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