A 3-liter bottle weighs around 2,6 kg when full. Approximately 1060 grams out of that is oxygen (this will vary between individual bottles). We weigh the bottles simply to make sure that they are full.

Now, we need to calculate the duration of the bottle for our climb.

The flow control measures usage in liter/minute, so it is easier to calculate in liters per minute, rather than counting on the consumption of grams.

A 3-liter bottle holds about 720 liters of oxygen.

During the climb, you will vary the flows between 1, 2, 3 or 4 l/m. Even though it is possible, don't go higher than flow 4. Oxygen becomes toxic and dangerous at that rate. The flow set by you will determine the duration of your bottle.

We stated that the bottle holds 720 liters of oxygen. If you breathe from it at the rate of 2 liters a minute - the 720 liters will be gone after 360 minutes. That’s 6 hours.

Should you use 3 liters a minute, the bottle goes in 240 minutes, or 4 hours. At 4 liters a minute, the bottle consequently empties in 180 minutes/3 hours. On the other hand, a flow of one only, will last the bottle for 720 minutes - 12 hours.

When and how much?

Most climbers start using oxygen at C3.

The main reason being that it will give you a good night's sleep (on flow 1 liter or less) before the long, strenuous summit attempt. Also, you will try out the gear and get used to it well in time. Using oxygen to Camp 4 gained Thomas only one hour of speed, compared to when he went without. Yet, it left him less worn for the summit attempt.

Furthermore, you might have to stay at C4 for 2 nights if the weather is bad and you need to preserve your strength. Staying in shape by using oxygen from C3 is a good strategy towards your optimal chance of a summit. The use of oxygen below C3 is of minimal benefit.

Sherpas will start using oxygen at C4. At that altitude you should use a rate of one l/m while resting/sleeping. If you are weak, don't hesitate to put it up to two liters until you feel better. The main reason for weakness is however not lack of oxygen but lack of fluid. Try to drink 1 liter of water upon your arrival to camp, and another 3 liters before the climb. Bring 2 liters of HOT liquid for the climb, keep it in your down suit - never in the pack (it will freeze in no time).

When leaving for the summit, you should calculate for at least 18 hours, to be on the safe side. Head out at around 11 PM, arrive at South Summit by 7 AM and the main summit at 9 AM. This is considered a very good time. If the snow is just little deeper and the wind just a little higher, you could just as easily summit at 12 or even 2 PM. Never ever summit later than 2 PM! Allow 4 to 6 hours for descent.

If staying one hour at the summit, we are looking at a time span of 15 to 21 hours. If your average use of oxygen is 2,5 l/m, you’ll need at least 4-5 POISK bottles to be sure.

But how much do climbers actually bring?

Well, considering that up to one third of all climbers get into oxygen trouble up there, they are either bringing to little or using too much.

So, let’s say that you bring (or are provided with) only 2-3 bottles of oxygen (funny we should use THAT figure...).

If you use 2 liters a minute, 2 bottles will last you for 12 hours.

3-9 hours without oxygen, on the climb down, when you are the most tired – will be a chilling fact!

Sure, perhaps you’ll be really disciplined and use only one l/m at some parts, but that will just call for 3 l/m on the hard parts, since you’ll get worn. You will still end up on the average 2 liters a minute consumption. Expect many hours without oxygen on your climb down - just when you are at your worst! This is the time when climbers die most often.

3 bottles would rather be the minimum and that is what the Sherpas normally bring for themselves. If you are an experienced and altitude strong (which is not equivalent to sea level strong) climber, three bottles could be OK.

Most people should however bring 4 bottles and still be careful with the usage. With an average of 2,5 liters that will last for 19 hours. In most cases that will bring you up and down safe.

We try to bring 5 bottles each if possible. That gives us a good margin of safety. The extra bottle is only going to Balcony (8500-m/28000 ft) for rescue. High up, we’ve been able to offer climbers in trouble spare oxygen because of this and that is a nice feeling too. It is just a spare bottle, but could mean the difference of your or your friend's survival.

Finally, an important fact to be remembered in all this is also that bottles occasionally brakes on summit attempts.
 

Running out of oxygen

Fighter pilots, who reach altitude of 8000-m/26000 ft without supplementary oxygen, will fall into coma within 4-8 minutes.

For you likewise, running out of oxygen at a very high altitude means big, big trouble. It's like going in to a wall. You can not compare yourself with someone climbing without oxygen for the entire climb. They have adjusted their bodies gradually, whilst yours will have a shock.

If you are on the ridge (8750-meter/28700 ft) and the oxygen is gone, your chances of returning alive are slim and the chance of returning without frostbite almost nonexistent.

Don't trade US 250 against your life! Bring an extra bottle and save yourself or someone else.

The cost

To be safe, we need around 2-3 bottles from C3 to C4 (including sleep), 2 bottles at C4, 4-5 for the summit push and two spares. That totals 12. If you are up for another attempt you need at least another eight. That totals twenty.

The cost for this will be around US 300 each bottle or US 6000 in total. Invest this money and sell what you are not using upon going home.