From Rg-uf/m,…to Rgu-2,… And Beyond

FROM RG-UF/M,…TO RGU-2,… AND BEYOND
By: Dan Dunfee

BACKGROUND
Many years ago, I routinely worked in rebreather (Rebreather) diving gear. In years after leaving the Navy my diving-associated work focused on developing some open circuit (OC), mixed-gas, SCUBA gear for light salvage and technical diving. However, in that period I built a couple of Oxygen-only (O2) rebreathers for inland cave and recreational diving. A few years ago I was re-infected by a rather bad case of rebreather-itis. … and you know how that goes….

I selected the old, East German, RG-UF/M (RGU) O2 rig as a simple, 'low-cost starter kit' to get re-involved with Rebreather's. The RGU has quite a following in Europe and Scandinavia, including substantial modifications for mixed-gas diving. The original RGU design had already benefited from much study, modification (modding) , and reporting-out by other homebuilders such as Dave Sutton, Ken Swain, and Tom Rose in the US… and George Kamarinos, Lothar Weidinger, Uwe Lessman, Martin Klokosch, and others elsewhere.
Several of these are chronicled in the following URL:
RG-UF/M page

However, it appeared that this unique little unit, shown below, still offered a challenging opportunity for a more thorough makeover than had been done before.

Several situations intervened to delay my really 'getting started' on the project, including an accident while shooting underwater video at Coco Island, off Costa Rica. The aftermath of that accident kept me 'grounded' from serious diving, and preoccupied with staying alive for a couple of years…but that's another story. By the time I actually got started on the project hardware, my conceptualizations had already moved far beyond just doing a major makeover of the basic RGU. However, after considerable vacillation, I finally decided on a phased program, where Phase 1 would focus on a major makeover of the basic unit. This in hopes that newcomers to homebuilding, who felt driven by costs, circumstances, or special interests, might benefit from any significant additional learnings that an extensive makeover might provide. Phase 1 maximizes the use of the original RGU componentry, hopefully to minimize costs to anyone who wanted to use all, or part, of this conceptual approach for their own 'low-cost Rebreather starter kit'. Subsequent phases will further refine the baseline RGU-2 O2 Module, and ultimately integrate it into a soft-chassis-mounted, mixed-gas mCCR, with Oxygen partial pressure (PO2) monitoring, integral buoyancy control, and bailout gas supply. This integrated unit is dubbed the D105.

After a couple of false starts, the train started moving down a track that I really liked.Altho I'm a credible conceptual designer, I've no substantial component fabrication equipment or skills,... and so must get customized componentry built mostly through collaborative efforts with others, or direct purchase of the parts and services. It seems to take 'forever' to get something from initial concept to hardware-in-hand.

During efforts to arrive at what is now the basic RGU-2 Oxygen Module design, several people made contributions ranging from moral support to making some components which didn't survive into the present configuration. These include Ken Swain in the US, and Ajren Van Krimpen and David Dekker in the Netherlands. My thanks to them for their support. Two merit special thanks, and their ongoing contributions are imbedded in the present configuration:

• Ken Swain (Rebreather World Skipbreather), who has in-depth expertise in the original RGU, …including its very innovative... but quirky,.. Integrated pressure and constant mass flow (CMF) regulator. Ken has been consultant, devils advocate, and design critic.
• Bill Sewell (Rebreather World CstmWrks), who has worked with me, thru several of my design iterations for the single, chest-mounted counterlung. Bill does especially good Latex rubber fabrication work, and for reasonable prices. Ken and Bill are both past-masters at their own homebuilding projects.

PHASE 1 RGU-2 OXYGEN MODULE
This posting will present an overview of the basic RGU-2 Oxygen Module, compare it to the original RGU rig, and summarize major areas of RGU-2 improvements. Later, incremental postings will provide more details on present and future RGU-2 improvements, including some teardown details, and on development of the integrated, mixed-gas, D105 mCCR recreational rig.

The D105 is intended as a 'recreational' rig…..simple, compact, lightweight, conveniently travelable, and intended for the sheer recreational enjoyment of the 'diving experience'…not for long, deep, cold technical diving, or deep-penetration wrecking or caving. The pictures below show the original RG-UF/M compared with the RGU-2 O2 Module. At this stage, they are analoguous and directly comparable.


