The term “vacuum” was defined by physicists many years ago, and the DIN standards now provide a generally applicable definition, but application engineers make a distinction between various forms of vacuum:

• Ultrahigh vacuum: Pressure range: < 10-7 mba
  Applications: Scattering of metals as dust, vapour deposition and electronbeam smelting.

• High vacuum: Pressure range: 10-3 to 10-7 mbar
  Applications: Smelting or annealing of metals and production of electronic tubes.

• Medium-high vacuum: Pressure range: 10-3 to 1 mbar
  Applications: Degassing of steel, production of incandescent lamps, freeze-drying of food or drying of plastics

• Low vacuum: Pressure range: between 1 mbar and ambient air pressure
  Applications: This is the range in which handling-technology applications operate. In practice, vacuums in the low-vacuum range are specified as percentages, i.e. the vacuum as a proportion of the ambient air pressure.
  In this range, the workpiece material plays a decisive role for vacuum handling:
  Materials with airtight surfaces (such as metals, plastics) are generally handled with a vacuum of 60% to 80%. In the case of porous/permeable materials (such as cardboard, pressed chipboards), the vacuum which can be achieved, which the basis for further calculations, must first be determined by experiments. As a rule, however, such materials are handled with a vacuum of about 20% to 40%.

How is vacuum measured?

For technical applications, it is naturally important that the medium “vacuum” can be evaluated and thus measured. However, when specifying a vacuum, the various types of specification and the various units of measurement must be taken into account.

How can a vacuum be specified?

Specification as a relative value

In practice, vacuums in the low-vacuum range are specified as relative values: the vacuum is specified relative to the ambient air pressure. The value thus always has a negative sign, since the ambient air pressure (atmospheric pressure) is the reference value and is assumed to be 0.

Specification as an absolute value

In scientific applications, and in the medium and high vacuum ranges, the vacuum is specified as an value referred to an absolute vacuum (such as that which exists in empty space). The value is always positive. The conversion tables below show the relationship between absolute and relative pressures and references to other commonly used units of measurement.

Which units of measurement are used?

The official unit of measurement for pressure and vacuum is the Pascal [Pa]. Technical applications also use the multiples kilopascal [kPa = 1,000 Pa] and megapascal [Mpa = 1,000,000 Pa]. A further common unit is the hectopascal [hPa = 100 Pa], since this simplifies conversion from and to the “old” unit millibar [mbar]:

1 hPa = 1 mbar

However, the Pascal is still not used widely in practical applications. For this reason, all vacuum values in this catalogue are specified in bar, mbar or %.

The “%” specification is typical for relative specification of the performance of a vacuum generator, since it is not affected by the actual ambient air pressure (see page 1.7, Atmosphere).

Various other units of measurement are used in other countries. Some of these are included in the following table.

What is the basis for the calculations in this catalogue?

All calculations and forces specified in the design data in this catalogue are based on a vacuum of 60% (–600 mbar).

Power required for vacuum generation

High vacuums require a disproportionally high energy input. Increasing the vacuum from –0.6 to –0.9 bar involves a force increase by a factor of 1.5, but the evacuation time and the energy required increase by a factor of 3! For handling technology, this means that excessively high vacuums cannot be generated economically and that it is better to seek a solution which works with a lower vacuum.

The atmosphere and its effects on vacuum technology

The atmosphere is the layer of air which surrounds the Earth. This layer of air is several kilometres thick and its weight exerts a corresponding pressure – called the atmospheric pressure – on the ground and everything on it. The weight of a column of air above a ground area of 1 m2 is about 10.000 kg.

Since the atmospheric pressure depends on the height of this column of air, it varies with the altitude. As can be seen in the diagram below, the atmospheric pressure at sea level is 1.013 mbar. At the Schmalz headquarters at an altitude of 600 m, it drops to 938 mbar and at an altitude of 2000 m it is only 763 mbar. This reduction of the atmospheric pressure with increasing altitude also affects working with vacuum, as can be seen in the following example:

Ambient pressure at 2000 m = 763 mbar

Ambient pressure in the SCHMALZ headquarter (altitude 600 m) = 938 mbar

Ambient pressure at sea level (0 m) = 1.013 mbar (standard atmospheric pressure in accordance with DIN 1343)

The atmospheric pressure drops as the altitude increases. This also reduces the maximum pressure difference which can be achieved and thus the maximum holding force of a suction pad. The atmospheric pressure drops by 12.5 mbar for every 100 m increase in altitude. A vacuum pump which can generate a vacuum of 80% (or –800 mbar) at sea level will be able to generate a vacuum of only 610 mbar (80% of the ambient pressure of 763 mbar) at an altitude of 2000 meters. The holding force of a suction pad operated with this vacuum will be reduced accordingly.