Vacuum

The quality of a vacuum refers to how closely it approaches a perfect vacuum. A perfect vacuum is a volume of space that is essentially empty of matter. In practice, achieving complete emptiness is impossible. Even outer space is not completely empty—quantum mechanics explains why.

Vacuum quality is commonly divided into several categories: low vacuum, medium vacuum, high vacuum, ultra-high vacuum, extremely high vacuum, and outer space. In this terminology, the construction of composite parts using vacuum processes operates in the low to medium vacuum range—typically no better than about 99% of atmospheric pressure. The remaining categories refer to vacuum qualities between roughly 99.999% and almost 100% of atmospheric pressure.

When discussing vacuum infusion, I often use terms such as “high vacuum”, “full vacuum”, or “deep vacuum”. What I usually mean is a vacuum level approaching about 99% of ambient pressure— for example, around 29.5 inHg or roughly 1 bar. That sounds impressive, but in strict vacuum terminology this still falls under what is classified as medium vacuum.

Now that we know this, let’s immediately forget it again. For practical purposes, I prefer to speak of “full” or “high vacuum” to mean achieving the best possible vacuum that a given vacuum pump is capable of. Most rotary vane vacuum pumps can reach an absolute pressure of around 50 mbar (about 1.5 inHg absolute) or better.

At this point, I can already hear the question: “But what about pulling 29 inHg or more? That’s what I read everywhere on websites about vacuum bagging, resin infusion, and vacuum pumps.”

And that is exactly where misunderstandings begin. I will try to clarify this in the next chapters.