Workshop:
Vacuum Metrology for Industrial Environments
27 June 2013
QUALITATIVE AND QUANTITATIVE
METHODS FOR LEAK TESTING
LEAK TESTING: definition
UNI 8450 - February ’83
Non-Destructive Testing
• “Examination conducted to verify
compliance of materials or
components respect to specified
requirements, using techniques that do
not harm the characteristics or
performance."
Definition according ASNT
Non-Destructive Testing
• “Non-destructive testing is to determine
the physical conditions of an object
avoiding to compromise the functions
for which the object was constructed.”
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Leak Testing
The leak testing should not be understood
as an abstract search of an unrealizable leak
from the value of zero.
It is an intelligent check within the
production process.
It is finalized at a non-statistical control,
but continuum control of the compliance
of products with the requirements of
tightness.
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The unit of measurement of gaseous flow: mbar·L/s
The fluid flow
• is expressed for example in cm3/s or in L/min and more.
The gaseous flow
• is expressed for example in mbarL/s or in Pam3/s and more.
Pressure
Varation
Volume
Time
Leakrate
in mbar·L/s
1 mbar
1L
1s
1
= 100
1 mbar
0,1 L
10 s
0,01
= 10-2
0,01 mbar
0,1 L
100 s
0,00001
=
10-5
TIGHTNESS
mbar·L/s
Water tight
10-2
Vapor Tightness
10-3
Oil and Gasoline Tightness
10-4
Bacteria Tightness
10-5
Virus Tightness
10-6
“Absoluta” Tightness (technique)
10-10
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The Correct Approach
The correct approach to the choice of method for the
leak testing:
1.
4.
Define the requirement of the leak testing
2.
Calculate the allowed limits of the leak
3.
Choose the suitable method of leak testing
Compare the different economic solutions
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Methods for Leak Detection
PRESSURE CHANGE METHOD
• pressure decay
• pressure rise
BUBBLE TEST (immersion or covering)
• Identified only for the localization of the leaks
ULTRASONIC DETECTION
• spontaneous
• induced
LEAK TESTING WITH TRACER GASES
• thermal conductivity
• mass spectrometry
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Pressurize the
piece in test
and then
check the
pressure
decay.
Reduce the
pressure in the
piece (by
pumping in
vacuum) and
then check the
pressure rise.
METHOD 2
METHOD 1
Pressure Change Method
absolute pressure transducer
The
measurements
can be carried
out with:
• direct measurements ΔP,
differential pressure transducer
• ΔP measured of the piece and a reference volume
together and then individually.
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Pressure Change Method
A pressurized or
evacuated component,
of known volume, have
to be monitored:
Pressure of gas
Temperature
(internal) of gas
Duration of test
Case II
Case I
No or minimal variation
of temperature:
You do not have
significant variations in
the volume and therefore
of the pressure during the
test. From pressure
variation (Δp) measured
in the time interval (Δt)
and known volume (V) of
the piece in test, it is
possible to calculate the
value of gas flow:
q = (Δp / Δt) x V
Substantial variation of
temperature:
significant variations in the
the pressure during the
test. The rate of leak is
calculated as following:
q = (P1 - (P2 x (T1/T2))x V)/Δt
P1
P2
T1
T2
V
Δt
initial pressure
final pressure
initial temperature
final temperature
volume
duration of the test
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Pressure Change Method
Advantages:
• economic system, it is sufficient a
pressure gauge for rough measurement;
• use of only compressed air;
• no "memory" at the end of the test.
Disadvantages:
• strong dependence of the temperature: the
variations during the test could invalidate the
measurements;
• different volumes in the test pieces, with the
same measurement time, affect the results;
• measurement of lower flows require longer
times.
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Bubble test
In the immersion or covering with liquid of a
part, the bubbles are formed in the area of the
leaks.
The bubbles are produced when the internal
pressure exceeds the forces of surface tension
of the external liquid.
The size of the bubbles and the emission
frequency depend on:
• the type of gas flow,
• the size of the leak,
• the surface tension of the liquid.
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Bubble test
Identified only for the localization of the leak
Using liquids with
a low surface
tension, it is
possible to detect
flows from 1/50
to 1/100 of the
smallest
detectable flow
using the bubble
test in water.
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Bubble Test
Application
(vacuum)
For the localization of
leaks on different
types of food
packaging, from
coffee pods to the
confectionery
products, such as
chocolates.
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Ultrasonic Detection
The propagation of
ultrasound occurs by
vibration of molecules
in the fluid and from the
fluid are absorbed in
the inverse function of
the density or
aggregation state.
Ultrasound propagate
not much in gas, air and
steam, but better in
liquid. In vacuum it does
not propagate.
The directionality of ultrasound is a fundamental
characteristic, while the intensity is an inverse function of
source-sensor distance.
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Ultrasonic Detection
The characteristics of directionality and decay facilitate
the localization of the source of the detected wave of
ultrasound.
The method of ultrasonic detection is divided in :
Passive type
Active type
• when the emissions are
spontaneously emitted by
the pieces in test,
• when controls are carried
out with the assistance of
ultrasound artificially
generated.
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Ultrasonic Detection
Advantages:
• absence of tracer gases;
• use of only compressed air;
• no "memory" at the end of the test.
Disadvantages:
• the commercial devices have a high sensitivity
(adjustable) but they do not detect microleaks in the
absence of turbulence. It is necessary to use
ultrasounds generated artificially;
• all the phenomena of mechanical friction, of turbulent
motions of fluid moved by the pressure difference
emit spontaneously ultrasound.
