a vacuum package tester is capable of providing data by which the internal pressure and volume of residual gas of a flexible wall vacuum package exposed to atmospheric pressure may be calculated . a container partially filled with liquid is surrounded by a deformation - absorbing enclosure . a vacuum pump reduces the pressure in the head space above the liquid , thereby causing a submerged package to expand . pressure gauges record the respective pressures in the head space when the package expands by first and second known volumes . from this data , the internal pressure and volume of residual gas of said package , when exposed to ambient pressure and temperature , may be calculated . in use , the invention also performs an improved method .