Accurate leak testing is critical for industrial safety, environmental protection, and quality assurance in manufacturing. Whether you are testing HVAC systems, automotive components, or medical devices, understanding the flow rate of a gas leak is essential. This calculator utilizes the Pressure Decay Method, widely regarded as the standard for non-destructive leak testing.
The Pressure Decay Formula
The leak rate ($Q$) is calculated based on the change in pressure over a specific time duration within a fixed volume. The governing formula derived from the Ideal Gas Law is:
Q = (V × ΔP) / (t × P_std)
Where:
V = Internal Volume of the part or system under test.
ΔP = Pressure Drop ($P_{start} – P_{end}$) observed during the test.
t = Time duration of the measurement phase.
P_std = Standard Atmospheric Pressure (used to normalize the flow rate to standard conditions).
Key Metrics Explained
SCCM (Standard Cubic Centimeters per Minute)
SCCM is the most common unit for quantifying gas leaks. It represents the volume of gas flow normalized to standard temperature and pressure conditions. This allows engineers to compare leak rates regardless of the testing pressure or temperature.
Pressure Drop (ΔP)
This is the difference between the initial pressure at the start of the test and the final pressure at the end. A larger pressure drop over a short time indicates a larger leak (or a very small volume).
Factors Affecting Accuracy
When performing pressure decay leak testing, consider these variables to ensure accurate results:
Temperature Stability: According to the Ideal Gas Law ($PV=nRT$), a change in temperature will cause a change in pressure even if no leak exists. Ensure the part has stabilized to ambient temperature before testing.
Volume Estimation: The calculated leak rate is directly proportional to the volume. If you underestimate the internal volume of the piping or vessel, the calculated leak rate will be artificially low.
Seal Creep: In flexible parts, the expansion of seals or hoses under pressure can mimic a leak (pressure drop) or mask a leak.
Typical Acceptable Leak Rates
Waterproof Electronics: ~1 to 5 SCCM.
Automotive Fuel Systems: ~10 to 20 SCCM.
Industrial Casting: ~50 to 100 SCCM depending on the fluid application.
Gas Piping: Often requires near-zero measurable decay over long durations (e.g., 24 hours).
function calculateLeakRate() {
// 1. Get Input Values
var volInput = document.getElementById('systemVolume').value;
var p1Input = document.getElementById('startPressure').value;
var p2Input = document.getElementById('endPressure').value;
var timeInput = document.getElementById('testTime').value;
// 2. Get Selected Units
var volUnit = document.getElementById('volumeUnit').value;
var pressUnit = document.getElementById('pressureUnit').value;
var timeUnit = document.getElementById('timeUnit').value;
// 3. UI Elements
var errorDisplay = document.getElementById('errorDisplay');
var resultsArea = document.getElementById('resultsArea');
// Reset UI
errorDisplay.style.display = 'none';
resultsArea.style.display = 'none';
// 4. Validation
if (volInput === "" || p1Input === "" || p2Input === "" || timeInput === "") {
errorDisplay.innerText = "Please fill in all fields.";
errorDisplay.style.display = 'block';
return;
}
var V = parseFloat(volInput);
var P1 = parseFloat(p1Input);
var P2 = parseFloat(p2Input);
var t = parseFloat(timeInput);
if (V <= 0 || t P1) {
errorDisplay.innerText = "Final Pressure cannot be higher than Initial Pressure (unless the system is gaining pressure, which is not a leak).";
errorDisplay.style.display = 'block';
return;
}
// 5. Normalization (Convert everything to a Base System)
// Base System: Volume = Liters, Pressure = Atmospheres, Time = Minutes
// Convert Volume to Liters
var volInLiters = 0;
switch(volUnit) {
case 'liters': volInLiters = V; break;
case 'gallons': volInLiters = V * 3.78541; break;
case 'cc': volInLiters = V / 1000; break;
case 'cf': volInLiters = V * 28.3168; break;
case 'm3': volInLiters = V * 1000; break;
}
// Convert Pressures to Atmospheres (atm)
// 1 atm = 14.696 psi = 1.01325 bar = 101325 Pa = 101.325 kPa
var p1InAtm = 0;
var p2InAtm = 0;
function toAtm(val, unit) {
switch(unit) {
case 'atm': return val;
case 'psi': return val / 14.6959;
case 'bar': return val / 1.01325;
case 'pa': return val / 101325;
case 'kpa': return val / 101.325;
default: return val;
}
}
p1InAtm = toAtm(P1, pressUnit);
p2InAtm = toAtm(P2, pressUnit);
// Convert Time to Minutes
var timeInMinutes = 0;
switch(timeUnit) {
case 'minutes': timeInMinutes = t; break;
case 'seconds': timeInMinutes = t / 60; break;
case 'hours': timeInMinutes = t * 60; break;
}
// 6. Calculation Logic
// Formula: Leak Rate (Std L/min) = [Volume(L) * (P1(atm) – P2(atm))] / Time(min)
// This assumes Standard Pressure is 1 atm.
// Q_std = (V * deltaP) / t (since P_std is 1 atm in these units)
var deltaP_Atm = p1InAtm – p2InAtm;
var leakRate_SLPM = (volInLiters * deltaP_Atm) / timeInMinutes;
// 7. Convert Results to specific output units
// SCCM (Standard Cubic Centimeters per Minute) = SLPM * 1000
var sccm = leakRate_SLPM * 1000;
// CFM (Cubic Feet per Minute)
// 1 L/min = 0.0353147 ft3/min
var cfm = leakRate_SLPM * 0.0353147;
// Yearly Loss (Estimate in cubic meters)
// SLPM * 60 (min/hr) * 24 (hr/day) * 365 (day/yr) / 1000 (L to m3)
var yearlyM3 = (leakRate_SLPM * 60 * 24 * 365) / 1000;
// Delta P in original unit
var deltaP_Original = P1 – P2;
// 8. Display Results
document.getElementById('resDeltaP').innerText = deltaP_Original.toFixed(3) + " " + pressUnit;
document.getElementById('resSCCM').innerText = sccm.toFixed(2);
document.getElementById('resLPM').innerText = leakRate_SLPM.toFixed(4) + " SLPM";
document.getElementById('resCFM').innerText = cfm.toFixed(5);
// Format yearly loss slightly differently based on magnitude
if(yearlyM3 < 1) {
document.getElementById('resYearly').innerText = (yearlyM3 * 1000).toFixed(2) + " Liters / Year";
} else {
document.getElementById('resYearly').innerText = yearlyM3.toFixed(2) + " m³ / Year";
}
resultsArea.style.display = 'block';
}