As the season progresses, if you’re like me, you have your eye on a certain dive you’d like to get to or a wreck that you’ve never dived.
When we’re planning our dive, we take into account many different considerations, including the depth, time, gas, tides, currents, equipment, special skills and certifications that might be needed to complete the dive in a safe manner. One thing that sometimes gets overlooked on these occasions is the amount of gas that will be required and sometimes it’s the limiting factor of our dive.
Let’s take the popular local wreck of the Aeolian Sky as our example. The “Sky” lies at a depth of 30m and makes it the ideal dive for an Advanced Open Water Diver with the Deep Diver and Enriched Air specialties. If we dive the wreck on air we get a NDL of 20mins, with Enriched Air or Nitrox 32% we get another 10mins and a NDL of 30mins (giving us a PPO2 of 1.28) but we might ask ourselves if we can complete the dive using a standard 12ltr cylinder?
The first thing we need to know is our Surface Air Consumption Rate or SAC. Some computers may give you a SAC rate, this is normally shown as a Bar/min eg. 0.8. In this case, we simply multiply this figure by the volume of our tank. 0.8 x 15 = 12ltrs/min. We can also work it out by finding our average depth for a dive, the dive time and the volume of gas we consumed during that dive. So, if we had a 12ltr cylinder and used 150 bar during a dive, we used 12 x 150 = 1800ltrs of gas. Lets say our dive time was 55mins and our average depth
for the dive was 12mtrs (2.2ata), we would then divide 1800 by 55mins and then divide by our depth in atmospheres. So, 1800/55 = 32.7 / 2.2 = 14.8. This then gives us a SAC rate of 14.8ltrs / min. By doing this for every dive we get a good average over time and for the sake of our example we shall say that we have a SAC rate of 15lts/min (Remember that this is the surface rate, so at 10 mtrs we would be using twice as much and at 20m three times and 30m four times).
If we are planning to dive the Aeolian Sky at 30m our rate of consumption at depth would be 15ltr x 4 or 60ltrs/min. A full 12ltr cylinder filled to 232bar gives us 2784ltrs of gas. We also want to get to the surface with 50bar in reserve. We may want to start our ascent when we were on 80bar (for this example) giving us gas for our ascent and the safety stop etc. So, we subtract our 80 bar from the 232 starting pressure. 232-80=152 or 1824ltrs of usable gas. We know that we breath 60ltrs/min, this gives us 1824/60 = 30mins.
As it happens, we could just about complete this dive on a 12ltr cylinder but we should also take into account the gas we need for our buoyancy device (BCD or wing) and the dry suit inflation. If you wanted to add further safety into your dive planning you could follow the same rule cave and technical divers follow and use the rule of thirds. One third of your gas for the descent and bottom time, one third for the ascent and the final third for reserve. With this in mind and using the above example we have 232/3=77.3 x 12 = 927.6ltrs/60 (our
SAC)=15mins bottom time before starting the ascent.
Hopefully, you’ve found this interesting and fairly straight forward to follow even though the maths can be tricky at times. If you have any questions please either talk to me on the boat or talk to one of the instructors at Dorset Diving. I hope it helps in your dive planning.
Have a safe season.
Mark - PADI MSDT Instructor.
Read all about The Sky here