Pressure & Vacuum

Atmospheric pressure is 1.0332 kg/cm2. So at 1.0332 kg/cm2 pressure we don't feel any pressure. In fact in practical purposes reference point for measuring pressure is atmospheric pressure and is called gauge pressure. So value of gauge pressure is 1.0332 kg/cm2 lower than absolute pressure.

Above atmospheric pressure is called gauge pressure and below atmospheric pressure is called vacuum pressure or simply vacuum. In other words vacuum is negative gauge pressure. Positive gauge pressure presses and negative gauge pressure pulls. It can be compared with compressive stress and tensile stress for materials. Incidentally the unit for both pressure and stress is same.

It applies to human psychology too. When we have too much work load, we say- "I'm pressed". But when we think too much without end result, we say- "I'm tense".

Suction is not the lack of anything. It is the movement of matter due to a pressure gradient. It can also occur at high pressures, as long as there is a pressure differential, so has nothing to do with low pressure, and most certainly has nothing to do with vacuum (although the two terms are often wrongly seen as interchangeable). You can have suction without vacuum, and vacuum without suction. The following is an extract from Wikipedia, which I think also defines suction rather well:

"Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner.

The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter."

So, suction is the movement of matter due to a pressure gradient. It has nothing to do with pressure itself, only with differential pressure.

Vacuum is also not necessarily the lack of anything. For example, it would be considered a vacuum condition when a vessel has an internal pressure of, say, 500mbara. Yet this is a positive pressure, it is just that it is less than atmospheric pressure. Vacuum is specifically the pressure range between zero pressure and atmospheric pressure.