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Glossary of Vacuum Terminologies


Absolute vacuum: A state where pressure is completely absent. Like absolute zero (0 ?K), it is a theoretical value, used to define the zero point for many vacuum scales.


Backing pump: A vacuum pump (diaphragm or rotary vane) that is used at the outlet of another vacuum pump to keep it at a reduced pressure.

Bar: A unit of pressure, defined as 1,000,000 dynes/cm2.

Bourdon gauge: A Bourdon gauge is a common dial vacuum gauge. Pressure measurements are made relative to atmospheric pressure and as such can be skewed. Precision is also impaired as the gauge functions purely mechanically. The prime advantages of a Bourdon gauge are low cost and rugged construction.


Calibration: In reference to a diaphragm pump, calibration is the alignment of the diaphragms to provide maximum performance without directly contacting the pump head. If the diaphragm contacts the pump head it can cause excessive noise and wear. If it does not come close enough to the pump head, vacuum performance suffers.

Coarse vacuum: See rough vacuum.

Cold trap: A device used on a pump inlet to protect pumps from chemical vapors when operating in the fine vacuum range. Cold traps lose their effectiveness when operated in the rough vacuum range, such as that used in most laboratory applications. Chemistry design diaphragm vacuum pumps do not require cold traps to protect them from chemical vapors. Often diaphragm pumps will be fitted with an outlet condenser, to recover solvents and protect the user and environment from chemical vapors.

Chemistry-HYBRID pump: A vacuum pump that combines both rotary vane and diaphragm technology. A two-stage diaphragm pump is mounted to the outlet of a two-stage rotary vane pump. Because the outlet of the rotary vane pump is at a reduced pressure, many chemicals that would ordinarily condense inside the pump are now pumped out via the diaphragm pump, where they can be easily condensed. By keeping the oil chamber at reduced pressure, the water vapor tolerance is much greater, the pump oil stays cleaner, pump life can be extended, and some applications may not need cold traps.


Diaphragm pump: A pump that uses two valves and a moving diaphragm to move substances through it, analogous to the way a heart functions. Diaphragm pumps require no oil for their function. Diaphragm pumps operate in the rough or coarse vacuum range down to approximately 1 mbar.


Fine vacuum: Pressures in the range from 1 mbar to 10-3 mbar also known as medium vacuum. Pressures in this range are usually produced by either rotary vane or roots pumps. The most common laboratory application requiring a pressure this low is freeze drying.

Flow curve: A plot of the flow rate of a pump at various pressures. This curve reflects the true performance of a pump, throughout its range of operation, better than the two end points of the curve—atmospheric pressure and ultimate vacuum—that are typically used for pump specifications. Due to the nature of vacuum, a flow curve should be plotted on a log-log scale. When comparing flow curves, make sure that the graph scales match. A well designed diaphragm or rotary vane pump should have
a steep curve near the ultimate vacuum level, indicating high flow rate until close to the ultimate vacuum. A higher flow rate in the operating vacuum range will allow the pump to reach the desired vacuum faster.

Flow rate: The flow rate is the amount of air or other gas that can be passed through a pump, measured at or near atmospheric pressure. On diaphragm and rotary vane pumps, the flow rate falls off under vacuum, dropping to zero at the ultimate vacuum. Since vacuum pumps operate at a vacuum and not typically at atmospheric pressure, the flow curve gives a truer indication of pump performance under real life conditions.


Hysteresis: As used in vacuum controllers, the acceptable range of pressure around the target value before the solenoid valve reopens. This is analogous to the way a thermostat operates to control temperature. Some hysteresis is necessary in a conventional vacuum controller/solenoid valve arrangement to avoid continuous opening and closing of the solenoid valve.


Inlet catchpot: A flask mounted at the inlet of a vacuum pump to help protect the pump from particles that might come from the application. It also collects any liquids that may have condensed in the vacuum line that could impair performance.


Medium vacuum: See fine vacuum.

Membrane pump: See diaphragm pump.

Millibar (mbar): A unit of pressure defined as 1/1,000 bar, or 1,000 dynes/cm2. Atmospheric pressure at sea level is approximately 1,013 mbar. One hectopascal (hPa) is equal to one millibar.

