How I built my breather - by Jason Rogers

Enclosure in a case Proper Insulation Active gas heating The Scrubber Counterlungs

Hoses Mouthpiece Diving Semiclosed Sensor Package

The design for this hasn't really gelled in my mind yet. I'm still kicking around ideas, and so I won't put anything up here till I get something a bit more concrete

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Second Loop

As you can see from the photos, I've chosen a horizontal scrubber layout. This was for two reasons, the first was ease of construction and repacking and the second was to provide room for a second back mounted scrubber, without the breathing hoses getting tangled up in one another as would happen with two vertical scrubbers.

I have to make some tests on the projected arrangement, but currently I'm envisaging moving the existing scrubber up higher on my back, and placing a similar unit in the same location that the current unit occupies. Instead of using over the shoulder counterlungs for the second loop, I'm intending to try a cummerbund style of counterlung, at about the same level on the unit as the existing scrubber.

There are several problems associated with idle loops. Heating the loop, and ensuring that the loop always contains a breathable mixture are amongst them. On a cold water dive, kick starting the loop may not be possible, but I'll have to have a shut off mouthpiece before I can start trials anyway!

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Shut off Mouthpiece

This is one of the items that I've found most difficult. Rodney's is a work of art, and without any doubt, the best CC mouthpiece I've ever seen, and probably the best ever built.

I've tried to replicate it in plastic, as I really don't like the idea of using Aluminium with caustic alkalis anywhere present, but so far, I've created a series of rather lovely, but quite useless leaky bits of plastic pipe.

You're getting sick of this, but I'll update this when I've achieved something!

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Enclosure

You can see Rodney's enclosure in the photos. It works brilliantly well, for the type of diving he's interested in doing. He can easily fit two 18 litre cylinders, one each side, or he can fit two 2 litre cylinders, what ever he needs! The beauty of this is that the bailout and drive cylinders are one in the same, and so he has a tiny, strong enclosure for just the delicate parts of the breather.

On the other hand, I'm still trying to get the second loop idea to fly. So I'll need a large, and light enclosure, that will provide mounts for both sets of counterlungs, and contain the two scrubbers, and four small cylinders that I'll need.

Currently, I'm thinking of finding a suitcase in ABS and removing the locks and hinges, and replacing them with short rubber straps. Counterlungs, and harnesses can mount on the "bottom" and the lid will just fit over in a sort of clamshell type of way. So far I haven't gone far with this idea. I've got to get the second loop going, so I'll have a better idea of just what I am going to have to fit inside the enclosure. No point in getting the enclosure right, and then finding that the stuff won't fit!

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Proper insulation

The current loop is more or less bare to the water. This reduces the efficiency of the scrubber material, and promotes condensation of water inside the loop. While there is a lower temperature in the loop, there is a net movement of water out of the diver, and into the loop. Neither of these are desirable.

The large flat counterlungs in my design are very difficult to insulate effectively. I'm going to try gluing sheets of neoprene to the surface of the counterlungs, but I don't expect this to make a great deal of difference.

What I'm currently doing is gluing closed cell foam to the outside surface of the scrubber. At the time of writing, the scrubber is about half covered, and I'll update this when I get it completely covered and take it for a paddle.

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Active gas heating

This would really be the ideal solution to the problems of heat loss, and loss of scrubber efficiency that result from very deep diving. I've not got much further yet than the idea that I'd like to have active heating.

Problems I see so far are:

Making it intrinsically safe. An oxygen fire inside the loop is guaranteed excitement! Putting it in the right location. Preventing it from over heating the gas, and injuring the diver. Preventing it from restricting the gas flow inside the loop. Making it waterproof. Making it controllable. Setting it up so that if it fails, the diver has an alternative option to deal with the loss of heat from the loop. Making it so that the current flow inside the heater will not disturb the readings from the sensor package.

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The Scrubber

In order to build something simple, and quick, I chose to use the good old standby, PVC sewer pipe.

Easy to fashion, and easy to join, reasonably strong, and quite light, with a wide range of fittings available off the shelf to make construction easy.

I chose to make mine horizontal, for a variety of reasons, and this is how I went about it.

