Low vacuum pump applications and selection: some considerations
Two-stage rotary vane pumps and the even higher (oil diffusion, turbo molecular, ion getter) vacuum pumps are selected considering pumping speed and ultimate vacuum in terms of removing some of the remaining very small fraction of gas molecules from a space.
The weight of the earth atmosphere exerts a pressure of 1.033 kg /cm². So a negative pressure of that would be a perfect vacuum. The difference between a mediocre vacuum pump (all diaphragm pumps give a relatively poor vacuum, but they are certainly not useless) and the very highest vacua is small in terms of kg/cm². High vacuum pumps remove a fraction of one percent of the remaining molecules. That no longer represents any real change in the weight of the atmosphere. So a vacuum of 0.8kg/cm² is good for lifting and filtering, but not sufficient to even back a turbo molecular pump.
To obtain faster filtering or to lift objects it is best to compare vacua expressed in kg/cm². Any vacuum will exert some suction and so get some flow. Here is a conversion of our three diaphragm pumps - giving capacity in kg/ cm² which gives you a practical comparison of three diaphragm pumps suction power.
- VFV-30 Diaphragm Vacuum Pump, 300 mmHg 0.408 kg / cm²
- VFV-60 Diaphragm Vacuum Pump, 600 mmHg 0.816 kg / cm²
- VF3-220 Oil-less Diaphragm Vacuum/Pressure Pump 0.881 kg / cm²
The biggest pump gives little more negative pressure, but that pump has a much greater pumping speed (litres/min or m3/hour) than the other two pumps. So that pump would offer little advantage when filtering and only excel if lifting involves much leakage. Obviously the larger pump is much better for pumping on larger volumes - like a vacuum oven.
However, if the application is vacuum infiltration and involves low boiling point and particularly hot materials (resins), then a small pump more easily avoids boiling.