Molded diaphragm: A diaphragm made by molding elastomer permanently to a pushrod. Due to the tolerances in the process, a pump using molded diaphragm will need to be calibrated after a diaphragm change. Molded diaphragms are also less flexible than the thinner diaphragms made from elastomer sheets.


Oil filter: Some rotary vane pumps can be fitted with an oil filter, much like that on a car, to remove particles and polar substances from the oil. This can extend time between oil changes and pump life. Not to be confused with an oil mist filter.

Oil mist: A fine mist of pump oil that is generated at the outlet of a rotary vane pump during normal operation, as much as 2cm3/hr/m3 at standard temperature and pressure. This mist can pose a health hazard as well as a safety hazard after settling on surfaces. High flow rate, shallow vacuum applications tend to produce more oil mist than lower flow rate applications.

Oil mist filter: A device fitted to the outlet of a rotary vane pump that collects much of the oil mist generated by the pump.

Outlet catchpot: An outlet catchpot collects liquid that is blown out of the pump, or from an outlet condenser.

Outlet condenser: A condenser fitted to the exhaust port of a diaphragm pump to recover solvents that would otherwise be released into the atmosphere. Because the exhaust of a diaphragm pump is at atmospheric pressure, condensing of solvents can be achieved at much higher temperatures than if performed under vacuum, as is the case with a cold trap.


Piezoresistive gauge: The transducer in a piezoresistive gauge changes resistance with pressure, which is measured electronically. The resistance is highly reproducible, and independent of the gas. The transducer can also be made very chemically resistant. The operable range for piezoresistive gauges is from slightly above atmospheric pressure through the rough vacuum range.

Pirani gauge: A Pirani gauge measures pressure by thermal conductivity of the gas being measured. Because of the method used, there are two potential problems with Pirani gauges. One is that the pressure measurement is dependent upon the gas used. Pirani gauges are calibrated using pure nitrogen, and will be inaccurate by as much as 20% with other gasses. The second problem is the transducers are not corrosion resistant. Despite these problems, Pirani gauges are quite useful in the fine vacuum range, and eliminate the hazards of mercury manometers.

PTFE: A highly chemical resistant flouropolymer used in chemistry design diaphragm vacuum pumps and other laboratory equipment designed for aggressive environments. PTFE manufactured by DuPont is given the trademark Teflon®.

Pump stage: A basic unit of a pump, whether diaphragm or rotary vane. By increasing the number of stages in a pump, within certain limits, a pump can obtain a deeper ultimate vacuum. Diaphragm pumps will have one, two, three or possibly four stages, while a rotary vane pump will usually have one or two.


Rotary vane pump: A traditional design of vacuum pump that requires the use of oil for a seal as well as a lubricant. Use of a rotary vane pump for chemistry applications requires protective measures, such as a cold trap, to protect the pump from chemical vapors that contaminate the oil and degrade lubricating effectiveness. Rotary vane pumps operate best in the fine vacuum range between 1 mbar and 10-3 mbar. The deeper vacuum provided by a rotary vane pump is not required for most laboratory applications except freeze drying.

Rough vacuum: Also known as coarse vacuum, this range covers pressures from atmospheric to 1 millibar. Most laboratory vacuum requirements are in this range. Rotary vane pumps are inefficient in this range.


Torr: A unit of pressure, formerly millimeters of mercury (Hg). Named after Evangelista Torricelli, Italian mathematician and physicist. Atmospheric pressure at sea level is approximately 760 torr.


Ultimate vacuum: The point at which the flow rate of a pump reaches zero. When used in an application, a vacuum pump will never actually reach the ultimate vacuum, but reach equilibrium somewhat above it, due to a combination of the vapor flow from the application and leakage. Of two pumps with identical ultimate vacuum specifications, the pump providing a higher flow rate just above the ultimate vacuum will obtain a deeper vacuum than one with a lower flow rate in application. This can be determined by looking at the flow curve.


Vacuum: Pressures lower than atmospheric pressure. Contrary to popular belief, there is no such thing as "suction." A higher pressure will tend to push into an area of lower pressure until there is an equilibrium. See also "Absolute vacuum."