Air Tightness Test, is a test to verify the tightness of the structure by means of air pressure difference.
Figure 1: U-Tube
Air testing should be carried out at a stage sufficiently close to the completion of the structure or after a repair period. The air test should take place when all hot works, attachments, outfitting or penetrations which may affect the strength or tightness of the structure have been completed, and before any ceiling and cement work is applied over inspected seams.
For all manual or semi-automatic erection welds and all fillet welds tank boundary connections including penetrations, the final coating is to be applied after air testing. For other welds, final coating may be applied prior to air testing provided the Surveyor is satisfied with the condition of welds.
All boundary welds, erection joints, and penetrations including pipe connections are to be examined under a pressure differential not less than 0.15 bar (0.15 kgf/cm2, 2.2 psi) with a leak indicating solution (a soap solution).
A U-Tube with a height sufficient to hold a head of water corresponding to the required test pressure is to be used for checking the testing pressure (Fig. 1). The cross sectional area of the U-Tube is not to be less than the pipe supplying air. The yard’s Circulating Air supply is constant and the height differential of water end to the max U-Tube height, denotes the tank’s air pressure.
Welds which cannot be air tested shall be tested by other effective methods of tightness testing such as vacuum testing or compressed air fillet weld testing.
Officer of the Watch 
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