Vacuum Unit and Conversion
Hardly any other physical dimension have as many different units as vacuum units do. The reasons for this are both the difference between the metric and imperial measurement systems and the various reference values. The absolute zero for pressure (the absolute vacuum) is common to all units. The following table contains the most common dimensions and their conversions into the pascal SI unit. In vacuum handling technology, the pressure units of pascal (Pa) as the SI unit, bar (bar) and the percentage value (s.u.) have become established in Europe.
Relative data for pressure (vacuum units):
In vacuum technology, negative values are normally used for the units specified in the table above (e.g. -600 mbar). Negative pressure is not physically possible. Rather, the negative values are to be understood as the pressure difference from the ambient pressure. Assuming an ambient pressure of 1,000 mbar (just below standard atmospheric pressure), the -600 mbar relative pressure specified above corresponds to an absolute pressure of 400 mbar. In this case, you would often speak of a vacuum of 600 mbar. The relative data is particularly significant for calculating the carrying capacity. The differential pressure (amount of the relative vacuum) is the only piece of data that can be used for calculation as it provides carrying capacity data that does not depend on the attitude of the ambient pressure.
The percentage values are often used for the industrial vacuum suction cup and vacuum lifters and are to be understood in a similar way to the relative data. They compare the underpressure level to the ambient pressure and specify the percentage of real vacuum that is actually reached. At a 60% vacuum, the example above corresponds to an absolute pressure of 400 at 1,000 mbar ambient pressure and therefore a relative pressure of -600 mbar.
At 5,500 m above sea level, the atmospheric pressure is only half as high on average. Therefore, a percentage value for the vacuum refers to a starting value that is half as high at this altitude. In this case, 60% vacuum corresponds to a relative pressure of only -300 mbar as the ambient pressure is only around 500 mbar. This situation must be considered even when using vacuuming handling technology at low altitudes because the carrying capacity also is reduced when the vacuum is lower. Therefore, percentage values must sometimes be taken with caution.