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When you’re diving, it’s important to be constantly aware of how much air you’re using. This is known as surface air consumption, or SAC.
We created a calculator to allow you to determine your surface air consumption. It’s a great tool for planning your dives and being aware of how much air you consume.
In this blog post, we’ll discuss what SAC is and why it’s important for scuba divers. We also provide a SAC calculator for your use.
Surface Air Consumption Calculator
What Is Surface Air Consumption Rate
Surface Air Consumption (SAC) is a measure of your rate of gas usage expressed in pounds per square inch (psi). It is sometimes called surface gas consumption (SGC). For divers who use the metric system, it is reported in bars of pressure.
Knowing your surface air consumption is an important part of being a safe and experienced diver. It enables you to accurately plan dives, by understanding how much air you can expect to use at varying depths – without the pressure influence.
Learning your SAC isn’t too time consuming either. Most solo diving and technical courses will include it in their training programs.
Importance of Knowing Your Air Consumption Rate
It’s very important to know your air consumption rate when scuba diving for several reasons. These include:
- You can calculate how long your air will last at your planned depth.
- You can precisely determine the minimum tank reserve pressure for a dive. Experienced divers are often shocked to discover that deep dives often require a greater reserve pressure (up to 1,000 psi) for their dive buddy’s safe ascent.
- For the safety and success of decompression diving, it is essential to know your air consumption rate so you can calculate how much air you need for each stop.
- You can evaluate a diver’s stress level by comparing their air consumption on a dive to their baseline usage rate. If your air consumption rate is notably higher than usual and you find yourself using 500 psi in five minutes of diving at 45 feet, when typical conditions dictate only 200 psi should be used, this could potentially signify an issue.
- If a diver is displaying seemingly normal breathing, but their gas usage increases faster than usual, then it could be indicative of a large leak. In addition to an increased rate of air consumption, heightened resistance while inhaling and exhaling can also suggest that the regulator needs servicing.
How to Determine Your Air Consumption Rate
Method 1 – Plan a dedicated dive to measure your SAC
The best way to determine your SAC is to do a dive dedicated to measuring how much air you use. This allows you to eliminate variables, such as swimming against a current, and collect accurate data.
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- Follow these steps to determine your surface air consumption rate:
- Before conducting the test make sure the tank is properly cooled down in water.
- Descend to a depth that can be maintained accurately and consistently for at least 10 minutes. Ideally, this is 33 feet in saltwater.
- Record your tank pressure before the test.
- Swim for a predetermined amount of time, usually 10 minutes at your normal swimming pace.
- Record tank pressure after the test and optionally perform the same test while resting or hovering as well as swimming quickly to obtain data for “resting” and “working” states.
- Calculate your air consumption rate using the SAC rate or RMV rate formula.
- Repeat the test a few times to ensure accuracy and reliability of results.
Results can be compared with standard or published Air Consumption Rates as well as personal goals to evaluate progress and performance.
Measuring a diver’s air consumption rate using this method provides more reliable data due to its controlled and constant depth settings. However, it is important to remember that real-life situations rarely match up exactly with test data.
Remember, SAC and RMV rates obtained from either method should only ever be used as guidelines.
Method 2 – Use data from normal scuba dives
- Before getting ready for your scuba dive, it’s essential to make sure that your tank is completely cooled down. This should be done for a few minutes before submerging into the water.
- Record the starting pressure of your tank using a slate.
- After completing the dive, record the final pressure of your tank on the surface before it can warm up in the sun.
- A dive computer or watch should be used to determine the total dive time and average depth of your dive. This information is necessary to accurately calculate your SAC rate or RMV rate.
- Once all the relevant information has been obtained, plug it into the appropriate equations.
Divers gravitate towards this air consumption calculation method because it is based on realistic dive experiences. Although the rate generated from this approach is an average of the entire dive, it will never be as precise as conducting a dedicated dive as outlined above.
By taking the average of many calculations conducted over several dives, a diver can make an accurate estimation of their air consumption rate.
Calculate Your Surface Air Consumption Rate
You can use the following equation to calculate your surface air consumption rate.
