Have Enough Gas? Is That 12L Really Suitable For 30m Divers?

I am going to assume, everyone reading this understands SAC and RMV rates, if not please check out my blog:
https://grahamsavill.wordpress.com/gas-management-and-planning/

Here we are going to look at some calculations to work out how much actual ‘Safe Time’ you have at different depths with various size cylinders, as well as looking at how far that 3L pony may get you. As per the blog above, I now apologise, as I am a UK diver these calculations are all using Litres, Meters and Bar.

First let me clarify ‘Safe Time’, where in an emergency you can ascend at a rate of 10 meters per minute, stop and hold a 3 minute safety stop at 5m, and surfacing with 50 bar. Anything else I would class as unsafe which has a higher chance of ending with an injury and maybe lead to a trip to the chamber (No one wants that).

For these calculations I have used a few assumptions in addition to the safe criteria above;
SAC of 20 (Average for most recreational divers, newly qualified can be in the 25 to 30 range giving even less time).
1 minute at depth to address the issue. This might sound like a long time, but if you have an out of gas situation on a wreck with a strong tide, you may need to also launch a DSMBs.
SAC multiplied by 3 to take into account the way the body reacts to a stressful situation. (In some divers it may double, but studies have shown it can increase to 4 times as much especially with inexperienced divers).

So below I have put a table together with 4 categories and each category as 3 Cylinder sizes 10, 12 & 15 litres. On the left going down we have the various depths. The figures in the table show maximum bottom. The green is a Safe Accent from the bottom buddy breathing, completing a safety stop and having a 50 bar reserve. Orange is if you ascended and skipped the stop (Not to much of an issue but not ideal). Light red you ascended completed a safety stop, but would have no air left (This is getting dangerous, slightly longer at the bottom, at stop or not come up as quick you would run out of air before you reached the surface). Red is if you skip the stop and surface with no reserve. The worrying thing about this, is how many divers have you seen do a 20 to 30min dive to 30m on a single 12 (There is a popular wreck near me, where this happens all the time). Looking at the chart below after 17 minutes they would not be able to perform a safe ascent while buddy breathing and most likely run out of air before reaching the surface.

As from the table above, this is why it’s extremely good practice that if you are diving below 20m having an alternative air supply can greatly improve the safety of the dive. I do know some operations hang a spare cylinder from the boat, however this is only any good if you can make it back to the shot line. This could be an issue if you were far away from it, or like the wreck I mention above that has a huge tide, will take you away from it.

Now I do see a number of divers who carry a 3 litre pony, which is a vast improvement on the above, but again a lot of these divers are unaware of their limitations. Below is a table broken down similar to the one above but shows if you will surface with air left in your tank or if you would be short. These are similar to the calculations above, however only calculated for 1 diver breathing off it. For an example if you tried to surface from 30m and wanted to perform a safety stop you would be 360 litres short. However even if you skipped the safety stop, you would still be 90 litres short.

Of course if you are with a buddy, the pony would get you out of a tricky situation and easily get you to a depth where you could then switch to your buddy’s alternative air source/octopus and safely ascend. This is one of the reasons, when venturing below 25 and especially 30m you should really be considering a twin configuration setup such as backmount or sidemount. What I hope people take away from this is this size pony is not really big enough to give you that a safety net you may think it does below 25m.

Again thank you for reading the post and as always, I do love a good discussion about my views.

G-SAV