Vacuum Measurement & Control
We have put together a glossary to help guide you through the vacuum terminology we use today. This terminology helps explain
the principles, concepts, components and functionality of vacuum gauges. When purchasing a vacuum gauge this glossary will be
helpful in identifying how we can help you.
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​Term​​​
​Definition
​Pressure

​Force exerted on the surface of an object per unit area. There are different terms related to measuring pressure which correspond to defined reference points. These include terms such as absolute pressure, gauge pressure, and differential pressure.
Absolute Pressure​
An absolute pressure measurement Pabs is referenced with respect to absolute vacuum, i.e. a volume of space free of all gas molecules. As an example, 30 psia (pounds per square inch absolute) is a pressure that is 30 psi above vacuum.
​Gauge Pressure
​Gauge pressure measurements are referenced with respect to the ambient atmospheric pressure. As an example, 30 psig is 30 psi above ambient atmosphere (typically 14.7 psia at sea level). In this example, 30 psig is 44.7 psia.
​Differential Pressure
​The pressure difference between any two points in a system.
​Positive Pressure
​Pressure within a system that is greater than the surrounding environment.
​Negative Pressure
​Pressure within a system that is less than the surrounding environment.
​Partial Pressure
​Refers to the partial pressures exerted by the constituent gases comprising of a mixture of gases
​Atmospheric Pressure
The pressure exerted by the Earth’s atmosphere at sea level. The atm is a defined unit of pressure
(1 atm = 760 Torr = 1.01325 bar = 1013.25 mbar ~14.7 psia).
​Vacuum
​Loosely defined as any pressure below ambient atmospheric pressure.

### Vacuum Gauge Terms:

​Term​
​Definition
​Direct Vacuum Gauges
Gauges that measure the force exerted by gas molecules directly through interaction between gas molecules and a sensor. The force is often conveyed through the use of a membrane (as in the case of a capacitance manometer or piezoresistance gauge), or the use of a tube (Bourdon).
​Indirect Vacuum Gauges
​Gauges that measure a physical property, such as thermal conductivity or ionization rate, of gas molecules to determine the pressure in a vacuum system.
​Active Vacuum Gauges
​A gauge containing both sensors and circuity capable of sending analog or digital signal to indicate vacuum measurement. Active vacuum gauges do not require a separate control instrument.
​Accuracy
​Accuracy is the deviation of a reading when compared to a standard. The deviation or discrepancy is often expressed as a percentage, between a measured value and the actual value.
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​NIST Traceable Calibration
​Calibration using references (metrology) that have a calibration trail to standards residing at NIST (National Institute of Standards & Technology). A NIST Traceable Calibration can include all documents (i.e. a paper trail) linking the calibration back to NIST standards.
​Precision
​Precision refers to the consistency in agreement between successive measurements. Note that an instrument giving precise readings cannot be assumed to be accurate.
​Vacuum Gauge Tube
​Precision thermal sensing device designed to provide accurate measurement of vacuum. Also referred to as a “thermocouple gauge tube” or just “tube”.
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​Thermocouple Gauge
​An indirect vacuum gauge that measures the thermal conductivity of particular gases using a temperature sensing technique to determine the pressure in a vacuum system.
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​Reference Tube
​A reference tube is an evacuated, sealed vacuum gauge tube accurately marked at a specific pressure. It electrically simulates a pressure reading equivalent to our most popular vacuum gauge tube families. The actual pressure in the glass tube is not equal to the labeled pressure.
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​Piezoelectric Gauge
A piezoelectric gauge is a direct vacuum (or pressure) instrument used to measure vacuum/pressure. In a piezoelectric gauge, the sensor contains piezoelectric material whose electrical properties, such as resistance, change as stress is applied. A piezoelectric gauge measures the mechanical stress imparted on a thin membrane which in turn changes the stress on the piezoelectric material thus causing a change in electrical output signal. The electrical output signal corresponds to the pressure.
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​Pirani Gauge
​An indirect vacuum gauge that measures the pressure-dependent thermal conductivity of the particular gas in a vacuum using a heated element, such as a wire or thin-film membrane. The heated element is part of a resistance bridge. The temperature, and thus the resistance of the heated element in the vacuum changes as the pressure changes. By measuring the electrical behavior of the bridge, the pressure in the vacuum can be determined.
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​Ionization Gauge
​A broad category of indirect vacuum gauges in which the instrument, along with its sensor, creates and then measures the ion density within a volume to determine the pressure. Ionization gauges include the hot-filament ionization gauge and cold cathode gauges.
​Hot Filament Ionization Gauge
​An indirect vacuum gauge used to measure pressure in a vacuum system in which a heated element thermionically emits electrons. The electrons make several passes through a volume and may collide with a gas molecule and create a positive ion. By measuring this positive ion current, an indirect measurement of the pressure in a vacuum can be obtained. In general, the positive ion current I+ is directly proportional to the gas pressure (P) and is given by the equation I+ = K*Ie* P, where Ie is the electron current (in the mA range), and K
is a constant.
​​Cold Cathode Gauge​
​An indirect vacuum gauge used to measure pressure in a vacuum system. The cold cathode gauge is a type
of ionization gauge in which a crossed high voltage electric field and a magnetic field set up a discharge. The
resulting positive ion current is proportional to the pressure in the vacuum system.

### Vacuum Gauge Units:

​Term
​Definition
​Torr
A unit of pressure used to measure vacuum on an absolute scale. One Torr corresponds to a column height of 1 mm of Hg. (760 Torr = 1 Atm).
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​mTorr ("millitorr")
​A unit of pressure used to measure vacuum on an absolute scale. Also called a “micron”. One mTorr corresponds to a column height of 1 μm (micron) of mercury. (1 Torr = 1000 mTorr).
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​micron
​A unit of pressure used to measure vacuum on an absolute scale. Also called a “mTorr”. One micron corresponds to a column height of 1 μm (micron) of mercury. (1 Torr = 1000 micron = 1000 mTorr).
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​bar
​A unit of pressure used to measure vacuum on an absolute scale. 1 bar is the nominal pressure of atmosphere at sea level. Bar and more frequently mbar, are popular units in Europe. (1 bar = 1000 mbar = 100,000 Pa = 100 kPa)
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​mbar ("millibar")
​A unit of pressure used to measure vacuum on an absolute scale. A mbar is one thousandth of a bar. The mbar is often the unit of choice for weather maps and is used by meteorologists. The mbar is popular with vacuum users in Europe.
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​Pascal (Pa)​​

​A unit of pressure used to measure vacuum on an absolute scale. The Pa is the SI (International System of Units) derived unit of pressure. It is defined as one newton per square meter. The Pa is popular with vacuum users in many parts of Asia. (101,300 Pa = 101.3 kpa = 1 ATM)