Posted by Chris on March 08, 2004 at 18:42:01:
In Reply to: Re: Solo Dive Ques./ Pony Tank Backup posted by seahunt on March 07, 2004 at 21:35:16:
To decide on how big a pony to use you should measure your at rate surface air consumption rate.
Put a regulator on your tank, and sit down and rest for about 15 minutes, but carefully timed as ttest (in minutes). Read your tank starting Pstart and ending Pend presures.
The tank you use will have a rated volume Vtank at a rated pressure Ptank.
Your at rest surface air consumption rate will be:
Rate(0) = (Vtank / Ptank) * (Pstart - Pend) / ttest
The rate you get will be your lowest air consumption rate. Your at rest rate at depth (d in feet) will be:
Rate(d) = Rate(0) * (1 + d/33)
Now that you know this you can calculate how long a pony tank will last a depth if you know the volume of your pony tank Vpony and rated pressure Ppony
First you must calculate the usable (or effeective) volume Vpeof the tank, since most regulator will not operate will if the tank pressure falls below 300 psi.
Vpe = Vpony * ( Ppony - 300) / Ppony
To find out how long a pony will last at depth for the at rest
tpony(d) = Vpe / Rate(d)
No remeber this time is based on an at rest case, in a stressful situation your air consumption rate may be 3 times greater.
When you do these calculations, you will find out you may need a much bigger pony tank. You will also find out that a spare air is virtually useless.