--1.43 System Compatibility for Gas Sharing

1.43 System Compatibility for Gas Sharing

As the population of rebreather divers has grown, there's been a few divergent paths evolved on gas flow paths that make in-water or even on-site gas sharing a bit complicated. It would be advantageous for subsystem level configurations to become standardized to enable cross-unit compatibility within a dive team. Of critical importance is cross-compatibility of offboard gas distribution systems. All members of a dive team should be able to share or utilize any other members' offboard gas supply for bailout/gas sharing purposes. Similarly, if multiple gas depots are staged during an extended range excursion, all cylinders must be suitably configured to incorporate into the rebreather configuration and with ease.

 

During the course of diving’s evolution, a variety of quick disconnect fittings have been adopted. The one that is most familiar is the ‘standard’ BC tip used on your inflator mechanism which opens a Schrader valve in the inflator hose. Many manufacturers have developed proprietary inflator tips, only to confuse the consumers and create a mishmash of tips and corresponding coupling/hoses on workbenches and on dive gear. Likewise, in commercial diving, a number of higher flow quick-disconnect (Q/D) fittings and couplings are used, though these are not frequently used in recreational or technical diving.

 

The need for offboard gasses, and diving as a team will surface sooner than later, and the question of which fitting to use and how to configure these into your unit ends up being a tantalizing and overwhelming task given the lack of an industry-wide accepted best practices or consensus amongst equipment manufacturers.

 

So, where to start?

 

As of this writing, there are several camps, each with its own advantages and disadvantages. AP Diving sells a proprietary system. DeltaP/VR uses a QC6 style fitting, some use a QC4, KISS folks use an Omniswivel version, still others use a standard BC inflator, and so on. While the goal of this section is not to criticize any of these methods, here we present an argument for a highly versatile and best compromise of features for rebreather diving incorporating the ‘CEJN’ style quick-disconnect components.

 

CEJN-style quick disconnect components

 

Some background: CEJN (pronounced ‘sane’) is a manufacturer of hose coupling systems for fluid and gas applications. They made their first appearance in diving with that ‘other’ fitting found on drysuit inflator valves. Both Poseidon Unisuits and Viking drysuits have incorporated this type of fitting through several decades of product development. If you dive one of these suits, your first thought was probably, Ugh, how annoying-this fitting doesn’t work with any of my normal inflator hoses!  Today, several additional drysuit manufacturers use this type of industry-standard tip, and compatible parts have been incorporated into BC inflation mechanisms and Q/D assemblies for breathing gas supplies. For example, Apex uses this style fitting on their BC’s. Ocean Reef, the manufacturer of full-face masks, sells a Q/D assembly for their masks featuring true CEJN parts.

 

A CEJN-style male tip commonly found in diving systems. Note the large bore suitable for high volumes required for primary life support.

 

A look at the CEJN catalog will be overwhelming. Several of their fittings are listed as suitable for breathing gases, however, these are geared towards SCBA units or other HAZMAT supplied air systems. CEJN Series 220 and 221 are what is used for diving applications. Components in Series 220 and 221 are interchangeable and likewise are compatible with those used in diving from Poseidon, Viking, Apex, and Ocean Reef. One should note however, that 220 nipples are mild steel. Series 221 are stainless and brass, thus best suited for in-water applications. Both 220 and 221 Series female couplings are constructed of non-corrosive materials and suitable for in-water use.

 

Why use CEJN-style fittings & assemblies?

The original and very simple advantage, stemming from use on drysuits, is the ease of connecting and disconnecting the fittings, even while under pressure and with heavy gloves on. The knurled collar on the Series 221 female coupling makes use very simple. These female couplings are self-checking, just like a standard inflator hose, allowing them to be disconnected underwater without gas loss.

 

The next consideration, and perhaps most important if designing a system from scratch, is providing ample gas flow to any second stage demand systems: a BOV or ADV in this case. Standard BC inflators are not designed to supply breathable volumes of gas and thus are not suitable for offboard gas addition to primary life system second stages. True CEJN 220/221 fittings, or those from Ocean Reef, are specifically designed to provide sufficient gas flow to a second stage. This is much less critical if only feeding your peripheral rebreather devices such as MAVs. However, if your BOV is fed from a manifold, there should be no restrictive fittings in line from the gas source; this includes non-return valves (see note on flow coefficients).  By using high flow fittings throughout, there is no confusion, and an offboard whip can be plugged into the rig or directly to your BOV without concern.

Note: Any and all fittings used in a gas supply system must have flow coefficients (cV) suitable for supplying a breathable flow of gas to second stages. Creating restrictive flow paths leading to second stages will cause inadequate breathing gas supply, especially during arduous breathing cycles associated with bailing out under stress. This could escalate the breathing pattern and increase the chance of a hypercapnic event.

