In an apparatus for controlling a gas-rise pattern in a vacuum treatment process a gas inlet is operatively connected with a mass-flow-controller MFC ; said MFC being again operatively connected via a first valve with a vacuum chamber and in parallel via second valve with a vent-line. The inventive HHPA derivatives are useful as nucleating andor clarifying agents for such thermoplastics, are practical and easy to handle. This time difference is derived by computing the cross correlation between the two reflections.