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for being
detected.
so to induce the
tracer gas to
move through
possible leaks
Tracer Gases
submit the
particular to
be tested to a
pressure
difference
The process of
leaktesting with
the use of tracer
gases considers
to
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Tracer Gases:Thermal Conductivity
These devices are
mostly portable types
as probes.
The different gases
are detected
according to their
different thermal
conductivity.
These characteristics varies with dependence from molecular size and speed.
They can be used with different gas or gas mixtures, since each of them
produces a different thermal conductivity respect to the air.
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Tracer Gases:Thermal Conductivity
Through a probe, the gas is conveyed, with a small
fan or a small pump, on a hot filament maintained in
thermal - energy equilibrium.
The variation, although minimum, made ​by the gas,
is used as a signal for the determination of the leak.
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Tracer Gases:Thermal Conductivity
Advantages:
• low cost;
• possibility to operate with different gases;
• high maneuverability and transportability.
Diasdvantages:
• use only in sniffer mode;
• difficulty in determination of the leaks in terms of
volumetric flow;
• gas extraneous to the process can affect the
measurements.
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Tracers Gases:
Mass
Spectrometry
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Tracers Gases:
Quartz Capillary
In this leak detector, the detection system of the tracer gas (helium) is
constituted by a membrane in quartz, permeable only for helium and not other
gases present in the atmosphere.
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The tracer gas mostly used is HELIUM because of its chemicalphysical characteristics:
is non-toxic, has no taste and no smell,
does not pollute, does not disturb the ambient,
is chemically and physically inert (is non-combustible
and non-oxidizing),
is present in atmospheric air in a minimum quantity
(5 ppm).
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Symbol: He
Atomic number: 2
Atomic mass: 4
From the greek: hêlios (sun)
Year of discovery: 1895
Discoverers: Sir William Ramsey, Nils
Langet , P.T.Cleve
Inert gas, odorless, colorless and tasteless. It is the second most abundant element in the universe
and the sixth in the atmosphere.
It is located in deposits of natural gas (presence up to 9%) and in air (5.2 x 10-4 in parts). Helium of
endogenous origin comes from the decay of radioactive elements present in the subsoil.
Used in balloons, in breathing mixtures for deep water, in thermometers, in the leak testing
methods.
Through the evaporation of liquid He you can reach temperatures of 0.5 K.
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Test Methods
with Mass
Spectrometry
[UNI EN 1779:2004 – B.4]
ADVANTAGES
• It is the most simple and intuitive method.
• Low-cost, you can operate with high Dp, up
to the limits of safety.
Test System
with Probe Sniffer
DISADVANTAGES
• Requires high attention and
experience of the operator.
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Test Methods
with Mass
Spectrometry
Test System by
Spraying
[UNI EN 1779:2004 – A.3]
ADVANTAGES
• It is a valid and safe industrial process for
quality testing.
• It is easy to carry out, and it is useful with
great volumes.
DISADVANTAGES
• The ΔP is limited to 1 bar, high
consume of helium gas.
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Test Methods
with Mass
Spectrometry
Integral Test
Method
[UNI EN 1779:2004 – B.6]
ADVANTAGES
• It is the most complete and versatile
method.
• It is allowed to test pieces at high ΔP
and the result is the sum of the leaks
present.
DISADVANTAGES
• It does not furnish the localization of
the leaks;
• High cost.
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It is firstly possible distinguish two different types for the leak testing:
A) Leak Detection Methods (QUALITATIVE), generally controlled by
operator, with localization of leak:
•
•
•
•
•
Method in Water with pressurized piece (visual inspection);
Method with Bubble (soap solution) with pressurized piece (visual
inspection);
Method with Ultraviolet Lamps with pressurized piece (visual
inspection);
Method with Hot Air (visual inspection at infrared);
Method with Dielectric Variation on plastic components (ionic
system at high voltage).
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B) Leak Detection Method (QUANTITATIVE) with indication of Good,
Reject Piece and value of leakrate:
•
•
•
•
•
Measurement with presurized gas (Helium);
Measurement the flow meter of the pressurized piece;
Measurement of the pressure differential between the pressurized
piece and the reference sample;
Measurement of the pressure decay of the pressurized piece;
Measurement “compliance” of pressure increase in vacuum bell.
The instrumentation of this type allows you to monitor in time any
drifts of quality, if it is applied throughout the production.
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Metrological
Laboratory
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Gas Injection
System
Pumping System
LT-SP-1
Pumping System
LT-SP-2
Leak in
Calibration
Pressure Gauge
LT-TP-1
Reference
Capillary Leak
Mass
Spectrometer
Reference
Capillary Leak
Scheme of
calibration system
Reference
Capillary Leak
Reference
Permeation
Leak
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The calibration requires a calibrated device:
REFERENCE LEAK
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JRP14i – Joint Research Project - Work Packages 2 –
Optimization of the process of measurement and leak testing in
an industrial environment.
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JRP14i – Joint Research Project - Work Packages 2 –
Optimization of the process of measurement and leak testing in
an industrial environment.
Study and analysis of the main parameters of the leak
detector for refrigerants.
Manufacturing of Capillary Leak with Gas Refrigerants
(respect to atmosphere).
Study of the behavior of the flow of Gas Refrigerants through
capillary leaks.
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