Glossary

Here you can find explanations of important technical expressions on the topic of leak testing.

Zeltwanger - Leak testing glossary

Absolute pressure

(symbol p; unit Pa)

Pressure indicated by an absolute pressure gauge.

[According to DIN-EN 1330-8]

Absolute pressure gauge

Instrument measuring pressure with reference to a zero, corresponding to the pressure of empty space.

[According to DIN-EN 1330-8]

Accumulation test

Leakage test where a tracer gas is accumulated in a known volume. After a specified period of time, the partial pressure increase of the tracer gas is measured. The leakage rate can be obtained by calibrating the partial pressure rise with that due to a known leak or comparing the measured concentration with a known concentration.

[According to DIN-EN 1330-8]

Aperture leak (orifice)

Conductance leak having a diameter much greater than the leakage path length.

NOTE: It may be considered as an opening in a very thin wall.

[According to DIN-EN 1330-8]

Atmospheric pressure

Pressure of the atmosphere at a specified place and time (standard atmospheric pressure is 101325 Pa).

[According to DIN-EN 1330-8]

Back pressurizing test

(bombing test; pressurizing-evacuation test)

Test in which sealed objects are subject to bombing prior to testing them in a vacuum chamber.

[According to DIN-EN 1330-8]

Balancing phase

See also:
Calming phase

Bell

See also:
Tight chamber

Bell test

See also:
Hood test

Bombing

See also:
Back pressurizing test

Bubble test

Test used to detect leaks by the immersion of the object in a test fluid or covering its outer surface with a surfactant (foaming) solution. The pressure difference across the walls of the object is high enough that leaks are shown by bubble formation.

[According to DIN-EN 1330-8]

Calibration

The setting of the sensitivity or response factor of a leak detector, so that the display change is equal to the leakage rate of a test leak.

Calibration leak

Device which provides a known and repeatable gas flow rate under specified conditions, used for the setting of leak detectors.

[According to DIN-EN 1330-8]

Calming phase

With a leak test using a pressure change method the calibration phase allows the system the necessary time to minimize undesirable contributions with respect to the pressure change which can be attributed to an effective leak. Typical disturbances are the thermal effects due to inhomogeneity of the temperature in the gas and the test item, and the physical effects due to evaporation or condensation of vapors in the test item. The duration of the calibration phase is strongly dependent on the test item and the ambient conditions and is usually determined empirically.

Capillary leak

Feed leak, wherein the diameter in relation to the length is small.

Chamber test

See also:
Hood test

Concentration

(symbol nB)

Ratio of the number of atoms or molecules of a given constituent in a mixture to the total number of atoms or molecules. For gases this is equivalent to the volume fraction

(symbol c).

[According to DIN-EN 1330-8]

Conductance

(for fluid flow in a duct, or part of a duct, or an orifice) (symbol C, U; unit m3/s)

Throughput divided by the difference in mean pressures prevailing at two specified cross sections or at either sides of the duct or orifice, assuming isothermal conditions.

[According to DIN-EN 1330-8]

Conductance leak

Leak which consists of one or more discrete passages, including porous areas, through which a fluid may flow.

[According to DIN-EN 1330-8]

Counterflow leak detector

Mass spectrometer leak detector where the tracer gas enters through the backing line into the outlet port of the high vacuum pump and the partial pressure of the tracer gas is measured on the high vacuum side of this pump.

[According to DIN-EN 1330-8]

Dead volume

Volume of the leak test system adding to the volume of the test object.

[According to DIN-EN 1330-8]

Direct flow leak detector

Mass spectrometer leak detector where the tracer gas from the inlet line enters the high vacuum side of the pump system and the partial pressure of tracer gas is measured on the high vacuum side of the pump.

[According to DIN-EN 1330-8]

Filling

See also:
Filling phase

Filling phase

In this phase the test item is filled to a level equivalent to the test pressure. The duration of the filling process (filling time) is dependent on the volume of the test item and the pressure.

The filling phase may be preceded by a pre-filling phase. This preparation phase serves to pre-stress and mechanically stabilize the test item and to minimise the thermal effects of the test gas. Duration and pressure value are determined empirically. Typically the pre-filling pressure is 10-20% higher than the filling pressure.

Filling pressure

See also:
Filling phase

Filling time

See also:
Filling phase

Flow rate

(Mass: symbol: qM; unit kg/s

Particles: symbol: qN ; unit s?1

Molar: symbol: qv; unit mol/s)

The rate at which a quantity of gas passes through a given cross section of the system. The flow rate gives the number of moles, particles or amount of mass moving per unit time. The flow rate, already stated above, is expressed in moles per second, kilograms per second or s?1 (for particles).

For gases, the volume flow rate (Volume: symbol qV; unit m3/s) is a measure of quantity only at specified conditions.

[According to DIN-EN 1330-8]

Gauge pressure: pressure

Indicated by a pressure gauge measuring with reference to the atmospheric.

