Doing some future equipment planning for dives to 130m - still yet to get the MOD3 training but like to be prepared equipment wise.
If you are planning to plug in DIL from off board tanks, how many would you allow for? 2, 3, 4? One at a time of course.
My thinking is that for a deep dive like that, you only need two, one for the bottom phase and one for a normoxic range.
The reason to plug off board is to allow one to supply heaps of gas to the BOV in the event of a CO2 hit. Also interested in others reasons.
Comments please.
Hi
I've cut and pasted the text below from another thread I responded to regarding diluent. Some of this might therefore not directly relate to your query but is I feel relevant.
In essence the overarching configuration principle I apply is to draw an absolute ‘firewall’ between the primary life support system (CCR) and the secondary life support system (OC). There are numerous elegant engineering ‘pluming’ options to interface the two life support systems enabling the selection of on-board / off board supplies etc. However despite their attraction, such solutions introduce potential for operator error with potentially life critical implications. The use also of different diluents in an effort to reduce in-water decompression again is fraught with user error potential and the resulting decompression is based upon a breathing loop inert gas component of best guess values. Therefore I recommend remaining with a common diluent throughout the dive drawn from the on-board diluent cylinder. However, like the majority of CCR divers, for failure mode management I retain the option of being able to access off board gas via the diluent and oxygen manual add vales.
Looking at the DAN CCR fatality analysis data; inappropriate gas is by far the single largest disabling injury occurring on over 50% of fatal incidents. No surprise considering the very dynamic nature of the breathing loop and the increased potential of encountering hypoxia, hypercapnia and hyperoxia when using a rebreather. Now introduce a gas supply system that provides the ability to readily access inappropriate gas via the weakest link in the chain, i.e. operator error and you have now significantly increased the probability of encountering inappropriate breathing gas - the single largest rebreather disabling injury. Safety logic asks why adopt such a system? Does the convenience of gas switch blocks and associated ‘plumbing’ justify the increase in risk? Only the individual can answer that, however “seems like a good idea” is not an analysis of risk and so suggest a simple but well considered Failure Mode Effects (FME) and Human Error Analysis (HEA) is applied to each configuration option.
In essence, keep it simple, keep it safe.
Rgds
Paul
I am of the opinion that diluent should only be used to make up loop volume, wing inflation, temporarily lowering or elevating PO2 during emergency procedures (dil flush) and to conduct a cell check (dil flush). Whether using normoxic or hypoxic diluents, my reasons I do not use diluent for bailout are:
1) 2 or 3 litre cylinders hold insufficient gas for emergency use. The concept of sanity breaths from a diluent cylinder during an emergency I believe to be highly questionable, particularly during a hypercapnic event (put a 3 litre cylinder on a breathing machine, press to 40m for example, crank up the breathing rate to even moderate RMVs and watch the gas depletion - it will scare you). Increase the depth and RMV and it will terrify you.
2) Breathe down the diluent and you have lost wing inflation for routine buoyancy or for emergency use (yes you can access other gas supplies if you have wing inflation hoses fitted to the first stages, this however is additional task loading and likely a 'bridge too far' if sinking with little or no control over the descent rate).
3) If your diluent is hypoxic, breathing it will knock you out at the surface / shallow water if you have to bailout (it is frequently the case that things go wrong either at the beginning or end of a CCR dive in the shallows or when at the surface - the danger zone for hypoxic gases). For the same reason no one would start an open circuit dive breathing a hypoxic mix, I do not start a CCR dive with my BOV connected to a hypoxic mix.
I like to keep things very simple, my Meg BOV has a swage lock type high flow male Quick Connect (QC) fitting on the end of the hose that runs along the exhale breathing hose (left side) and then down the outside of the exhale counterlung. All my side mount bailout / deco gases have swag lock type high flow female QCs fitted to the first stage. I then follow gas switch protocols as per open circuit hypoxic trimix, i.e start my descent with my BOV connected into a 'travel gas', usually my 60% deco gas. At 15m I then disconnect the QC and connect into my bottom mix bailout gas, check the functionality of the BOV by taking 5 full breaths on open circuit and then continue my descent. On bottom I again confirm the functionality of my BOV before committing to the dive. On the ascent the protocol is reversed.
This set up completely disassociates inappropriate gases from the BOV supply (assuming you switch gases at the correct depth) and avoids complex manifold plumbing and the potential for inadvertently placing the valve in the wrong position. In addition it permits the demand valve to perform as designed by avoiding routing gas through numerous gas valve internal direction changes that can have a significant detrimental effect on gas flow at peak demand at depth.
The Meg BOV demand valve components appear to be from an Apex TX50. As such I expect the Meg BOV to perform as well as the Apex demand valves I have on my bailout and deco gas cylinders. Using Ali 80's for bailout / deco I see little reason to have to switch from the BOV to the stand alone demand valves I also have fitted to the bailout / deco cylinders unless the BOV becomes un-useable for some reason (caustic solution perhaps).
Rgds
Paul
