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CONCLUSIONS REGARDING ALTERNATIVE METHODS OF PUMPING HELIUM The "proper" way to do it is a booster pump of course. Haskel will sell me one in the UK for about 6000 UKP that's not even O2 ready, or 7500 UKP for one that will pump O2 (although they recommend a separate one for helium, but then I guess they would). I may not have made clear that this is a personal application, I'm not running a store or project, so a booster pump is out of the question on financial grounds. It was commented (by Richard Pyle I think) that cascading 3 or 4 cylinders needn't actually waste huge amounts of gas, which is true, but even 20-30 bar remaining represents a 15-20 percent gas loss is the cylinder only started at 137 bar (as British Oxygen Helium cylinders do). Air Products cylinders, at 250 bar reduce this waste to only 10 percent or so but if there's a way to feed the cylinder gas almost directly into the inlet of a standard oil-free compressor then it would be possible to use virtually all the gas in a cylinder (just leave them a couple of bar, OK?), no hassles with cascading, no need to keep as many cylinders (or just one larger one, instead of several smaller ones) and at virtually no extra cost. The list concensus (with a couple of exceptions...see below) was that it's no real problem. One person replied that they thought there would be a problem with the gas flow out of a cylinder at 1 bar keeping up with the compressor's input demand but to my mind this shouldn't be a problem. After all, if you just open the valve on any cylinder, the gas pressure is about 1 bar straight away (once you get away from the immediate area of the valve outlet) and if you think about it a cylinder in such circumstances will empty much faster than it can be refilled by a smallish compressor. A 5 cu ft / min compressor will take over 20 minutes to pump 104 cu ft of gas, but a 104 cu ft cylinder will empty in a fraction of that time if the valve is opened fully. (I hope these numbers make sense, cu ft are not my "natural" units.) Therefore a single cylinder should be able to keep up with most small compressors (which is what I have) with no problem. If you have a bigger compressor, just feed it from a manifolded bank. You shouldn't need a huge inlet reservoir (one person claimed to be using an atmospheric bag the "size of my bedroom" (perhaps he's a mouse). All you need is a small reservoir to even out the compressor's first stage demands, which are intermittent due to the piston cycling. You can go straight from the helium cylinder's valve, through a suitable hose to the reservoir bag. If you're willing to monitor the process continually, quite a small gas-tight bag should do the trick, you just have to adjust the cylinder valve to stop the bag bursting or collapsing. Obviously it's _absolutely essential_ that no air can leak into the system or be drawn in by venturi effect while pumping the helium so the system must be properly sealed, tested and the whole compressor/hose/bage/etc. system purged with helium before filling commences. Read the Deep Tech article about homebrew and follow the caveats in there. Like the man said, "Do it right or don't do it". Another correspondant noted that Helium is harder to compress than air and thus may make the compressor run hotter. Also Helium will apparently affect the lubes rubber components, causes them to dry out faster. For both these reasons keeping the compressor in good shape will be important (as it is anyway). Having said all the above, a couple of people wrote that their actual experiences of putting the above into practice (although not with exactly the same system described), were not that good. Harold Gartner remarked that (like Rich Pyle) his 4-tank cascade didn't actually leave much gas wasted and the problem of regulating gas flow from the helium cylinder as the compressor's inlet demands changed, was a real pain, his conclusion - "Throw out gas that you don't use, don't mix until you're ready to dive in the next few days and use a cascade system as Rich Pyle". His system described an attempt to manually regulate the helium gas pressure and using a vacuum gauge to keep the inlet supply to the compressor at 1 atm. Harold's own post has more details. Perhaps the bag system would help overcome this problem. Anyone going to DEMA this year can stop by the SofTek R&D booth, where they will have a fully automated gas mixing station. It takes air, O2, and He and produces compressed air/NOAAI/NOAAII or custom mix based on MOD, ppO2, and END. Their mixer uses a continuous blend low pressure mixing as an input to an oil free compressor. Very nice too. Finally, name dropper that I am, Jim Bowden apparently used a portable compressor to pump Helium during one of the spells at Zacaton. Many thanks to all those who took the time to reply. Regards --------------------------------------------------------------------------- | Paul Chapman EMail paul@de*.de*.co*.uk* | ---------------------------------------------------------------------------
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