Gas Boosting
There is a problem with mixed gas diving that most places are only equipped to blend gases by partial pressure filling. This means that to get an elevated oxygen mix you bleed a measured amount of oxygen from a supply tank into the required cylinder and then pump air in on top. This works well with low nitrox mixes. For example to make 32% you add 32bar of oxygen to an empty tank then in pump air to 230bar. You can do this easily until your supply tank gets below 40bar and then you have to swap in a new one.

This is not so OK when you want 80% or even pure oxygen. While the supply tanks may be big, the standard 'J' size is 50L and stands nearly as tall as I do, they are normally just 200bar so the best you can fill my 7L stage with 100% from empty is 175bar. The next time guy in the queue gets 153bar and so on. There are tricks to improve the situation if you have several supply tanks by drawing as much as you can from your lowest pressure tank first but it still rapidly becomes a problem if there is a long line of teckies waiting for a fill. You get used to walking out of the shop with 120 bar and feeling lucky. This may be OK with a 7L stage where you plan for a margin but with a 3L rebreather tank it's not so happy as this is your fallback deco gas in case something breaks.

There is an additional problem with helium in that it all has to come from the supply tank - at least there is some oxygen in the air you pump. To make a 40% helium mix to 230bar you need 91bar of helium so if the supply tank is below 99bar it isn't going to make it, even on my tiddly little 3L rebreather tank. BUT 98bar is virtually HALF FULL! Again you can play the bank trick and use several cylinders but it is easy to get stuck with sending a lot of expensive devil gas back because you can't get at it easily.

What you need is an oxygen pump but compressed oxygen is nasty stuff at the best of times and pumping it is bad news. I won't bore you with the horrible details as if you are even basic nitrox trained you know the gist and if you have a blender ticket you know the full horrors. Let us just say that this is my oxygen pump.

It is a Haskel AG-75. Haskel are the big name in boosters with Aerospace quality but sadly with Aerospace prices to match. I bought it second hand because they are about five thousand quid new. Incidentally the whole thing is 40cms end to end and weighs about 10 kilos.

I am told the AG-75 might not be the ideal model for dive gas boosting but it was available at a price I was prepared to pay and the specification certainly suits me. What you are looking at is a gas powered piston engine. That's the cylinder on the left in the top photograph with the big green label. It is run on up to 10bar and it drives the narrower gas pump on the right with a 75:1 pressure increase. With its rating of 150psi drive pressure it could give 11250psi output (in the old units on the label) that is 10.34bar in to get 775bar out.

Now admittedly 775bar could destroy virtually any piece of HP diving equipment I own but 10bar is a convenient feed being the interstage pressure of most regs. So to take advantage of that I fitted the drive inlet pipe with the nipple from a drysuit inflator. To run it I couple up any cylinder to a first stage that has a drysuit feed, clip the feed onto the inlet and ease the pillar valve open and off we chuff. Interestingly Haskel warn you not to boost oxygen beyond 345bar. (Ho Ho Ho).

The rest of the plumbing: The first thing I did was to dismantle my old Subaqua Products blending whip and reuse the parts - that provided a one way valve, the 2 meter blue inlet hose, various fittings and a nice slow feed valve. This got me started but still left me a few bits short but I phoned them and ordered the parts from their catalogue including the fancy 300bar swivels that connect the flexible hoses to the pump. Then off to the Swagelok place in Burgess Hill who looked at my pile of bits and helpfully worked out what I needed to connect A to B and finally down the scuba shop for the Apex drysuit nipples to replace the one I pinched from the spare drysuit inflator.

Well the dry suit nipples are only almost the right thread so they needed to be sawn back a bit and then I ordered a complete new set of Haskel O-rings and undertook a complete strip and rebuild under oxygen clean rules. This involved changing some of the parts to oxygen service bits as I was not quite sure of everything and also removing the external pilot modification. Sadly the end cap, the bit with the high pressure one way valves, was obsolete with no spares available and that meant it needed to be swapped out for the current version. Finally the new hoses were connected up and we were ready.

To use it I couple the donor cylinder to the inlet hose - the blue one. This has a one way valve and a nice needle valve so I can feed gas slowly - considered a good move with oxygen. I use a 300bar DIN fitting here but I have a 300bar female to Bullnose adapter to fit the Air Products 50L 'J' supply tanks I use when I'm at home. Then I connect the outlet, orange, to the recipient tank again by 300 bar DIN. I open the recipient valve to check its starting pressure then the donor which, if it has sufficient pressure, will just feed through the needle valve and the pump. Once they are equal I couple the cylinder that is going to power things through the drysuit feed and use its pillar valve as a throttle control.

