Day 9 – Under Pressure

There are two words that should never be used together in the same sentence, ‘Vacuum Pressure’. There is no such thing, a vacuum describes an absence of pressure within a closed/sealed space such as a vacuum bag.

The ‘pressure’ created on the surface of a composite laminate which has been placed under a vacuum bag, is totally dependent upon two factors;

• Pressure: the total amount of ambient air pressure caused by the weight of the air in the atmosphere, bearing upon the outside surface of the vacuum bag.
• Vacuum: the degree of vacuum measured in inches of mercury, i.e. - 28.5” Hg (or in millibars – metric) that has been created within the sealed area of the vacuum bag.

The ‘differential’ between the ambient air pressure and the degree of the vacuum created in the bag is the ‘resultant’ pressure applied to the composite laminate. A whole bunch of factors will affect this!

It’s complicated. But all is explained in detail in ACT’s composites training courses.

QUESTION: if you placed a Vacuum Gauge (as pictured above) into a sealed chamber where you have created a vacuum of 27” Hg, what would the reading be on the gauge?

• Answer on Day 10