Principles of Bubble Testing
In leak testing by the bubble test technique, a gas pressure differential is first established across a pressureboundary to be tested. A test liquid isthen placed in contact with the lower pressure side of the pressure boundary. (This sequence prevents the entry andclogging of leaks by the test liquid.) Gas leakage through the pressure boundary can then be detected by observation offormed in the detection liquid atthe exit points of leakage through thepressure boundary. This technique provides immediate indications of theexistence and location of large leaks, 10-3to 10-s Pa-m3-s-1 (10-2 to lo-4 std cm3·s-1).Longer inspection time periods may be needed for detection of small leaks, 10-5to lQ-6 Pa·m3-s-1 (10-4 to 10-5 std cm3-s-1),whose bubble indications form slowly.
In bubble tests, the probing medium isthe gas that flows through the leak due to the pressure differential. The test indication is the formation of visible bubbles in the detection liquid at the exitpoint of the leak. Rate of bubble formation, size of bubbles formed and rate of growth in size of individual bubbles provide means for estimating the size of leaks (the rate of gas flow through leaks).
Bubble Test Liquids
In leak testing by the bubble test technique, a gas pressure differential is first established across a pressureboundary to be tested. A test liquid isthen placed in contact with the lower pressure side of the pressure boundary. (This sequence prevents the entry andclogging of leaks by the test liquid.) Gas leakage through the pressure boundary can then be detected by observation offormed in the detection liquid atthe exit points of leakage through thepressure boundary. This technique provides immediate indications of theexistence and location of large leaks, 10-3to 10-s Pa-m3-s-1 (10-2 to lo-4 std cm3·s-1).Longer inspection time periods may be needed for detection of small leaks, 10-5to lQ-6 Pa·m3-s-1 (10-4 to 10-5 std cm3-s-1),whose bubble indications form slowly.
In bubble tests, the probing medium isthe gas that flows through the leak due to the pressure differential. The test indication is the formation of visible bubbles in the detection liquid at the exitpoint of the leak. Rate of bubble formation, size of bubbles formed and rate of growth in size of individual bubbles provide means for estimating the size of leaks (the rate of gas flow through leaks).Bubble Test Liquids
Bubble test techniques for detecting or locating leaks can be divided into three major classifications related to thetechnique of using the test liquid.
1. In the liquid immersion technique,the pressurized test object or system is submerged in the test liquid. Bubbles are then formed at the exit point of gas leakage and tend to rise toward thesurface of the immersion bath.
2. In the liquid film application technique, a thin layer of test liquid is flowed over the low pressure surface of the test object. An example of this solution film leak test is the well known soap bubble technique used byplumbers to detect gas leaks. Films of detection liquid can be readily appliedto many components and structuresthat cannot be conveniently immersedin a detection liquid. For detection ofsmall leaks, this liquid should form a thin, continuous, wetted film covering all areas to be examined.
1. In the liquid immersion technique,the pressurized test object or system is submerged in the test liquid. Bubbles are then formed at the exit point of gas leakage and tend to rise toward thesurface of the immersion bath.
2. In the liquid film application technique, a thin layer of test liquid is flowed over the low pressure surface of the test object. An example of this solution film leak test is the well known soap bubble technique used byplumbers to detect gas leaks. Films of detection liquid can be readily appliedto many components and structuresthat cannot be conveniently immersedin a detection liquid. For detection ofsmall leaks, this liquid should form a thin, continuous, wetted film covering all areas to be examined.
3. The foam application technique is used for detection of large leaks in which the applied liquid forms thick suds or foam. When large leaks are encountered, the rapid escape of gas blows a hole through the foam blanket, revealing the leak location.
Pressure Control in Bubble Testing
Subclassifications of these basic techniques of bubble testing refer todifferent techniques for controlling thepressure differential acting across thepressure boundary. Several techniques are used to raise the pressure differential andso to increase the rate of gas leakage and the rate of formation of bubbles.
1. Pressurize the interior volume of thetest object or system before and duringthe leak test. Internal gas pressure should be applied across the pressure boundary before test liquid contacts the external surface. This tends to prevent entry of liquid into leaks, which might possibly clog the leaks togas flow. Protection against hazards ofoverpressure must be provided.
2. Control the heating of sealed test objects and small components tocause internal gas expansion. This increases the pressure differential andcauses outward gas flow throughpossible leaks in the pressure boundary.
3. Apply a partial vacuum above thesurface of the test liquid (immersion liquid or solution film). This reduces external pressure to the pressure boundary. The resultant increase inpressure differential across the system boundary acts to cause gas flow through any leaks that are present.
Pressure Control in Bubble Testing
Subclassifications of these basic techniques of bubble testing refer todifferent techniques for controlling thepressure differential acting across thepressure boundary. Several techniques are used to raise the pressure differential andso to increase the rate of gas leakage and the rate of formation of bubbles.
1. Pressurize the interior volume of thetest object or system before and duringthe leak test. Internal gas pressure should be applied across the pressure boundary before test liquid contacts the external surface. This tends to prevent entry of liquid into leaks, which might possibly clog the leaks togas flow. Protection against hazards ofoverpressure must be provided.2. Control the heating of sealed test objects and small components tocause internal gas expansion. This increases the pressure differential andcauses outward gas flow throughpossible leaks in the pressure boundary.
3. Apply a partial vacuum above thesurface of the test liquid (immersion liquid or solution film). This reduces external pressure to the pressure boundary. The resultant increase inpressure differential across the system boundary acts to cause gas flow through any leaks that are present.