerous because most will not be detected at the surface, even though they may become very important at depth.

The trials included 20 pre-breathe tests of five minutes for each condition. A Inspiration Evolution Plus rebreather was used with:

  • Normal scrubber
  • Partly failed scrubber
  • Absent scrubber
The results were enlightening:
  • With a normal scrubber, nobody terminated.
  • With a partly failed scrubber, 2/20 terminated.
  • With an absent scrubber, 15/20 terminated which means that 25% of divers DID NOT TERMINATE.
A pre-breathe was felt useful to check the function of oxygen addition system but a duration of less than five minutes is felt adequate.
Read the study for more interesting points made:
Deng C, Pollock NW, Gant N, Hannam JA, Dooley A, Mesely P, Mitchell SJ –
The five-minute prebreathe in evaluating carbon dioxide absorption in a closed-circuit rebreather: a randomized single-bind study
Diving and Hyperbaric Medicine 2015; 45:16-24