Howard Payne(Offline)

Well sort of. I spent a very pleasant day at Vobster yesterday helping Clare Gledhill put some more hours on her RB80 in preparation for some bigger sea dives.

She had phoned me a few weeks earlier and asked if I fancied it and I jumped at the chance. I was curious about the RB80 and wanted to see how compatible an OC DIR diver was with someone on the unit.

She pointed out that my "Instructor level awareness" made me an ideal babysitter. I pointed out that "Howard Payne fails to save Clare Gledhill from rebreather fatality" didn't look good on my resume and consequently she was defintely making it back alive even if I had to drag her out of the crushing works between my teeth on a breath hold wearing nothing but a pair of speedos

Several things were clear as she took my though her pre dive preparation of the unit:

Firstly that she treats the unit with a healthy mixture of suspicion and respect. In spite of it being extremely simple, robust and well made and her knowing its operation inside out - there is absolutely no rose tinted love affair going on with a new toy. In the water I was to see the reasons for that.

The unit is entirely compatible with DIR OC diving. The long hose, the bungeed backup, isolator manifold, lights, dumps and all the stuff that we're totally familiar with are all there and work in the normal way. That was very reassuring. We tried a few S drills and V drills on the dives and with the exception that the loop has to be shut and the BOV removed to switch to bungeed backup and the lonhose needs to be unclipped rather than removed from the mouth - everything is the same. The extra steps are not trivial - but no big deal.

The ability and readiness to switch between open circuit and closed circuit is very reassuring as well. As we practiced ascents and descents and various drills - as much time was spent on OC as CC.

I settled into a routine of checking to see that the bellows were going in and out through the hole in the bottom of the unit; making sure that a blast of bubbles was there every few breaths and that the Swagelock connector block which is clipped to the side of the light cannister and routes backgas and offboard gas into the unit was OK. With no electronic controls to worry about - if all this was OK - my buddy was fine.

What is more worrying is the huge drop in FO2 in the loop at shallow depths. Clare was diving 36% rather than the usual 32% we would use for this dive . She explained that whilst I was right to think previously that you only get a very small drop in O2 levels on an RB80, this was only the case at depth. When shallow the density of gas in the loop is much less and much more of the Oxygen fraction in the mix is used up between injection cycles. A drop of of nearly 10% in FO2 is nearer the mark as you get closer to the surface.

Personally I think a rebreather with the ability to have such a marked swing in O2 content - even without all the usual electronic considerations - really should have some sort of PPO2 monitoring?

Even with 36% backgas, we had to switch to an O2 deco cylinder at 6m to stop the loop going hypoxic. Alternatively Clare pointed out that she could have just switched the loop to OC - but the gas switch was good practice. I authorised the switch just as we do OC - but the cheater that feeds gas into the switchblock from the otherwise normal configured OC deco bottle has to be threaded through the crotchstrap and routed correctly around lightcords and long hoses. In short it's a right faff which has to be done blind. Clare managed it fine and then plugged the offboard gas into the switchblock and disconnected the back gas feed. It's time consuming and tricky - you almost need to re-authorise the switch with the amount of time it takes to route the hose.

A definite extra complication to diving an RB80 - but one that gives you a massive number of options for driving the unit from almost any gas source.

Clare bemoaned the deterioration in her trim and buoyancy when diving the RB compared to her ability to be point perfect on OC. She asked for feedback and I explained that at one stage on the ascent she was 2.7 degrees off horizontal for about 10 seconds and she missed one buoyancy stop momentarily by about 10cm for 10 secs. Some people are just never content

The conclusion that we drew after a few dives was that I would happily buddy up with someone on an RB80 for a Trimix or any other dive and that as the ultimate progression of the GUE / DIR system - it is not without significant complication and extra task loading - but at the same time - entirely compatible with the rest of DIR diving as a holistic system

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LCF(Offline)

I've had the experience of diving with my GUE instructor when he was doing some shakedown stuff with his RB80. Although I didn't know enough about the unit to do any critiquing of how it was working, we did practice S-drills with him on the dive, and with the exception of the rather dramatic toss of the loop, they were the same S-drills I was used to.

The system's pretty solid, in that it accepts all these upgrades without changing the basics. One of the things that keeps me a pretty committed DIR diver.

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Kevrumbo(Offline)

Kind of like this Lynne? I concur & like the way how this is based on and differs very little from the same immediate action S-drill we all learned in Fundies (and which is why I'm reluctant to train on any other rebreather system). . .

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Cyrille(Offline)

Hi Howard,

My instructor had a shallow water black-out when diving the RON last year... and all of this despite the fact that he is a very experienced GUE tech diver... He had to be ressucitated and taken to the hospital. Fortunately he survived the ordeal.

I have difficulties to understand why having a ppO2 meter would not be DIR...

If we follow this logic, then we should not use SPG's to read the pressure from our cylinders, should we?

