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Change the answer mode for this tool by selecting pressure loss (ΔP), test volume (V), test duration (t) or leak rate (QL) as the parameter to calculate instead
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User Guide
This tool will calculate any parameter from the formula for leakage rate for a pressurised enclosed space Q=ΔPV/t, which includes the difference in pressure from the start to the finish of the leak test, the total volume of the space being tested, the elapsed time of the leak test, and the average leakage rate.
Formulas
The formulas used by this leakage rate calculator to determine each individual parameter are:
ΔP = QL · t / V
V = QL · t / ΔP
t = ΔP · V / QL
QL = ΔP · V / t
Symbols
- ΔP = Pressure loss
- V = Test volume
- t = Test duration time
- QL = Leak rate
Pressure Loss (ΔP)
This is how much the pressure changes during a leak test.
Test Volume (V)
This is the total internal volume of the vessel being tested.
Test Duration Time (t)
This is the time taken for the pressure to drop during a leak test.
Leak Rate (QL )
This is a measure of the rate of gas loss from a test volume over the duration of the leak test. It is intended to provide a measure of the proportion of the total gas that has escaped, so that the same pressure loss for a larger sized volume would result in a larger leak rate value.
The following leak rate units can be used with this calculator:
- millibar litres per second (mbar·L/s)
- pascal cubic metres per second (Pa·m³/s)
- pascal litres per second (Pa·L/s)
- standard litres per minute (atm·L/min, slm, slpm)
- standard cubic millimetres per second (atm·mm³/s)
- standard cubic centimetres per second (atm·cm³/s, sccs)
- standard cubic centimetres per minute (atm·cm³/min, sccm)
- standard cubic centimetres per hour (atm·cm³/h)
- standard cubic metres per minute (atm·m³/min)
- standard cubic feet per minute (atm·ft³/min)
- standard cubic feet per year (atm·ft³/yr)
- micron of mercury litres per second (µHg·L/s, lusec)
- micron of mercury cubic feet per minute (µHg·ft³/min, µHg·cfm)
- micron of mercury cubic feet per hour (µHg·ft³/h)
- torr litres per second (Torr·L/s)
Help
Inclusion of atmospheric pressure in leak rate calculations
When calculating leak rate using the formula QL = (ΔP ⋅ V)/t, do I need to account for atmospheric pressure? My manual calculations don’t match standard leak rate calculators when I use units like SCCM.
This is a frequent point of confusion that stems from the difference between the fundamental physical formula for leak rate and the definitions of standardized volumetric flow units commonly used in industry.
The formula QL = (ΔP ⋅ V)/t is the correct fundamental equation for determining leak rate based on the pressure decay method. When using consistent base units (e.g., Pascals for pressure, cubic meters for volume, and seconds for time), the resulting leak rate QL is expressed in absolute units like Pa⋅m3/s. In this form, atmospheric pressure is not a required parameter.
The discrepancy arises when you need to express the leak rate in standardized volumetric flow units, such as SCCM (Standard Cubic Centimeters per Minute) or SLM (Standard Liters per Minute). The key term here is “Standard.” These units normalize the volume of gas that has leaked to standard conditions, specifically standard atmospheric pressure (1 atm, which is 101325 Pa or approximately 14.7 psi).
Therefore, to correctly calculate a leak rate in a standardized unit like SCCM, the pressure loss value (ΔP) used in the formula must also be expressed in the corresponding standard pressure unit, which is atmospheres (atm).
If you measure the pressure loss (ΔP) in a different unit, such as Pascals (Pa), millibars (mbar), or pounds per square inch (psi), you must first convert this value into atmospheres before applying the formula. This is achieved by dividing your measured pressure loss by the value of one standard atmosphere in those same units.
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