 

The CEJN website describes their Series 221 nipples and couplings are listed in their ‘breathing air’ section with the following technical data provided:

 

 

 

One should note, however, that the above technical data is true for CEJN 220/221 Series parts only. While Ocean Reef and Apex produce CEJN-style fittings that are fully compatible with true CEJN parts, their internal bore diameter is slightly different. In these cases, the parts are suitable for breathing gas, per Ocean Reef and Apeks specifications when used with their respective hardware only.

 

A final advantage to CEJN style fittings is that they can be used throughout your rig including offboard whips, at manual add valves and for BC/suit inflation. This consistency used throughout the rebreather gas distribution configuration makes it possible to access any gas supplies at any point in your equipment. This eliminates confusion and anxiety in addressing any number of failure modes. This also allows for full bypass of the rig itself to directly feed the BOV in a full boom scenario.

 

A note on flow coefficients

 Normally, the coefficient of flow (cV) means nothing to divers, and frankly, it should not be a concern for the consumer of tested off the shelf assemblies and complete systems. However, as CCR divers start to make custom hoses, adding inline shutoffs, pivoting adaptors, quick-connects, and non-return-valves flow resistance can greatly reduce gas flow, producing an undesirable, or even unbreathable supply of gas to the diver.

 

For discussion, a starting point in understanding adequate flow is needed. CEJN 221 Series parts allow a flow rate of 580 l/min at surface pressure (1 ATA). Another variable is considering pressure drop across the fitting itself or other restrictive components in the gas flow path. Pressure drop across CEJN 221 Series fittings is stated as 0.5 bar. 

 

The most widely used characteristics of valves and various adapters in the industry is the flow coefficient, or cV-value (coefficient of volume). The cV value is defined as the number of gallons of water per minute that can flow through an orifice with a pressure-difference of one psi. You can also use these values to calculate the flow of gasses. The gas fitting supplier Swagelock has produced an online calculator for this purpose which can be found here:

 
http://www.swagelok.com/tools/CV_Calculator.aspx

 

 

When using the above values for CEJN 221 Series products, where:

Pressure_1 P1 = 6.0 bar (max rated inlet P for CEJN
Pressure_2 P2 = 5.5 bar (P with reduction)
Temperature = 4 C
Media = Air
Flow = 580 l/min

The calculated cV is 0.81.

 

So as a rule of thumb, very crudely, components with a cV higher than 0.8 will provide flow better than a CEJN-coupling.

 

Running multiple fittings and components in line will result in multiple pressure drops. Knowing the cV values of your components would allow for a rough estimate of the resulting pressure-drop. The objective should be to supply your second stages (BOV, ADV) with an inlet pressure and volume suitable for their proper operation through any number of inline valves and bends and maintain sufficient gas flow for breathing.

 

In short, the fewer restrictions in line with critical components, such as a BOV, the better. It is possible to have adequate flow at the surface, but dangerously high resistance at depth, which could result in exacerbated carbon dioxide problems.

 

Female Offboard Subassemblies

There are a number of compatible CEJN-style female hose fittings. Hoses with this coupling can be attached directly to an offboard gas supply's 1st stage, as they are self-checking valves. CEJN Series 220 females are seawater safe, as are CEJN 221 females. Drysuit hoses from Poseidon/Viking are also suitable whips however the 3/8-24 UNF fitting terminal end should be checked for bore size since they can be restricted as a preventive measure for runaway drysuit over-inflation. The whip provided from Ocean Reef is however suitable for breathing. A female assembly is also produced by the company Hansen, which is sold as the female counterpart to the Apex CEJN-style fittings. Each of these mentioned female fittings are cross compatible with all of the above-mentioned male fittings. 

 

 

A CEJN-style coupling commonly found throughout diving systems. The black ring is a locking collar to prevent accidental disconnection.

 

The Bottom Line – What’s Most Important

Nothing will provide more knowledge and experience than spending time with your rebreather unit. Understanding gas flow pathways is critical in making any rig modifications. Beyond the components, I am emphasizing that the following fundamental considerations be evaluated and incorporated into any new subsystem design:

 

●       Ensure that all fittings in-line with gas supplies that feed a BOV or second stage provide sufficient gas flows for the device.

 

●       When planning to incorporate multiple diluent gasses, all auxiliary functions must be served by the gas that is turned 'on', to avoid any confusion as to what is feeding each auxiliary item.

 

●       Always utilize a diluent gas that is suitable for breathing in open-circuit mode at any given depth. Should a hypoxic diluent be utilized, one should be certain that the BOV is always fed by a gas that is breathable at depth.

 

●       Standardize your rig by utilizing cross compatible components at all points where you might want to connect an offboard whip or may require access to a redundant gas supply.

 

●       By utilizing a high flow CEJN male fitting at the BOV, you can easily bypass your rig and plug offboard whips directly to the BOV. This is particularly useful is you need to access offboard O2 for open-circuit decompression.

 

●       Ensure that all members of your team are using the same, recognizable configuration, and most importantly, compatible components!