Mechanical seal users' repeated demands for "100-% sealing" is not only a physical impossibility – it also runs counter to the interests of prolonged mechanical seal life. Leakage usually escapes from mechanical seals as vapour or mist in quantities of several cm³/h, i.e. without forming any visible (drops of) leakage from mechanical seals that look to be leak tight. This (normal) state for a large number of mechanical seals is often referred to erroneously as zero leakage.