1.41 Diluent Side Gas Distribution Components and Functions
The high-pressure diluent gas is regulated to 125-150 psi (typical first stage regulator intermediate pressure) and then distributed to a diluent manifold. The manifold distributes gas to the following components:
- ADV (automatic diluent valve) – automatically injects diluent into the breathing loop as counterlung volume is compressed with descent. The ADV is fitted with an isolator to shut it down in the event of a stuck open (free-flow) failure.
- MAV (manual addition valve) – allows the diver to manually inject diluent to make up breathing loop volume and dilute or flush the breathing loop as needed. The MAV flow path should be positioned such that it flushes the oxygen cell face.
- BOV (bailout valve) – allows for immediate open-circuit breathing of the diluent supply in the event the rebreather atmosphere becomes unbreathable. The BOV may be an integrated type, or a stand-alone second stage fit to the diluent manifold.
Again, each component should have the ability to be isolated in the event that the component fails ‘open’. The ‘open’ failure mode would result in a potentially catastrophic gas loss and/or the requirement to ‘feather’ the diluent cylinder valve to introduce diluent to the breathing loop in short but controlled bursts. Providing a means to isolate each component ensures continued system function without having to feather the entire diluent supply or completely bail out of the rebreather. For protection during varying combinations of diluent component isolation, the diluent first stage is fitted with an overpressure relief valve.
In the community, there is varying logic on which peripherals are supplied from the primary diluent and which may be supplied otherwise. I have found that supplying everything from a single source diluent makes for very easy gas identification and verification and lessens any potential confusion during a gas switch. My only exception would be the need to supply a drysuit when it is disadvantageous to supply the suit with a helium breathing mixture and an air or argon alternative would be beneficial. In that instance, a separate cylinder of drysuit inflation gas would be introduced.
Multiple diluent supplies may be utilized for flexibility in diluent input via a three-way ball valve that feeds the primary diluent manifold. My preferred onboard diluent supply is typically an AL13 cylinder of air, which is preserved for use at or near the surface and then using an offboard cylinder for primary diluent or 'drive gas'.
The ball valve selected 'up' (shallow) is for onboard gas. Selected 'down' (deep) is for offboard gas.
The ball valve is a critical but overlooked safety component for mixed-gas dives, as it ensures that only one diluent gas is supplying all diluent devices, and the diluent selected is appropriate for the given depth, particularly mitigating risks of hypoxia. The ball valve is positioned at the diver’s left chest or central on the chest with diluents being plugged and played with quick disconnect fittings should additional offboard supplies be required. The diver dives with the onboard diluent selected, then switches to the offboard supply at the predetermined gas switch depth. The onboard AL13 is a sufficient volume for use at or near the surface as open-circuit bailout should an issue arise during diver ingress or egress and is always a normoxic or hyperoxic gas that is breathable at or near the surface without risk of hypoxia. This is particularly useful when conducting more complex dives with multiple offboard cylinders carried as the diver can enter the water on air diluent, get situated, and then switch to the required offboard hypoxic diluent at a depth where it is safe to breathe. Likewise, upon surfacing, extraneous hypoxic cylinders and equipment can be ditched, leaving sufficient air diluent to address shallow water bailout at this terminal stage of the dive. The risk of hypoxia is virtually eliminated from the standpoint of introducing hypoxic diluent into the loop or inadvertent open-circuit access. In most cases, hypoxic diluents used within rebreather depth ranges can be breathed at 20fsw. This marks an opportune stop during the descent, during an s-drill for instance, to make the intentional gas switch.
Naturally, as we consider diluent strategies, all dives should consider full open-circuit bailout requirements and configure the system accordingly. Done correctly, all gas carried by the diver onboard or offboard should be available to diver in both open-circuit and closed-circuit modes.