--1.42 Oxygen Side Gas Distribution Components and Functions

1.42 Oxygen Side Gas Distribution Components and Functions

The onboard oxygen supply is also an AL13 (AA 2L) cylinder containing medical or aviator grade oxygen. Only pure oxygen (typically supplied at 95 to 98%) should be utilized in this cylinder.  Thirteen cubic feet (2 L @ 150 bar or 300 free liters) provides a sufficient supply for oxygen metabolism of an average adult male working diver consuming 1 liter per minute for 300 minutes (5 hours) of dive time regardless of depth.

This cylinder provides the best match for this unit’s configuration with a conventional 5-pound scrubber (equating to one hour per pound. In this regard, the unit is well matched as a 5-hour rebreather with conservatism favoring the scrubber. The oxygen supply can be used as the gauge to replenish both consumables.

The high-pressure oxygen supply is regulated to 125-150 psi (first stage regulator intermediate pressure allowed to adjust with depth) and then distributed to the following components:

1. Needle valve or orifice – provides for passive introduction of oxygen to the breathing loop at a diver selected flow rate (typically +/- 1 liter per minute).

2. MAV (manual addition valve) - allows the diver to manually inject oxygen to boost loop oxygen partial pressure as needed. 

 Each component can be isolated in the event that the component fails ‘open’. The ‘open’ failure mode would result in a potentially catastrophic gas loss, a potentially fatal spike in oxygen partial pressure, and/or the requirement to ‘feather’ the oxygen cylinder valve to introduce oxygen to the breathing loop. Much like the diluent side distribution, providing a means to isolate each component ensures continued system function without needing to completely bail out of the rebreather.

I am partial to the oxygen supply being configured such that the diver can access oxygen in open-circuit mode by cross connecting the oxygen supply whip to the 3-way ball valve that permits offboard gas access. In a shallow water emergency, access to open-circuit oxygen may be useful to buffer decompression requirements, and it makes good intuitive sense to have ready access to every ounce of gas carried when it comes down to a real emergency. Imagine how it would feel to be staring at 20-30 minutes of required decompression (open circuit) remaining at 20fsw and have no diluent gas to work with, but plenty of oxygen that you just can’t get to!

Additional staged oxygen cylinders, or integration of offboard oxygen supplies, can be added for specialized primary or bailout circumstances.

Why Use a Needle Valve?

Oxygen delivery is among the fundamental sources of controversy amongst rebreather designers and between end-user factions. There are two basic methods – an electronically controlled system via a solenoid, and/or mechanical or manual control via flow through an orifice. This orifice may be supplied with a constant intermediate pressure, resulting in constant mass flow, or a constant number of oxygen molecules regardless of depth. This is depth limited based on the supply pressure of the oxygen regulator’s fixed intermediate pressure.

Diving a constant mass flow system with an orifice is simple but requires modification of the oxygen first stage and is essentially ‘set it and forget it’, meaning the oxygen flow cannot be changed during the dive. If the diver is working hard and consuming more oxygen, the balance must be made up with manual gas injections. Further, the orifice itself is a very small bore and can become clogged from salt or corrosion over time. This can be mitigated with simple maintenance and should not sway the end-user; however, I believe needle valves mitigate this somewhat and should be considered as a viable alternative.

Needle valves are gaining popularity and should be considered seriously as viable oxygen injection tools through future development projects. Needle valves may be supplied with either a fixed intermediate pressure or variable intermediate pressure. For the latter, manual controlled systems can forgo orifice depth limitations when used carefully. For any mechanical system, the ability for the diver to manually adjust pO2 with a MAV is necessary to bump up oxygen if metabolism exceeds the pre-set orifice flow rate.

I am an advocate for using a high precision needle valve supplied by a standard pressure compensated first stage regulator (no depth limitation). Needle valves allow the oxygen flow to be adjusted during the dive, either up or down, based on the diver’s working metabolism. By monitoring pO2 and the rate of required manual oxygen injections, the diver can easily determine if ‘a little more or a little less’ oxygen may be a beneficial adjustment. Further, being able to actuate the needle valve routinely at the surface to effectively open the orifice debris or corrosion can be blown through the system. The needle valve’s internals should be maintained, just as with an orifice, however in the field, any blockage can be easily flushed out without disassembly.

Care must be taken in selecting a needle valve. There are numerous makes/models that provide flow rates within the range of desirable for rebreather diving, say 0.5 to 2 liters per minute nominally, and are also built with materials suitable for seawater use and oxygen compatibility. The trick is needle sensitivity. Fully open to fully closed in a just a few turns would be incredibly difficult to fine-tune, particularly with thick gloves on. I am an advocate of precision needle valves with a minimum 10 full turns open. My various design-build systems include a product from the company Aalborg, which is 15 turns open, and provides a flow rate of 0.7 LPM at the mid-point (about 7 turns open). From there, I can adjust up or down as needed based on my metabolic consumption needs.

Now, I am also an advocate of using standard first stages, oxygen cleaned of course, that remain depth compensated. This eliminates any need for first stage modification and does not require a higher-than-normal IP to dive deep as it would to maintain constant mass flow with a smaller orifice.

Some argue that this is a dangerous configuration, since oxygen flow will increase with depth unless the needle valve is adjusted downward (effectively reducing the orifice size). For discussion, I evaluated flow rate change with variable inlet pressure on the bench, along with numbers of turns on the needle valve. Since downstream pressure does not factor into constant flow through an orifice [until downstream pressure = inlet pressure], this is not a factor for evaluating needle valve performance and a bench test is adequate. We recorded flow rates over increasing inlet pressures and with different needle valve turns.

Based on the figure, we see that only three turns closed would be required to adjust for diving from the surface to a depth of 500fsw. Three turns is hardly a difficult thing to manage and easily accounted for with good tactile reference to the needle valve knob. Likewise, on ascent, opening the valve three turns when returning from this depth is a negligible burden. In shallower water, say up to 200fsw, only a single turn closed or even a half turn closed is all that is required to maintain our target flow of 0.8 to 1.0 LPM.

With a sensitive needle valve, I believe that these are viable options for manual rebreather systems and can provide for significant range extension without the limitations of a constant mass flow orifice. Proper education is required, as is discipline, to react to pO2 fluctuations, though one should recognize that less effort will be placed on needle valve adjustments than buoyancy adjustments during a dive, and that is easily engrained during day one of basic scuba training.