I got 150 mm pipe, and cut it slightly shorter than the distance from one shoulder to the other. This was to allow room for the end fittings, and have it align with the shoulder counterlungs.

A 150 mm end cap was bought, and a hole made in the centre, to mount the "through hull" fitting, (aka skin fitting). The through hull fitting had the moulding ridges removed, and the hose barb cut off. Rubber gaskets were made, and they went between the THF and the end cap. The gaskets had a light smear of silicone sealant on them as well (silastic). The end cap was then siliconed onto the pipe.

At the other end of the pipe, a screw on inspection cap was fitted, again with silicone. This provides a way into the scrubber pack to change absorbent.

The actual screw on cap received a similar treatment to the end cap.

Then another bit of 150mm tube was cut slightly shorter than the first, and then slit lengthways. This was put over the whole lot, and glued to the end cap and screw part of the inspection cap. This is because the smell of the PVC glue takes years to dissipate, and the silicone glue that was used in it's place is not strong enough.

The result was a tube with caps on both ends, one removable, and 1 1/4 BSP parallel threaded tubes sticking out.

To this was fitted a 90 degree female elbow on both threaded tubes and then I fitted a male hose barb on both elbows.

It's finished, and now must be packed.

A circular piece of coarse plastic mesh was cut out, so that it would just fit inside the pipe. This had a perforated 100mm end cap cable tied to it and it was dropped into the tube. Then a layer of open cell foam, and another layer of plastic mesh. Then the absorbent material was put in, and another layer of mesh, and then foam and then mesh-100mm-end-cap-assy. The layer of absorbent is such that the endcap sits proud of the top of the pipe, and the whole lot is compressed when the inspection cap is screwed on.

The scrubber is shaken, and if there are any sounds of rattling material, it needs more absorbent.

This ends up making a pretty large scrubber, with a smallish surface area to insulate. The 90 degree bends increase the work of breathing, but you have to turn the gas at some point, and it compares well with "there and back" designs which force a single 180 degree turn on the gas.

150mm area is about the minimum I'd go for, but it's OK, due to the effects of the twin counterlungs, which will be explained in that section!

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The Counterlungs

Twin counterlungs, but why?

Twin counterlungs double the number of pulses of gas that flow through the scrubber, and halve the gas speed.

Consider a single counter lung. It can be up or downstream of the scrubber, but when you inhale, all the gas has to follow one path, and when you exhale all the gas has to follow one path. In one of these cases, that path will include the scrubber, and all the lungfull of gas will have to pass though the scrubber, in half the breathing cycle.

Now consider twin counterlungs. Due to the hydrostatic pressures involved, the level of gas in each lung will be very close to the same at all times. Hence, when you exhale, half the gas goes directly into the exhalation bag, and half into the inhalation bag via the scrubber. When you inhale, half the gas that you inhale, comes direct from the inhalation bag and half comes from the exhalation bag, via the scrubber.

This has some real advantages. The gas flow through the scrubber travels at half the velocity, and so the energy used to drive it through is only 1/4 of the amount used if it went through in higher speed pulses. This allows me to get away with things like 90 degree bends on the scrubber pack.

Also the absorbent material is far more effective if it is given a bit of time to work on the gas. With twin counterlungs, the gas gets to stay in the scrubber for longer, and so you're not working the absorbent as hard, and it lasts longer, and you have a lower CO2 level in the inhaled gas. With a single counterlung, the full lungfull of gas passes through the scrubber pack, and the absorbent only has the time while it's "passing through" to work on it.

The actual material I chose is Layflat PVC hose, also known as "fire hose". It is reasonably pliable but also very tough. It is designed to have fittings put on it, and seal by compression, so closing the ends off with clamps works very well. It's flat, and designed to roll up while flat, and so it bends over the shoulder nicely. It's reinforced, and so fittings such as drysuit inflator valves can be installed very easily. It's rated "Food Grade" so it won't poison me.

The tee piece serves double duty. Mounted as you can see in the photos, on top of the counterlung, it has inside a dividing wall, across the flow of gas. This directs the gas into the counterlung, and there the water separates from the gas, forming an effective water trap.

