What kind of gas supply would you recommend?
As other posters have alluded to, your initial question breaks down into two sub-questions.....
1. What sort of gas volume will I need?
The volume of gas that you carry can determine your dive duration in respect of your air consumption. To answer this question you must;
A. Calculate your Surface Air Consumption. This is a figure respresented in Liters or PSI per minute. Firstly you must record you air consumption at a set depth, for a set time. You can then use this
ONLINE SAC CALCULATOR. There is also a useful Scubaboard thread
HERE.
B. Utilise your known SAC rate to predict your gas requirements for a future dive at a given depth for a given time. You need to calculate your gas consumption rate at your planned depth. The formula is
SAC Rate x (Depth + 33)/33. You can then multipy this number by the time (
in minutes) that you plan to dive for to provide a total gas requirement. Alternatives, you can determine the volume of gas available and deduct the consumption rate from this (allowing for a reserve) to provide a predicted gas duration. An 'appropriate reserve' should follow the guidelines stipulated by your training agency (
usually 500psi/50bar), but you may wish to adopt the
Rule of Thirds and/or
Rock Bottom gas management principles.
C. If you wish to dive to a certain depth, for a certain time, then you will need to multiply your gas consumption rate (at that depth) by the dive time, then add a suitable reserve, to determine your minimum gas requirements. This can then be compared against the gas volumes provided by differing cylinder options.
2. What sort of gas mixture will I need?
The mixture of gas that you utilize can determine your dive duration in respect of your no decompression limit (NDL). The use of nitrox mixtures allows extended bottom times.
To determine your optimum gas mixture you must:
A. Find the richest nitrox mixture that can be utilised at the deepest depth you will dive to. The optimum Eanx is calculated by dividing the max PPo2 (1.4) by the ata. There is a (metric) nitrox 'best mix' calculator
HERE
B. Utilise the Equivalent Air Depth (EAD) formula so that you can obtain your No Decompression Limit (NDL) and plan your dive on air tables. Alternatively, you can plan directly on suitable Nitrox tables or utilise a nitrox-capable Dive Computer. There is a (metric) EAD calculator
HERE
Here is the EAD Formula:
FO2 = Fraction of Oxygen (i.e. the nitrox %)
FSW = Actual Depth in Feet Sea Water
THE PERFECT SOLUTION
Having found your answers to both of these questions, you must then compare your results to provide the
best workable solution. At this stage, you must also consider any dive duration limitations that may be imposed by your buddy or team.
To achieve your overall solution, you need to compare the maximum dive durations allowed by both your
Gas Volume and your
Gas Mixture. You should aim for both factors to provide you with
approximately equal Dive Durations.
There is little point going to the expense and trouble of obtaining a nitrox mixture allowing you a 60 minute NDL, when your gas supply only provides 35 minutes of gas supply.
Likewise, there is little point in obtaining a large capacity cylinder/s that provide 60 minutes of gas supply, when your NDL will be limited to 40 minutes.
And, of course, there is no point going to any of this trouble if your buddy is, regardlessly going to dive with on Air with an AL80 and suck it dry in 20 minutes.... or if the dive charter limits the dives to 30 minutes...