The vacuum pressure required will depend on the specific application.
For normal engine operation at idle speed, the vacuum reading should be between 14 in. and 22 in. Hg. When the throttle is quickly opened and closed, the vacuum should drop below 5 in. and then rebound to 23 in. or more.
For operations that use vacuum to move liquids, such as vacuum filtration or liquid aspiration, a vacuum no deeper than 100 mbar is typically sufficient. This represents about 90% of the total pressure differential available, considering atmospheric pressure at sea level is about 1000 mbar.
Evaporative applications often require greater vacuum depths, closer to the 2 mbar range. The specific vacuum depth needed will depend on the solvents involved, temperature, and other conditions of the application. Careful calculation is necessary to determine the specific depth and pump required.
In terms of vacuum pump basics, there are different pressure ranges:
- Rough/Low Vacuum: 1000 to 1 mbar / 760 to 0.75 Torr
- Fine/Medium Vacuum: 1 to 10-3 mbar / 0.75 to 7.5-3 Torr
- High Vacuum: 10-3 to 10-7 mbar / 7.5-3 to 7.5-7 Torr
- Ultra-High Vacuum: 10-7 to 10-11 mbar / 7.5-7 to 7.5-11 Torr
- Extreme High Vacuum: < 10-11 mbar / < 7.5-11 Torr
For furnace performance, the ultimate vacuum for a diffusion pumped furnace is in the 10-6 Torr range, while for furnaces using mechanical pump/blower combinations, the ultimate vacuum is 20 Microns Hg or better. The operating vacuum with a diffusion pump is in the 10-5 Torr range, and without a diffusion pump, it is in the 50-60 Micron Hg range. The pump-down time to high vacuum cross-over is approximately 10 minutes, and the pump-down time to 1 x 10-4 Torr is approximately 15 minutes. The furnace leak rate should be less than 5 Microns per hour.
When comparing vacuum systems with and without cold traps, the ultimate pressure in the vessel being pumped is around 10-6 to 10-7 Torr without a cold trap. By using a cold trap, the ultimate pressure can reach 10-9 Torr, but it may lower the pumping speed by about 40%. The capacity of the pumps may also be reduced by as much as 50% at low pressures when using cold traps.
In evaporative applications, it is important to have a pump with an "ultimate vacuum" that is somewhat less than the vapor pressure of the substance being evaporated. For example, if the vapor pressure of water at room temperature is about 20 Torr, a pump with an ultimate vacuum of less than 20 Torr would be needed to effectively move vapor from the application.
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