There are some big differences between the oxygen system used on Everest by International Mountain Guides and that used by all other groups.  IMG uses their own oxygen tanks and regulators and currently almost everyone else on the mountain uses Poisk.  This difference was a big concern for me when choosing IMG in 2006, but I didn’t know much information at the time to make an educated decision. 

Where are the systems made and filled?
Neither Poisk nor IMG's system is local to Nepal.  IMG’s bottles are made by a factory in California and after they are used, they are returned to the US for filling and reuse.  Poisk bottles are made in St. Petersburg, Russia and after used they are shipped back to St. Petersburg for inspection, refilling and reuse.

So what's the big difference?
The bottles are the major difference between IMG’s system and the one made by Poisk.  IMG’s are larger and heavier than Poisk bottles but they do hold more oxygen.  According to this information provided to me by IMG in late 2005, each IMG bottle holds 1800 liters of oxygen when full.  According to the technical data available from Poisk’s web site, each one of their bottles holds only 1280 litres at +50C.  So why does everyone choose to use a Russian oxygen system, when you could get a bigger one from the US?  I believe it’s because of the weight.  A full IMG bottle weighs 7 kg whereas a full four liter Poisk bottle weighs 3.5kg,  - that's half as much.  So this means that you can carry two Poisk bottles for the same weight as one IMG bottle and have the redundancy benefit of two bottles.  Plus two Poisk bottles will have 2560 liters of oxygen in them, compared with only 1800 liters in the IMG cylinder.  This is over 40% more; it’s a significant extra amount for no extra weight at all.  I think this is why Poisk is still the system used by almost everyone on the mountain.  (See the discussion below for more information about the change in volume of oxygen at low temperatures.) 

There must be some benefits of the IMG system?
The argument promoted by IMG to me in 2005 was that their system requires fewer bottle changes and because no one else uses the same bottles, there is less chance of theft, which is true.  It is true that there are less bottle changes with IMG, because with Poisk you normally change on the South Summit as well as the Balcony, whereas with IMG you only change on the Balcony.  As for the theft issue it’s difficult to quantify, but here are my thoughts.  Theft is unlikely to happen while you are climbing (from C4 to the summit) because you & your Sherpas will be carrying with you all the bottles you need.  The main camp where theft could happen is C4.  Perhaps a climber who had planned to climb without oxygen or someone who needed to stay up there for another night took one of yours.  It’s a slim chance and I would recommend bringing more bottles than you plan to use anyway for emergencies.  You can easily resell them for about half price back in Kathmandu.

Why do I like Poisk?
From my thinking, the big benefit with Poisk is redundancy for no extra weight.  I can tell you that sitting just below the South Summit in 2006 with an empty IMG bottle and no spare at all, I would have given anything to have been using Poisk and to have a spare bottle available to me.  We are all given two kidneys, two eyes, two ears, two lungs, two legs and two arms for a reason.  For those unfamiliar with the events last year, my Sherpa, Dasona changed my bottle at the balcony, and it was supposed to last me all the way to the summit and back down.  Three hours later, just below the balcony I suddenly found the going get incredibly tough and couldn't go on any further.  I sat down for a brief rest - Dasona was ahead and he continued on.  Soon I was totally alone.  About half an hour later another IMG Sherpa, Danuru, descending from the summit saw me sitting just below the South Summit and generously offered me his oxygen bottle, saying that he was able to go down without it.  I consider myself very lucky that it was an IMG Sherpa that came past that day.  Had it been someone else from another team offering me a spare Poisk bottle, but not strong enough to go without themselves, I wouldn’t have had the regulator or correct fitting to attach my mask.  Hopefully they might have had some spare gear that they could have also loaned me, but as you can see it’s all starting to get very reliant on other people, which is not something you can count on that high up when everyone is struggling themselves to stay alive. 

I still don’t know what caused my bottle to be empty in 2006, but a similar experience is written about in Kevin Flynn’s book, "Mt. Everest: Confessions of a Peak Bagger", where an IMG bottle being used by an IMG Sherpa mysteriously ran out and the Sherpa had to turn around below the Balcony.  Possibilities in my case could be that the bottle wasn't full, that the flow rate wasn't right, that the bottle leaked or that the regulator leaked.  I have my theories, but that's all they are.  It's important to note that to the best of my knowlege, out of the 16 climbers with IMG in 2006, plus all the climbing Sherpas, no other person had an issue with their IMG oxygen equipment.  

