Helium mass spectrometer
Encyclopedia
A helium mass spectrometer is an instrument commonly used to detect and locate small leaks. It was initially developed in the Manhattan Project
during World War II to find extremely small leaks in the gas diffusion process
of uranium enrichment plants
. It typically uses a vacuum chamber
in which a sealed container filled with helium is placed. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer.
. It is accomplished by ionizing
a gas sample containing helium, pushing the sample through a magnetic field
, and collecting the helium ion
s as they emerge. Since helium ions exit along a different path from all other ions, collection of helium is reasonably simple. The current produced by the helium ion flow is used to drive an ammeter
. Often audio alarms, and visual display on leak detection system to give information and warnings about leak levels. Ionization
, separation and collection takes place within a spectrometer tube, which is the heart of the system.
Helium is used as a tracer
because it penetrates small leaks rapidly. Helium has also the property of being non-toxic, chemically inert
, inexpensive to produce, and present in the atmosphere only in minute quantities (5 ppm). Typically a helium leak detector will be used to measure leaks in the range of 10−5 to 10−12 Pa
·m
3·s
−1.
under vacuum, and collide with gas
molecules, creating within the chamber ions quantitatively proportional to the pressure in the ion chamber. These ions are repelled out of the ion chamber, under vacuum, through the exit slit, by a repeller field. The combined electrostatic effect of the repeller, exit slit, focus plates, and ground slit collimates the ion beam
so that it enters the magnetic field as a straight "ribbon" of ions.
for leak detection.
and air conditioning
, automotive parts, carbonated beverage containers, food packages and aerosol
packaging, as well as in the manufacture of steam products, gas bottles, fire extinguisher
s, tire
valves, and numerous other products including all vacuum
systems.
If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered.
It still requires the part to be tested to be connected to a helium leak detector. The outer surface of the part to be tested is sprayed with a localized stream of helium tracer gas.
If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered. Thus correlation between maximum leakage signal and location of helium spray head will allow the operator to pinpoint the leaky area.
This test method applies to a lot of components that will operate under pressure: airbag canisters, evaporators, condensers, high-voltage SF6 filled switchgear.
First the device under test will be exposed for an extended length of time to a high helium pressure in a "bombing" chamber.
If the part is leaky, helium will be able to penetrate the device.
Later the device will be placed in a vacuum chamber, connected to a vacuum pump and a mass spectrometer. The tiny amount of gas that entered the device under pressure will be released in the vacuum chamber and sent to the mass spectrometer where the leak rate will be measured.
This test method applies to implantable medical devices, crystal oscillator, saw filter devices.
This method is not able to detect a massive leak as the tracer gas will be quickly pumped out when test chamber is pumped down.
This mode of operation is frequently used to locate a leak that has been detected by other methods, in order to allow for parts repair.
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
during World War II to find extremely small leaks in the gas diffusion process
Gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238 . By use of a large cascade of many stages, high separations...
of uranium enrichment plants
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
. It typically uses a vacuum chamber
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
in which a sealed container filled with helium is placed. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer.
Detection technique
The leak detection technique depends on the separation of helium from other gases in a vacuumVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. It is accomplished by ionizing
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
a gas sample containing helium, pushing the sample through a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
, and collecting the helium ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s as they emerge. Since helium ions exit along a different path from all other ions, collection of helium is reasonably simple. The current produced by the helium ion flow is used to drive an ammeter
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
. Often audio alarms, and visual display on leak detection system to give information and warnings about leak levels. Ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
, separation and collection takes place within a spectrometer tube, which is the heart of the system.
Helium is used as a tracer
Tracer
Tracer may refer to:* Histochemical tracer, a substance used for tracing purposes in histochemistry, the study of the composition of cells and tissues...
because it penetrates small leaks rapidly. Helium has also the property of being non-toxic, chemically inert
Inert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
, inexpensive to produce, and present in the atmosphere only in minute quantities (5 ppm). Typically a helium leak detector will be used to measure leaks in the range of 10−5 to 10−12 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
·m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
3·s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
−1.
- A flow of 10−5 Pa·m3·s−1 is slightly less than 1 ml per minute at Standard conditions for temperature and pressureStandard conditions for temperature and pressureStandard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
(STP). - A flow of 10−13 Pa·m3·s−1 is slightly less than 3 ml per century at STP.
