Vacuum Gauges and Instruments Information
Vacuum gauges are devices for measuring vacuum or sub-atmospheric pressures. A vacuum is a space in which the pressure of a gas is low compared to the atmospheric pressure. The measure of vacuum is associated with pressure. Vacuum gauges and instruments are used in conjunction with vacuum sensors to monitor and control the vacuum pressure in a system.
Vacuum Gauge Technologies
Vacuum gauges use several different technologies to measure vacuum in an environment.
Low vacuum can be measured with devices using mechanical deflection.
Piston technology uses a sealed piston/Cylinder to measure changes in pressure.
Mechanical deflection uses an elastic or flexible element to mechanically deflect with a change in pressure, for example a diaphragm, Bourdon tube, or bellows.
Piezoelectric pressure sensors measure dynamic and quasi-static pressures. The bi-directional transducers consist of metalized quartz or ceramic materials which have naturally occurring electrical properties. They are capable of converting stress into an electric potential and vice versa. They are very rugged but require amplification circuitry which can be susceptible to shock and vibration.
MicroElectroMechanical systems (MEMS) are typically micro systems manufactured by silicon surface micromachining for use in very small industrial or biological systems.
Vibrating elements (silicon resonance) use a vibrating element technology, such as silicon resonance.
Variable capacitance pressure instruments use the capacitance change results from the movement of a diaphragm element to measure pressure. The device uses a thin diaphragm as one plate of a capacitor. The applied pressure causes the diaphragm to deflect and the capacitance to change. The deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit.
Strain gauges (strain-sensitive variable resistors) are bonded to parts of the structure that deform as the pressure changes. Four strain gages are typically used in series in a Wheatstone bridge circuit, which is used to make the measurement. When voltage is applied to two opposite corners of the bridge, an electrical output signal is developed proportional to the applied pressure. The output signal is collected at the remaining two corners of the bridge.
Manometers are usually made of a transparent U-shaped tube and are 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. An advantage of using manometer type vacuum gauges is that the pressure readings are independent of the type of gas.
Bourdon vacuum gauges are composed of a tube which is bent into a circular arc. The inside of the tube is connected to the vacuum system and the end of the tube bends as the external pressure changes. The end of the tube is also connected to a pointer which moves on an indicator dial as the pressure changes, similar to a bimetallic strip
Bourdon tube. Image Credit: efunda.com
Medium to high vacuums need to be measured using thermal and molecular devices.
Thermocouple gauges measure changes in the thermal conductivity of a residual gas within a gauge tube. The pressure readings for this device are dependent on the type of gas. Thermocouple gauges include a filament, power supply for the filament, and moving coil meter for displaying the pressure. The amount of heat lost depends on the gas pressure. There are several designs of the Pirani gauge. One design includes using two plates with different temperatures. The amount of power spent for heating is the measure of gas pressure. Another design uses a single plate to measure the thermal conductivity of gas by heat loss to the surrounding area.
Thermocouple gauge. Image Credit: National Instruments
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. Cold cathode devices draw the electrons from the electrode surface by a high potential field.
For more information, read Engineering 360's guide on How to Select Vacuum Sensors.
Vacuum gauges have a display to allow the user to monitor the vacuum pressure of the system. Types of displays include:
- Analog -- Analog meters are simple visual indicators using a dial.
- Digital meters -- Digital meters are visual indicators with numerical valves.
- Cathode ray tube (CRT) -- CRTs are commonly found in computer monitors.
- Liquid crystal display (LCD) -- LCDs are semiconductor light sources using electrons which recombine with electron holes within the device and releasing energy in the form of photons.
Multi-line video display -- Video displays allow the user to watch and record live feeds of pressure changes in the system.
- Single scale devices display pressure in only one set of units.
Dual scales devices display pressure in two sets of units on the same dial face.
Specifications for vacuum gauges include:
Vacuum range is the span of pressures from the lowest vacuum pressure to the highest vacuum pressure.
Vacuum pressure ranges. Image Credit: Oerlikon Leybold, Inc.
Operating temperature is the full-required range of ambient operating temperatures. Temperature and pressure in a system are directly related to each other. If the temperature of the closed operating environment increases, the pressure in the system will increase. In order to prevent equipment damage, it is important to know the extreme temperature ranges of the area.
Accuracy is the difference between the true value and the indication expressed as a percentage of the span. It includes the combined effects of method, observer, apparatus and environment.
Media is the term used to describe the material that surrounds the area of vacuum. Some vacuum gauges measure the pressures of liquids. Others measure the pressures of solids. Devices are also available that are rated for hazardous duty or for unlisted, specialized or proprietary materials.
Vacuum Gauge Standards
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).
Vacuum Gauge and Instrument Application
Vacuums are used in many industrial applications such as automotive, nautical, research and development, and manufacturing. They may be used to keep materials moving through the system, or to keep the work area clean of pollutants. Gauges and instruments, such as sensors, are an important component to ensure the proper function and safety of the system and equipment.
Other typical applications include chemical and petroleum refineries, pharmaceutical, offshore drilling and production, paper mills, fertilizer, etc.