Vacuum pressure gauge
From a pressure gauge is expected primarily the measurement and display of pressures. In practice, there are two different measuring methods that are completely different from each other. In a relative pressure measurement, the manometer determines the difference to the prevailing atmospheric pressure at the measuring point. This pressure is therefore also dependent on the weather and the level of the measurement above sea level. When measuring the relative pressure, a parameter of 1013.25 mbar at zero height is assumed. In industrial and control engineering, a relative pressure measurement usually suffices.
However, it is slightly different in the reverse direction, the negative pressure or vacuum. Here we have a negative pressure in relation to the relative pressure of the height zero meters, which can reach up to the absolute vacuum at 0 bar. For a vacuum, so-called absolute pressure measuring devices are always used. They are primarily to be found in vacuum pumps or at corresponding in, for example, the process or food technology. In these areas, a precise vacuum measurement must be maintained for certain processes.
Negative measuring range
Compared to pressure gauges, a negative pressure range is displayed on a vacuum gauge. Also, the scale is counter-clockwise ausschlagend mounted and read accordingly. The measuring range extends from 0 (right scale side) to normally – 1.0 (left scale side). Some vacuum gauges also offer 2 measuring ranges and scale values. Usually in black color, the vacuum range is displayed from 0 bar to -1.0 bar. A second and red scale indicates the vacuum range from 0 psi to -15 psi.
The tube spring commonly used in vacuum pressure gauges is made of copper. However, the corresponding spring mechanism does not react to the expansion, but to the shrinkage. If the Bourdon tube contract with increasing negative pressure, this linear movement is transmitted to a spring mechanism and displayed. Rarely, but in special measuring environments unavoidable, a bourdon tube made of quartz glass is also used. This is always used if the measuring medium would attack the commonly used materials such as copper.