[According to DIN-EN 1330-8]

Helium leak detector

Leak detector that responds to helium (4He) tracer gas.

[According to DIN-EN 1330-8]

Hood test

Over-all test in which the object is placed in a soft enclosure at atmospheric pressure.

NOTE 1: If the object is under vacuum the enclosure is filled with tracer gas and the leak detector is connected to the inner volume of the object.

NOTE 2: If the object is pressurized with the tracer gas, the test is performed with the sampling probe inserted in the hood.

[According to DIN-EN 1330-8]

Ideal gas; perfect gas

Gas obeying the relationship pV = nRT exactly
where: n = m/M
p is the pressure;
V is the volume;
m is the mass of the gas;
M is the molecular mass;
R is the ideal gas constant;
T is the absolute gas constant.

[According to DIN-EN 1330-8]

Integral leak

See also:
Total (integral) leakage rate

Integral test

See also:
Total (integral) leakage rate

Leak

In NDT technology: hole, porosity, permeable element or other structure in the wall of an object capable of passing gas from one side of the wall to the other by the effect of pressure or concentration difference across the wall.

[According to DIN-EN 1330-8]

Leak calibrator

The term leak calibrator refers to a device in which the leakage rate can be adjusted using a precision needle valve depending on the pressure.

Similar to a test leak the unit is used for checking leak test apparatus / leak tightness test apparatus.

Leak rate

See also:
pV-throughput

Leak testing

(also known as leak tightness testing, leakage testing, leak testing or leak detection)

Leak testing is a nondestructive test method in which leaks in a test item are determined by the flow of gas or liquid through leaks in a test item and the obtained value, converted to the actual leakage rate under operating conditions, is compared with the maximum allowable leakage rate.

Leak testing is always a comparative measurement.

See also:
Leaktight

Leak testing - absolute pressure method

See also:
Leak testing - relative pressure method

Leak testing - differential pressure method

The differential method (DD method) is an improvement on the relative method. In this method the test item and an additional reference volume are filled with compressed air in the filling phase. A differential pressure sensor is connected between the test item and the reference volume. After the system is separated from the pressure supply and balanced the pressure difference between test item and the reference volume is determined over time. This value corresponds to the pressure change in the test item and from it the leakage rate can be calculated. Since the pressure difference between the test item and the reference volume is very small regardless of the test pressure, the pressure range of the differential pressure sensor can be much smaller than the test pressure. This enables a typical resolution of 0.1 Pa and even smaller in special cases.

The differential pressure method is used when high resolution is required at higher pressures. The technical structure is more complex than with the relative pressure method.

Leak testing - direct flow method

In the direct flow method (DF method) the test item is subject to pressure directly from a mass flow sensor. The gas flow through the leak is measured directly.
The process is very fast and particularly suitable for large leakage rates.

Leak testing - reference flow method

The reference flow method (MF method) is a variant of the differential pressure method which is particularly suitable for large volumes. If the test item is very large and the leak very small an extremely long measurement time is required to produce a detectable pressure drop. With a mass flow sensor (typically a thermal sensor),   a leak can be measured directly without waiting until a measurable change in pressure develops. The design is the same as with the differential pressure method, however, a mass flow sensor replaces the differential pressure sensor. Once the device is under test and the reference volume is filled, the mass flow sensor is activated (bypass valve). A leak in the test item will cause a compensation flow between the reference volume and the test item. This flow is immediately detected by the sensor. The measured mass flow is not exactly equivalent to the test item leakage; rather it is dependent on the ratio between the test item and the reference volume.

The resolution is determined by the sensitivity of the mass flow sensor.

Leak testing - relative pressure method

The relative pressure method (RD method) is the basis for all pressure change tests.   It allows a compact design and is characterized by high reliability as well as a wide measuring range. In this method the test item is filled to the pressure and is then shut off from the air supply using a valve. The pressure change over time is measured using a relative pressure sensor. A single sensor is used to measure the pressure change and the test pressure. The measuring range of the sensor corresponds to the maximum test pressure. The pressure change is dependent on the test volume and the test pressure. The pressure change becomes more prominent with decreasing test volume. The resolution of this method is dependent on the measurement range of the sensor (maximum test pressure). This method typically enables a resolution of 1 Pa.

Leak testing with air

A leak testing method using air as the test medium. Depending on the method of detection used reference is made to "pressure change test," a "bubble test" or "flow testing". 

Leakage rate

pV-throughput of a specific fluid which passes through a leak under specific conditions.

[According to DIN-EN 1330-8]

Leaktight

Free from leaks according to a given specification.

[According to DIN-EN 1330-8]

Mass spectrometer leak detector (MSLD)

Leak detector in which the sensing element is a mass spectrometer, adjusted to respond only to the tracer gas.

[According to DIN-EN 1330-8]

Measurement phase

In a leak test device (relative pressure or differential pressure method) the measuring phase is the time interval over which the pressure drop is measured.

