IT TURNS OUT THAT DIVING with a rebreather isn’t difficult. My Poseidon Discovery MkVI course went as smooth as silk, thanks to instructor Martin Ainsworth from the Blue Lagoon, so the following weekend I took my unit to a local lake and went for a dive.
Being a newbie and a good boy, I didn’t just swim around enjoying myself. I ran some practice drills, including bailing out to the built-in open-circuit demand valve.
It’s easy – you turn a lever beside the mouthpiece, and instead of breathing gas from the loop, you’re breathing gas from the diluent cylinder. Or, in my case, water from the lake.
“Goodness me,” I thought, “it never did that on the course”, and elected to go back to the loop rather than drown.
Trying again produced the same result, so I thumbed the dive, surfaced and sent a worried text to Martin, certain that there was a fundamental problem with the unit, and ready to drive back to the shop for serious repair work.
Martin’s a good guy. He rang me back in a few seconds to suggest very gently, and without audible sarcasm, that I check the exhaust valves of the bail-out reg.
They were full of weed from the lake, allowing water to flood into the mouthpiece.

LESSON 1: If there’s a problem it isn’t necessarily the rebreather, so don’t just throw away everything you learned diving open-circuit.

Very slightly embarrassed, I cleared the weed and went back in. At the end of the second dive I sent up my DSMB, the string of which promptly leapt from the reel and enthusiastically tangled itself around the top of my rebreather.
Back on dry land, even more embarrassed and possibly swearing a bit, I asked a passing diver to pull the line free for me. He took one look at the unit, said he’d only just started diving and didn’t know anything about rebreathers, and ran away.

LESSON 2: Rebreathers can scare people. Some can’t see past the unit, even when the problem is something as basic as a trapped piece of string. So when you dive with an open-circuit buddy, you’re probably going to have to do some educating.

A month or so later, I was suffering from excessive lake immersion syndrome, so I booked a Red Sea liveaboard to put in some more hours on the unit.
Slightly to my surprise, it didn’t take me long to assemble my unit on the dive-deck – certainly no longer than it took the open-circuit lot to build their single-cylinder rigs.
I might even have been pointing this out, in a superior rebreather diver’s tone of voice, as I started the unit up. I certainly wasn’t paying attention. It failed the automated start-up process, and shut itself down.
“Goodness me,” I thought, “it never did that on the course”, and I tried again.
It failed again. This time I was watching, and saw that it had failed test 49 with failure code 49.
Poseidon supplies a list of test and error codes. 49/49 is the test to detect leaks in the breathing loop, but I didn’t need the list. Now that I was paying attention I could hear air escaping from the exhale side of the loop, and could see the counterlungs deflating.
I shouldn’t have needed an automated test to pick this up. My Poseidon may run the test as part of the power-up sequence but it can be done manually, just as it is on every other unit on the market. I had been far too casual assembling the unit, but the electronics had saved my sorry backside.
Mentally kicking myself, I undid the hoses from the exhale lung, re-lubricated the O-rings, put it all back together and restarted the unit.
It failed again. Same test, same error.
So I did the hoses and O-rings again, starting to worry that something had somehow been damaged on the flight out, despite the rebreather having been in a hard suitcase.
It failed again. I was about to strip and reassemble the exhale side yet again when a single-cylinder open-circuit diver pointed to a different joint in the exhale side of the loop, and asked if it was meant to be leaking gas.

LESSON 3: Lookin’ cool wiv my bereaver is far less important than concentration.

LESSON 4: The electronics are there to catch your stupid mistakes before they become life-threatening, but best to catch them yourself first.

LESSON 5: Jenny is a wonderful human being with far better hearing than me! Or, if you prefer, even open-circuit divers can have their uses.

Mumbling to myself a bit, I undid the joint, re-seated and lubricated the O-ring, put it all back together and the unit sailed through the pressure test. Then it failed a gas calibration test.
Starting to feel ill, I vaguely remembered something mentioned on my course – not in a formal theory session, but when we’d been chatting about diving the unit I had.It was a story about a diver on a Red Sea liveaboard who had been given a diluent cylinder containing nitrox 32, not air.
I removed the hired “air” diluent cylinder and checked the contents. Nitrox 26.
Half an hour later, the diluent cylinder had been refilled with air and, much to my relief, the unit had turned on and pre-breathed very nicely.

