The Molecule Counters - Part One: Vacuum Gauges
What is a Vacuum Gauge?A vacuum gauge is an instrument for measuring pressures below atmospheric pressure. There are many types, each designed for a specific function. Some of the more common types of vacuum gauges are shown below, listed in order of descending pressure range:
Manometer – A relatively simple device that usually consists of a tube or column filled with a liquid. The pressure is found by measuring the column height or the difference in heights of several columns.
Thermal conductivity gauge – These devices operate on the principle that heat transported by gas molecules can be related to gas pressure. A heat source causes changes in surface temperature (or in the heating power required to maintain constant temperature), and this is related to the pressure of the system. Various types of thermal-conductivity gauges are distinguished according to the method of indicating the surface temperature (that is, the way in which the wire temperature is measured). The most common types are Pirani gauges (typical range: 1 torr to 10-5 torr) and thermocouple gauges (typical range: 1 torr to 10-3 torr).
Knudsen (radiometer) gauge (typically accurate to 10-6 torr) – These devices measure pressure in terms of the net rate of transfer of momentum by molecules between two surfaces maintained at different temperatures and separated by a distance smaller than the mean free path of the gas molecule.
McLeod gauge (typically accurate to 10-6 torr) – The principle of operation involves measuring the pressure of a gas by measuring its volume twice, once at the unknown low pressure and again at a higher reference pressure.
Ionization gauge (typically accurate to 10-9 torr and beyond) – These devices have a means of ionizing the gas molecules and a means of correlating the number and type of ions produced with the pressure of the gas. Various types of ionization gauges are distinguished according to the method of producing the ionization. The common types are hot-cathode gauge (typical range: 1 x 10-2 to 1 x 10-10 torr) and cold-cathode gauge (typical range: 1 x 10-2 to 1 x 10-11 torr).
A Little HistoryEngineers first became interested in vacuum measurements in the 1600s, when they noted the inability of pumps to raise water more than about 30 feet (9 m). The Duke of Tuscany in Italy commissioned Galileo to investigate the “problem.” Galileo, among others, devised a number of experiments to investigate the properties of air. After Galileo’s death in 1642, Evangelista Torricelli continued the work that included vacuum-related investigations and the invention of the mercury barometer. He discovered that the atmosphere exerts a force of 14.7 psi (101.3 kPa) and that, inside a fully evacuated tube, the pressure was enough to raise a column of mercury to a height of 29.9 inches (760 mm). The height of a column of mercury is therefore a direct measure of the atmospheric pressure. The value of 1/760th of an atmosphere is called a torr, in honor of Torricelli. Today, manometers (Fig. 1) come in a variety of forms to suit particular application needs.
Pressure is simply defined as a force per unit area, and the most accurate way to measure air pressure is to balance a column of liquid of known weight against it and measure the height of the liquid column so balanced. The units of measure commonly used in the U.S. are inches of mercury (Hg), using mercury as the fluid, and inches of water column (W.C.), using water or oil as the fluid.
The U-tube manometer (Fig. 1A) is the most common type of manometer today because the difference in height between the two columns is always a true indication of the pressure regardless of variations in the internal diameter of the tube. With both ends of the tube open, the liquid is at the same height in each leg. When positive pressure is applied to one leg (Fig. 2), the liquid is forced down in that leg and up in the other. The difference in height (h), which is the sum of the readings above and below zero, indicates the pressure. When a vacuum is applied to one leg (Fig. 2), the liquid rises in that leg and falls in the other. The difference in height, “h,” which is the sum of the readings above and below zero, indicates the amount (or degree) of vacuum.
Pressure RangesThe correct choice of gauge depends on knowledge of the working principles of the gauge, the range of pressures it can measure and its accuracy over the required range. These factors have been determined by experience (Fig. 3), and there is a vacuum gauge for every pressure range.
For low vacuum ranges (higher pressures) between atmospheric and 10 torr, Bourdon tubes, bellows, active strain gauges and capacitance sensors are all suitable measurement devices.
For mid-range vacuum requirements – those in the 10-2 to 10-3 range – there are several choices including the capacitance manometer, a good choice for more accurate measurements, and the hot cathode ion gauge.
For intermediate vacuum applications (between 10-2 and 10-4 torr), capacitance manometers are the best in terms of performance but are also the most expensive. The lowest-priced gauge is the thermocouple type, but its error is the greatest. Digital Pirani gauges represent a good compromise solution with accuracy between that of capacitance and thermocouple sensors.
For ultrahigh-vacuum service (≥10-6 torr), either cold cathode or Bayard-Alpert hot cathode gauges are used. There is some concern over accuracy and/or stability, and both require frequent calibration.
Dictionary of Vacuum Gauge Terms[5, 6]Absolute pressure – Pressure measured above the zero value of a perfect vacuum designated psia (pounds per square inch absolute).
Atmospheric pressure – The pressure exerted by a mercury column 760 mm high at 0°C under a standard acceleration of gravity (980.665 cm/sec2) – 14.7 psi at sea level.
Boyle’s Law – One of the gas laws, Boyle’s law states that pressure and volume in a gas are inversely proportional (assuming constant temperature and mass).
Gauge pressure – Pressure measured at atmospheric pressure as a reference point. Gauge pressure is designated psig (pounds per square inch gauge).
Millimeter of mercury (mm Hg) – A unit of pressure defined as that pressure that will support a column of mercury one millimeter high.
Pressure – The force per unit area a gas exerts. Common units are torr, millibar, microns, psia or millimeters of mercury.
Torr – A unit of pressure defined as 1/760th of an atmosphere.
Vacuum – A space filled with gas at a pressure less than atmospheric pressure. Vacuums are classified as rough (760 torr to 1 torr), low (1 torr to 10-3 torr), high (10-3 to 10-6 torr), very high (10-6 to 10-9 torr) and ultrahigh (10-9 and above).
Part Two will talk about the types of vacuum gauges in more detail. IH