The left hand or exhalation bag has the O2 add valve, a drysuit inflator valve, and the right hand bag has the over pressure dump valve (an adjustable drysuit dump) and the auto diluent add valve, a scuba regulator, that has been attached by a short length of corrugated BC hose to a fitting from a BC. The exhaust port of the regulator has been blocked up, and it's adjustable, to vary the level of gas in the bags. This is not an ideal position for the valve, and the diaphragm has been preloaded shut by putting a block of open cell foam under the diaphragm.

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The Hoses

The breathing hoses come from a powered air purifier, from 3M, and the hoses from the tees to the scrubber are third party replacement radiator hoses. They're stiff, but they hold the counterlungs properly in place on the shoulders. Not much to tell about the hoses really, but the 3M hoses have to cure for a long time to rid them of the smell of plastiziser.

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The Mouthpiece

This has to be changed on my unit (see the section on plans for the future for more details)

The current mouthpiece is a remove-and-die unit, incorporating no shut off system. The reason that I built my own, rather than using an old twin hose one is that the old ones had very narrow bore, (around 19mm) which made them unsuitable for the depths I'm planning on using the unit at, and also the poor availability and high cost of spare parts for old twin hose regulators. Nemrod in Spain is the only company making them now, and they have no Australian distributor.

The actual mouthpiece is a 25mm female tee (25mm is the nominal bore as stupidly described in the "metrification" of Australian plumbing supplies, it's actually 1 inch BSPT which is about 1 1/4 inches) That has a couple of exhalation valve carriers from Auer BA sets fibreglassed into the ports of the tee, and a PVC pipe to thread adapter that has been heated and squashed, screwed into the middle port of the tee. It has a groove cut into it, and a scuba mouthbit cabletied to it.

It works surprisingly well, i.e., it works, and that surprised me!

The hoses only just fit over the ports on each side, and are "rolled" on, rather the way one puts on a condom, and then cable tied in place.

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Diving Semiclosed

Till I get the sensor pack finished, and also during the testing phase of the sensor pack, I'll be diving the set semiclosed. (Except of course for the initial dives, which I did on CC O2)

These are the methods that I'm using. I've never seen this type calculation mentioned anywhere, and of course, it's really quite untested. If you're dumb enough to follow my lead, then you'll be doing the gene pool a favour by dying, as you most probably shall.

I'm using two methods of calculating the lung dumps for semiclosed diving. Note, I have NEVER measured the O2 levels in the set during a dive, so the following is based purely on my speculations. If you do it, then do your own speculations, and expect, as expected, to snuff it at any moment during the course of diving the set.

The first method applies to shallow diving, where consumption of the O2 in the loop will trigger addition of more diluent.

When diving this way, I calculate the MOD of the driving nitrox based on a max PPO2 of 1.8 (This is higher than recommended by almost every agency in the world) I then work out how much N2 will accumulate as I consume the O2 at 3 litres/minute. (I'm currently quite unfit, and I don't' think my VO2 comes anywhere near 3 litres/minute, if you are fit, then you need to substitute a higher figure for this).

For example, diving with NTX 60. This is made up of 2 parts nitrogen, to 3 parts oxygen. If I consume 3 litres of O2 every minute, then I need to dump 2 litres of "waste" nitrogen every minute. (In actual fact, one needs to dump slightly higher than this if really consuming 3 litres/minute of O2, or else the "dump" would be pure N2, and the diver would be dead)

My lung tidal volume is 3 litres, so on the surface, I would dump once every minute.

At 3.3 metres, my lung's tidal volume is 4 litres absolute, and so if I dump once every 2 minutes when below 3.3 metres, I'm dumping the 2 litres/minute that I need to.

At 10 metres, my lung tidal volume is now 6 litres absolute, so I need only dump once every 4 minutes in order to dump the 2 litres/minute

At 17 metres, my lung tidal volume is now 8 litres absolute, so I need only dump once every 5 minutes.

with SC diving, the diver needs to do a flush before ascending to assure himself that there is sufficient O2 in the loop for the lower pressure near the surface.

Still, even with flushing and mask clears and such, this is a very efficient way to use drive gas. Since the flow is based on dumping, rather than incoming gas, the amount of gas coming from the cylinder is reduced if the workload isn't high.

