Because vacuum technology spans 15 orders of magnitude (103 mbar – 10-12 mbar, or 760 torr – 10-12 torr), there is no single vacuum gauge that can cover the full range. Consequently, a number of different gauge technologies have been developed in order to measure vacuum within ranges of common interest. These various gauge technologies can be broken down into those which measure the pressure directly and those that measure the pressure level indirectly.
Direct-reading gauges are so called because they measure pressure directly – that is, by measuring the force exerted by a gas over an area. Direct-reading gauges exist that can measure accurately from atmospheric pressure down to the range of 1 mbar/torr – 10-3 mbar/torr. The lower measuring limit depends upon the size and sensitivity of the particular gauge. Popular direct-reading gauge technologies include U-tube manometers, Bourdon tubes, and capacitance manometers. A major advantage of direct-reading gauges is that they can be gas independent; that is, accuracy is not impacted by the type of gas being measured.
However, as the pressure level decreases below this level, it becomes necessary to use indirect-reading gauges. These gauges are referred to as indirect-reading because, at low pressure, it is not possible to accurately measure the force exerted by a gas. Instead, these gauges measure a pressure dependent property of the gas, such as the thermal conductivity or density. The Pirani gauge is a common technology based on thermal conductivity; these gauges are most accurate in the range of 10 torr – 10-3 torr. Ionization gauges, such as hot cathode or cold cathode (Penning) gauges, cause gas molecules to ionize. The resulting ions then pass through a circuit resulting in an electric current that is proportional to the density of gas molecules. Depending upon the exact design of the ionization gauge, optimal accuracy can be obtained between 10-3 torr and 10-9 torr.
In future posts, we’ll review how some of these specific gauge technologies work in more detail.
VACUUBRAND offers a range of vacuum controllers and gauges designed to provide accurate, chemical-resistant measurement and control in the rough and fine vacuum ranges (atmosphere to 10-3 torr). If you have questions about measurement or control, please email us.