MOD 8

(In the quest for the perfect rebreather)

Just because this works for us doesn't mean it will work for anyone else!

Rebreathers can and do contribute to diving fatalities. 

Dive safe

We in this article refers to myself and my normal dive partner, Dennis Price.

Due to many problems over a period of years, we felt it was time to re-address our dive platform.  We have had hose failures (in a cave!), we have had flooding issues from time to time.  As addressed in earlier generations we have tackled each problem in turn, from leaking mouthpieces to a too complicated look and cluttered rig. 

Dennis felt we did not need SPG's on the rebreather itself.  It either worked or it didn't.  After a lot of  hem hawing about and some thinking and analyzing our diving practices I reluctantly came on board. We got rid of them.  (O2 supply and diluent.)  Our rational, particularly on the O2, is that as we monitor the PO2 in the loop, if we lost our O2 the PO2 would drop and we would have to go into some type of bailout mode.  The most likey failure point is the HP hose.  The same with the diluent.  You either had it or you didn't.  If not, we would switch to offboard supply or bailout on open circuit.  In either case the dive is turned.  We check the O2 and diluent supply immediately before each dive.  To do this we replaced the HP hose and SPG with a check valve used in the paint ball industry.  Of course every change has its unintended consequences.  It seemed to work well, that is until we did some dives.  We would enter the water with the O2 supply off.  Unbeknownst to us the check valve would let a little water enter the 1st stage before we turned it on.   After a couple of dives we encountered orifice problems and upon disassembly discovered water in the ist stage.  .  The solution was simple enough, we installed caps on the check valves after repairing the units.

Typical consumption of O2 with a 13 cf (2L) bottle is about 1 lpm or 2cf/hr.   Diluent use is about twice that depending on buoyancy and depth changes.   

The dynamics of rebreather diving (and breathing) is very different from OC, (open circuit).  While breathing OC we would not consider not using an SPG.  On verification of your supplies before each dive we know barring a small leak, which we monitor on our safety check, we know we have sufficient supplies for the dive. 

The latest modifications have gone a long way in simplifying and streamlining the unit.  The left collumn shows the latest configuration and the right is the original system.   The latest mod is, as if not more so, user friendly and has more capabilities than the original.  Not shown is we can side mount at least one stage on the left side and then sling normal stages inboard of that.  All our stages are equiped with the hydraulic connector used to plumb in diluent.                               Due to continuous problems with the leaking standard  OPV (Over pressure valve, a type of dry suit exhaust valve), we replaced the standard OPV with OPV's from a lift bag. Over pressurization is dealt with by exhaling from the loop.  It was felt that we needed the OPV in the event we forgot to turn of the O2 injection.         We permamently plumbed in the O2 injection system to avoid contaminants blocking the small O2 orifice.  We use a 0,076 mm (0.003 in) orifice.   We got rid of the ADV as it was prone to free flowing and leaking.  We like the MDV (Manual Diluent Valve) better and it is more reliable.   We eliminated more hoses by teeing the inflator hose with the bailout regulator.   We found we could also top off our supplies through the pressure check ports without disassembley of the unit cylinders. (Just a small mod to the 1st stages)                   The PO2 monitor housing was remade for wrist mounting.  See PO2 gauge page.         Instead of the anti-collapse rings Draeger uses, which always seemed to move and then the hose would collapse and make a dive most unfriendly, I inserted a 45 degree fitting into the end of the hose.  No more problems.	PO2 gauges are now on the forearm.	From the front this unit seemed to be intimidating to divers.
	Argon bottle is mounted on the inside bottom under the plate.
	Stiff OPV form a lift bag	O2 sensor placement
	O2 injection system	Manual diluent system
	Regulator T	Detail overview of components
	Flow check	Modification to a loose hose attachment. It is a piece of inner tube strtched over hose end.  It has stopped the bad connection that new connectors or hoses would not fix.
	Wrist mounted dual PO2 monitor	Predive pressure check

Last update 15 Dec 2010