Both units are worn as stand-alone, chest-pack O2 rebreathers. As previously said, a key objective of Phase 1 was maximum use of original RGU componentry, which met the design concept objectives. This to minimize the total RGU-2 Module costs when it is used as an 'Rebreather starter kit'.

BREATHING LOOP
Except for the totally new single counterlung (CL) and the extensively modified scrubber canister (SC), discussed below, most of the other breathing loop components are original stock items, or modest modifications of them. The original RGU has a large, uncomfortable mouthpiece bit, and a rubber plug instead of a dive / surface valve (DSV). In the RGU-2, the original mouthpiece bit was replaced by a smaller, comfortable one. A custom DSV spool and lever was fitted into the machined mouthpiece shell.


Scrubber
In the original RGU, the gas path is from the exhale hose, thru the exhale tube, which runs centrally thru the length of the scrubber, then turns 180 degrees, back thru the scrubber pellet bed, to the CL, and thru the inhale hose to the diver. The original scrubber is non-refillable. Several homebuilders have fashioned their version of a refillable scrubber. These typically retain most, or all, of the original parts, and simply separate the bottom end cap at a splitline near the bottom of the scrubber. A new, watertight O-ring gland seals the splitline upon reassembly. A positive watertight seal is critical since the scrubber is exposed to the ambient water.


The RGU-2 scrubber also retains essentially all the original scrubber parts, including the original 1 Kg scrubber charge, and is also made refillable thru a separable endcap. However, in the RGU-2 the scrubber is embedded within the CL, so the gas flow path is redirected. The flow path is now from the exhale hose, thru the scrubbers central exhale tube directly into the single CL. From the CL the flow is thru the scrubber bed to the inhale hose, to the diver. This is accomplished by extending the scrubber central exhale tube to project out thru the scrubber bottom cap. The bottom cap is also 'ported' to allow return flow from the CL back thru the scrubber pellet bed and inhale hose to the diver. Since the RGU-2 scrubber is embedded within the CL, a watertight splitline seal is no longer required and a common Critical Failure Mode is eliminated from the system. A more detailed pictorial teardown of the scrubber will be posted when the most-recent modded hardware becomes available.

Counterlung
The original RGU 'horsecollar' CL was replaced by a rectangular molded Latex 'chestpack', which also encloses all but the top end of the scrubber. The original over-pressure valve (OPV) is relocated to the very lower-left corner of the CL to serve two purposes… as a normal OPV, and also as a dump valve for clearing of any serious CL flooding. The CL has ~ 6 Liters of free volume with a charged scrubber in place. This about doubles the original RGU free volume, which many divers consider inadequate.

Reconfiguring and relocating the CL moves the hydrostatic centroid of the CL closer to the breathing centroid of the divers lungs. It also has the major effect of changing from a mostly- negative, to a mostly-positive hydrostatic breathing head. The positive hydrostatic head helps drive the gas thru the scrubber during the inhale cycle. Thesefactors reduce the perceived work of breathing (WOB) particularly during the inhale cycle. Embedding the scrubber within the CL substantially insulates it from the water, and increases scrubber efficiency by running it at a higher temperature. This is especially beneficial in cold-water diving. The degree of efficiency improvement remains to be quantified.

The counterlung molding is a very simple lay-flat configuration, without the need for paneled sidewalls or gusseted construction. The CL 'lays' very close to the chest in the deflated condition, and 'rides' very well in the inflated condition, with less of a sense of 'chest clutter' to the diver than typical over the shoulder (OTS) CLs. With its minimum molded thickness of 0.060 in., the tempered latex CL bag is sufficiently 'bulletproof' as to not require a protective rigid shell or cover except for deep-penetration wreck or cave diving.

The redirected gas flow path, which results with the RGU-2s 'embedded scrubber' configuration, makes it highly tolerant of non-catastrophic flooding. An absorbent 'sponge' is located in the bottom of the CL to absorb condensate and the normal 'goop' that comes down the exhale tube into the CL. This CL configuration can tolerate up to about 2 Liters of flooding, in almost any swimming attitude, before beginning to flood the scrubber bed. Such major flooding can readily be 'dumped' thru the combination OPV / Dump valve simply by 'hugging' the CL, or injecting some breathing gas.

