Method for leak testing and leak testing apparatus

#The present invention is generically directed on a technique for leak testing closed and filled containers, whereby the filling material comprises at least one liquid component.

#Leak testing techniques according to which closed containers are introduced in a test cavity which, after having sealingly been closed, is lowered in pressure by a suctioning pump are known. If the container is not leaking, then once a predetermined pressure has been reached in the test cavity and thus in the surrounding of a container to be tested, this pressure will be kept substantially constant. If a leak is provided in an area of the container, wherein air is entrapped, a flow of air out of the container will lead to a rise of the surrounding pressure. If a leak is present in the area of the container where filling good is entrapped, the question whether such leak will lead to a significant rise of the surrounding pressure is largely dependent on the kind of filling good as of its viscosity, whether solid particles are present in the filling good and, obviously, on the largeness of the leak.

#Different approaches have become known to accurately detect leaks at such product-filled containers, irrespective whether the leak is present in an air entrapping container area or in a container area covered with filling good. One such approach which is the topic of the co-pending European patent application EP-A-0 791 814 and the U.S. patent application Ser. No. 08/862,993 proposes to provide an impedance measurement, specifically a resistance measurement, just adjacent to the outer wall of the container by means of an electrode arrangement: As soon as liquid emerges from a leak it will contact a respective pair of impedance measuring electrodes and lead to a significant change of impedance measured between such electrodes.

#The present invention will now additionally be described with the help of figures showing specific and today preferred examples of realising the present invention. Such figures show:

#FIG. 1: qualitatively the dependency of vapour pressure from temperature of a liquid;

#FIG. 2: schematically an inventive test apparatus operating according to the inventive method;

#In FIG. 1 there is qualitatively shown the course of vapour pressure pV(T) in the pressure versus temperature diagram. At a predetermined temperature Tx a liquid starts to evaporate when the respective vapour pressure pVx is reached. Above the vapour pressure course the material is liquid, below the material is gaseous.

#According to FIG. 2 an inventive apparatus comprises a test cavity 1 with a sealingly closable cover 3. A vacuum pump 5 connected to the test cavity 1 which may be a drag pump or a rotational piston valve pump or a diffusion pump or a turbo vacuum pump as a turbo molecular pump. This depends on the degree of vacuum which shall be established within cavity 1. Further, there is provided a vacuum pressure sensor 7 as e.g. a Pirani sensor, which measures the pressure prevailing in the test cavity 1. At least one closed container 9, which is filled at least to some extent with a filling product containing at least one liquid component is introduced through opened cover 3 into the test cavity 1 which is then sealingly closed. By starting operation of vacuum pump 5 the surrounding of container 9 and thus the intermediate volume V of test cavity and container 9 is lowered.

#According to FIG. 3 starting at ambient pressure po the pressure in volume V is lowered at least down to the value PV which accords to the vapour pressure of the liquid component within the filling good of the container 9. It is advisable to select a vacuum pump 5 which enables to evacuate the test cavity 1 down to a pressure which is at least one, preferably two and even more preferred three decades lower than the vapour pressure pV of the liquid content of the filling product.