In the past several different tracer gases have been used to find leaks and to measure leak rates, for example Ammonia or halogen gases. But since the leak test method with the tracer gas helium and the detection of this gas with a mass spectrometer was developed, all other methods became less significant compared to the superior sensitivity of the helium method. Also the measurement with a mass spectrometer ensures, that only helium and no other gases can influence the test.
Some properties of tracer gases
Leak Test with helium
Leak search and leak rate measurements with the tracer gas helium and sensing of this noble gas with mass spectrometer leak detectors has won such a large significance that this method can be marked as the most important leak detection method nowadays.
The Design Principle of Helium-Leak detectors
The following sketch shows the principle of a helium-leak detector, reduced to the essential components.
Tracer gas leak location techniques
The vacuum method
The pumps built into the leak detector evacuate the test object. If, by spraying helium to the test object, traces of helium come into the vacuum, a helium partial pressure is produced, which is given by the amount of incoming helium and the amount pumped away by the pump. The spectrometer tube measures this partial pressure. The pumping speed of the involved pump is included as a constant to the measurement and multiplication of the measured partial pressure by the pumping speed gives the leak rate [mbar.l/s]. By calibration of the leak detector, this multiplication is done electronically and the reading of the meter is then calibrated in leak rate.
With a gas spay probe a fine helium beam is directed to the critical areas from the outside. If a leak is present helium is pumped to the mass spectrometer of the leak detector and rapidly displayed on the leak rate meter. The response time, which is a function of test object volume divided by the pumping speed, must be in the range of seconds. Longer response times make it difficult to locate the leak. With a fine movement of the spray probe is searched for the maximum helium signal. There is the location of the leak. It is recommended to start the leak search at the top of the system, moving slowly to the bottom.
Otherwise some helium could raise to higher positions and if there would be a leak a singal would be displayed, irritating the operator, who is pointing the probe to a much lower position.
The sniffer method
The test object is filled with the tracer gas with a pressure higher than atmospheric pressure. From the oustside a sniffer probe which is connected to the leak detector, is moved over the critical areas. If a leak is present, helium comes out and is sucked in to the sniffer probe and through that to the leak detectors mass spectrometer. The leak rate meter will show a display of the signal. When the maximum leak rate display is found, the leak is located. This method has two advantages and two disadvantages compared to the vacuum method. Sniffing for the leak location shoud start at the bottom of the test object and slowly move to the top.
a.) The response time is only given by the lentgh of the sniffer probe, regardless how large the volume of the test object is.
b.) The sensitivity of the method can be increased by increasing the tracer gas pressure.
a.) The leak rate cannot be measured exactly, because one never knows, if all the tracer gas escaping from the leak, is completely sucked in by the sniffer probe.
b.) The sensitivity of the test is limited by the helium content in the air (5 ppm), which produces a background
Integral Leak Test Methods
The integral leak test is the measurement of the total leak rate of the whole test object. It can be carried out with two methods:
1.) from the inside to the outside
2.) from the outside to the inside
This methods do not give information´s about the leak location.
Integral leak test "outside - in"
The test object is evacuated by the helium leak detector. The outside of the test object is covered by some envelope or bell jar which is filled with 100% helium. With a bell jar the 100% filling can be achieved by evacuating the bell jar prior to the helium filling. At an envelope, which cannot be evacuated because it is weak (for example some plastic foil) the 100 % filling can only be achieved by flushing helium in and measure at an outlet opening the helium concentration.
If a leak is present, helium will come in and will reach the spectrometer which then displays directly the leak rate. With this kind of measurement the responsetime is insignificant (contrary to the leak location techniques) Possible the leak rate signal will increase slowly. The operator has to wait until the leak rate meter shows a stable leak rate.
Integral leak test "inside - out"
The test object is filled with 100% helium and placed under a belljar. The bell jar becomes evacuated by the helium leak detector. If a leak is present, wherever it is located, helium pures out and will be detected an displayed by the leak detector. The response time is given by the rest volume of the bell jar divided by the pumping effective pumping speed on the bell jar.
Large test objects, which cannot be placed under a bell jar, can be enbveloped by a plastic foil. With a sniffer probe the increase of the helium concentration under the foil is measured. Prior to the test the snifferprobe must be calibrated to measure helium concentration. This test can have long waiting times. The foil should be metalized for avoiding helium loses due to permeation. A detailed description of this procedure, including sample calculations, is given on the seminar on Leak Detection and leak test methods (see under products)
Leak detection with Hydrogen
In the past, leak detection with the search gas hydrogen was of little significance, because of the dangers involved with this gas. In the meantime a semiconductor sensor has been developed which can measure hydrogen concentrations down to 0,5 ppm. The hydrogen content in the atmosphere is also only 0,5 ppm (ten times lower than the background of helium). Therefore it becomes interesting to search for leaks with a test gas mixture of 5% hydrogen and 95% nitrogen. This gas mixture is neither inflammable nor explosive.
The use of hydrogen as a future source of energy has already initiated applications such as fuel cells, hydrogen motors etc. which are filled with & driven by this gas. Thus the use of hydrogen as a sensor is an ideal solution for leak testing. Mass spectrometer leak detectors can be calibrated for hydrogen, but a simpler solution is to employ the semiconductor sensor, which detects hydrogen without the need for vacuum.
For leak location with this method only the sniffer method is possible and for integral leak test only the method inside out is possible, as been described under helium integral test for large test objects.