Safe Diving Practice and Problem Solving Part 2 – Gas is Time, Effective Planning

Any problem is solvable with enough time, and in diving Gas (Air) is time. As long as you are breathing you can deal with what is going on. There are 2 aspects to ensure you have enough gas for any situation:

• ·         Gas Planning and Management
• ·         Emergency Procedures and Redundancy

Let’s first explore as Planning and Management. During this blog I use Gas which maybe Air, Nitrox or Trimix depending on the dive you are planning. Firstly Gas Planning. It is important to be sure that you have planned a dive that is actually possible to do. Many divers do dives that I class as ‘Suck it and see dives’, where you dive until you reach a set bar level. This is ok for shallow dives and with greater experience you can start to judge how much you breathe on any particular type of dive. However if you want a more accurate method then you can employ a technical diving method.

First you need to know how much gas you have, and with the metric system it is nice and easy.

Liters of Gas (ltr) = Cylinder Capacity (ltr) x Cylinder Pressure (bar)

For example a full 12 ltr cylinder at 230 bar

Liters of Gas (ltr) = 12 ltr x 230 bar

Liters of Gas (ltr) = 2760 ltr

The next stage is to know how much you breath, this is known as you Surface Air Consumption (SAC). To work this out accurately swim at a constant depth for 10 min and measure how much you breathe in bar and then convert this to how much you breathe at the surface:

SAC = (Pressure of Gas Breathed (bar) x Cylinder Capacity (ltr)) / (Time (min) x Pressure of Depth (bar))

For example you swim at a steady pace at 20m for 10 min and breathe 20 bar of air from a 12 ltr cylinder:

SAC = (20 bar x 12 ltr) / (10 min x 3 bar)

SAC = 240 / 30 = 8 ltr/min

Another way to calculate a quick average SAC is to take the average dive depth from your computer and use the total gas pressure used through the whole dive and the total dive time.

For example your average depth is 15.5m and the dive was 40min. In that time you breathed 80 bar from twin 12 ltr cylinders (24 ltrs total):

Avg. SAC =( 80 bar x 24 ltrs) / (40 min x 2.55 bar)

Avg. SAC =1920 / 102 = 18.8 ltr/min

Now you know how much gas you have and how much you breathe you can work out how long you can dive with redundancy gas left for emergencies. In Tech diving we use the rule of thirds: 1 third to dive away from the entry point, 1 third to get to the exit point and 1 third in reserve for emergencies.  In No Decompression diving people tend to plan to leave around 50 bar in the cylinder at the end of the dive. Whatever safety margin you decide it should reflect the type of dive that you are on. Here is how to calculate how much time you have on a dive. First work out how much gas you have and the amount of safety margin you want and then divide the useable gas by your SAC at depth.

For example you have a 12 ltr cylinder with 230 bar and you are planning a 20m dive and you know your SAC is 18 ltr/min. You want to end the dive with 60 bar how long can you dive for.

Cylinder Capacity = 230 bar x 12 ltr = 2760 ltr

Safety Margin = 1 – (Reserve Gas Pressure / Start Gas Pressure)

Safety Margin = 1 – (60 / 230) = 0.74

If you are using the Rule of thirds the Safety Margin is always 0.67

Gas for the Dive = Capacity x Safety Margin

Gas for the Dive = 2760 x 0.74 = 2042 ltr

Max Dive Time = Gas for Dive / (SAC x Depth Pressure)

Max Dive Time = 2042 ltr / (18 ltr/min x 3 bar) = 2042 / 54

Max Dive Time = 37.8 min

Obviously this s an average dive estimate so if you will be working harder than normal then account for this by increasing you SAC. In a stressful situation it is easy for a SAC rate to double if not triple!

If you use this set of rules then as far as a dive plan goes you should never run out of gas. During the dive is when gas management becomes key. You need to keep a regular check on you SPG throughout the dive and have a set turn pressure at which you will end your dive. Also you should be able to tell if you are using your gas more quickly than expected and shorten your dive accordingly. Trust me, no dive is ever that good that it is worth pushing past your planned end gas pressure.