The term "watertight" describes colloquially a tightness requirement that is specified for a component and its application. The precise description of the requirement is made on the leakage rate. For the term "watertight" this would be 10⁻² mbar l/s (using the test medium air).
To define a leak requirement, information such as test pressure, the medium for testing and the maximum allowed leakage rate must be known. For example, if the test is set to the limit of 10⁻² mbar l/s using the medium air, there will be no water leakage through the leak in dense components - but permeability to gases or less liquid media (such as gasoline or oil) may still be possible. The tightness requirement of 10⁻² mbar l/s or "watertight" cannot be reliably and reproducibly fulfilled with commercial ultrasonic technology in normal industrial environments. For further explanation see FAQ L3 (Minimum detectable leak rate using air-borne sensors).