The basic rules associated with good gas planning are reasonably simple:

Rule One: Your SPG is for confirmation of information you already have tucked away in your head. What this means is divers should start their dives with a comprehensive understanding of their gas consumption requirements for the dive! Furthermore, at any point during the dive, they should be able to make a pretty accurate guesstimate of exactly how much gas they have consumed and how much remains in their cylinder(s).

Rule Two: It's a wise diver who comes home (or arrives at her safety stop) with one third of their starting gas pressure in reserve. This refers to diving within the Rule of Thirds, which is an operational trick used extensively by experienced divers to make sure they have appropriate gas in reserve to finish their dives should things go wrong! This boils down to allotting one third of their available gas volume for the outbound journey, one third to come home and one third for the diver to deal with an emergency, such as getting lost, over-staying the NDL or helping another diver

with lesser gas management skills. While this practice may sound overly conservative to the average sport diver, it is an essential part of good gas management, and a great practice to adopt "right out of the box."

Rule Three: People who cheat rules one and two make poor dive buddies and even poorer solo divers!

Key Concept

SPG confirms gas plan doesn't replace it.

One third of starting volume is a very sensible reserve for most emergencies.

Calculating Surface Air Consumption:

The calculations needed to work out how much gas will be used during a particular dive begin with plugging some sort of constant into the gas management plan. That constant is the diver's SAC rate. SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest. The SAC rate for an experienced diver, such as one enrolled in an SDI Solo Specialty, will have leveled out over time and be about the same from season to season. Some things, such as cardio fitness, body mass, and age will affect a diver's SAC over time, but for all intents and purposes, we can look at it as a constant: it remains the same from day-to-day, week-to-week, month-to-month.

Although SAC rates vary from diver to diver, and depend on things like size, fitness and sex, a great number of divers have SAC rates that are reasonably close to each other. A commonly used default SAC is about 14 litres or half a cubic foot per minute, and we'll be using this figure in the examples contained later in this chapter.

You will calculate your personal SAC rate during this course. There are several ways to calculatet it. Many divers take an average from past dives and rework the figures showing consumption at a depth during a dive to extrapolate what their consumption would have been at one atmosphere on the surface. However, this method has a basic flaw: it actually gives the Surface Respiratory Minute Volume (SRMV) of the dive the SAC. Without clogging our minds with acronyms let's just say that the figure one arrives at using this method is not a SAC rate because it has built into it some allowance for things like workload and "mental" stress that are part of an actual dive. These outside factors need to be subtracted to arrive at a true SAC figure. Why? Simply put, if one uses the volume based on a SRMV calculated using consumption on easy dives where everything went well and the diver was in familiar surroundings, the volume calculations for a stressful, hardworking dive will be unrealistically light. Conversely, if the SRMV figure used is based on hard dives where the diver was stressed by things like poor vis and cold, and swimming against a heavy current, the volume calculations for an easy dive will be unrealistically heavy.

If you have calculated a "SAC rate" based on gas volumes derived from actual dives without factoring out workload, stressors such as temperature, visibility, current and related issues, I'd suggest multiplying it by 60 percent to arrive at a more realistic figure.

A better, more accurate method of calculating SAC is to actually measure how much gas is breathed while on the surface; let's say sitting watching TV or reading, and using that value as a baseline. Hook up

a tank of known volume and take a reading of its starting pressure. Breathe from it for 10 or 20 minutes and measure the difference between the starting and ending pressures. Calculate how many litres or how many cubic feet this pressure drop represents and divide that figure by the number of minutes to find the average consumption per minute, which of course will be your SAC. Make a note of that and use it in all future gas calculations.

Here are examples using metric and imperial units to help explain the process.

Cylinder A is a 12 litre steel cylinder, with a starting pressure of 175 bar. Diver Jack pops a regulator in his mouth, plugs his nose (to ensure accuracy) and starts working on a simple Sudoku puzzle. After 20 minutes, he is close to completing his second puzzle and the pressure gauge on his cylinder reads 150 bar. The pressure difference is 25 bar, therefore the volume of gas consumed is 25 multiplied by 12, which works out to 300 litres. Diver Jack's SAC is 300 litres / 20 minutes or 15 litres / min. Diver Jack has a SAC rate just a fraction above average.

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