LESSON 6: Pay as much attention to the idle chatter as to the formal instruction when you do your course. And always analyse your gases.

Next morning was scheduled to start with the check dive at 8am. I arrived an hour early and the unit fired up without a problem. Of course it did.

LESSON 7: If you’re prepared for problems, you won’t have them.

Starting to feel more optimistic about life, I dropped happily into the water. Straight away I could taste salt. This was new, but as all my training had been in quarries I didn’t worry.
About half an hour into the dive I could hear a little bit of water in the exhale hose, and post-dive I emptied about a cup of water from the exhale lung, but once again I didn’t worry.
I had the same taste of salt and the gurgling in the exhale hose on the next dive, on the Carnatic at Abu Nuhas. Then, on the safety stop, I realised that the work of breathing seemed to have gone up a bit. I still wasn’t worrying – I had another challenge coming up.
Looking up, I could see a big swell at the surface. That’s not unusual at Abu Nuhas but it means that the RIB will be heaving and lurching, and as soon as you surface you’re washed toward the reef, so it’s a good idea to de-kit quickly.
I’ve seen divers clinging desperately to the RIB with one hand, pulled around by the heaving boat as they struggle out of their kit.
As I made my final few metres of ascent, I undid all the straps and clips on my harness and, as my head broke surface, I closed the mouthpiece and reached back with both hands to pull the unit up and over my head. I was fully de-kitted in three seconds, give or take. I got the idea from a DIR video on YouTube.

LESSON 8: Don’t be afraid to do things differently. Sometimes there is an easier way. Or, if your prefer, even DIR divers have their uses.

Back aboard, I emptied lots more water out of the exhale lung and decided to open the scrubber canister. It was half-full of water.
I couldn’t understand it. The pre-dive pressure tests had been fine, so the loop had to be OK, so where could the water be getting in
Then I remembered reading online about a unit flooding because of a split mouthpiece. Was that why I’d had salt water in my mouth
Not quite, but close. The cable-tie holding the mouthpiece on was loose. On open-circuit it wouldn’t have mattered – I once did a full season with a mouthpiece that came off every second dive – but it damn near drowned the rebreather.
If I hadn’t opened it up when I did it would have been goodnight electronics, hello repair bill. If I’d survived another dive.

Lesson 9: Given the way I’ve looked after my open-circuit kit, it’s a miracle I’m still alive. Rebreathers are less forgiving – everything has to be tickety-boo if the unit is to do its job.

Lesson 10: If something odd happens, investigate at once. Unlike scabs, rebreathers don’t get better if you don’t pick at them.

Lesson 11: Not all postings on the Internet are rubbish. Read the forums and learn about rebreathers and your own unit in particular.


A couple of days later we were diving the wreck of the Ulysses off the northern edge of Gubal Seghir, a site well known for rapid currents. It requires a negative entry and rapid descent to get into the lee of the wreck, which is what I did.
I had my unit set for an O2 partial pressure of 1.3 bar at depth and the PO2 rose as I descended, exactly as it should – except that it kept going up and up until it briefly registered 1.5 bar, before falling back to 1.3 as I metabolised the oxygen.
It wasn’t a problem, but the inevitable result of a rapid descent. However, Internet experts say it can’t happen on a Poseidon rebreather because it works unlike every other unit on the market.
Wrong. “Ye cannae change the laws o’ physics, Cap’n,” as Scotty may have said.

LESSON 12: Most of what’s posted on the Internet is rubbish. Believe it when you’ve seen it for yourself. But always bear in mind Lesson 11!

And after that, all was well. My Poseidon Discovery MkVI recreational rebreather was fabulous. To be fair, it always had been – the issues were all my fault.
My friend Ginny always says “you don’t know what you don’t know”. She doesn’t dive and has never even seen a rebreather, but she’s right.
I wonder what I’ll find out tomorrow

LESSON 13: You never know it all!