Canning and Fish Packaging Technology

Unit 3 - Unit operations in canning

3.2.3.4.Methods available for measurement of can vacuum

• By the use of piercing type vacuum gauge.
• Flip vacuum testing equipment.
• Light spot method of Shiger and Kimera.
• By measuring Newton’s rings formed by the curved lid and a diffraction grating.

However, the most widely used method is by the use of piercing type vacuum gauge which is similar to pressure gauge which is Bourdon tube which opens out or closes down depending on the pressure and the extent of the movement shown by a needle (as in clocks) over a dial calibrated either in kg/cm2 or lb(Pounds)/square inch or mm of Hg to read the a vacuum directly. This is a destructive test and can has to be discarded.

Flip vacuum test is another test to measure vacuum inside a can. In this method can is placed inside a bell jar fitted with vacuum gauge and also to a vacuum pump. Air inside the bell jar is gradually withdrawn till the can end (lid) suddenly flips out and at that instant vacuum gauge reading in taken. However, vacuum gauge reading includes not only the vacuum inside the can but also the resistance offered by the tin plate resistance.

i.e., flip vacuum = can vacuum + tin plate resistance (Tr)

By using piercing vacuum gauge, can vacuum can be found out. The difference between flip vacuum and can vacuum gives tin plate resistance and this can be used for further tests on cans of same size made from the same tin plate as the resistance depends on diameter of the can, tin plate thickness and other mechanical properties. From all subsequent readings, the tin plate resistance (Tr) if deducted, gives real vacuum in cans established as in piercing type vacuum gauge by flip vacuum test.