1. English units SAC rate formula
Equation 1: [{(psi start – psi end) x 33} ÷ (depth + 33)] ÷ time in minutes = SAC rate in psi/min
The variables used in the equation are:
- psi start = tank pressure in psi at the beginning of the test period (method 1) or the dive (method 1).
- psi end = tank pressure in psi at the end of the test period (method 1) or the dive (method 2).
- time in minutes = the test period (method 1) or the total time of the dive (method 2)
- depth = the depth maintained during the test period (method 1) or the average depth during the dive (method 2)
2. Metric SAC rate formula
Equation 2: [{(bar start – bar end) x 10} ÷ (depth + 10)] ÷ time in minutes = SAC rate in bar/min
The variables used in the equation are:
- bar start = the tank pressure in bar at the beginning of the test period (method 1) or the dive (method 2).
- bar end = is the tank pressure in bar at the end of the test period (method 1) or the dive (method 2).
- time in minutes = the test period (method 1) or the total time of the dive (method 2)
- depth = the depth maintained during the test period (method 1) or the average depth during the dive (method 2)
What is Respiratory Minute Volume Rate
Respiratory Minute Volume (RMV) rate is a measure of the air that a diver consumes in one minute on the surface. RMV rates are expressed in cubic feet per minute for English units or liters per minute in metric.
When planning a dive, the diver needs to take into consideration the size of their tank and its working pressure when calculating air consumption. To do this they start with RMV rate as it doesn’t depend on what type or size of tank is being used and then converts that figure to an SAC rate which accurately reflects how much air will be consumed during the dive.
Calculate Respiratory Minute Volume Rate
Put the SAC rate and other required information into the formula provided below to quickly figure out your RMV rate. Calculating metrics in this way is far easier than doing it for imperial RMV rates.
Calculating RMP using English units
Step 1: To determine the tank conversion factor for your data collection, you’ll need to record the volume (in cubic feet) and working pressure (in psi), which will be indicated on the tank neck.
Equation 3: Tank conversion factor = Tank volume in cubic feet ÷ working pressure in psi
Step 2: To calculate your imperial SAC rate, simply multiply it by the tank conversion factor.
Equation 4: RMV rate in cubic feet/minute = Tank conversion factor x SAC rate
Use SAC Rate to Estimate How Long Your Air Will Last
When planning a dive, you can estimate how long your air will last if you know your SAC rate.
Calculate your SAC rate specifically for the tank you intend to use. When you’re working with English units (psi), make sure to divide your RMV rate by the individual conversion factor of your tank for maximum accuracy. By using this, you can easily determine the Specific Area Consumption (SAC) rate for any tank of your choice.
How to Use Less Air While Scuba Diving
Streamline your profile
Make the most of your underwater journey by packing smartly and utilizing simple techniques to streamline your movement. Stow what you need in pockets, attach gauges and an octopus regulator onto a BCD, secure hoses close to the body for convenience – then reduce drag while finning by bringing arms together or clasping hands behind you with fins tucked neatly inside your tank’s slipstream. With these easy tricks, even small efficiencies can lead to big benefits!
Optimize your buoyancy and trim
You can get the perfect swimming profile by fine-tuning your BCD and weight distribution. With a little planning and persistence, you can find a balance that keeps you in good horizontal form – no more Mack truck image!
In addition to looking great under water, having less air in your BC means even more oxygen for breathing during dives. Give yourself an advantage over other divers: learn how buoyancy control and proper trim work together for maximum performance.
Fix air leaks
You should thoroughly inspect your gear before each dive. A seemingly small air leak from a gauge, octopus or BCD inflator can deplete your air quickly.
Don’t forget about your mask. Constant water clearing while diving will lead to unnecessary loss of your air.
Final Take
Knowing your surface air consumption rate and using tips to reduce air consumption can help you become a more efficient diver. With the right planning and practice, you can master your RMV rate and be able to accurately calculate your SAC rate for any dive situation.
This will give you the confidence to safely and comfortably plan dive profiles. Good luck and Happy Diving!
Scuba Diving Calculators
Check out our other scuba calculators.
- Tank Air Volume – calculate the volume of air in a pressurized scuba tank.
- Calories Burned While Scuba Diving – determine how many calories you burn during your dives.
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