Scuba Air Consumption Rate Calculator (SAC & RMV)
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Understanding Your Air Consumption
Calculating your air consumption is critical for safe dive planning. This calculator provides two key metrics:
- SAC Rate (Surface Air Consumption): This tells you how much pressure (PSI) you breathe per minute, normalized to the surface. However, this number is specific to the tank size you used.
- RMV (Respiratory Minute Volume): This converts your SAC rate into a volume measurement (cubic feet per minute). This is a universal number that you can apply to any tank size to predict how long your gas will last.
The Math Behind the Calculator:
SAC = ((Start PSI – End PSI) / Minutes) / ((Depth / 33) + 1)
RMV = (SAC * Tank Volume) / Working Pressure
function calculateAirConsumption() {
// 1. Get input values
var startPsi = document.getElementById('startPressure').value;
var endPsi = document.getElementById('endPressure').value;
var avgDepth = document.getElementById('avgDepth').value;
var time = document.getElementById('diveTime').value;
var tankVol = document.getElementById('tankSize').value;
var workPress = document.getElementById('workingPressure').value;
// 2. Validate inputs
if (startPsi === "" || endPsi === "" || avgDepth === "" || time === "" || tankVol === "" || workPress === "") {
alert("Please fill in all fields to calculate accurate rates.");
return;
}
startPsi = parseFloat(startPsi);
endPsi = parseFloat(endPsi);
avgDepth = parseFloat(avgDepth);
time = parseFloat(time);
tankVol = parseFloat(tankVol);
workPress = parseFloat(workPress);
if (startPsi <= endPsi) {
alert("Starting pressure must be greater than ending pressure.");
return;
}
if (time <= 0) {
alert("Dive time must be greater than 0.");
return;
}
// 3. Perform Calculations
// Calculate Gas Used in PSI
var gasUsedPsi = startPsi – endPsi;
// Calculate Average Depth in Atmospheres Absolute (ATA)
// Formula: (Depth in feet / 33) + 1
var ata = (avgDepth / 33) + 1;
// Calculate SAC Rate (PSI/min normalized to surface)
// Formula: (Gas Used PSI / Time) / ATA
var sacRate = (gasUsedPsi / time) / ata;
// Calculate Tank Factor (Cubic feet per PSI)
var tankFactor = tankVol / workPress;
// Calculate RMV (Cubic feet per minute)
// Formula: SAC Rate * Tank Factor
var rmv = sacRate * tankFactor;
// 4. Update UI
document.getElementById('resGasUsed').innerHTML = gasUsedPsi.toFixed(0) + " PSI";
document.getElementById('resATA').innerHTML = ata.toFixed(2) + " ATA";
document.getElementById('resSAC').innerHTML = sacRate.toFixed(1) + " PSI/min";
document.getElementById('resRMV').innerHTML = rmv.toFixed(2) + " cu ft/min";
// Show results container
document.getElementById('sacResult').style.display = "block";
}
Why Calculate Your SAC & RMV Rates?
One of the hallmarks of an experienced scuba diver is the ability to accurately predict gas consumption. While new divers often rely solely on their SPG (Submersible Pressure Gauge) during a dive, advanced divers, divemasters, and technical divers calculate their Surface Air Consumption (SAC) rate and Respiratory Minute Volume (RMV) to plan dives safely.
SAC Rate vs. RMV: What is the difference?
While these terms are often used interchangeably, they refer to slightly different metrics depending on the context of the dive planning:
- SAC Rate (PSI/minute): This measures how much pressure drops in your specific tank per minute, adjusted as if you were at the surface. It is useful only if you continue to use the same size tank. If you switch from an Aluminum 80 to a Steel 100, your SAC rate in PSI will change because the internal volume of the tanks differs.
- RMV (Cubic Feet/minute): This measures the actual volume of gas you breathe per minute. This is the "Gold Standard" metric because it is independent of the tank. Whether you are using a tiny pony bottle or double 100s, your lungs consume the same volume of gas.
How to Use These Numbers for Dive Planning
Once you know your RMV (e.g., 0.6 cu ft/min), you can estimate exactly how much gas you need for any dive depth and duration.
Example Scenario:
You plan to dive to 66 feet (3 ATA) for 30 minutes. Your RMV is 0.6 cu ft/min.
Gas Needed = RMV × Pressure (ATA) × Time
Gas Needed = 0.6 × 3 × 30 = 54 cubic feet.
Knowing this, you can determine if an Aluminum 80 (which holds roughly 77 cubic feet) provides enough reserve gas for the dive.
Factors Affecting Air Consumption
If your calculated rate seems high, consider these factors which often increase air consumption:
- Buoyancy Control: Struggling to maintain neutral buoyancy wastes energy and oxygen.
- Workload: Swimming against a current or kicking hard increases metabolic demand.
- Cold: The body burns calories (and oxygen) to generate heat in cold water.
- Stress: Anxiety leads to rapid, shallow breathing (hyperventilation).
- Depth: While SAC/RMV normalizes for depth, the psychological effect of deep water can sometimes alter breathing patterns.
We recommend tracking your air consumption over 10-20 dives in different conditions to find your reliable average. Use this calculator after every dive to log your performance.