Pressure Measurement
16Pages

Technical Notes ■ Measuring Pressure ■ Units of Measure Sub-atmospheric pressures are measured in several units, including: Torr (also called millimeters of mercury, mmHg), milliTorr (mTorr but also called micron, μ ), inches of mercury (" Hg), millibar (mbar), and pascal (Pa). In the U.S., three units are in common use: micron as the unit for pressures reached by backing pumps, Torr for high vacuum and UHV pumps, and inches of mercury for coarse vacuum pumps. In Europe, millibar is the common unit for all pressure measurements. Japan uses the pascal unit, but often has Torr as a secondary unit. Most authors of scientific/technical papers are urged to use the SI unit pascal, and some do. The units are derived from: • Pascal—the force of 1 newton (1 kg accelerating at 1m/sec./sec.) acting on 1 m2 • Millibar—1,000 times the force of 1 dyne (1g accelerating at 1cm/sec./sec.) acting on 1 cm2 • Torr—1/760 times the height of a mercury barometer under “standard” atmospheric pressure Pressure Measurement • MilliTorr or micron—1,000th of 1 Torr • Inches of Hg (vacuum)—1/29.92 times the height of a mercury barometer under “standard” atmospheric pressure (taking atmospheric pressure as 0" Hg) • Inches of Hg (weather forecasts)—1/29.92 times the height of a mercury barometer under “standard” atmospheric pressure (taking no pressure as 0" Hg) Pressure Ranges There is no “universal” gauge that can measure from atmosphere to UHV pressures (a dynamic range of 1015). There are, essentially, three mechanisms used in pressure measurement and the one chosen depends on the pressure range and the residual gases in the vacuum. The typical arrangement of two gauges covering the range of interest between 10 and 1 x 10-9 Torr leaves a poorly covered band at pressures widely used in sputtering, etching, CVD, etc. Fortunately, the precise measurements needed between 10-1 and 10-3 Torr for reproducible processing can be made by adding a third gauge—the capacitance manometer. When choosing a gauge, in addition to pressure range, other features should be considered: the gauge’s pumping speed; how it is affected by radiation, magnetism, temperature, vibration, and corrosive gases; and the damage caused by switching it on at atmospheric pressure. These subjects are discussed in comprehensive vacuum texts such as John F. O’Hanlon’s A User’s Guide to Vacuum Technology (see page 17-20 to order). ■ Vacuum Gauges ■ Mechanical Gauges A gas’s pressure is the sum of all the individual forces caused by each atom or molecule colliding with a surface at any instant. Mechanical gauges register this total force by monitoring the surface’s movement against the (restoring) force trying to keep the surface in its original place. Because mechanical gauges respond to molecular momentum only, they measure pressures of any gas or vapor. They can be very accurate or inaccurate depending on how the movement is registered. McLeod This gauge, though seldom used, is employed mostly as a primary calibration standard for other gauges. In effect, a large known volume of gas at unknown pressure is captured in a glass bulb and compressed by raising the mercury level until the gas is confined in a small, closed capillary of known volume. Because the ratio between the original and final volumes is known and the final pressure can be measured, the original pressure is calculated by Boyle’s law (P1 x V1 = P2 x V2). McLeod gauges are particularly useful in the 1 Torr to 10-4 Torr range but, because of the compression, cannot be used to measure vapors. Bourdon When a closed-end, curved, oval cross-section, copper alloy tube is connected to the vacuum, atmospheric pressure bends it to a greater or lesser degree, depending on the internal pressure. The mechanical force moves an indicator needle through a geared linkage. Bourdon gauges are used primarily in high-pressure measurement (most commonly attached to regulators on gas cylinders), but variations are built to indicate pressures from 0" Hg to 30" Hg and are used for freeze drying, “house” vacuum systems, vacuum impregnation, etc., where the major concern is whether vacuum exists rather than its accurate measurement. Mechanical Gauges have liquid or solid diaphragms that change position under the force of all the gas molecules bouncing off them. These gauges measure absolute pressures unaffected by gas/vapor properties. Unfortunately, this type of gauge is ineffective below 10-5 Torr. Gas Property Gauges measure a bulk property, such as thermal conductivity or viscosity. They are dependent on gas composition and are effective over limited pressure ranges below approximately 100 Torr and above 10-4 Torr. Ionization Gauges For high vacuum and UHV measurements, charge collection is used. The residual gas molecules are ionized by electrons and the resulting ion current measured. Although such gauges will ionize vapors as well as permanent gases, their response depends on parameters other than ionization potential, making accurate total pressure measurement difficult in gas mixtures. Ionization gauges cover the pressure range from 10-4 Torr to 10-10 Torr. Piezo-resistive pressure sensors are typically comprised of a silicon wafer that is machined on a surface that makes the crystal into a suitable deflecting diaphragm when subjected to a normal stress (pressure). The thickness of the silicon crystal at its minimum section is the primary factor that determines the pressure range of the gauge from 1,500 to 0.1 Torr. As the diaphragm deflects under pressure, the resistances of the piezo-resistive elements change in value, causing the Wheatstone bridge network to move out of balance. Applying a voltage to this bridge produces an output voltage that is proportional to the applied pressure. If the elements are of equal resistance, there will be a zero