1. Section 7 Other leak detectors
LEAK DETECTION
Section 7 Other leak detectors
Section Title Slide
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2. Ion Pump as a Leak Detector+
Anode cell
Cathodes A
Electron
Ion B
Notes
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3. Internal View of the Helitest
Platinum Heater
Ion Pump 0.2L/sec
Probe
High Voltage Supply
Sampling Pump
Notes 1/
This slide was prepared by Varian Vacuum Technologies for their Helitest product. 2/
Helitest is based on a technology called Selective Ion Pump Detector
This is the correct name for the HeliTest principle of operation. 3/
Operating Principles
A quartz membrane (capillary) is connected to an ion pump working in UHV condition. The quartz permeability for Helium is very high but the permeability for all the other atmospheric gasses is negligible. A change of the Helium concentration in the atmospheric side of the capillary can be detected as a change in the ion current (pressure). 4/
Sensor components
Varian 0,2 Liter /sec ion-getter pump
Capillary quartz (0,4x0,3 diameter - 100mm length)
Getter pad
Platinum resistor
HV card V 5/
Helitest specifications
Sensitivity : about 2 ppm
Response time : 3 sec.
Start-up time : less than 2 min.
Weight : Kg
Continue battery operation : 4 hours
Clean up time : Min 4 sec, Max 2 min. 6/
Interface
Autoranging
Autocheck procedure
Audible alarm
Autozero
Dual sensitivity
Calibration mode
Clean up mode
Battery check
Optional RS232 communication
Optional analogue outputs 7/
HeliTest has a high sensitivity...but other benefits are: High discrimination for He signal: it is 10 times less sensitive to Hydrogen; Portability; Easy to use
Current Amplifier
MICROPROCESSOR
Display
Keyboard
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4. Vacuum Gauges as Leak Detectors
Simple vacuum gauges are used for leak detection applications such as furnaces for heat treatment or freeze drying chambers.
Factory Scale System
Chamber
Vacuum Gauge
Notes 1/
Leak detection in vacuum systems usually relies on measurement. Since most industrial and R&D vacuum systems have at least one vacuum gauge present, recording the normal readings and setting some action levels can form a useful and rapid indicator of gross leakage problems without the expense of dedicated sophisticated leak detectors. 2/
The operators of such systems may set dial gauges with the normal vacuum readings marked on the gauge glass, or preset alarm points on pressure sensing displays such as strain gauges and capacitance manometers. Another very useful method of determining the likely presence of leakage is to record the pump down time to reach a level, and comparing this with the normal. This may be done manually or automatically under process control software. 3/
Likewise, the integrity of a sealed vacuum system can be assessed when it has been pumped out and then isolated from the means of pumping, by recording the leak up rate and comparing this with the normal expected rates. This simple method is very widely used in industry, but can require long times to establish low level leakage validation, because the normal outgassing leak up rate to be expected for the internal vacuum surface will mask such leakage. 4/
Another problem with using internal gauges can be their gas dependency, since uncontrolled changes in residual gas composition can influence the apparent leakage rate. A well know example of this is the effect of argon gas in Pirani gauges, which can apparently read low pressure at atmospheric argon pressure if preset to air calibration; by contrast helium and hydrogen gases cause thermal gauges to show atmospheric when at a few mbar vacuum. Of course this very effect is the basis of the Pirani or thermocouple gas sniffer used for overpressure leak testing. Most high vacuum gauge types have a gas dependency because they are based on ionisation or thermal phenomena, so they too can have a limited but very useful role in assessing the state of vacuum system integrity using tracer gas. 5/
Many workers use their vacuum pump characteristics to determine the state of the vacuum system: the ion pump current is a typical measure of the residual gas pressure, while cryo-pumped systems may be overloaded by argon or helium under certain circumstances. Observation of unusually high backing pump pressure is another leak test.
There are other gauge types that can be used for leak detection with tracer gases: the spinning rotor gauge type depends on gas viscosity, while it is less well known that the phenomena of thermal transpiration can be exploited with special capacitance manometer configurations and helium or hydrogen gas around backing pump pressures.
