Glossary of Vacuum Terms

Absolute Pressure – A term used in engineering literature to indicate pressure and temperature that have zero as a base point. Absolute vacuum has 0″ of HgAb as perfect vacuum. Absolute temperature is usually measured in Rankine where 0 degrees is the complete absence of heat.

Absolute Zero — The zero point on the absolute temperature scale (0 degrees R or K).

Absorption – The binding of gas in the interior of a solid (or liquid).

ACFM/CFM – Actual pumping speed, at a given pressure, in cubic feet per minute. Actual Cubic Feet per Minute or Cubic Feet per Minute.

Adsorbate – A material which takes in gas by adsorption.

Adsorption – The condensing of gas on the surface of a cooled solid.

Air-Inlet Valve – A valve used for letting atmospheric air into a vacuum system or chamber. Also called a vacuum breaker or vent valve.

Antisuckback Valve – A valve or other device that prevents the migration of oil and air from a vacuum pump into the system, when the pumps stops and the system is under vacuum.

Atmospheric Pressure —The pressure of the atmosphere at a specified place and time. The normal or “standard” atmosphere has been defined as the pressure exerted by a mercury column 760 mm in height at 0 degrees C.

Backstreaming —The back flow of the working fluid of a pump upstream toward the vessel being evacuated.

Belt-Drive Pump —A pump with the motor drive provided by a belt. A pulley is attached to the pump shaft. The ratios of the diameters of the pump and motor pulleys determine the actual rotational speed of the pump.

Cold Trap or Condenser – A vessel designed to hold a refrigerant, or cooled by coils in which a refrigerant circulates, inserted into a vacuum system to condense on vapors present in the system.

Compression Ratio – The ratio between the outlet pressure and the inlet pressure of a pump for a specific gas.

Compound Mechanical Pump – A mechanical pump having two or more stages of compression.

Condensation Rate – The number of molecules which condense on a surface per square cm/second.

Conductance – The actual capacity of a piping system in CFM. Conductance in a vacuum system can be limited by line size and configuration.

Degassing – The removal of gas and vapors from a substance under vacuum.

Direct-Drive Pump – A pump with the motor drive provided by a direct coupling to the pump rotor shaft. The rotational speed of the motor is the rotational speed of the pump.

Displacement – The geometric volume swept out per unit time by the working mechanism of mechanical pumps at normal rotational speeds. Also called free air displacement. This value, being theoretical, is not usable by end-users. It is mainly a standard used by vacuum pump manufacturers.

Foreline – Vacuum line connecting the vacuum system to the inlet of a vacuum pump.

Gas – Gas is defined as the state of matter in which the molecules are practically unrestricted by intermolecular forces so that the molecules are free to occupy any space within an enclosure. In vacuum technology the word “gas” has been loosely applied to both the permanent (non-condensable) gases and vapors, or condensable gases.

Gas Ballast – The venting of the compression chamber of a mechanical pump to the atmosphere to prevent condensation of condensable vapors within the pump.
High Vacuum – Vacuum range from 10-3 to 10-6 Torr.

Ideal Gas – A gas which obeys Boyle’s Law and has zero heat of free expansion (Charles’ Law). Also known as a perfect gas.

Impedance – The reciprocal of the conductance. Also called resistance.

Implosion – The rapid inward collapsing of the walls of a vacuum system or device as the result of failure of the walls to sustain the external atmospheric pressure.

Inlet Pressure – The total pressure measured at the inlet of a vacuum pump.

Isolation Valve – A valve that seals off a vacuum system from the vacuum pump when the pump is off.

Leak – A hole or porosity in the wall of a vacuum system. It is very important to maintain vacuum system leaks due to the subsequent loss of vacuum that can occur.

Leakage Rate – The rate of flow of a gas through a leak. Pressure rise per unit time.

Low Vacuum or Rough Vacuum – Vacuum range from atmospheric pressure to 29.88″ Hg gauge (1 Torr).

McLeod Gauge – A liquid mercury level vacuum gauge that measures the pressure in a vacuum system.

