Rebreather Prebreathe Myth-busting with Dr Simon Mitchell

In this deep‐dive episode of Pod Diver Radio, host Joe Cocozza sits down at BTS 2015 with Dr Simon Mitchell (University of Auckland anesthesiologist, technical diver, and DAN Rolex Diver of the Year) to dissect his landmark rebreather prebreathe study. Mitchell walks us through the experimental design used with CCR divers on Inspiration units: Three loop conditions — normal scrubber, no scrubber, and a partial scrubber bypass (missing spacer O-ring) Blinded subjects instructed to perform a standard 5-minute prebreathe exactly as on a dive boat Termination criteria based on self-recognized CO₂ toxicity symptoms He then breaks down the results and implications for real-world CCR operations: 25% of divers failed to detect complete scrubber absence during prebreathe 90% failed to detect a partial scrubber failure, despite measured inspired CO₂ ≈ 20 mmHg All divers showed objective hypercapnic responses (increased ventilation, visible distress), but many did not recognize or report symptoms—highlighting how CO₂ narcosis itself impairs judgment Mitchell explains why the traditional 5-minute prebreathe cannot be relied on as a CO₂ scrubber integrity test, why partial failures are especially insidious (tolerable at rest, dangerous under workload), and how this should change the way tech divers think about: Scrubber packing discipline and O-ring checks Checklist-driven CCR assembly The emerging role—and limitations—of CO₂ sensors in rebreather loops Risk management for deep, high-workload CCR dives If you dive a closed-circuit rebreather, teach CCR, or write SOPs for tech operations, this episode is a must-listen, grounded in peer-reviewed data from Diving and Hyperbaric Medicine (March 2015) and explained by one of the leading voices in diving science.