Unit Systems
The COMSOL Multiphysics software supports the following unit systems:
Metric Unit Systems
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SI units, the International System of Units (SI, Système International d’Unités). This is the default unit system (sometimes also called MKS). For a list of SI units in COMSOL Multiphysics, see SI Base, Derived, and Other Units.
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CGSA units. The CGS system uses centimeter, gram, and second as basic units of length, mass, and time, respectively. The remaining basic units are identical to the SI units. The CGS unit system gives nice values for small lengths, masses, forces, pressures, and energies when working on a microscale and with weak electromagnetic forces. The derived units of force, pressure, and energy have well-known and widely used names: dyne, barye, and erg, respectively. CGSA adds ampere as the basic unit for electric current. For a list of CGSA units, see Special CGSA Units.
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Electromagnetic units (EMU). This system is based on Ampère’s law, which defines the unit of electric current once you select an appropriate value for the constant C. When dealing exclusively with magnetic effects, it is convenient to set C = 1. If CGS units are used for the remaining basic dimensions, the current unit is called an abampere, and the corresponding coherent unit system is called electromagnetic units. Unique names for derived units have been introduced by prefixing the SI name with ab-. For a list of EMU units, see Special EMU Units.
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Electrostatic units (ESU). Based on Coulomb’s law for the force between point charges, ESU uses a unit of charge called the statcoulomb with CGS units for length, mass, and time. From there, the statampere, or franklin, and other derived units of the electrostatic unit system follow. For a list of ESU units, see Special ESU Units.
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MPa units. For stationary structural mechanics, where the density does not appear in the equations, it can be convenient to use a system where newton and megapascal (hence the name “MPa system”) are naturally derived units of force and pressure, respectively. Keeping the SI unit for time, the basic units of length and mass become millimeter and tonne. Except for the force and pressure units, other derived units are nameless. For a list of MPa units, see Special MPa Units.
English Unit Systems
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Foot-pound-second unit system (FPS units). The original foot-pound-second system seems to be the absolute system using the pound as a unit of mass. This version of the FPS system is in agreement with the IEEE standard (the pound is a unit of mass and not of force). The natural derived unit of force is the poundal. For a list of FPS units, see Special FPS Units. In addition to the FPS unit system, which uses Fahrenheit as the temperature unit, there is also an FPS (Rankine) unit system, which uses Rankine as the temperature unit, which is useful in models where a temperature unit without offset is needed. The FPS (Rankine) system can be useful in models that use formulas intended for absolute temperatures such as exp(-Ea/(k_B_const*T)) or material properties such as the coefficient of thermal expansion with a unit one over temperature ([1/K], for example) that expect to be multiplied with a temperature difference.
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British engineering units. An alternative to the standard FPS system is the British engineering unit system (also called gravitational foot-pound-second system or foot-slug-second system). Here, the pound force is the natural unit of force, which causes the introduction of the mass unit slug such that a pound force is a slug-foot per second squared. For a list of British engineering units, see Special British Engineering Units. In addition to the British engineering units system, which uses Fahrenheit as the temperature unit, there is also a British engineering units (Rankine) system, which uses Rankine as the temperature unit, which is useful in models where a temperature unit without offset is needed.
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Inch-pound-second unit system (IPS units). It is possible to define varieties of the FPS and British engineering systems based on the inch instead of the foot as basic unit of length. This gives rise to two distinct inch-pound-second systems: the absolute IPS system (just called IPS) and the gravitational IPS system. For a list of IPS units, see Special IPS Units. In addition to the IPS unit system, which uses Fahrenheit as the temperature unit, there is also an IPS (Rankine) unit system, which uses Rankine as the temperature unit, which is useful in models where a temperature unit without offset is needed.
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Gravitational IPS units. This alternative IPS unit system considers the pound a unit of weight rather than a unit of mass. For a list of Gravitational IPS units, see Special Gravitational IPS Units. In addition to the Gravitational IPS unit system, which uses Fahrenheit as the temperature unit, there is also a Gravitational IPS (Rankine) unit system, which uses Rankine as the temperature unit, which is useful in models where a temperature unit without offset is needed.
Other
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None. No units appear in the settings, which can be useful in nondimensionalized (de-dimensionalized or dimensionless) models.
If you want to use another unit system than SI as the default unit system for new models, you can choose the unit system to use from the In new models list under Unit system on the Model Builder page in the Preferences dialog box.
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Using Units
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Setting the Unit System for Models