TDI Extended range + Normoxic Trimix Course, June 2017

I took this course primarily to access the deeper wrecks in the English Channel and to extend my skills under the concept of doing hard stuff to make the simple stuff easier.

Last year I took the "Advanced Nitrox, Decompression Procedures and Helitrox" course (aka Helitrox) with Mark Powell. This was a superb introduction to longer diving where a single stage cylinder is used to accelerate one's decompression times. It's limited to 45 metres using a light Trimix (Helitrox) up to 20% helium to reduce the narcosis felt at that depth. Planning on a Helitrox dive is about catering for the failure of the single stage -- the lost gas times.

Unexpectedly, Helitrox really changed my attitude to the NDLs (non-deco limits) which are absolutely drummed into you as a PADI recreational diver -- and probably all other agencies too. The number of times I've dived off a RIB where the chant "no deco" came out (what's the point of the safety stop then, and how do you stop on-gassing...). Following on from this course I could easily do an hour's bottom time at 30 metres when diving a twinset -- a vast improvement over the 30 mins on Nitrox 32, or 20 mins on Air.

The Extended Range and Normoxic Trimix course increases the maximum depth to 60 metres and uses two stage cylinders, typically containing 50% and 80% oxygen. Backgas is normoxic, so needs more than 18% oxygen with any amount of helium -- as much as you can afford! The course is very much a follow-on from the Helitrox course.

The major difference between a 60 metre dive and a 45 metre dive is the increase in risk due to the depth. 60 metres is a very long way down, 200 feet in old money. For comparison, each floor on a high-rise building would be about 3 metres, so that makes 60 metres about 20 floors.

At 60 metres, you won't make it to the surface if you do something stupid. And if you do, you will be bent. At 60 metres there's always a decompression schedule and it's roughly 2:1, so 30 mins on the bottom means roughly 60 mins of deco, compared with 45 metres where it's roughly 1:1. The decompression gasses carried in the stages are very poisonous at depth due to their oxygen content and pressure.

This means that you're going to be spending a lot of time on stops; you must follow a plan/computer; you'll need a lot more gas; and you must be able to switch between stages and clean up afterwards, stowing hoses, etc.

Owing to the nature of deeper diving, you'll most likely be diving in mixed groups including rebreather (closed circuit - CC) divers. They like Open Circuit (OC) divers as we swim around carrying lots of gas which they could use for a "bailout". Mocking aside, rebreather divers carry just enough bailout gas for themselves to get to the surface and they need one cylinder of gas which works on the bottom, so they've only got one decompression gas, whereas an open-circuit diver has three types of gas -- one on the back, and two deco gasses on the front -- and an OC diver carries lots of reserve gas.

Kit requirements are standard technical kit with two large Ali 80 stage cylinders for both OC & CC. For OC, it's a standard manifolded twinset rig with long-hose. It's possible to use other configurations, but this is up to the discretion of the instructor. You'll need a backup mask, two SMBs, two depth computers, spools/reels, etc.

The theory component of the course goes into a lot of detail about decompression models, comparing different gradient factor settings and bubble vs dissolved gas models. Read Deco for Divers. Then read it again. There's lots of work calculating MODs (maximum operating depth) for oxygen; also the END (equivalent nitrogen depth) for the helium.

Planning's done using, in our case, Multideco. An excellent program which includes the two main models (Buhlmann and VPM).

The course is run over 4 days with 6 dives and will include both open and closed circuit divers.

The original plan was to do the first two days at NDAC and the second days diving out of Dartmouth. Alas the wether interceded, so four days in NDAC.

Dive 1 was a review of the course pre-requisite Helitrox skills. So down to the 6 metre platform hovering motionless at the bar -- one of the skills is being completely still for a timed 60 seconds. Then it's valve shutdowns; removing the mask and replacing with the backup mask; out of gas donations (give and receive); switching to a stage cylinder; and send up an SMB.

Personally I've been practicing these skills for months now in preparation for this course, so for me this was easy street -- I know there's plenty of challenges coming later.

We had another person on the course who'd not been practicing in advance and who really struggled with the valve shutdowns. These skills definitely need to be mastered in advance. That person left the course on the second day.

The second dive was the descents and ascents. Down to 22m, put up the SMBs, then up to 12m at 10m/min, stop for 1 min, up to 9m and switch to the first stage, stop, then up to 6m and switch to the second stage.

Two stages are a real faff until you get used to them. Kitting up is seriously painful as they're sodding heavy (~18kg each stage). In the water the ali80's are good, pretty neutral and pretty easy to handle. For me the new skill was cleaning up and being efficient about switching from one stage to another whilst maintaining buoyancy and trim. Was so glad of those endless practice sessions this year!

Day two was more ascents and stage switching, intertwined with lots of bubble-gun action. This is where the instructor will swim behind you (Mark Powell's very good at sneaking up behind you!) and fire off a bubble gun where you then need to efficiently shut down the valves to find the problem, then either fix it, or thumb the dive. The preferred action is to always isolate first unless you know where the problem is. Loosing all your gas at 60 metres is definitely not a good thing.

My rebreather diving buddy took the brunt of all the failures: cells reading high (dil flush or bailout); cell voting failed; running it as a manual semi-closed rebreather; running it as an oxygen rebreather, plus loads of other things which a mere OC diver can only watch in awe. The rebreather torments continued throughout the course!

The blindfold comes out and have to do ascents with a buddy on a shot line where the buddy knocks your hand up, down or squeeze to stop. Then its a blindfolded stage deployment; up again, then a blindfolded stage switch. Actually it's easy as you've less to worry about! Just concentrate on keeping your trim & buoyancy, and doing the switches. Quite fun really.

