Vacuum testing (tightness)

Vacuum (box) testing is a leak-detection method that proves the tightness of a weld seam where only one side is accessible, such as a tank-bottom fillet or a deck plate joint. A clear-topped box with a soft seal is placed over the soap-solution-coated weld and evacuated; a leak path draws air through and shows as bubbles inside the box. It checks tightness, not strength, and suits welds that cannot be hydrostatically or air-pressure tested from the far side. The class testing schedule accepts vacuum-box testing as an alternative tightness test for certain boundaries. The vacuum level and a calibrated gauge govern sensitivity; the operator works the box along the seam in overlapping steps.