Sac Rate Calculator

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SAC Rate Calculator (Specific Air Consumption Rate)

The Specific Air Consumption (SAC) rate is a crucial metric for scuba divers. It measures how much air you consume per minute at a given depth, adjusted for surface equivalent pressure. Understanding your SAC rate helps you better manage your air supply during dives, plan dive times, and improve your overall safety and enjoyment underwater. A lower SAC rate generally indicates better air conservation techniques.

The formula for calculating SAC rate is:

SAC Rate = (Starting Cylinder Pressure – Ending Cylinder Pressure) / (Time in Minutes)

However, to compare air consumption across different depths, we need to convert the pressure readings to their surface equivalents. This is done using the depth. Every 10 meters (or 33 feet) of depth adds one atmosphere (ATA) of pressure to the ambient pressure. So, at 10 meters, the pressure is 2 ATA (1 ATA from the surface + 1 ATA from the water), at 20 meters it's 3 ATA, and so on.

The formula for Surface Equivalent Air Consumption (SAC) Rate is:

SAC Rate (L/min or cu ft/min) = [(Starting Pressure – Ending Pressure) / Pressure at Depth] / Time in Minutes

Where:

  • Starting Pressure: The pressure in your cylinder at the start of the measurement (e.g., in PSI or Bar).
  • Ending Pressure: The pressure in your cylinder at the end of the measurement (e.g., in PSI or Bar).
  • Pressure at Depth: The absolute pressure at your current depth. This is calculated as 1 ATA (atmospheric pressure at the surface) + (Depth in meters / 10) or 1 ATA + (Depth in feet / 33).
  • Time in Minutes: The duration of the measurement in minutes.

Calculate Your SAC Rate

Meters Feet
function calculateSACRate() { var startPressure = parseFloat(document.getElementById("startPressure").value); var endPressure = parseFloat(document.getElementById("endPressure").value); var depth = parseFloat(document.getElementById("depth").value); var depthUnit = document.getElementById("depthUnit").value; var timeMinutes = parseFloat(document.getElementById("timeMinutes").value); var resultDiv = document.getElementById("result"); resultDiv.innerHTML = ""; // Clear previous results if (isNaN(startPressure) || isNaN(endPressure) || isNaN(depth) || isNaN(timeMinutes)) { resultDiv.innerHTML = "Please enter valid numbers for all fields."; return; } if (startPressure <= endPressure) { resultDiv.innerHTML = "Starting cylinder pressure must be greater than ending cylinder pressure."; return; } if (timeMinutes <= 0) { resultDiv.innerHTML = "Time must be greater than zero."; return; } var pressureAtDepth; if (depthUnit === "meters") { pressureAtDepth = 1 + (depth / 10); } else { // feet pressureAtDepth = 1 + (depth / 33); } var pressureDifference = startPressure – endPressure; var sacRate = (pressureDifference / pressureAtDepth) / timeMinutes; // Displaying the result, assuming pressure is in PSI. If Bar, the interpretation might differ. // For simplicity, we'll display the raw rate. A more advanced calculator might include unit conversion. resultDiv.innerHTML = "Your SAC Rate is approximately: " + sacRate.toFixed(2) + " PSI/min"; }

Understanding Your SAC Rate

Once you have your SAC rate, you can use it for several practical applications:

  • Air Planning: Knowing your typical SAC rate at various depths allows you to estimate how long your air supply will last. For example, if you have a 2000 PSI tank and your SAC rate at 20 meters is 30 PSI/min, and you need to stay at that depth for 30 minutes, you'd consume 30 PSI/min * 30 min = 900 PSI. You'd also need to factor in air for ascent and safety stops.
  • Dive Time Calculation: You can work backward to determine the maximum time you can safely spend at a particular depth based on your air supply and SAC rate.
  • Improving Air Consumption: If your SAC rate is higher than average, it's an indicator that you might be able to improve your buoyancy control, reduce unnecessary finning, and relax more effectively underwater. Practicing these skills can lead to a lower SAC rate and longer, safer dives.

Example Calculation:

Let's say you complete a dive:

  • Starting Cylinder Pressure: 3000 PSI
  • Ending Cylinder Pressure: 1500 PSI
  • Depth: 15 meters
  • Time of Measurement: 20 minutes

First, calculate the pressure at depth:

Pressure at Depth = 1 + (15 meters / 10) = 1 + 1.5 = 2.5 ATA

Now, calculate the SAC Rate:

SAC Rate = [(3000 PSI – 1500 PSI) / 2.5 ATA] / 20 minutes

SAC Rate = [1500 PSI / 2.5 ATA] / 20 minutes

SAC Rate = 600 PSI / 20 minutes

SAC Rate = 30 PSI/min

This means that, on average, this diver consumes 30 PSI of air for every minute spent at 15 meters depth (when adjusted for surface equivalent pressure).

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