How Much is a Fathom? Understanding the Fathom MK III CCR and Its Capabilities

The Fathom MK III CCR (Closed Circuit Rebreather) has proven itself through rigorous real-world testing. Notably, Fathom CCRs were instrumental in the 16.5-hour world-record dive (26,930 ft/8,208 m penetration) at Cathedral Canyon. They have also been utilized in numerous extended dives, lasting 10 to 14 hours, at locations such as Manatee, M2, and Lineater. Exploration dives in Weeki Wachee Springs, exceeding depths of 400 ffw/121 mfw, further demonstrate the Fathom CCR’s capability.

While the Fathom CCR excels in deep and long-duration dives, its compact size and lightweight design, combined with its ease of use, make it an ideal choice for divers who travel.

Note: Training is mandatory for purchasing the Fathom MK III CCR. Please contact one of the FATHOM CCR instructors at https://www.fathomdive.com/training/ for more information.

Fathom MK III CCR: Detailed Specifications

To better understand the fathom and the capabilities of the equipment used for such dives, let’s examine the specifics of the Fathom MK III CCR. Here’s a breakdown of the technical specifications:

Dimensions: 7 in (178 mm) diameter x 16 in (406 mm) height
Weight: 30.2 lb (13.7 kg) without cylinders or scrubber material

Head Unit

Elimination of batteries within the head removes any risk of toxic chemicals contaminating the breathing loop.
A fully potted and submersible head design prevents corrosion on vital life-support components.
A splitter/isolation board allows for dual monitoring of oxygen sensors for redundancy.
Hardwired leads and coaxial SMB connectors on oxygen sensors replace vulnerable Molex connectors.
Minimal headspace reduces static loop volume for better gas management.
Oxygen addition on the exhaust side ensures proper gas mixing within the scrubber before reaching sensors and the diver.
Diluent addition on the inhale side, over the #1 sensor, allows for immediate cell verification against a known gas.

Breathing Loop

Back-mounted counterlungs offer excellent work of breathing (WOB) in all positions. They also keep the frontal area clear for mounting stage bottles and minimize the diver’s profile.
Counterlungs can be removed without removing the backplate to facilitate loop cleaning.
Low-profile T-pieces use stretch (front) and rigid (back) breathing hoses.
A Shrimp BOV (Bailout Valve) enables rapid bailout without removing the mouthpiece.
Quick-disconnect bayonets make cleaning and inspecting the one-way valves easier.

Mounting Hardware

Kent Tooling 316 SS onboard cylinder mounts provide a secure, low-profile attachment.
Kent Tooling 316 SS STA backplate mount.

Scrubber

Available with either a 5 lb (2.3 kg) or an 8.8 lb (4 kg) radial scrubber.
Inside-to-out radial flow ensures maximum scrubbing efficiency.
The black Amalgon canister insulates the scrubber 400 times better than aluminum.
Integrated water trap with chammies to absorb condensation.
Latchless head, canister, and bottom streamline the profile and eliminate broken latches.

Oxygen MAV/Needle Valve

The needle valve utilizes a constant mass flow (CMF) orifice design for simplicity, eliminating IP adjustment problems or blockage risks.
316 SS Swagelok needle valve offers precise flow control.
A fully recessed manual oxygen addition button prevents accidental oxygen injection.
Non-depth compensated Apeks DS4 1st stage with a stronger spring allows for adjustment from 200-290 psi (14-20 bar) IP for 400-600 fsw (122-183 msw) maximum depth.

Diluent MAV

Fully recessed manual diluent addition button prevents unwanted gas injection.
Female QC-6 plugs into offboard bailout/diluent.
Bypass port directs gas directly to the BOV or ADV.

PO2 Monitoring

Shearwater Petrel 3 primary display for real-time oxygen partial pressure monitoring.
An improved HUD (Heads-Up Display) provides a reliable secondary PO2 monitor with automatic activation in water, automatic brightness adjustment, voltage indication on start-up, and a visual alarm feature. Alternative alt text: Fathom Mk III CCR heads-up display download icon.
Optional Shearwater NERD 2 replaces the HUD as a secondary display (dual AK cables).

Fathom: Downloads and Resources

For further information and detailed specifications, you can download the following documents:

FATHOM Mk III CCR Spec Sheet: Download (Last updated: March 10, 2025)
FATHOM Mk III CCR User Manual: Download (Last updated: February 29, 2024) Alternative alt text: Fathom Mk III CCR User Manual download icon.

In Conclusion

The Fathom MK III CCR is a versatile and reliable rebreather suitable for both recreational and technical diving. Its robust design, combined with advanced features and a focus on safety, make it a popular choice among experienced divers pushing the boundaries of underwater exploration. The “How Much Is A Fathom” question extends beyond simple measurement; it embodies the pursuit of deep-sea knowledge and the equipment that makes it possible.