Understanding Enriched Air Nitrox

Gas Monitoring and Tracking Gas Consumption (Bonus Section)

Regarding your air consumption rate, you should consider keeping track of your consumption rate. With the use of SAC, Surface Air Consumption along with the rate you document your consumption at depth may be used to calculate time at depth before you must ascend for future dives. I add this recommendation here because it is relative. We will not go over consumption rate in this article. I thought you should be introduced to it for your own knowledge. I had added some detail for SAC at the end of this article for your reference.

Your buoyancy, trim, weights, breathing technique, currents, equipment, health, environment, your competitiveness with matching or beating your fellow divers rate, experience, and other considerations all factor into your gas consumption rate. Your log book should have most of those details so you may reference them at a later date.

Most recreational divers know over time whether they are coming up short on bottom time or gas volume. You may hear people around the dive site asking their friends and buddy’s, How much air did you come up with? Usually to show off if they came up with more or kept their mouth closed if less usually in good fun and to tease. Do not ever let someone convince your you are performing bad on gas management into making poor decisions. You will get better over time and with more experience.

Tracking your gas consumption can be burdensome for some and the attention to detail for others. If knowing your gas consumption is something that you will consider, there is no time like the present to begin. If you are unsure, I suggest you begin. It will add to your knowledge base, assist you in conversations, provide input when calculating gas needed at depth, provide clarity and much more.

You could collect just the starting and ending pressure and the difference between the two extremes. What you will get is a very general SAC value or you can go further in your data collection and collect numbers for the amount of time and gas consumed at each of the specific depths during the course of the dive.

I am not here to tell you which way is best for you. Provided below is a basic review of what to track and how to track information. It is up to you decide how much to use.

To keep track of your SAC, you will need to collect some basic information and perform a little math. A spreadsheet or a program found on the Internet can help but a pen and paper will for sure.

The resulting figures are the average rate of gas consumed per minute based on an Aluminum 80cf cylinder filled to 3000psi and having consumed 2000psi during the course of the specific dive. Change the tank size, the working pressure, the exertion, equipment, etc. for example and your SAC rate changes. Makes you wonder if you should be separating your SAC rates based on all the variables listed above.

Surface Air Consumption, SAC
To get the total cubic feet of gas consumed, to the cylinder size used, use the following formula, For example on an 80cf cylinder filled to 3000psi:

((80 cubic feet x 2000 psi) / 3000 psi) = 53.3 cubic feet of gas consumed

To get the cubic feet of gas consumed per minute, use the following formula
(((80 cubic feet x 2000 psi) / 3000 psi) / dive time)

53.3 cubic feet / 20 minutes = 2.6 cubic feet / minute

Note the above and below examples are the Average over the course of the 20 minute dive.. Most likely the diver did not spend the whole 20 minutes at one specific depth. realistically, you are at various depths during the course of a dive.

If it is the psi vs. cf the formula would be:
((Air consumed during dive/dive time) / ATA) = SAC psi per minute

((psi_consumed / DiveTime) / ATA) = psi per minute 

((2000psi/20 minutes) / 3) = 33.33 psi per minute SAC

((2000psi/20 minutes) / 2) = 50 psi per minute SAC

As you can tell, the lower the SAC rate the better.

Take this one step further to calculate your Respiratory Minute Volume, RMV. The main difference is the RMV rate is independent of tank size. You may use RVM to calculate volume based on variable sizes of tanks. If one dive is on AL80 and another on LP95, the volume of the two cylinders are not the same. Knowing your RMV, you can apply it to the specific volume of cylinder you plan to use.

Let me provide an example. If you are able to drive 200 miles on a gas tank with a volume of 20 gallons, changing the volume of the tank changes the number of miles you may travel. Using a 30 gallon tank, you would expect to go 300 miles. 1/3 farther than on a 20 gallon. The quantity of gas used per mile didn’t change, just the size of the tank.

To turn your SAC into RMV, simply factor in the size of the tank you were using by adding a tank conversion factor.

Imperial: Divide the tank volume, in cubic feet with the tank operating pressure then multiply it with your SAC.

( 80cf / 3000 ) * SAC

Using the above SAC rate of 33.33 with an 80cf Aluminum and working pressure of 3000psi:

RMV=( 80 / 3000 ) * 33

RMV=0.88cf/min

For a steel 95cf cylinder with a 2440 working pressure:

RMV= (95 / 2440) * 33

RMV=1.28cf/min

Sounds a bit daunting, all these numbers and different ways to calculate to have to learn. Just keep learning. Perform some internet searches and read more about it. Practice until you feel comfortable and by then you may find it is something you will perform and log.

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