Mean Free Path – The average distance a gas molecule can travel before colliding with another gas molecule. The mean free path is dependent on the density of the gas and the diameter of the molecule.

Mechanical Pump – A pump which moves the gas by the cyclic motion of a system of mechanical parts such as pistons, eccentric rotors, vanes, screws, valves, etc.

Micron (Micron of Mercury) – Unit of pressure equal to .001 Torr. Also known as a millitorr.

Millimeter of Mercury – Unit of pressure equal to 1 Torr.

Non-Condensable Gas – Permanent gases inside a vacuum system, not to be confused with vapors.

Oil Separator – An oil reservoir with baffles to reduce the loss of oil by condensation in the exhaust.

Outgassing – The escape of gas from materials within a vacuum system. Can be a limiting factor in the ultimate pressure obtained.

Oxygen Service – VERY IMPORTANT: Evacuation of oxygen by a vacuum pump at partial pressures above that of normally occurring oxygen in the air or 21%. Vacuum pumps for O2 service must be specially prepared or explosion could occur!

Partial Pressure – The pressure due to a gas or vapor component of a gaseous mixture. For example, in atmospheric pressure, nitrogen has a certain partial pressure that makes up a large percentage of the total pressure.

Process – The operation for which the vessel is being evacuated.

Pumpdown Curve – A graph representing the relationship between pressure and time. Used to determine the time required to achieve the desired operating pressure in a system with a given pump.

Pumping Speed – The volume of gas per unit of time which the vacuum pump is able to remove from the system. Pumping speed is often expressed in CFM (Cubic Feet per
Minute), L/M (Liters per Minute), or L/S (Liters per Second).

Rate of Rise – The timed rate of pressure increase during a given interval in a vacuum system which is isolated from the pump by a valve.

Roots Blower Pump – A rotary blower pump having a pair of inter-engaging impellers. This type of pump is usually used in conjunction with a backing pump to increase the pumping speed of a vacuum system over a certain pressure range.

Roughing Time – The time required to pump a given system from atmospheric pressure to the holding pressure for a vapor pump or other medium or high vacuum pump. The
usage here is roughing pump, roughing time, rough out the system, etc.

SCFM – Volume flow referenced to standard conditions. Standard cubic feet per minute.

Standard conditions are sea level pressure (760 Torr) and 68 degrees F.

Stage – Operating unit of a vacuum pump. By combining stages in series a higher compression ration can be achieved.

Throughput – The quantity of gas in pressure-volume units at a specified temperature flowing per unit of time across an open cross section of a pump or pipe line.

Torr – Unit of pressure equal to 1 mm of Mercury.

Time of Evacuation – The time required to pump a given system from atmospheric pressure to a specified base pressure. Also known as pumpdown time or exhaust time.

Total Pressure – Sum of all the partial pressures in a gas mixture.

Trap -An accessory used condense vapors present in the vacuum.

Ultimate Pressure – Lowest attainable pressure in a vacuum system. In a vacuum pump, the lowest pressure that can be attained with that pump. Ultimate pressure is limited by the pumping speed of the vacuum pump and the vapor pressure of the sealing fluid, among other factors.

Vacuum – Any pressure in a chamber that is less than the ambient atmospheric pressure.

Vacuum Cooling – A process for lowering the temperature of a material by subjecting it to vacuum conditions to cause vaporization of a liquid.

Vacuum Drying – The removal by evaporation of liquid from a substance in a vacuum. When the liquid is water, the process is sometimes called vacuum dehydration.

Vacuum Gauge – Any instrument used to measure pressure in a vacuum system. Called manometers, diaphragm, thermocouple, pirani, McLeod, etc.

Vacuum Manifold – Part of the vacuum system piping, an enclosure with ports so that a number of vacuum processes can be operated simultaneously.

Vacuum System – A system designed for the manufacture of a product or the operation of a process. Consists of a vacuum pump or pumps, vacuum chamber, interconnecting piping and a variety of other components.

Vapor – A substance in gas phase which is condensable at ambient temperatures.

Vapor Pressure – The partial pressure exerted by a vapor.

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