How to choose gases for deep dives by Volkart Spahr

To remember 
VOLKART SPAHR 
He had a paradoxical ambolia after surfacing from a normal dive and passed away in a decompression chamber in Egypt. (05/11/2011)

(This article was posted to www.diversaround.com)

There are some rules we should follow:

1. max pPO2 for the bottom gas = 1.4 bar, on dives deeper than 100msw better less (1.0 - 1.3)

2. max END = 40 msw or less (40msw is ok for the Red Sea, for cold water and bad viz I would choose 25msw)

3. Every gas with a pPO2 of less than 1.0 is basically useless for decompression

4. For the switches: the next gas should not increase the N2 content by more than 10 percent points

5. Switch to next gas on a pPO2 of 1.6

Easy? Easy!

Let's do some calculations on an example for a 120msw dive!

On 120msw we have 13 bars of ambient pressure. So the gas we breath should have a total pressure of 13 bars, means: 1.3 bars of O2 (rule 1) + 3.95 bars of N2 (rule 2, we are in the Red Sea) + x bars of He. pPHe = 13 - 1.3 - 3.95 = 7.75

To get the percentages, we need to divide all the numbers by 13 and we are back on the surface (ambient pressure of 1 bar):

O2: 1.3/13 = 0.1 = 10%

N2: 3.95/13 = 0.308... let's say 30%

He: 7.75/13 = 0.596...let's say 60%

Now we have our bottom gas. it will be a 10/60

What will be the next gas?

According to rule #3 we need to figure out, on which depth we will have a pPO2 of 1.0:

1.0/0.1 = 10 bars = 90msw

No we need a gas for 90 msw

First we need to calculate the O2-content:

1.6/10 = 0.16 = 16%

According to rule #2 we need at least 39.5% He (3.95/10 = 0.395 = 39.5%)

But: according to rule #4 we should have an N2 content of max 40%.

We can play a little bit with the numbers, but personally, I wouldn't take a gas with let's say more than 42% of N2.

So we will have a gas with 16% O2, 42% N2 and 42% He which will be a 16/42

What will be the next gas?

Where will this last gas reach a pPO2 of 1.0?

1.0/0.16 = 6.25 = 52.5msw

1.6/6.25 = 0.256 = 25.6% O2, so let's say a 30% (at a certain point, we should use standard gases, even when we are for a moment under a pPO2 of 1.0).

Max N2 content = 52% (rule#4).

100 - 30 - 52 = 18% He, the gas will be a 30/18 (or 30/20)

For the next switch I would say goodbye to the He. As experience shows, a switch from a gas of max 30% He to a gas with no He can be conducted w/o any problems.

So we will have our switch on 1.6/0.3 = 5.3bars = 43msw.

Next gas:

1.0/0.3 = 0.333 = 23msw.

To stay with standard gases, I would use a 50%, switched on 22msw.

If you would go on like this, you would've a lots of tanks to carry.

Personally, I would use the bottom gas as mentioned above (10/60), the 16/42 as a deep travel gas, the 30/20 as travel mix for descending (and, of course, also while ascending), the 50% at 22msw and O2 at 6msw. In this case you need support divers to supply you with air for the airbreaks (don't use a gas which contains He for airbreaks). But for a dive to 120msw you will need support divers anyway to have backup gases ready for each switch depth (ok, maybe not for the first switch, but to have a backup gas in a 70 meters range is very nice...)

Nice diving...cu somewhere in this wet world, Volkart