Is our Recreational breathing gas planning not safe enough? Part 1
Cozmo Neiger Dec 12, 2022
Is our Recreational breathing gas planning not safe enough? PART 1
This is a subject that is hotly contested by all. I have been diving for a long time now and I have heard a lot of different views on this topic and the most resounding non-Tech response is: We are to monitor our gas and once we are at 500 psi we need to leave the bottom and return back to the surface or plan to make it back to the boat/surface with 500 psi left. I have heard this from both new divers and ones that have been doing it for 30+ years. The training agencies minimum that is taught is that you should plan to have 500 psi at the surface and is usually preached by dive operators. Also there is the trend especially with divers that rent tanks or tanks that are included in dive trips (Air or Nitrox) who feel that they paid for all the gas in the tank so they are going to try and use it all.
I have found once this is taught it seems to get ingrained in the recreational mind set, and it seems that they are not willing to consider more conservative diving practices.
Before we go off the deep end we are not here to say you are doing it wrong and you have to change. We are just going to explore the topic a little to provide another look at it so you as a diver can use this info to make your own decision about how you want to dive.
First let's look at a little of the background on how we got to the 500 psi reserve mind set. When we took our open water diver course we are taught that you are to plan your dive and dive your plan to stay within the recreational no-stop/no-decompression time and depth limits, do a safety stop (if you can) between 10-30 ft for a min of 3 minutes and to make it to the surface with 500 psi in your tank.
Now it’s hotly contested by some including training agencies with the labels non-stop or no-decompression time in reference to depth and bottom time dive tables in the recreational diving world. That’s for another day. Today we are going to just cover a recreational scuba dive within the Open Water diver limits and a diver can ascend with no stopping at no more than 30ft per min.
Divers out there these days come in all shapes, sizes and all levels of health that we might want to always plan our dives with a safety/decompression stop at 20 ft for no less than 3 minutes to allow the body to off gas some of the inert gas before breaking the surface.
So now let’s get on with it and look at a typical open water dive to 60 ft with planning to be back at the surface of the water with 500 psi left in our tank. We need to do a little gas planning that is not really taught in your typical open water class (we will skip a lot of the math to keep it simple and we will give you the data without the work if you want to take a course see here). We will be doing this dive on air with the good old Aluminum (AL) 80 which you can find everywhere. There is one misconception with the AL 80 and they are not 80 cu ft they average about 77 cu ft dependent on manufacturer. For our calculations we will be using a surface consumption rate (SCR) of .8 cu ft per minute. This is a good average for a newer open water diver at a moderate work load. We will also be using a SCR of 1.4 for a diver dealing with an emergency because when stressed our body protects itself by releasing adrenaline, blood pressure increases and blood flow increases to our muscles to be prepared for the fight or flight response which also increases breathing.
So let’s take a look. For illustrative purposes we will be doing a square profile as our plan so we will dive our plan.
Dive 1:
Will be a typical recreational dive where there is no emergency with a 3 min safety stop @ 20’
Depth: 60ft Time: 25 minutes Gas used: 58 cu ft
We leave the bottom when our pressure in our tank is @ 765 psi or 19.87
The Ascent and 3 min safety stop uses 6.87 cu ft
We reach the surface with 500psi or about 12 cu ft left in the tank
So what was our total cu ft used?
- Bottom Phase 58 cu ft
- 60’ to 20’. 2.35 cu ft
- 20’ stop 3.84 cu ft
- 20’ to 0’ .68 cu ft
- Total dive 64.8 cu ft
- Reserve 12.13 cu ft
- Total needed 77 cu ft = one Aluminum 80
Now that seams fairly reasonable right; but that is if all goes well. What if you have an emergency. During my own dive planning I always bring along Murphy and his law and he likes to act-up at the worst point of a dive. So we will plan the first dive again as if our buddy has a total out of air emergency at the end of the dive phase (@60’ @ 25 min) and now each one of our two divers are breathing @ 1.4 sac rate.
Will they have enough breathing gas to make it back to the surface? Let’s take a look.
Dive 1 w/emergency:
Depth: 60’ Time: 25 minutes Gas used: 58 cu ft
Diver 1 checks his pressure gauge and sees that they are at the planned pressure to end the dive ( 765psi ) and then turns to his buddy to indicate its time to end the dive and is interrupted by the buddy grabbing the regulator out of his mouth. The buddy then indicates he is out of air. It takes the two about 30 seconds to get situated and be ready to go to to the surface. Now remember the max accent rate is 30’ per min and should try and make a safety stop @ 20’. Also remember once the emergency starts both divers are breathing their gas @ 1.4 SAC rate and either diver has never had to do this outside of training.
So what was our total cu ft used in this emergency?
- Bottom Phase 58 cu ft
- Link up and get ready to ascend 30 sec @ 60 3.9. cu ft
- Ascend from 60’ to 20’ @ 1.33 min 8.23 cu ft
- 20’ for 1 min 4.47 cu ft
- Ascend from 20’ to 0’ 2.4. cu ft
- Total for this dive 77 cu ft = Aluminum 80
Umm looks like there is just enough gas volume just to make a 1 min safety stop and make it to the surface and then inflate their BCs orally. They made it to the surface but with no breathing gas left in their tanks. Dependent on where you are diving or the condition of the sea state you may need to go back under the surface (deeper than a snorkel) to make it back to say the boat or the shore.
For me just enough does not make me warm and fuzzy. In Part 2 we will look at how to plan this dive with the Rule of Thirds.
