Vacuum Leak Rate Calculator

Calculate the leak rate of a vacuum chamber using the pressure rise method. Enter your system volume, pressure differential, and test duration below.

Understanding Vacuum Leak Rate Calculation

Maintaining vacuum integrity is critical in semiconductor manufacturing, mass spectrometry, and surface science. A "leak" allows gas to enter a vacuum system, causing the pressure to rise and potentially contaminating the process. The Pressure Rise Method (or Vacuum Decay Test) is the most common way to quantify this leak rate without using tracer gases like helium.

The Formula

The leak rate ($Q$) is calculated based on the Ideal Gas Law. Assuming the temperature remains constant during the test, the formula is:

Q = (V × ΔP) / t

Where:

• Q = Leak Rate (throughput of gas)
• V = Volume of the vacuum chamber and connected piping
• ΔP = Pressure Rise (Final Pressure – Initial Pressure)
• t = Duration of the isolation test

How to Perform the Pressure Rise Test

1. Evacuate: Pump the system down to its base pressure.
2. Isolate: Close the valve connecting the pump to the chamber. The chamber is now isolated.
3. Measure Initial: Record the pressure immediately ($P_1$) and start a timer.
4. Wait: Allow the system to sit for a set duration ($t$). This allows gas from leaks or outgassing to accumulate.
5. Measure Final: Record the pressure ($P_2$) at the end of the duration.
6. Calculate: Input the values into the calculator above.

Interpreting the Results

It is important to distinguish between a real leak (air entering from outside) and outgassing (desorption of gas from internal surfaces). Both cause pressure rise.

• Real Leaks: The pressure rise is usually linear over time.
• Outgassing: The rate of pressure rise often decreases over time as the surfaces deplete their adsorbed gases.

To differentiate, you may need to perform the test again after a longer pump-down time. If the calculated $Q$ decreases significantly, outgassing is likely the dominant factor.

Common Unit Conversions

Vacuum technology uses various units depending on the region and industry. This calculator provides the following conversions automatically:

• mbar·L/s: Common in Europe.
• Pa·m³/s: The SI standard unit (1 mbar·L/s = 0.1 Pa·m³/s).
• Torr·L/s: Common in the USA (1 mbar·L/s ≈ 0.75 Torr·L/s).
• sccm: Standard Cubic Centimeters per Minute (1 mbar·L/s ≈ 59.2 sccm).

function calculateLeakRate() { // Get inputs var volume = document.getElementById('sysVolume').value; var time = document.getElementById('testTime').value; var p1 = document.getElementById('initPressure').value; var p2 = document.getElementById('finalPressure').value; var errorDiv = document.getElementById('vlrError'); var resultsDiv = document.getElementById('vlrResults'); // Reset display errorDiv.style.display = 'none'; resultsDiv.style.display = 'none'; // Validation if (volume === "" || time === "" || p1 === "" || p2 === "") { errorDiv.innerText = "Please fill in all fields."; errorDiv.style.display = 'block'; return; } var volNum = parseFloat(volume); var timeNum = parseFloat(time); var p1Num = parseFloat(p1); var p2Num = parseFloat(p2); if (isNaN(volNum) || isNaN(timeNum) || isNaN(p1Num) || isNaN(p2Num)) { errorDiv.innerText = "Please enter valid numbers."; errorDiv.style.display = 'block'; return; } if (volNum <= 0 || timeNum <= 0) { errorDiv.innerText = "Volume and Time must be greater than zero."; errorDiv.style.display = 'block'; return; } // Delta P Calculation var deltaP = p2Num – p1Num; if (deltaP < 0) { errorDiv.innerText = "Final Pressure must be greater than Initial Pressure for a leak test."; errorDiv.style.display = 'block'; return; } // Calculation Q = (V * dP) / t // Base unit: mbar * L / s var q_mbarls = (volNum * deltaP) / timeNum; // Conversions // 1 mbar L/s = 0.1 Pa m3/s var q_pams = q_mbarls * 0.1; // 1 mbar L/s = 0.750062 Torr L/s var q_torrls = q_mbarls * 0.750062; // 1 mbar L/s = 59.21 sccm (Standard T=20C, P=1013.25 mbar) var q_sccm = q_mbarls * 59.21; // Formatting Helper function formatSci(num) { if (num === 0) return "0"; if (num 1000) { return num.toExponential(3); } return num.toFixed(4); } // Display Results document.getElementById('resDeltaP').innerText = formatSci(deltaP) + " mbar"; document.getElementById('resQ_mbar').innerText = formatSci(q_mbarls); document.getElementById('resQ_pa').innerText = formatSci(q_pams); document.getElementById('resQ_sccm').innerText = formatSci(q_sccm); document.getElementById('resQ_torr').innerText = formatSci(q_torrls); resultsDiv.style.display = 'block'; }