To take the above example, if the diver is consuming 0.5 litres/minute of O2 (typical rates for a diver at rest) then he is dumping 2 litres/min, and consuming 0.5 litres/min for a total of 2.5 litres/minute that will come from the diluent cylinder.

Oxygen levels can be worked out by looking at how much nitrogen comes in and how much gas is dumped. In this case, the quantity of N2 is 2.5 X 0.4 or 1 litre. Since one litre of N2 comes in, and two litres are dumped, the other one litre is O2, or in other words, the loop FO2 is 0.5

If he's working hard, and consuming 2.5 litres/min of O2, and dumping 2 litres, then 4.5 litres will be drawn from the diluent cylinder.

To work out the dump gas (and so the loop gas) FO2; 4.5 X 0.4 gives 1.8 with the remainder being 0.2 litres of O2 or 10% Fine if at depth, but you can see the need for a flushthrough to raise the loop FO2 up closer to the drive gas if working hard.

you employ this system, you need to work out your lung tidal volume, and your own max VO2 and add in a margin for oxygen that goes out with the dump gas.

Remember that the more you dump, the safer you are from hypoxia.

In a practical diving situation, working hard, I increase the amount of dumping, it's still very little gas. Also, I tend to switch from one "per minute" rate to another slightly deeper than calculated.

is compares with systems that add through a metered dosage system. They must assume that the diver is working flat out all the time, and usually add enough gas so that a diver consuming 3 litres/min of gas will still have more than 20% O2 in the loop at all times.

So it's (Block of gas added each minute) - 3 litres of O2 equals (a gas with more than 20% O2 left in it)

For 60% oxygen drive gas, that's the final gas supply having the original 40% of N2, unchanged, and 10% O2 left unconsumed, (40% + 10% = 50%, and this 50% refers to the percentage of gas left over each minute after the 3 litres of O2 have been taken out. "10%" in this case, means 20% of the remaining gas)

the remaining gas is 50%, this lets us know that the three litres of O2 consumed is equal to 100% - 50% or (also, but only in this case) 50%

Since 3L = 50%, we can say that 100% = 6 litres, and that's how much drive gas must be added in a normal semiclosed system.

in a dumping system, the flow from the cylinder varies between about 2.5 litres/minute to about 6 litres, depending on workload, and in a dosage system, the flow is constantly 6 litres/minute.

That's all a little garbled, (so sue me!) If you want to get it really right, and a bit neater, do the Atlantis course, as that should teach you about calculating flows for different mixtures on a dosage unit. Or alternatively, sit down with a paper and pen and draw little blocks of gas that look like this:

|______________80% N2________|__20% O2___|______3L O2___|

|______________40% N2________|_____________60% O2_______|

It's a much easier way of visualising what's going on than the forgoing mess!

The second way of dumping gas is used when deeper than the point where consumption of the O2 in the loop will reliably trigger the addition of gas from the diluent cylinder. This seems to be something like 30 metres.

It works like this,

My tidal volume is around 3 litres.

If I dump 3 litres from the loop, and the drive gas gives a PPO2 of 1.6 at the depth I'm diving, then 1.6 X 3 litres of O2 will enter the loop to replace the gas I've just dumped.

If I do this dump once per minute, and at the same time, I'm consuming 3 litres per minute of oxygen, then you could say that I've taken one lot of 3 from 1.6 lots of 3, giving me 0.6 remaining.

In other words, the PPO2 of the gas will fall from 1.6 to 0.6 if I consume 3 litres of Oxygen from the 3 litres of gas.

u'll need to check how your tidal volume compares with your O2 consumption rate. I know people with a 6 litre tidal volume, and they could dump once every two minutes.

Still, for me it's easy, and it beats counting breaths!

u'll also need to remember that if the PPO2 of your drive gas is lower than 1.2, then you're going to be taking too much O2 out to support life, unless you start increasing your dumpage rate.

Let me just reiterate the warning at the top of this page.

This stuff is untested, I'm trying it out to see how it goes, and I may well die doing so. Even if it doesn't kill me, it may kill you, because your breather, lungs and O2 consumption rates will be different to mine. Back to top

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Last updated Yesterday. 1999 by Scott Leimroth ©