If you've been diving for any amount of time, you may have heard the term "SAC rate" thrown around on the beach or boat deck. What is SAC rate? We will tell you not only what it is, but why you should know it and how to calculate it.
SAC stands for Surface Air Consumption. Your Surface Air Consumption rate is a measurement of the amount of air you consume while breathing for one minute, on the surface. These values are given in the same unit of measurement you would use to measure the air in your cylinder, i.e. psi in the United States (as we use the metric system) and bar in most the rest of the world. Note, however, that your SAC rate is tank specific, meaning that it only applies to the exact size of the cylinder you will be using on your dive. For example, if you regularly dive with an 80 cubic foot tank but switch to smaller 60 cubic foot cylinder for your next dive, you would need to recalculate your SAC rate.
Before we get into the specifics on how to calculate your SAC rate, there are a couple of main reasons to discuss as to why you would want to know it as a recreational diver.
Why know your SAC rate:
1. Planning your dive
You want to know your SAC rate before diving to plan how long you would be able to stay underwater at your planned depth based on the amount of air you have available. Technical divers want to know their SAC rate to calculate how much extra gas they will need to carry with them for a decompression stop.
2. To notice gear malfunctioning issues
Another reason why you, as a recreational diver, may want to know your SAC rate is to identify a potential gear malfunction early before it becomes a major issue. If you notice that you are breathing at your normal, calm rate but are going through your air more quickly than usual, this may indicate there is a leak in their scuba system, perhaps with their BCD, regulator hose, or cylinder valve. This would allow you to decide to terminate the dive early before the loss of air could become a major problem.
How to determine your SAC rate?
One fast way to calculate your SAC rate is by using the automatic SAC rate calculator in the DiveNerd website. However, if you want to do all of the math yourself here is what you will need:
- The average depth of your previous dive
- Total amount of air you consumed (i.e. beginning pressure minus ending pressure)
- Total length of dive (in minutes)
STEP 1 - Calculate: total psi used / total dive time = psi per minute used
STEP 2 - Adjust for pressure of the dive: (depth of dive / 33 ft.) + 1 = atmospheres absolute of pressure (ATA)
STEP 3 - psi per minute used (STEP 1) / ATA (STEP 2) = SAC rate in psi/minute
Learning your SAC rate and applying it to each dive you do makes you a better prepared and more aware diver. If you want to learn more, check out the great article on Estimating Your Air Consumption by Alert Diver.
