A leak test is performed to verify that a component intended for use with a gas or liquid is free of defects that could cause the gas or liquid it contains to leak. This test is particularly important when the component being tested is intended for use with explosive, flammable, or toxic gases or liquids, or is used in environments where a defect could lead to hazardous contamination.

In non-destructive testing, a leak is a porous area, a hole, a gas-permeable area, or another structure in the walls of a test object through which solids, liquids, or gases can enter or escape unintentionally due to a difference in pressure or concentration.

A leak test can detect:

• Molding defects, such as cracks and porosity
• Welding defects
• Adhesive defects
• Assembly errors (missing gaskets, lack of adhesives, for example)
• Defects in technical components (e.g., cut sealing lips, scratches on sliding rods)

Yes, leak tests can also be performed manually, and examples of manual methods for detecting leaks include the following:

• Immersion of the part, pressurized internally, in water to check for the formation and/or emergence of bubbles
• Wetting the exterior of the internally pressurized component with soapy solutions to check for the formation of bubbles
• Use of ultrasonic probes to identify the area where gas released into the atmosphere generates ultrasonic waves
• Use of fluorescent tracer gases to pressurize the component and UV lamps to detect fluorescence

These manual methods are not capable of measuring the leak but do allow for its location to be identified.

The technologies used to perform an automatic leak test can vary depending on the component being tested and the magnitude of the leaks to be detected.

• Measurement of absolute pressure decay after pressurizing the test part
• Measurement of differential pressure drop, after pressurizing the test part and a reference sample known to be leak-free in parallel
• Measurement of direct flow using a flow sensor,
• Measurement of the pressure increase in a containment volume and collection of the gas used to pressurize the test part
• Pressurization with helium or another tracer gas and monitoring via mass spectrometer to detect the escape of gas molecules
• Pressurization with ionized gas and monitoring of conductivity between the interior and exterior of the component using

There are also specialized techniques for testing small, fully sealed components that involve the use of gases such as Krypton-85, a radioactive isotope: the component is placed in a small airtight cavity; the volume inside the chamber and outside the component under test is pressurized with the gas for a
certain period of time; the component is then removed and its radioactivity is monitored, which will be greater the more gas has managed to penetrate inside.

In the absolute pressure decay test, the system measures and evaluates the relative pressure drop (or vacuum) (i.e., relative to ambient barometric pressure) inside the previously pressurized test specimen. 
With the differential pressure decay test, however, the system measures and evaluates the differential pressure (or vacuum) between that present inside the test specimen and a second reference channel, both of which have been previously pressurized to the test pressure. 
The first system has an insurmountable technical limitation: as the test pressure increases, the system’s resolution and sensitivity decrease, since no transducers or converters exist capable of providing the necessary resolutions; for example, to ensure a resolution of 0.001 mbar at 8 bar, 8 million resolution points would be required—equivalent to 23 bits of actual resolution and stability. 
This limitation does not apply to the differential solution, since even at high common-mode pressures (i.e., the pressure to which both the test specimen and the reference channel are simultaneously brought), differential pressure transmitters with a very low measurement range—and thus high resolution—can be used; for example, one could easily operate at 10 bar with a differential transmitter having a full-scale range of 20 mbar, which guarantees a measurement resolution of 0.001 mbar.

Selecting the correct connecting hose is critical for optimizing the performance of the leak test system.
Always use high-pressure hoses made of RILSAN PA12 or a material with superior performance, and avoid extra-flexible polyurethane or silicone hoses.
At low pressures, the pressure drop caused by the hose could significantly reduce the air flow and result in long fill times; in such cases, if the hose’s volume is not significant compared to that of the test piece, it is advisable to increase the hose size to achieve a higher air flow rate.
Similarly, at high pressures, it will often be possible to use hoses with a smaller cross-sectional area.

For a given volume of leakage, the measurable pressure drop is inversely proportional to the volume of the component under test. Therefore, detecting leaks is straightforward in small-volume parts, whereas it becomes difficult and, in some cases, ineffective as the volume increases, since extending the measurement time to evaluate the pressure drop over a longer time interval also results in a greater influence of heat exchange phenomena that can alter the test result.

Temperature has a significant effect on the behavior of gases, which are governed by the ideal gas law:

P × V = N × R × T

where T denotes the absolute temperature expressed in degrees Kelvin.
A temperature change from 20 °C to 20.2 °C, or from 293.15 K to 293.35 K, under conditions of constant volume and gas moles results in a pressure increase of 0.068%, which might seem negligible, but at 1 bar this amounts to 0.68 mbar, a value much greater than the resolution at which many leak detectors operate and therefore capable of altering the test results.
With the data from the example, a small leak of 0.68 mbar could be completely masked by a temperature increase of just 0.2 °C.

Generally, a component that must meet watertightness specifications includes the following requirements in the drawings:

• Test pressure
• Measurement time
• Maximum leakage limit, expressed as pressure drop or volumetric leakage

When expressed as a pressure drop, the limit is typically given in Pa, Pa/s, Pa/min, mbar, or mbar/s—that is, in a pressure-time unit of measurement. When expressed as a volumetric flow rate, the limit is typically given in cc/min, cc/h, ml/min, ml/h, l/min, or l/h—that is, in a volume-time unit of measurement. In Anglo-Saxon countries, it is very common to express flow rates in scfm, or Standard Cubic Feet per Minute.

There is no difference: in the International System (SI), the unit of volume is the cubic meter (m³), but non-SI units such as the liter (l) and its submultiple, the milliliter (ml), are also accepted.
cc and ml are equivalent because cc stands for cubic centimeter (cm³), and therefore 1 ml = 1 cm³.

The abbreviations scc and ncc stand for Standard Cubic Centimeter and Normal Cubic Centimeter, respectively, and refer to the standard conditions: STP or NTP.
The standard conditions STP (Standard Temperature and Pressure) for a gas are defined by IUPAC and are 0 °C (273.15 K) and 1 bar (100 kPa).
The most recent definition of normal conditions (NTP) established by NIST, however, specifies 20 °C (293.15 K) and 1 bar (100 kPa).
Therefore, scc or ncc refers to a volume of 1 cm³ of gas under STP conditions (0 °C / 1 bar) or NTP conditions (20 °C / 1 bar).
There are also other standard reference conditions: ISO 13443 specifies 15 °C (288.15 K) and 1 atmosphere (101.325 kPa), while standard ambient conditions (SATP) are often used in chemistry and are defined as 25 °C (298.15 K) and 1 bar
(100 kPa).

Converting a volumetric flow rate to the equivalent pressure drop (or vice versa) requires knowledge of the total volume of the test circuit:

F ml/min = 0.0006 × V cm³ × Δp/Δt Pa/s
Δp/Δt Pa/s = F ml/min / (0.0006 × V cm³)

Therefore, a leak of 1 ml/minute from a volume of 1 liter (1000 cm³) results in a pressure drop of:

1 ml/min / (0.0006 × 1000 cm³) = 1.667 Pa/s = 100 Pa/minute = 1 mbar/minute

Our converter can do the calculation for you!