The test specimen, connected to the helium leak detector, is slowly traced with a very fine stream of helium from the spray pistol, aimed at likely leakage points (welding seams, flange connectors, fused joints), bearing in mind the time constant of the system as per Equation 5.8 (see Fig. 5.14). The volume sprayed must be adjusted to suit the leak rate to be detected and the size and accessibility of the object being tested. Although helium is lighter than air and therefore will collect 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 helium 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 helium when the spray valve is opened (as this would “contaminate” the entire environment) it is advisable to install a choke valve to adjust the helium quantity, directly before or after the spray pistol (see Fig. 5.15). The correct quantity can be determined easiest by submerging the outlet opening in a container of water and setting the valve on the basis of the rising bubbles. Variable-area flowmeters are indeed available for the required small flow quantities but are actually too expensive. In addition, it is easy to use the water-filled container at any time to determine whether helium is still flowing.
Avoiding the “helium surge” when the pistol valve is opened.
a) Throttle hose or
b) Adjustable throttle valve ahead of the spray pistol.
Minimum helium quantity for correct display: Changing the setting for the throttle shall not affect indication.
The minimum quantity is always much smaller than one would set without a flowmeter (e.g. by listening for flow or letting the helium flow across moistened lips). The simplest check without a flowmeter: Letting gas bubble through water.
Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10‑7 mbar · l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too.
a: Integral leak detection; vacuum inside specimen
b: Local leak detection; vacuum inside specimen
c: Integral leak detection (test gas enrichment inside the enclosure); pressurized test gas inside specimen
d: Local leak detection; pressurized test gas inside the specimen
Note: In the composition of atmospheric air the relative humidity (RH) is indicated separately along with the temperature. At the given relative humidity, therefore, the air pressure read on the barometer is 1024 mbar.