What else did I like about Poisk?
I was really impressed with the build quality of Poisk bottles.  Of particular note was how they write the weight in grams of the full bottle on the orange plastic shrink wrapping.  (The bottle itself is a brown kevlar color and this orange wrapper is replaced each time the bottle is filled in the factory.)  I took some digital weighing scales that I bought for $10 on ebay (they are marketed as fish scales) and I was able to check that all the bottles still weighed the same at base camp as they did when they were filled in St. Petersberg.  I also rechecked them in Camp 2.  They were all fine.  If you read below you can see that pressure changes with temperature, so pressure isn't a good guide to tell when the bottle is full, but the weight is a 100% accurate guide. 

Whats not to like?
The weak point in Poisk's oxygen system (and any system in my opinion) is arguably the regulator - it's a complex piece of equipment so get a new one and take this along with you.  I liked it how the Poisk regulators had the year of manufacture stamped on them - choose a newer one.  Monitor the change in pressure over time as per the steps in this guide and then you'll know if the regulator is delivering the flow rate as stated on the regulator.  If you find its over or under, you can adjust the flow rate accordingly (ie. if 2.5 liters per minute is really 3 liters per minute).  Atilla and I did these calculations at C3 whilst we were resting there and we both found that our regulators delivered the stated flow.

What about the cost?
Price shouldn’t be a determinant when choosing the oxygen system you use on Everest.  Poisk is expensive and in my opinion IMG would be saving money using their system and avoiding Poisk.  The price charged by AT for a single 4 liter Poisk bottle was $420 – AT do include 5 bottles in the base fee, but you need more if you want to have plenty of oxygen and also if you want to have enough in reserve for a second attempt. 

AT provided me with a brand new, unused oxygen regulator to carry as a spare in case anything went wrong with my first one.  All climbers with Asian Trekking carried a spare. 

Masks
IMG purchased new TopOut masks in 2006, whereas as of 2007, AT still only offered Poisk masks.  Most climbers purchased their own TopOut masks for about $500, although this year AT said to me that they were considering buying TopOut Masks.  I haven't seen any climbers using the new Poisk masks that appear on the Poisk site.

What happens with the volume of oxygen within the cylinder as the temperature decreases?
I am a Civil Engineer, so forgive me if I get a little technical here, but I think it's important to understand how gases behave inside cylinders when you vary the temperature.  This behavior is roughly governed by the Ideal Gas Law.  Without going into all the technical details, although I am happy to supply them on request, the volume of oxygen inside the cylinder will change in proportion to the changes in temperature as measured in degrees Kelvin.  To convert from degrees C to degrees Kelvin, just add 273 to degrees C.

So a bottle at -25C will have about 23% less liters of oxygen in it than the same bottle at +50C (1-323/248).  This means that a Poisk bottle that starts out with 1280 liters at +50C, will have 982 liters of oxygen in it at -25C.  An IMG bottle with 1800 liters at +50C will have 1366 liters at -25C.  You can see this in action on the mountain - you'll notice a reasonably rapid drop in your pressure gauge as the temperature cools when the sun goes down.  This is because there is less volume of oxygen in the bottle now as it's colder.

For the purposes of my comparison between IMG's bottles and Poisk I have taken the volume of oxygen claimed by IMG.  In the document I received from IMG the volume of the bottle was stated at 525 cu inches and the pressure when full at 3000 psi.  This enabled me to calculate that there would be 1779 liters of oxygen in the bottle, however I have used their claimed figure of 1800 liters in my calculations as it's near enough and gives them the benefit of the doubt.

I don't have any data on the temperature that the pressure of IMG's bottles are measured, but I have assumed that its the same as that used by Poisk.  If the pressure of IMG's bottles (3000 psi) was recorded at +20C (ie. once the bottles have cooled down to room temperature after filling), then two bottles of Poisk would still have 30% more oxygen in them, instead of 44% more.  Either way it's still more, plus there is the extra safety of redundancy thrown in for no extra weight.