EDO-04 RMV Keyed Semi-Closed Rebreather

EDO-04 RMV Keyed Semi-Closed Rebreather
By londonsean69


General

The EDO-04 is an RMV keyed Semi-Closed Rebreather, made by STDE (Swiss Technical Diving Equipment). I chose this over a CCR, specifically a Classic KISS, because at my current stage of diving, I feel happier on SCR.



Operating Principles

The EDO is different to many other SCRs, such as the Draegers, the Azimuth etc. These are active addition SCRs, which work by constantly injecting a pre-determined amount of Nitrox. These rebreathers however, have a few failure modes that are not immediately apparent to the user. For example, if you are working hard, maybe swimming into a current, it is possible to over breathe the loop, using up more of the O2 than is available. This can lead to hypoxia.

The EDO is keyed to the breathing rate of the diver. The harder you work, the more you breathe, the more gas the Edo gives you. It is also possible to plug in any gas you want, thus greatly extending the range over which the unit can be breathed. However, the EDO has some failure modes of its own, one of which led to the death of diver.

If, for example, the mushroom valve into the exhale counterlung fails, then the O2 in the loop will be used up due to venting and fresh injection taking place.


Components

We will start from the bottom up. Counterlungs and dump valve


There are 2 bellows, one inside the other, that act as the counterlung. The inner bellow (exhale CL) is 1/10th the size of the outer bellow (inhale CL). Upon exhalation, both bellows fill up. On inhalation, the mushroom valve to the exhale CL closes, and mechanical advantage draws up the outer CL. This forces the gas contained in the exhale CL through the Apeks auto dump and out into the water, reducing the loop volume by 10%.


Plenum Chamber and Water Trap


After a few breaths, the loop volume is decreased enough that the plate on the bottom of the counterlung rises up and hits the pushrods. These pushrods then act upon the gas injectors, which are, in effect, modified second stages, and top up the loop volume with whatever gas is plugged in.

The pushrods sit in the plenum chamber, which incorporates a water trap and the counterlung vents through which the inhaled gas flows. The water trap’s design is simple, but effective. Any moisture in the loop flows directly down the tube in the centre of the scrubber, into the water trap, through the mushroom valve and directly into the exhale CL. On the next inhalation, this moisture is expelled. In theory, even a full flood is recoverable; I will be trying this in the pool very soon.


Counterlung Cover


This protects the counterlungs, as well as forming the base of the unit. This is made from anodised aluminum. This is one of my major gripes, the aluminum is quite soft. As I use a Halcyon metal cam-band, this has gouged lumps of the aluminum out.


Scrubber


On top of the water trap sits the spring plate, which acts upon the mesh screen to compress the sodalime, helping to prevent channeling.

The central tube screws into the head, which can be separated from the scrubber tube.


The Head


The head is machined out of solid Delrin, which gives it a nice rugged feel. As can be seen, there is a P-port between the inhale and exhale (red) ports, this was, apparently, missing on earlier versions. I am getting some PPO2 monitors for it as I write this, although I haven’t yet had a problem I have absolutely no idea what the O2 content in the loop is doing.


Hoses

The ends of the hoses are simple affairs, with a machined Delrin boss, which is o-ring sealed into the hose connectors on the head. These are a very tight fit in the KISS hoses, and as such are just cable tied on at the moment, although I just tested how strong the connection is, and I managed to lift the entire EDO, complete with 2x7l cylinders (about 25kg). The grey bits are from garden hose connectors, and why not? They are cheap, strong and available all over. I do not yet have hose weights, these are on order, and will be hidden under the blue Cordura hose protectors.


DSV

I went with the KISS OC/CC DSV from Jetsam. I feel this is a VERY good safety feature, especially considering I am learning as I go along. This is a very well made DSV, with just a simple twist that both shuts off the loop, and switches the loop to a normal OC reg. The hoses attach by simple hose clips; I may get some quick release connectors eventually.


Rigging


I have rigged the unit by creating triple bands from 2 pairs of AP Valves Buddy bands, and a halcyon single tank adaptor. The bands hold 2x7l Faber cylinders, although I am having a play with different cylinder sizes. I will, on deeper dives, use the backgas for bailout, and carry the drive gas in a side slung, as well as any deco gasses required, as per our DIR friends. The regs I use to feed the gas block are Apeks DST’s; these are to be replaced with DS4’s, which will allow much cleaner hose routing.

The whole unit is mounted to a stainless backplate with one-piece harness, mated to a Dive-Rite classic wing.


Gas Block


I purchased the optional gas-switching block, which allows me to plug in any gas mixture I choose. It is block of alloy drilled and tapped to accept Swagelok valves and fittings. I have also assembled low-pressure push fit hoses with the male connectors on the end. These are put on all deco and stage regs, enabling any reg set to be used either SCR or OC.

I would like to modify the block slightly by adding a 90-degree fitting to both the QC connectors, enabling one hose to run straight along the blue feed hoses, and one perpendicular, across my body.


Diving the Unit

The unit can be dived in a fashion similar to OC, although it is important to remember that the drive gas might not be safe to breathe at the surface. The warning systems are simple, if your gas supply to the unit is interrupted then you will breath down the loop, and the amount of gas available to breathe gets smaller and smaller, making it harder to draw a full breath.

If the mushroom valve fails then no gas will be vented, or injected. The gas injection and venting is audible, and you can feel the gas been injected. As such it is important to get in tune with the unit, so you know how it ‘feels’.


Cost

The base unit was 2900 Euros, to this you have to add hoses and DSV (750$ from Jetsam), gas block is optional (476 Euros), cylinders, wing, backplates; Swagelok hoses to feed gas block, PPO2 monitors. All in, including delivery charges, taxes and import duties I would estimate the total cost was approx. £3200.


Gas Efficiency

The perfect figure is 10:1, but with mask clearing, wing use etc. it is more like 7 or 8:1. However, I have only achieved about 5:1 so far. Part of this is because I am learning the unit; part of this is due to my profiles. You lose a lot of efficiency on up and down profiles, having to vent on ascent etc. It is much more efficient if you can get to depth and stay there for a while, like on a wreck. I have only done 2 wreck dives so far and got about 5:1, this will come down with use.


Summary

Many people have said the unit is too much for open water diving, and I would have been better with a KISS. I got the EDO because there are no active parts at all, purely mechanical. In it’s current configuration, it is a bit overkill for open water diving, swapping the 7’s for 5’s will lighten the load a bit, but as I intend to do deep Trimix, 2x7l will provide huge amounts of bailout gas, and 2x5 will cover the shallower stuff. There is always the possibility of using it in caves, which will improve the gas efficiency.

Please do not hesitate to contact me if you have any questions about the unit, and check out my website for more diving updates (when I get around to it).


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