What is Vacuum
Vacuum is the absence of matter within a defined space. This generally means that air has been evacuated from that space. The amount of air still remaining in proportion to the area determines the measure of vacuum. A perfect vacuum has no matter at all remaining
How Vacuum is Measured
Vacuum has two measurable properties, its negative pressure, or pressure differential, and volumetric flow. Vacuum Pressure or the pressure exerted by a denser outside atmosphere, is measured in a variety of units. Vac Cubes uses “hg, inches of mercury, as our standard unit although bar, torr (mmhg) and % vacuum are also commonly used. Vac Cubes uses CFM, as our unit of volumetric flow, other common units include m3/h and l/m. CFM is the number of cubic feet of air moved through a cross section in a minute.
How Vacuum Works
Vacuum works by using the pressure of our atmosphere to press down on a surface more so than the side with the vacuum can push back. For example a suction cup is pressed onto a piece of glass and holds the two together. It can also be seen as the tight film wrapped around packaged meats.
Since Vacuum is a measurement of atmospheric pressure it is important to understand how the air pressure around us works. Due to Earth’s gravity, everything with mass is pulled towards the Earth’s surface at 9.81m/s2 including the air. This makes the air pressure on the surface of the Earth similar to the water pressure on the bottom of the ocean. The lower you are to sea level, the higher the pressure. Likewise the higher the altitude, the lower the pressure. Atmospheric pressure at sea level is 14.7 PSI or 1013.25 mbar. At the peak of Mount Everest it is 4.6 PSI or 317 mbar. A vacuum pump will perform the same no matter the altitude or surrounding air pressure as a percentage of vacuum. For example a vacuum pump creating 90% vacuum at sea level has pulled the pressure of 1013.25 mbar down to 101.325 mbar. At Mount Everest, 90% vacuum is 31.7 mbar. However as the atmospheric pressure drops the holding force of the vacuum drops as well. This is because the differential in pressure is a subtractive measurement where % of vacuum is a proportion. 90% vacuum at sea level has the force of 911.925 mbar holding down the surface of the vacuum where at Mount Everest there is only 285.3 mbar.
The Venturi Effect
The venturi effect is based on the Bernoulli principle. It states that fluids at higher velocity have a lower pressure than fluids at a lower velocity. Vac Cubes vacuum pumps have been designed so that compressed air is constricted into a nozzle, which speeds up the air flow so that it exits the nozzle at supersonic speeds. Accordingly this air has a very low pressure. This bead of supersonic air is moving through a “vacuum” chamber on its way to the next receiver nozzle. The air in the vacuum chamber starts at atmospheric pressure which is much higher pressure than the fast moving bead of air. Air seeking equilibrium moves from the higher pressure area to the lower pressure area and continues out with the flow of air through the nozzle out of the pump.
Single Venturi vs. Multi Venturi
All venturi vacuum pumps require compressed air fed into a nozzle. The nozzle is a gradually restrictive orifice that slowly expands. This controls the amount of air that can pass through it as well as accelerating it. The air then leaves the first nozzle as a fast moving bead. This moves across a gap on its way into a second receiver nozzle. Across this gap is where vacuum is made. In a single venturi the second receiver nozzle exhausts out to atmosphere. In a multi venturi the second receiver nozzle continues the process over again and sends air across another gap to create vacuum. The air enters a third and then a forth nozzle before exiting the pump to atmosphere. By recycling the compressed air across multiple nozzles we can utilize the remaining energy to increase vacuum flow, thus making the multi venturi 3 times more efficient than the single venturi.