Over Pressure / Dump Valve
The original over-pressure valve is retained, but moved to the lower left corner of the CL, to also serve as a flooding dump valve. The cracking pressure of the OPDV is adjusted so that it will not free-flow, even in the fully-inverted, head-down orientation. This is done by removing and stretching the original valve spring until the desired cracking pressure is achieved


Collar Clamp
The collar clamp is a commercial, stainless steel worm clamp with a welded eye on each side of the scrubber. It serves 3 important functions,.. to :

• Ensure clamped, watertight, sealing of the CL 'neck' around the scrubber
• Provide anchor-points for the suspension neck strap (and in the future D105, the chassis mounting strap) clips… and
• Secure the top end of the catenary load distribution panel discussed below.

The positive top-to-bottom taper of the stock RGU scrubber minimizes the potential for the stretched, tight-fitting, CL neck to 'slip off' the scrubber, even with an inadequately tightened collar clamp. The top part of the overlength rubber CL neck is doubled back over itself to provide a protective clamping pad over the main neck seal to the scrubber.

Catenary Load Distribution Panel
The RGU-2 O2 Module has no surrounding case or rigid framework. This great flexibility makes the RGU-2 extraordinarily light and comfortable for the diver, and compact for traveling and storage. The catenary panel is made of rubberized nylon so as not to be stretchable. It is anchored by the collar clamp, runs behind the CL as shown in the backside picture below, and connects to the O2 cylinder shroud. This provides a primary load distribution path from the neck strap, thru the collar clamp and catenary panel to the cylinder shroud…without imposing substantial loads into the CL. The catenary panel shares its role of supporting and centering the O2 cylinder with the adjustable waist strap.


This backside view shows the Catenary Panel, and also the O2 feed line entering the CL thru a swiveled connection. At left-center in the photo is also the swiveled connection for the future mixed-gas diluent feed line.

Oxygen Cylinder
This Phase 1 posting shows the use of the original RGU 0.8 Liter X 200 Bar cylinder. These cylinders are plentiful, inexpensive, and are of good quality. They are imminently usable by homebuilders who can fill them at home, and/ or have booster pumps for field use. However, they are not US DOT or European CE rated, so will not be filled by most dive shops in most countries. As the RGU-2 Module goes forward it will alternatively use either a 1 or 2 Liter, DOT-rated O2 cylinder.

SUMMARY OF PHASE 1 IMPROVEMENTS TO DATE
The RGU-2 improvements to date, from the original RG-UF/M are:

• Added a DSV feature, a smaller, more comfortable mouthpiece bit, and an O2 cylinder pressure gauge
• The 11.3 lb / 5.1 Kg ready-to-dive weight is ~27% lighter than the original RGU
• Greatly increased the rig flexibility and compactness for diver comfort, and for rig traveling and storage
• Maintained the original 1 Kg scrubber charge. Increased scrubber efficiency by insulating the scrubber within the CL and running the scrubber hotter.
• Eliminated the need for positive watertight sealing of the refillable scrubber splitline. Eliminated a common Critical Failure Mode
• Reconfigured and repositioned the CL. Changed from mostly-negative to mostly positive hydrostatic breathing head, and decreased perceptible WOB, especially during the inhalation cycle
• About doubled the original RGU effective CL volume, which many divers consider inadequate. RGU-2 CL free volume is ~ 6 Liters , with charged scrubber in place.
• Markedly improved non-catastrophic flood tolerance,.. from essentially zero to about 2 Liters, in about any swimming attitude, before starting to jeopardize the scrubber bed. Major flooding can conveniently be 'blown' overboard thru the OP/Dump valve

NEXT-PHASE IMPROVEMENTS TO RGU-2 O2 MODULE

• Integrate on-hand DOT-rated 2 Liter cylinder
• Integrate on-hand Apeks DS4 1st stage, and a Clippard-valve-based CMF and bypass system. Some parts for the latter are already on-hand
• Start RGU 2 O2 Module integration into soft-pack BC chassis, with Atomic SS1 inflator / octopus

These will be reported-out in incremental / update postings.

Disclaimer: ReBreather diving systems are in fact complex life-support systems, which can, and do, kill divers, sometimes without warning. RBs are to be used only by properly trained and / or skilled divers, who are solely responsible for their own training and skill level. My posted info and data are solely to describe what I have done in my own developmental and experimental efforts, and not to encourage or instruct others in what to do or how to do it. Other persons use of this information is strictly at their own risk and responsibility, for which I am in no way liable.

Copyright © Ocean Quest, Dan Dunfee 2007 17.08.07

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