How to check for leaks using gauges, bubble immersion, foam spray tests
Leak detection methods without leak detector
Leak test using vacuum gauges which are sensitive to the type of gas
The fact that the pressure reading at vacuum gauges is sensitive to the type of gas involved (see "Indirect pressure measurement") can, to a certain extent, be utilized for leak detection purposes. Thus, it is possible to brush or spray suspected leaks with alcohol.
The alcohol vapor which flows into the device changes the pressure reading of the vacuum gauge since the thermal conductivity and ionizability of alcohol vapor vary greatly from the same properties for air.
The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete.
What is the bubble immersion test?
The pressurized test specimen is submerged in a liquid bath. Rising gas bubbles indicate the leaks.
Leak detection depends on the awareness of the inspecting person to a significant extend and induces the desire to increase the “sensitivity” by using increasingly higher pressures. Sometimes, however, this results in safety instructions not being observed. In case of small leak rates, this method is very time consuming.
The table below refers to the leak test of refrigeration systems which are operated with the refrigerant R134a. In case of these systems, the leak rate is indicated as grams of refrigerant loss per year (g/a). Water or petroleum-based oils are used as a test liquid (which may be heated or to which a surfactant may be added) The surface tension should not exceed 75 dyn/cm (1 dyn = 10–5 N).
Comparison of bubble immersion test with helium leak detector
| Loss of refrigerant R134a per year | Equivalent R134a leak rate at 25 °C | Time for 10 gas bubbles, Ø 2 mm | Detection time using helium leak detector |
| g/a | mbar · l/s | s | s |
| 430 | 3.32 · 10-3 | 3.2 | a few seconds |
| 86 | 6.63 · 10-4 | 15.8 | a few seconds |
| 21.5 | 1.66 · 10-4 | 63.1 | a few seconds |
| 17.2 | 1.33 · 10-4 | 1 min 48 s | a few seconds |
| 4.3 | 3.32 · 10-5 | 5 min 15 s | a few seconds |
| 0.86 | 6.63 · 10-6 | 26 min 19 s | a few seconds |
| 0.1 * | 7.70 · 10-7 | 3 h 46 min | a few seconds |
*) This leak rate represents the detection limit for good halogen leak detectors (≈ 0,1 g/a).
How does the foam-spray test work?
In many cases pressurized containers or gas lines (including the gas supply lines for vacuum systems) can be checked quite conveniently for leaks by brushing or spraying a surfactant solution on them. Corresponding leak detection sprays are also available commercially. Escaping gas forms soap bubbles at the leak points.
Here, again, the detection of smaller leaks is time-consuming and will depend greatly on the attentiveness of the inspector.
The hydrogen gas refrigeration systems used in power plant generators represent a special case. These are indeed sometimes tested in the fashion described above but they can be examined much better and at much higher sensitivity by sniffing the hydrogen escaping at leaks using a helium leak detector which has been adjusted to respond to (H2).
The Vacuum box test
As a variation on the spray technique mentioned above, in which the escaping gas causes the bubbles, it is possible to place a so-called “vacuum box” with a seal (something like a diver’s goggles) on the surface being examined once it has been sprayed with a soap solution.
This box is then evacuated with a vacuum pump. Air entering from the outside through leaks will cause bubbles inside the box, which can be observed through a glass window in the box. In this way it is also possible, for example, to examine flat sheet metal plates for leaks.
Vacuum boxes are available for a variety of applications, made to suit a wide range of surface contours.
Fundamentals of Leak Detection
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