Special sensors can be used for atmospheric pressure gas leak detection: these include chemical, optical, thermal, semiconductor, photo-ionisation, acoustic, holographic, biochemical and electron-capture detectors. The least sophisticated but widespread and very effective industrial leak test methods are bubbles and penetrating UV dyes but only rarely used in laboratories because of the materials involved. Product weighing at specified time intervals (days) is another common validation technique for packaged gas filled products like propellant cans and inhalers, but is not viable in many laboratory situations. The smaller the item the harder the validation, usually requiring the use of far more sophisticated methods such as helium mass spectrometer leak detection and residual gas analysis.
Diffusion, ejector or dry pumping combination
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© 2003, Steve Hoath

5. Vacuum Gauges and RGAs as Leak Monitors
A quadrupole mass spectrometer or RGA samples the residual gases and can monitor several masses individually. Spectrometer or RGA outputs can be used to signal specific gas type leakage.
Ionization gauges can be placed to directly measure the pressure.
Filament
Notes 1/
Quadrupole
Type of RGA
Collector
Grid
Ion source
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6. SF6 Leak Testing Sensor © 2004. IUVSTA 7.06 Gas discharge Inner port
Electrode
Electrical
Connection
To the power
Supply and
Signal display
Probe
Inlet
Capillary
The SF6 detector shown here is based on the Electron Capture Detection (ECD) principle invented by James Lovelock of Gaia fame. A argon gas carrier flowing through a chamber with a weakly () radioactive source of electrons is ionised and hence can produce a background current between the source and central electrode.
When SF6 gas molecules from a leaking test object pressurised with SF6 enter the probe inlet capillary, they get mixed with the argon carrier gas and reduce the background discharge current. This difference – as indicated by the display unit for the detection system – is proportional to the concentration of the SF6 test gas and thus to the size of the leak. This system has the major disadvantages of requiring a carrier gas and also a radioactive source, although it was until recently unchallenged for SF6 detection because of the sensitivity for testing high voltage equipment, e.g. insulated switchgear for power transmission and conversion systems and electrostatic accelerators.
Leak rates as low as 5 x 10-9 mbarl/s have been claimed for the best ECD detectors applied to SF6 leak detection.
(Weakly)
Radioactive
Source ()
Argon gas
Inlet port
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7. Bubble test and similar methods:
Table of fluid combinations used in leak tests by visual inspection
External fluid
Surface tension
10-4 N/m
Internal fluid under pressure
Minimum leak [10-6 mbarl/s]
Indication of leak by
Ether
1.7
Acetone
2.4
Methanol
2.3
H2 (hazard), He, clean air
Ethanol
2.2
Alcohol
0.5 (H2)
Very sensitive
Water
7.3
~10 (air)
Large, slow
Soap solution
Weak detergent
Phenolic paint 5
Discoloration white
HCl (corrosive) 50
Vapour clouds
CO2 (hood)
NH3
Clouds of NHCO3
Agar-agar solution (~65- 70°C)
H2O
Large leaks only
Wet external surface
Atmospheric air
Oil
>~0.5 mbarl/s
Notes 1/
This was taken from the 1st Edition Slide
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8. 85Kr as test gas – 85Kr decay scheme
10.6 year
0.46%
- 0.15MeV
99.54%
- 0.67MeV
 0.52MeV
Notes 1/
This slide is adapted from the 1st Edition slide and checked by the Table of Isotopes by Lederer & Perlman. 2/
Decay scheme of the radio-active (10.6 year half-life) isotope 85Kr. The majority of the decays are directly by beta radiation (electron emission) to the ground state of 85Rb, but a small fraction (0.46%) decay via an excited state with subsequent emission of a 0.517MeV gamma ray. Since this energy of gamma ray can penetrate solid walls of encapsulated test pieces, the presence of this gas within them can be used for determination of leakage rates.
This slide was taken from the 1st Edition but it has been redrawn to clarify that the nuclear transformations are due to electron emission beta radiation from the radio-active 85Kr to the different levels of stable isotope 85Rb.
The branching ratio to the excited state is low due to the much larger energy available for the beta radiation to the ground state and the low energy of the beta radiation to the excited state. The gamma ray is emitted promptly, i.e. without significant delay, once the excited state has been populated, so that the signal decays as 10.6year half-life.
85Kr
85Rb
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