If someone would dive a pure oxygen rebreather at max. 6m, I would understand that a ppO2 meter is not needed. But for SCR/CCR diving this is another story, especially during the ascent when reaching shallow waters.

I believe that buddy pairs must be consistent when it comes to decompression diving:
- either 2 divers using rebreathers
- or 2 divers on OC

Else, not having the same decompression schema, one may not be able to help the other in case of emergency.

Kind regards.

Cyrille

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JGrogan(Offline)

Cyrille,
The RB80 and it's clones allow for the use of an oxygen sensor. However, most people choose not to use them.

DIR does not preclude using an oxygen sensor, it just has a problem with relying on them. Sensors can be notoriously unreliable depending on their environment - temperature, humidity etc and so any unit that relies on a sensor (or group of sensors) to manage the level of oxygen in the loop fails some of the principals of DIR.

Shallow water blackout is due to not changing to an appropriate gas in shallow water. In fact, ideally, one should be on OC in shallow water.

The deco schedules using a RB80 and OC are identical, so no need to worry about that. I don't see a problem mixing RB and OC people so long as the OC people know about RB's.

Best,
John.

graham_hk(Offline)

I'll mirror John's comment about the lack of need for PO2 monitoring - know your gas (and particular its Hypoxia cieling/floor) and the PO2 is so stable there is no need for monitoring.


With this comment ... lets just say maybe for the unbendable Grogan (maybe I should sip expensive wine instead of chugging beer
)

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Howard Payne(Offline)

So if it's not an issue - how do we account for two highly respected divers having hypoxic incidents on the RB80 recently?

I'm not sh*t- stirring chaps - but it seems like there is a genuine issue here. Bearing in mind the tiny number of people diving the RB80 - if you multiplied that incident rate up to a unit as common as say a YBOD - it wouldn't look good would it?

I understand how you avoid this kind of incident by either being on OC or switching gas - but it would appear that this isn't happening occasionally. These guys are hugely experienced - but maybe a redundant HUD type failsafe would have helped them?

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Cyrille(Offline)

Hi,

I could not agree more...

The risk of shallow water blackout is real, and this is true for any rebreather. I don't believe that rebreathers are dangerous, once risks are known and mitigated.

At the end everyone choses the level of security that he wants... but as far as I am concerned I would not dive any rebreather (except. my O2 rebreather ) minus at least 2 independant ppO2 meters.

Should those meters "disagree", I'd thumb the dive and bail-out to OC. Period.

Kind regards.

Cyrille

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Clare Gledhill(Offline)

I'm a new RB80 diver but the recent incident made me think long and hard (I was there when it happened and don't take stuff like that lightly I assure you). If we deviate from proceedure the RB can be a dangerous thing indeed - but so can any RB or OC kit for that matter.

Switching to OC or richer gas shallow is a necessary part of RB80 diving, in the same way that we have to do when diving hypoxic back gas on OC. I've jumped off boats with hypoxic regs in my mouth, it's not proceedure to do so but I guess complacency made me make what was an essentially poor decision. Some have descended on SCR mode on in inappropriate gas using an RB80 or clone. That's not proceedure either.

Unlike Cyrille, I do believe that Rebreathers are dangerous. I have a gruding respect for the RB80 which has been designed as an intuative unit which is easy to get to know at a basic level. Just like OC you have to know your MOD and the bottom out fraction of O2 that your depth will give you. In addition to OC porceedure, you have to monitor depth to confirm that the FO2 does not drop below that which is acceptable. Adding bells and whistles which take away from this just seem to me to pass the buck from the diver to the unit which would lead to a greater possibility of failure. After all, we've seen enough CCR divers die after failing to turn on their O2 or dilluent - having posh alarms and buzzers doesn't safeguard from us accident it should merely confirm that these are not recreational tools and should not be used unless there is a training or a dive requirement for their use.

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Alastair(Offline)

I find it interesting we have experienced RB divers (John and Graham) saying you don't need a sensor. Once you've actually seen an RB80 taken to pieces and really understand what's involved you understand why it isn't needed.

The PP02 drop is predictable - you don't need a sensor to tell you what it is. The units failure modes are detectable by audible and physical alarms for the most part.

Incidents will happen whether you have sensors or not. Look at the ECCR's - multiple sensors, voting logic, redundant controllers. Yet people die on them - if sensors solve the problem then why would that happen? Simple answer is people make mistakes - no matter what you do you can't allow for the daft errors.

The solution is to keep things as simple as possible and train the appropriate responses. The RB80 is dangerous - much more so than OC and every time I dive it I don't doubt that. For that reason I don't see it as pinacle of GUE training. Rather its an off-shoot that is taken if the diving OC would be more dangerous than with an RB.

If you look at the dives Chris, Danny and Fred are doing they are accomplishing stunning dives all without a RB. No need to have one so they don't.

Cheers
Al

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