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.

1.) Advantages:

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.

2.) Disadvantages:

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