 

The duration of this interval is dependent on the test item and the limit values:
The larger the volume, the more time is needed to determine a discernible pressure drop.
The smaller the leak, the more time is needed to determine a discernible pressure drop.
The higher the desired accuracy, the more time is needed to achieve a stable result.

In the leak testing with the flow method this phase corresponds to the time interval in which the flow rate is read. The duration of the phase is very short and it has a negligible effect on the measurement results.

Measurement time

See also:
Measurement phase

Minimum detectable leakage rate

Smallest leakage rate that an instrument, method, system is capable of detecting under test conditions.

[According to DIN-EN 1330-8]

Minimum detectable signal

Output signal, due to the incoming tracer gas which is equal to the sum of the noise and the drift of the instrument. The minimum detectable signalis given in units of the instrument output, e.g. scale divisions, voltage.

[According to DIN-EN 1330-8]

Mole fraction

See also:
Concentration

Molecular leak

Leak of such geometric configuration and under such pressure conditions that gas flowing through it obeys the laws of molecular flow.

[According to DIN-EN 1330-8]

Normalized leakage rate

Leakage rate of a leak under standard pressure at one end and, at the other end, a pressure low enough to have a negligible effect on the leakage rate. The temperature is stated.

[According to DIN-EN 1330-8]

Partial pressure

(symbols pA, pB)

Pressure that would be exerted by a gas or vapour if it alone was present in an enclosure.

[According to DIN-EN 1330-8]

Permeability

Property of a non porous material to allow a substance to pass through under a partial pressure difference by a mechanism of solution/diffusion.

[According to DIN-EN 1330-8]

Pre-filling phase

See also:
Filling phase

Pressure change test

Leakage test in which the rate of the total pressure change, decay or rise, in an object is measured.

[According to DIN-EN 1330-8]

Pressure drop method

See also:
Pressure change test

Pressure drop test

See also:
Pressure change test

Pressure dye test

Leakage test in which a liquid containing a dye or fluorescent oil is driven by a pressure differential into the leak in the wall of the object to be tested and then detected by a visual inspection on the other side.

[According to DIN-EN 1330-8]

pV-throughput

(symbol qG; unit Pa ·m3/s)

Rate at which a volume of gas at specified pressure passes a given cross section of the system.

NOTE: If pV-throughput ? as in common practice in leak detection ? is

used to characterize the flow rate of gas, the temperature and molar weight or density are given additionally so that the flow rate can be calculated using the gas equation.

[According to DIN-EN 1330-8]

Reference leak

See also:
Calibration leak
Leak calibrator

Relative pressure

Pressure which is measured relative to a known reference pressure (typically atmospheric pressure).

See also:
Gauge pressure: pressure

Response time

The time, from the beginning of application of tracer gas until the signal reaches 90% of the equilibrium signal, obtained when the tracer gas is continuously applied.

[According to DIN-EN 1330-8]

Settling phase

See also:
Calming phase

Stabilization phase

See also:
Calming phase

Standard (calibrated) leak

Calibration leak, the leakage rate of which can be traced to a fundamental leakage rate standard.

[According to DIN-EN 1330-8]

Test conditions

For leak testing, the actual conditions of ambient temperature and pressure under which the test is performed.

[According to DIN-EN 1330-8]

Test leak

See also:
Calibration leak
Leak calibrator

Test port

Flange placed at the beginning of the inlet line used to couple the test object to the leak detector.

[According to DIN-EN 1330-8]

Test time

The test time is the duration of the entire test process. The measurement procedure consists of the pre-filling, filling, balancing, measuring and venting  phases. The test time is the sum of the duration of all phases. It is not to be confused with the measurement time.

See also:
Measurement phase

Throughput

See also:
pV-throughput

Tight chamber

A sealed encasement which fully envelops the test item during a leak test which can be pressurised or evacuated to subject the skin of the test item to a pressure difference.

Total (integral) leakage rate

Sum of all leakage rates from all leaks of an object, expressed as pV-throughput.

[According to DIN-EN 1330-8]

Tracer fluid

Fluid (gas, liquid) which can be detected, after it has passed through a leak, by a specific detector and thus discloses the presence of a leak.

[According to DIN-EN 1330-8]

Vent valve

Valve used to equalize the system pressure to the atmospheric pressure.

[According to DIN-EN 1330-8]

Venting phase

The leak testing unit subjects the test item to pressure for the entire duration of the procedure. Once the measurement phase is completed the test item can be vented. The venting time is dependent on the pressure and volume of the test item.

Virtual leak

Apparent (not real) leak, caused by slow release of sorbed or occluded gases from surfaces or bulk of material or from volumes partially trapped within the system.

[According to DIN-EN 1330-8]

Viscous leak

Leak of such geometric configuration and under such pressure conditions that gas flowing through it is ?viscous? in nature.

[According to DIN-EN 1330-8]

Water testing

See also:
Bubble test