Naturally we only pump a tiny bit of gas per stroke as the output pump is so slim but it takes in 20cc (0.02L) at the donor pressure per stroke and compresses it to the current recipient pressure by using 20cc*75 = 1.5L at one seventy fifth of the recipient pressure. In practice this means that taking a 12L cylinder at 100bar as donor and compressing a fill from it into my 3L rebreather tanks going from 100bar to 232bar uses about 150bar from my 20L twinset as power source and takes about 15mins if I do it slowly to stop the mix getting hot. Add to that that the whole rig is totally mobile and works in a field or a car park so it can go in the car and, provided I have got enough air in the twinset or can get a fill, I can decant right down to the bottom of my donor cylinders although that uses quite a lot of drive gas. At home naturally I have a big industrial compressor to provide 8 bar in bulk.

Well this means that I load the rebreather into the car, add a 12L tank with extra premixed diluent, a further 12L tank of pure oxygen (full!) and fill the twinset to power it all I'm set for four days of diving without ever touching a shop.

One point that needs to be mentioned is that the Haskel design does not totally exclude the drive gas mixing with the pumped gas. Looking at the schematics and the complexity of the piston seal you'd wonder how but I guess they are talking long term use here. If you are powering it from a compressor it needs to be oil free or at least nitrox clean. I suspect an occasional burst is no problem as the grease they supply to lube the driver side doesn't look very oxygen clean but I'll go by the manual just to be safe.

I can not speak too highly of Haskel's technical support and sales people. Unlike the current fiasco where you cannot buy spares for a simple scuba regulator they were happy to provide technical data, explanations and, although they definitely aren't cheap, they supplied all the bits I required by part number from their detailed drawings which rolled out of my fax. Realise that my AG-75 is twenty years old and I have restored it to what is to all intents and purposes new state.

What it is not
Finally let me clear up a few misconceptions. The Haskel is a booster not a compressor. If I start from the plain 8 bar air I have from my standard air tool compressor and try to pump that to top up a 100 bar bottle I get 20ccs of 8 bar so 160 surface ccs of gas per stroke. Hence, even in the 3L rebreather tanks, you only gain a twentieth of a bar per cycle so it's going to take forever to do a fill. To boost you really want 50 bar so you move one surface litre a stroke. We tried pumping from bottles less than that to try and save sending back the dregs of a J of expensive helium but it was a waste of time. Probably if you have a unit with a lot less compression ratio than mine, so you take a bigger input of gas each time you could make it worth while as a helium scavanger but never on air. I have four pumps here. The clean air 320bar 155L/min tank filler, a petrol driven 270bar unit, the oil free 8 bar 150L/min air tool system and the Haskel. They all have their place and they don't really overlap. <sigh> I never realised diving would be so expensive.

Notes on the strip down and clean.

One device I have added to my collection since I got the Haskel is the Gas Analyser.
"What?" you say. "An oxygen meter for Nitrox? A Trimix analyser?"
Nope. I have them but this is a box that feeds gas through Dräger tubes so you can measure the contamination levels.

Now Dräger tubes are little glass tubes that you first break the seals at each end on and then pass a measured quantity of gas through. Then you read off the scale on the front how much of the particular contaminant is present.
This one is made by Hamsworthy and it is basically a first stage in a box feeding a needle valve followed by one of those 'blow the ball up a tapered tube' flow rate meters and then a sealed unit to take the Dräger tubes. Add a timer that knows how long it takes to dose each tube and you have a portable compressor station measuring unit.

It seemed a good idea at the time but the tubes cost a bit. Still... One less excuse for killing myself.

One final trick while I was buying lots of compressed air fittings was to solve my perennial blowing up motorbike tyres problem. Another dry suit inflator nipple is mounted in the bottom of this little goody that I saw on eBay. Yes it's one of those garage tyre inflators that always hang off the machine which takes 20p. But you're wearing thick bike gloves, the 20ps are under all the water/wind proof kit you're wrapped in and it's raining. All this compressed gas I have hanging about the house and at last it's finally working for me.

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