Overview

The PhysicalQuantities package provides data types to represent physical quantities such as the area of a surface or the frequency of a repeating event.

In DEXPI, there is a distinction between two types of physical quantities: simple physical quantity types and application-dependent physical quantity types.

Simple Physical Quantity Types

Simple physical quantity types are characterized by a physical dimension, e.g., L² in case of area, and a set of units of measurement, e.g., m² (meter squared), cm² (centimeter squared) and some more for area.

For illustration, we discuss the information model for area:

• The data type NullableArea is abstract. It has two concrete sub types: Area is used for actual area values, and NullArea is the type of the explicit null value NULL_AREA.

• An Area has a mandatory numerical Value of type Double. The mandatory Unit is one of the literals of the enumeration AreaUnit, for example MetreSquared.

• Like any enumeration literal in DEXPI, a literal for a unit of measurement has a symbol, e.g., m² in case of MetreSquared. In addition, it is identified by a UN Code. For example, the UN Code of MetreSquared is MTK.

There are 12 simple physical quantity types:

• NullableArea

• NullableForce

• NullableHeatTransferCoefficient

• NullableLength

• NullableMass

• NullableMassFlowRate

• NullablePercentage

• NullablePower

• NullableTemperature

• NullableVoltage

• NullableVolume

• NullableVolumeFlowRate

Application-Dependent Physical Quantity Types

An application-dependent physical quantity type such as pressure is characterized by a physical dimension, e.g., L^-1MT^-2. For an application-dependent physical quantity type, there are specializations that are intended for different application areas. These application areas may have different physical or technical meanings. The allowed units of measurement may also differ among the application areas. For example, in case of pressure, DEXPI distinguishes between an absolute pressure and a gauge pressure.

To illustrate the information model for application-dependent physical quantity types, we consider pressure:

• The abstract data type NullablePressure has two abstract sub types that correspond to the two application areas: NullablePressureAbsolute and NullablePressureGauge.

• Each application area is modeled in the same way as a simple physical quantity type. For example, NullablePressureAbsolute has a concrete sub type PressureAbsolute for actual absolute pressure values. Another concrete sub type is NullPressureAbsolute, a singleton type whose instance NULL_PRESSURE_ABSOLUTE is a null value.

• A PressureAbsolute has a mandatory numerical Value of type Double and a mandatory Unit of type PressureAbsoluteUnit.

• Each enumeration literal of PressureAbsoluteUnit has a symbol and is identified by a UN Code. For example, in case of Bar the symbol is bar and the UN Code is BAR.

Note that the literals of PressureAbsoluteUnit of PressureGaugeUnit have the same names, symbols, and UN Codes. From a user’s perspective, this means that the same units can be used for an absolute pressure and for a gauge pressure. From the model’s perspective, these are still distinct literals.

See the unit enumerations for the application areas of frequency for the case when the allowed units actually depend on the application area. For example, ElectricalFrequencyUnit contains the literal Hertz, whereas RotationalFrequencyUnit and NumberPerTimeIntervalUnit have no literal with name Hertz.

There are 2 application-dependent physical quantity types:

• NullableFrequency

• NullablePressure