Internal spectrometer tube operation
In the spectrometer tube, the heart of the helium mass spectrometer, the electrons produced by a hot filament enter an ionization chamberIonization chamber
The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
under vacuum, and collide with gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
molecules, creating within the chamber ions quantitatively proportional to the pressure in the ion chamber. These ions are repelled out of the ion chamber, under vacuum, through the exit slit, by a repeller field. The combined electrostatic effect of the repeller, exit slit, focus plates, and ground slit collimates the ion beam
Ion beam
An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
so that it enters the magnetic field as a straight "ribbon" of ions.
Types of leaks
Typically there are two types of leaks in the detection of helium as a tracerTracer
Tracer may refer to:* Histochemical tracer, a substance used for tracing purposes in histochemistry, the study of the composition of cells and tissues...
for leak detection.
- Residual leak: A residual leak is a real leak that may be gross, or small, according to the sensitivity setting of the leak detector.
- Virtual leak: A virtual leak is the semblance of a leak in a vacuum system caused by slow release of trapped gases, as gases may adhere as pockets to the interior sides of a chamber. This can cause confusion to the operator as it may be a false indication of a present leak.
Uses
Helium mass spectrometer leak detectors are used in production line industries such as refrigerationRefrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
and air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
, automotive parts, carbonated beverage containers, food packages and aerosol
Aerosol spray
Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. This is used with a can or bottle that contains a liquid under pressure. When the container's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol or mist...
packaging, as well as in the manufacture of steam products, gas bottles, fire extinguisher
Fire extinguisher
A fire extinguisher or extinguisher, flame entinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations...
s, tire
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
valves, and numerous other products including all vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
systems.
Global helium spray - vacuum test
This method requires the part to be tested to be connected to a helium leak detector. The outer surface of the part to be tested will be located in some kind of a tent in which the helium concentration will be raised to 100% helium.- If the part is small the vacuum system included in the leak testing instrument will be able to reach low enough pressure to allow for mass spectrometer operation.
- If the size of the part is too large, an additional vacuum pumping system may be required to reach low enough pressure in a reasonable length of time. Once operating pressure has been reached, the mass spectrometer can start its measuring operation.
If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered.
Local helium spray - vacuum test
This method is a small variation from the one above.It still requires the part to be tested to be connected to a helium leak detector. The outer surface of the part to be tested is sprayed with a localized stream of helium tracer gas.
- If the part is small the vacuum system included in the instrument will be able to reach low enough pressure to allow for mass spectrometer operation.
- If the size of the part is too large, an addition pumping system may be required to reach low enough pressure in a reasonable length of time. Once operating pressure has been reached, the mass spectrometer can start its measuring operation.
If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered. Thus correlation between maximum leakage signal and location of helium spray head will allow the operator to pinpoint the leaky area.
Helium charged - vacuum test
In this case the part is pressurized (sometime this test is combined with a burst test, i.e. at 40 bars) with helium while sitting in a vacuum chamber. The vacuum chamber is connected to a vacuum pumping system and a leak detector. Once the vacuum has reached the mass spectrometer operating pressure, any helium leakage will be measured.This test method applies to a lot of components that will operate under pressure: airbag canisters, evaporators, condensers, high-voltage SF6 filled switchgear.
Bombing test (overpressure) - vacuum test
This method applies to objects that are supposedly sealed.First the device under test will be exposed for an extended length of time to a high helium pressure in a "bombing" chamber.
If the part is leaky, helium will be able to penetrate the device.
Later the device will be placed in a vacuum chamber, connected to a vacuum pump and a mass spectrometer. The tiny amount of gas that entered the device under pressure will be released in the vacuum chamber and sent to the mass spectrometer where the leak rate will be measured.
This test method applies to implantable medical devices, crystal oscillator, saw filter devices.
This method is not able to detect a massive leak as the tracer gas will be quickly pumped out when test chamber is pumped down.
Helium charged - sniffer test
In this last case the part is pressurized with helium. The mass spectrometer is fitted with a special device, a sniffer probe, that allows it to sample air (and tracer gas when confronted with a leak) at atmospheric pressure and to bring it into the mass spectrometer.This mode of operation is frequently used to locate a leak that has been detected by other methods, in order to allow for parts repair.
See also
- Mass spectrometryMass spectrometryMass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
- Tracer-gas leak testing methodTracer-gas leak testing methodTracer-gas leak testing method is a nondestructive testing method.-Types:Several tracer-gas leak testing methods exist among which the methods of choice are:* Helium mass spectrometer or helium leak detection, providing the highest sensitivity...
- Leak noise correlatorLeak noise correlatorA leak noise correlator is an electronic device used for Leak Detection and as a leak locator to find leaks in pressurized water or gas lines.Typically, microphones or acoustic sound sensors are placed in contact with the pipe, at two or more points, to record the sound emitted by a leak somewhere...