How are spray and sniffer techniques used for local leak tests

Leak detection techniques using vacuum leak detectors

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Vacuum method - spray technique

The test object connected to the vacuum leak detector is traced with a very fine stream of test gas from the spray pistol at likely leakage points (flange connections, welding seams, etc.) in an appropriately slow manner. The appropriate speed for this process is determined by the response time of the system. The test gas amount sprayed must be adjusted to suit the leak rate to be detected as well as the size and accessibility of the object being tested.

Although the test gas (hydrogen, helium) is lighter than air and will therefore accumulate beneath the ceiling of the room, it will be so well distributed by drafts and turbulence induced by movements within the room that one need not assume that test gas will be found primarily (or only) at the top of the room during search for leaks. In spite of this, it is advisable – particularly when dealing with larger components – to start the search for leaks at the top.

In order to avoid a surge of test gas when the spray pistol is opened, it is advisable to install a choke valve to adjust the test gas flow directly before or after the spray pistol (see Fig. 17 below). The easiest way to set the desired test gas flow is to submerge the spray pistol in a water/alcohol container and determine the setting based on the rising test gas bubbles. Water might clog the spray pistol. As an alternative also a container filled with alcohol can be used.

With helium leak detectors, it is also easy to detect the natural amount of helium in the atmosphere.

The natural amount of helium in the atmosphere amounts to 5·10^–4 volume percent ( = 5 ppm). If air enters the test object through a very large leak, the leak detector will therefore already detect helium gas flowing through the leak. The leak rate is then:

Display (helium from spray pistol) / 100 %
= Display (helium from atmosphere) / 5·10^-4 %

Display (helium from spray pistol) =1/(5 · 10^-6) · Display (helium from atmosphere)
= 2 · 10⁵ · Display (helium from atmosphere)

Fig. 17: Handling information for using test gases (e.g. helium)

Avoiding the "helium surge" when the spray-pistol valve is opened by means of throttle valve at the spray-pistol tip.

Minimum helium flow for correct display: Changes on the throttle valve setting must not affect the indication.

Simplest way to check the helium flow: Bubble test in a glass of water/alcohol

Positive pressure method - Sniffer technology

In case of this method, the test object is filled with test gas to an extend so that the partial test gas pressure in the test object is significantly larger than that around the test object. If possible, the test object should be evacuated prior to filling it with test gas.

The likely leak positions of the test object are traced with a sniffer tip in an appropriately slow manner. A typical tracing speed is 1 cm/s.

The sniffer tip is connected to the roughing pump via a long, thin line (length ⋍1 m, diameter ⋍1 mm).

Test gas which enters the sniffer tip is delivered to the leak detector by the roughing pump and then detected there by the mass spectrometer. Vacuum detectors by Leybold are able to “sniff” helium or hydrogen.

The sensitivity of the method and the accuracy of the localization of leaks depend on:

a) the type of sniffer unit used (sniffer tip + line),
b) the response time of the leak detector used,
c) the tracing speed
and
d) the distance of the sniffer tip from the surface of the test object.

The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes, it is possible to detect leak rates of more than 1·10–7 mbar·l/s. The limitation regarding the sensitivity for detecting helium is due primarily to the natural amount of helium in the atmosphere. For quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. In this case, the distance of the sniffer tip from the outlet of the calibration leak will be included in the calibration as well.

Fundamentals of Leak Detection

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