Vacuum Gauges and Instruments

Vacuum gauges are devices for measuring vacuum or sub-atmospheric pressures. They use several different technologies. Manometers are usually made of a transparent U-shaped tube and partially filled with a liquid such as water, mercury or oil. The relative amount of liquid displacement between the legs of the U indicates the excess of pressure exerted on one side or the other. Thermocouple gauges measure changes in the thermal conductivity of a residual gas within a gauge tube. These devices include a filament, power supply for the filament, and moving coil meter for displaying the pressure. Hot cathode ionization gauges initiate a constant electron flow from the cathode or electron source to the anode or electron drain. These electrons hit a pressure-dependent quantity of gas molecules, which become positive ions and cause a pressure-related current on the ion collector. Cold cathode ionization gauges are also available. Because they do not have active components such as hot filaments, cold cathode gauges can withstand sudden or prolonged exposure to high-pressure gases.  

Selecting vacuum gauges requires an analysis of display types, scale types and scale units. Some devices include analog or digital meters. Others include a cathode ray tube (CRT), liquid crystal display (LCD), or other multi-line video display. Single scale devices display pressure in only one set of units. By contrast, dual scales devices display pressure in two sets of units on the same dial face. There are many scale units for vacuum gauges. Examples include pounds per square inch (psi), kilo pascals (kPa), atmospheres (atm), bars, inches of mercury (inHg), centimeters of mercury (cmHg), inches of water (inH₂O), feet of water (ftH₂O), kilograms per square centimeters (kg / cm²), and ounces per square inch (oz / in²). 

Specifications for vacuum gauges include vacuum range, operating temperature, accuracy, and media. Accuracy, the difference between the true value and the indication expressed as a percentage of the span, includes the combined effects of method, observer, apparatus and environment. Typically, vacuum gauges use accuracy grades from the American Society of Mechanical Engineers (ASME) and Deutsches Institut für Normung (DIN), a German national organization for standardization. Examples include grades A, B, C, and D as well as grade 1A (1% full scale), 2A (0.5% full scale), 3A (0.25% full scale), and 4A (0.1% full scale). Some vacuum gauges measure the pressures of liquids. Others measure the pressures of solids. Devices that are rated for hazardous duty or for unlisted, specialized or proprietary materials are also available.   

Vacuum gauges are available with a variety of features, signal outputs, and communication protocols. Some devices include temperature measurement outputs or temperature compensation to prevent measurement errors due to changes in temperature. Other devices include audible or visual alarms, or an output switch that is compatible with transistor-transistor logic (TTL). Typically, the output voltage is a linear function of the measurement. Analog current is imposed on the output circuit proportional to the measurement. Serial interfaces for vacuum gauges include RS232 and RS485. Parallel interfaces are also available. Other communication protocols include DeviceNet, Fieldbus, Ethernet, and PROFIBUS.