by Tim Blömeke

Puerto Galera, Philippines, February 2024
In March last year, I trained on the Fathom Mk III CCR under Kelvin Davidson in Mexico. Since then, I’ve had opportunity to use it in different environments and circumstances, both for my own enjoyment and in my role as a dive professional. I recently completed my first 100 hours on the unit, and I thought this might be a good milestone to revisit my experiences.

Background

I’m an open circuit trimix instructor and tech dive guide working mainly out of Puerto Galera, Philippines. I’ve been a rebreather diver since 2017, and the Fathom is my second unit.

Once bitten, twice shy: Having experienced a number of reliability issues with the electronics of my first rebreather, I wanted to switch to a manual unit. At the time, I only knew of two – KISS and the now-discontinued Pelagian – and I spent some time looking at my options for either.

Why the Fathom?

Then I stumbled across a rebreather I hadn’t heard of before, the Fathom. Going by the specs, it ticked all of my boxes. and the design notes by Fathom Dive Systems CEO Charlie Roberson spoke to me in a loud and clear voice: Robust and minimalistic, with fully potted (sealed) wiring, coax sensor connectors, adjustable constant mass flow injection of O2, passive Shearwater electronics, back-mounted counterlungs, and an option for on-board bailout.

As chance would have it, I already knew my instructor: Kelvin Davidson, owner of Third Dimension Diving in Tulum, Mexico, whose facilities my partner Katia and I had used twice on cave diving trips. I contacted him to inquire about the unit and a training course.

Although the price tag is considerable and instruction is currently available only on the far side of the planet, I was sold. I took a big leap of faith and broke the piggy bank. My partner and I purchased a unit each and booked a trip to Tulum for training.

Training

Though technically a crossover class, training on the Fathom ended up being more of a re-training in rebreather diving in general. Although I had accumulated about 120 hours on my prior unit, I hadn’t dived on closed circuit in over three years and had gotten quite rusty. There are also considerable differences in how the two units handle, as well as in the overall concepts and methods of using a rebreather as taught by Kelvin (who was himself involved in the Fathom’s development) compared to my prior training.

In a word, I had my hands full for a while, but it was more than worth it in the end: I was doing proper cave dives on my new CCR. I’m deeply grateful to Kelvin and the Third Dimension team for their support and the lessons learned, and I can’t recommend these guys highly enough.

Change of scenery

For the first 40-odd hours, I had dived in cenotes, caverns, and caves, to a maximum depth of no more than 30 meters. After a three-month stint in Europe with virtually no diving, my next task was to learn how to use the Fathom in a different environment – my home turf, the tropical seas of the Philippines, with their occasionally strong currents and greater depths.

I began with a series of shallower tryout dives to fine-tune the configuration and get used to blue-water ascents before gradually extending my range, adding a scooter and more bailouts. At this point, I would like to give a shoutout to my friend and JJ CCR virtuoso Karl Hurwood, whose advice and encouragement have been very valuable.

Handling and operation

Despite its being a manual unit, diving a Fathom requires surprisingly little manual intervention once you get the hang of it. At depth, the adjustable constant mass flow design for oxygen injection (fixed-IP first first stage and needle valve) works remarkably well. I rarely have to touch the oxygen MAV at all, except during ascent and deco, when everybody has to do that anyway, including eCCR divers.

Since I opted for the version without an ADV (another once-bitten-twice-shy situation here), I’m a little busier during descent than my JJ-using buddies. However, managing descents has become natural pretty fast. It’s much like driving a stick shift vs. automatic transmission – yes, it takes a little bit of extra practice, but ultimately, the outcome is very similar.

While I wear a drysuit for bigger (by my standards) dives, the unit is lightweight enough to be dived comfortably even in a thin wetsuit when paired with aluminum cylinders. The buoyancy characteristics and balance of my configuration feel similar to those of an AL80 set of doubles.

The work of breathing is very low, comparable to open circuit. The on-board bailout configuration with manifolded dual diluent tanks (“GUE configuration”) eliminates the bottom bailout stage, which feels very clean and leaves the front of the body unencumbered.

These days (as of February 2024), I am making 90-minute trimix dives in the 60+ meter range on a regular basis and have started using my unit for work. I dive it every chance I get outside of work as well (i.e., when not teaching open circuit) and have been averaging about 15 hours per month lately.

Summary

I can’t describe the Fathom as anything but a workhorse, in the best sense of the word. It behaves in very predictable ways and is comfortable and intuitive to dive. The build quality is stellar, and there doesn’t appear to be much about the unit that can actually break down. Aside from the canister, head (fully sealed, no service required), and counterlungs, all other parts are generic and can be sourced on the open market. This reduces dependence on manufacturer service to a minimum – not a concern for everybody, but a pretty big deal where I dive.

I’ve had zero issues that I didn’t create for myself. Should any problems materialize, I’m confident that I can deal with them with the tools and tech skills that I have (which include regulator service but not electronics). I look forward to many more hours of hassle-free diving and would unconditionally recommend the Fathom as a rebreather for remote locations.

On a final, general note, I believe that the introduction of this unit marks the point where manual CCR technology has become fully mature. While eCCRs remain an area of active research and development, and there is some competition in the sidemount CCR market as well, I find it difficult to imagine anyone spending time and money on an attempt to improve on the Fathom, much less succeed at it. If a manual backmount CCR is what you want, then Fathom is the way to go.

My experience so far
in numbers
Time on the unit: 109 h
Deepest dive: 75 m
Longest dive: 190 min
Times I had to bail out: 0
Electronics problems: 0
Mechanical problems: 0
Lessons learned: yes
Sharks seen: lots

UPDATE (September 2025)
Revisiting this post a year and a half and well over a hundred hours down the road (250+), I stand by pretty much everything said above. The Mk3 continues to perform exceptionally well – so well in fact that I recently took the plunge and became an instructor on the unit.

In terms of problems, I had one issue with the handset, which was promptly taken care of by Shearwater. For completeness sake, I’ve also had to replace an O-ring.

As regards range, my biggest dive so far was in the context of a project to dive the USS Ommaney Bay, a WW2 escort carrier at the bottom of the Sulu Sea, at 115 meters of depth. No upgrades to the unit were required for these dives; the Fathom handles them in the out-of-the-box configuration.

TL;DR: If you’re looking for a rebreather that is up to the demands of big dives dives in remote locations and just keeps going, you couldn’t wish for anything better than the Mk3. Please feel free to reach out for questions regarding the unit or inquiries about training,