1.  Purpose: A suction/pressure unit can provide controlled pressure, vacuum, or both. They can be portable or fixed  Suction Units Uses  High volume Saliva evacuators in dental Surgery and ER for drainage of wounds and massive bleeding On "crash carts" for quick aspiration during emergencies

2.  Low volume  ENT-for minor surgery, ear irrigation, etc  Wards-thermatic units for slow drainage  Various lab and dental equipment; cell counters, vacuum mixer investor, porcelain furnace  Pressure Units Uses  Automatic BP cuffs  Nebulizers and portable ventilators  Impedance audiometers and tympanographs  Automated lab equipment to operate valves, pistons, etc

3.  Types of Suction/Pressure Units  Piston  Operates on principle of moving piston inside a cylinder  Valves (either cam operated or reed) control direction of air flow  Vary greatly in size  Can achieve extremely high volume and pressures  Usually require maintenance of oil level-although some are "oil less"

4.  Oscillating pumps  A rapidly oscillating rubber diaphragm replaces the moving piston  Small versions (aquarium pump style) are found in some cell counters  Larger versions are seen in dental lab  Can obtain fairly good vacuum and pressure  Virtually maintenance free Does require periodic replacement of diaphragm

5.  Rotary vane  Uses an eccentric rotor within a cylinder  Movable vanes keep tight seal  Air intake provides vacuum  Exhaust side supplies pressure  Can obtain very high vacuum  Very common throughout hospital (portable pressure/suction units)  Require periodic maintenance of oil level and cleaning of vanes

6.  Thermotic (Intermittent)  Uses principle of Charles's Law  Gas expands when heated  The gas contracts when cooled  Valves control direction of air flow  Used for vacuum applications only  Vacuum produced is cyclic-intermittent  Extremely low vacuum and volume  No moving parts and maintenance free

7.  Measurement  Vacuum  mmHg (millimeters of Mercury)  inches of Hg (Hg = Mercury)  cm of Hg (cm = centimeters)  inches of water  cm of water

8.  Pressure lb/sq in (pounds per square inch) Kg/sq cm (kilograms per square centimeter) mmHg inches of Hg  Flow rate cu ft/min liters/min liters/hr  Conversions - units of measurements within each category can be converted