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How is vacuum pressure measured?

The pressure ranges in vacuum technology and their characterization

It is common in vacuum technology to subdivide its wide overall pressure range – which spans more than 16 powers of ten – into smaller individual regimes. These are generally defined as follows:

Rough vacuum (RV) 1000 – 1 mbar 

Medium vacuum (MV) 1 – 10-3 mbar 

High vacuum (HV) 10-3– 10-7 mbar 

Ultrahigh vacuum (UHV) 10-7 – (10-14) mbar

Table IX Pressure ranges used in vacuum technology and their characteristics (numbers rounded off to whole power of ten)

This division is, naturally, somewhat arbitrary. Chemists in particular may refer to the spectrum of greatest interest to them, lying between 100 and 1 mbar, as “intermediate vacuum”. Some engineers may not refer to vacuum at all but instead speak of “low pressure” or even “negative pressure”. The pressure regimes listed above can, however, be delineated quite satisfactorily from an observation of the gas-kinetic situation and the nature of gas flow. The operating technologies in the various ranges will differ, as well. 

Atmospheric air

Prior to evacuation, every vacuum system on earth contains air and it will always be surrounded by air during operation. This makes it necessary to be familiar with the physical and chemical properties of atmospheric air. 

The atmosphere is made up of a number of gases and, near the earth’s surface, water vapor as well. The pressure exerted by atmospheric air is referenced to sea level. Average atmospheric pressure is 1013 mbar (equivalent to the “atmosphere”, a unit of measure used earlier). Table VIII shows the composition of the standard atmosphere at relative humidity of 50% and temperature of 68°F or 20°C. 

Leybold - Vacuum Fundamentals graphics

Table VIII Composition of atmospheric air. Note: In the composition of atmospheric air the relative humidity (RH) is indicated separately along with the temperature.

At the given relative humidity, therefore, the air pressure read on the barometer is 1024 mbar.

Table XVII Temperature comparison and conversion table (Rounded off to whole degrees)

In terms of vacuum technology, the following points should be noted in regard to the composition of the air: 

a) The water vapor contained in the air, varying according to the humidity level, plays an important part when evacuating a vacuum plant (see page on pumping gases – wet process). 

b) The considerable amount of the inert gas argon should be taken into account in evacuation procedures using sorption pumps.  

c) In spite of the very low content of helium in the atmosphere, only about 5 ppm (parts per million), this inert gas makes itself particularly obvious in ultrahigh vacuum systems which are sealed with Viton or which incorporate glass or quartz components. Helium is able to permeate these substances to a measurable extent. 

The pressure of atmospheric air falls with rising altitude above the earth’s surface (see Fig. 9.3). High vacuum prevails at an altitude of about 328,083 ft (100km) and ultrahigh vacuum above 1,312,335 ft (400km.) The composition of the air also changes with the distance to the surface of the earth (see Fig. 9.4). 

Fundamentas of Vacuum graphics

Fig 9.3 Decrease in air pressure (1) and change in temperature (2) as a function of altitude

Fundamentas of Vacuum graphics

Fig 9.4 Change in gas composition of the atmosphere as a function of altitude

Units of pressure and their definition

Pressure p (mbar)

Pressure of fluids (gases and liquids). (Quantity: pressure; symbol: p; unit of measure: millibar; abbreviation: mbar.) Pressure is defined in DIN Standard 1314 as the quotient of standardized force applied to a surface and the extent of this surface (force referenced to the surface area). Even though the Torr is no longer used as a unit for measuring pressure, it is nonetheless useful in the interest of transparency to mention this pressure unit: 1 Torr is that gas pressure which is able to raise a column of mercury by 1 mm at 32°F (0°C). (Standard atmospheric pressure is 760 Torr or 760 mm Hg.) Pressure p can be more closely defined by way of subscripts: 

Absolute pressure pabs

Absolute pressure is always specified in vacuum technology so that the “abs” index can normally be omitted.

Total pressure pt

The total pressure in a vessel is the sum of the partial pressures for all the gases and vapors within the vessel. 

Partial pressure pi

The partial pressure of a certain gas or vapor is the pressure which that gas or vapor would exert if it alone were present in the vessel. Important note: Particularly in rough vacuum technology, partial pressure in a mix of gas and vapor is often understood to be the sum of the partial pressures for all the non-condensable components present in the mix – in case of the “partial ultimate pressure” at a rotary vane pump, for example. 

Saturation vapor pressure ps

The pressure of the saturated vapor is referred to as saturation vapor pressure ps. ps will be a function of temperature for any given substance. 

Vapor pressure pd

Partial pressure of those vapors which can be liquefied at the temperature of liquid nitrogen (LN2).

Standard pressure pn

Standard pressure pn is defined in DIN 1343 as a pressure of pn = 1013.25 mbar. 

Ultimate pressure pend

The lowest pressure which can be achieved in a vacuum vessel. The so-called ultimate pressure pend depends not only on the pump’s suction speed but also upon the vapor pressure pd for the lubricants, sealants and propellants used in the pump. If a container is evacuated simply with an oil-sealed rotary (positive displacement) vacuum pump, then the ultimate pressure which can be attained will be determined primarily by the vapor pressure of the pump oil being used and, depending on the cleanliness of the vessel, also on the vapors released from the vessel walls and, of course, on the leak tightness of the vacuum vessel itself. 

Ambient pressure pamb

or (absolute) atmospheric pressure

Overpressure pe or gauge pressure

(Index symbol from “excess”)

Leybold - Vacuum Fundamentals graphics

Here positive values for pe will indicate overpressure or gauge pressure; negative values will characterize a vacuum. 

Working pressure pw

During evacuation the gases and/or vapors are removed from a vessel. Gases are understood to be matter in a gaseous state which will not, however, condense at working or operating temperature. Vapor is also matter in a gaseous state but it may be liquefied at prevailing temperatures by increasing pressure. Finally, saturated vapor is matter which at the prevailing temperature is gas in equilibrium with the liquid phase of the same substance. 

Table I Permissible pressure units including the torr 1) and its conversion.

  1. The torr is included in the table only to facilitate the transition from this familiar unit to the statutory units N · m-2, mbar and bar. In future the pressure units torr, mm water column, mm mercury column (mm Hg), % vacuum, technical atmosphere (at), physical atmosphere (atm), atmosphere absolute (ata), pressure above atmospheric and pressure below atmospheric may no longer be used. Reference is made to DIN 1314 in this context.
  2. The unit Newton divided by square meters (N · m-2) is also designated as Pascal (Pa): 1 N · m-2= 1 Pa. Newton divided by square meters or Pascal is the SI unit for the pressure of fluids.
  3. 1 torr = 4/3 mbar; fl torr = 1 mbar.
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References and sources

References, sources and further reading related to the fundamental knowledge of vacuum technology



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