Fathom Gemini Rebreather

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Fathom Gemini CCR Options

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FATHOM is the brainchild of Charlie Roberson, a cave diver and active explorer, who initially set out to produce equipment for his own exploration. As his dives became increasingly deeper and longer duration, Charlie, along with his teammates, recognized the need for a closed-circuit rebreather (CCR) with excellent work-of-breathing and long scrubber duration that would stand up to the rigors of cave exploration.

Unable to find an existing unit that met their requirements, Charlie started to design what would eventually become the FATHOM CCR. There are no easy decisions when it comes to rebreather design and every choice, every dimension, every o-ring, was debated endlessly with fellow divers. Ultimately, the final decisions were Charlie's and his background as a machinist in the US Navy and Hogarthian/DIR cave diver guided him towards simplicity and functionality. Refinements to the original design continue and the FATHOM Mk III CCR is the latest version.


  • Specifications
    • 5” (127 mm) dia x 13” (330 mm) tall
    • 20.2 lb (9.2 kg) without cylinders or scrubber material

  • Heads
    • No batteries inside head eliminates the possibility of toxic chemicals in breathing loop
    • Fully potted and submersible head eliminates the possibility of corrosion on sensitive life‑support equipment
    • Splitter/isolation board allows two devices to monitor oxygen sensors
    • Hardwired leads and coaxial SMB connectors on oxygen sensors eliminate multiple connections and fragile Molex connectors.
    • Minimal head space reduces static loop volume
    • Gas addition in exhale head insures proper gas mixing in scrubber before it reaches sensors and diver

  • Breathing Loop
    • Backmounted counterlung provides excellent work of breathing (WOB) in all positions while keeping frontal area free of clutter for stage bottle mounting and minimizing diver’s profile.
    • OPV in counterlung acts as a water dump and allows diver to dewater unit if needed.
    • Shrimp DSV with quick disconnect bayonets allow for easy cleaning and inspection of one-way valves
    › Optional Shrimp BOV allows for rapid bailout without removing mouthpiece

  • Scrubber
    • 4.8 lb (2.2 kg) dual axial scrubbers
    • Removable and swappable scrubber baskets
    • Black Amalgon canister insulates scrubber 400 times better than aluminum
    • Latchless head, canister and bottom streamlines profile

  • Dual MAV/Needle Valve
    • Needle valve incorporates the simplicity of a constant mass flow (CMF) orifice without the IP adjustment issues or the risk of blockage from debris.
    • 316 SS Swagelok needle valve for precise flow
    • Fully recessed manual oxygen addition button prevents unwanted oxygen addition
    • Non depth compensated Apeks DS4 1st stage with stronger spring allows for adjustment from 200-290 psi (14-20 bar) IP for 400-600 fsw (122-183 msw) maximum depth.
    • Fully recessed manual diluent addition button prevents unwanted gas addition
    • Female QC-6 plugs into offboard bailout/diluent
    • Bypass port routes gas directly to optional BOV

  • PO2 Monitoring 
    • NEW and improved HUD provides reliable secondary PO2 monitor
    › Reversible display
    › Automatic brightness adjustment
    › Voltage indication on start-up
    › Visual alarm feature
    • NEW Shearwater Petrel 3 primary display
    • Optional Shearwater NERD 2 replaces Petrel 3 as primary display

Fathom Gemini Rebreather User Manual

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