Pressure Decay Leak Rate Calculation

Pressure Decay Leak Rate Calculator

Results:

Pressure Drop: PSI

Leak Rate (sccs):

Leak Rate (cm³/min):

function calculateLeakRate() { var volume = parseFloat(document.getElementById('systemVolume').value); var time = parseFloat(document.getElementById('testTime').value); var p1 = parseFloat(document.getElementById('initialPressure').value); var p2 = parseFloat(document.getElementById('finalPressure').value); var resultDiv = document.getElementById('leakResult'); if (isNaN(volume) || isNaN(time) || isNaN(p1) || isNaN(p2) || time <= 0) { alert("Please enter valid positive numbers for all fields."); return; } var deltaP = p1 – p2; if (deltaP < 0) { alert("Initial pressure must be higher than final pressure."); return; } // Formula: Q = (V * ΔP) / (t * Patm) // Patm = 14.7 PSI for Standard Cubic Centimeters per Second (sccs) var atmPressure = 14.7; var leakSccs = (volume * deltaP) / (time * atmPressure); var leakCmMin = (volume * deltaP * 60) / (time * atmPressure); document.getElementById('resDrop').innerText = deltaP.toFixed(4); document.getElementById('resSccs').innerText = leakSccs.toFixed(6); document.getElementById('resCmMin').innerText = leakCmMin.toFixed(4); resultDiv.style.display = 'block'; }

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Understanding Pressure Decay Leak Testing

Pressure decay testing is one of the most widely used methods for non-destructive leak detection in manufacturing and engineering. It measures the drop in pressure within a sealed object over a specific period. This method is preferred for its speed, accuracy, and ability to be easily automated on production lines.

How the Calculation Works

The fundamental principle behind this calculator is the Ideal Gas Law. When the temperature remains constant, any loss in pressure within a fixed volume must be attributed to a loss of mass (a leak). The formula used to determine the leak rate in Standard Cubic Centimeters per Second (sccs) is:

Q = (V × ΔP) / (t × P_atm)

• Q: Leak rate (sccs)
• V: Internal volume of the part and test circuit (cm³)
• ΔP: Pressure change (Initial Pressure – Final Pressure)
• t: Test duration (seconds)
• P_atm: Atmospheric pressure (typically 14.7 PSI)

Standard Units of Leak Rate

In the industry, leak rates are commonly expressed in:

• sccs: Standard Cubic Centimeters per Second. This is the volume of gas leaking per second, corrected to standard atmospheric conditions.
• cm³/min (or sccm): Standard Cubic Centimeters per Minute. Often used for larger allowable leak rates.
• Pa·m³/s: The SI unit for leak rates, primarily used in scientific research.

Realistic Example Calculation

Imagine you are testing an automotive fuel tank with an internal volume of 1,200 cm³. You pressurize the tank to 5.000 PSI. After a test time of 20 seconds, the pressure drops to 4.992 PSI.

Step 1: Calculate ΔP = 5.000 – 4.992 = 0.008 PSI.

Step 2: Apply the formula: Q = (1200 × 0.008) / (20 × 14.7).

Step 3: Result: Q = 9.6 / 294 = 0.0326 sccs.

Factors Affecting Accuracy

Several variables can influence the results of a pressure decay test:

1. Temperature: Even a slight change in temperature during the test can cause pressure fluctuations that look like leaks or mask real leaks.
2. Volume Accuracy: You must include the volume of the hoses and connectors in your total system volume calculation.
3. Part Deformation: If the part being tested is flexible (like a plastic bottle), it may expand under pressure, causing a "false" pressure drop as the volume increases.
4. Stabilization Time: Air generates heat when compressed. You must allow a "stabilization" period for the air to cool to ambient temperature before starting the measurement timer.