Day three was a straight Helitrox dive to 45 metres using a wrist-slate for the plan. I hate those things. They're good in the water though as you've the plan, the longer plan, deeper plan, lost gas plans loosing 80% and 50%. More pain for the rebreather diver.

We then did the rescue skills. On to the top of the bus at 12ish metres, dump all gas from the wing and suit, then haul yourself up on an SMB. The unconscious diver lift was the tough skill though. I've not done this since my PADI rescue diver three years ago, which was done with recreational kit and no stops to worry about. This is a whole new level of hard as you need to control the other diver's kit and get this bag of wind to the surface with controlled stops. Hard, very hard.

Definitely will find another victim to practice on as this is a skill that needs practice.

Day four's the deep dive. 60 metres and it's up to the team to plan; the instructor observes. Ropes off was set at 10:30 (i.e. when the bus left!) and slack water at 11:00 (i.e. jump in).

The main planning was around balancing gas needs. Bottom time's controlled by the bubble-blowing OC diver as there's a restriction on backgas. The other restriction is the minimum gas requirements which is the gas required to get from the bottom to the first stop with two divers breathing from the same backgas at twice the normal rate. We calculated this as 100 bar of a twin 12. That's a lot more than the thirds required for a Helitrox course -- welcome to deeper diving. So we set the plan to be 30 mins at 60 metres or when I hit 100 bar. I chose to use 18/45 gas as I wanted a clear head for the expected problems.

Descending to 60 metres is quite a challenge. It's a long way down. The dive went well, although my SAC, normally under 15 litres/minute, went through the roof due to the stress of the dive and the expected problems -- which didn't come for me! So I thumbed the dive at 20 mins when I hit 100 bar.

The ascent went well. When we got to the first stop my buddy's rebreather 'failed', so he bailed out. We did our switches and carried up to to 6 metres where my buddy changed to oxygen rebreather mode. My deco gas 'failed', so I switched to my 50% and changed the computer to recalculate the stop.

A nice dive, sort of. Well as well as can be expected in a quarry and blowing £100 of gas in 20 mins.

Then came the exam. Questions of theory and gas calculations. TBH it was nice to have a sit down and enjoy something different.


I really enjoyed the course. Got a lot out of it in a much more 'workshop' manner; incrementally improving my skills. Loads of hints and tips which will make me a better diver.

I can't speak highly enough of Mark Powell as an instructor. A gentle firmness in his manner, his extensive teaching and subject matter experience showing though.

I suppose the main thing I learned is how much a rebreather changes diving, but how complex they are. I dare say my next course with Mark will be on a rebreather.

 

The day after the course we went diving in the sea off of Dartmouth on the Aldershot, an armed merchantman. 57 metres to the bottom.

To be brutally honest, the dive was very similar to many UK dives I've done at 30 metres! Dark, 3 to 5 metres vis, wreck... I was really disconcerted to see a beer can and carrier bag on the bottom. Only difference was the 50+ mins of decompression afterwards!

Definitely a case of the other man's grass is greener. Still, at least I can now play in his garden.

 

It's fairly straightforward at first, but it rapidly becomes difficult as you ascend and discover all sorts of nooks and crannies where the gas collects. For example the legs and arms with drygloves. On a rebreather there's the counterlungs to contend with, plus keeping their mouthpiece in place (unconcious).

I tried it from behind, using the stand and handles on the rebreather to hold him. This then gets silly after ascending a bit, and also you're using your left hand to control his elephant's trunk to dump and inflate, so it's hard to dump your own gas.

I'm going to try face-to-face next time; straddling them to keep them upright. That way I can use my right hand to control their buoyancy, and have my left arm to control mine.

The real issue though is what do you do if you find an unconsious diver and you've a significant deco obligation. At depth their prognosis isn't good; should you blow your deco and suffer the resulting injuries yourself, send them to the surface, or keep them hanging around whilst you do your deco. A really horrible choice that I hope I never have to make.

 

It's all too horrible for words. A deco obligation is referred to as a soft overhead environment, but as you get more loaded then your 'ceiling' becomes much harder; a long obligation bend isn't trivial and could well kill you.

There's been examples of this awful choice where people have opted to attempt to save their buddy who died and they ended up seriously ill. I'm sure this isn't as uncommon as we'd like to imagine.

As for chasing your buddy down whilst breathing a rich mix, ISTR it was Garf who posted that. Very interesting read. First thing is to switch back to your backgas.

 

Well... 32% has a PPO2 of 1.6 at 40 metres, so safe down to there (yes, within reason, hypercapnia, CNS loading...). You could go deeper briefly. But the narcosis would kick in along with stress and you're heading deeper into the very real danger zone of hurting yourself, you'd be chugging your gas and racking up your own deco obligations, not good.

ISTR Garf quoted 40m if on 32% which given all the other dangers seems very reasonable.

If on a deco gas, e.g. 50% or 80%, then you've probably got the option of switching to backgas and dropping down. By definition you're also on a deco dive too, so would be configured for deeper.

 

Not mandatory at all. Need to be big enough for the gas required, so pretty close for rebreather divers bailout.

Was nice to pick up an ali40 at about half the weight. One handed!

Interesting that on the boat on Monday everyone -- all 8 divers -- had Ali80s. OK, only two bubble blowers amongst them.

I'll bow to your experience, but thus far I find Ali80s to be nicer in the water than 7s when manipulating the stages as they're a bit buoyant.