[3] defines a quantity as: property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference. Hence the value of a physical quantity is expressed as the product of a numerical value and a unit of measurement. A physical quantity is a quantity that can be used in the mathematical equations of science and technology.

Examples of physical quantities are length, mass, and velocity. The value of a quantity is its magnitude expressed as the product of a number and a scientific unit of measure, and the number multiplying the unit is the numerical value of the quantity expressed in that unit of measure.

A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. Thus, the numerical value of a particular physical quantity depends on the unit in which it is expressed. Units are organized in unit systems like the International System of Units (SI), the Imperial System of Units (ISU) or the united states customary units (US)

In today's world most physical quantities are expressed in units of SI, however there are still areas where other unit systems are used. Table 1 shows the unit systems that are used by the Application Server for units of quantities. Note that not all quantities necessarily support all units of all unit systems.

Table 1: Unit systems used for units of physical quantities in Redbex

Property structures can contain properties of type quantity that are used to store physical quantities, i.e. the numerical value alongside the unit. The Application Server predefines available physical quantities along with the units that are available for storing values of properties of type quantity. New physical quantities, or units may be added when new feature types or sensor types are installed.

For each physical quantity the Application Server defines a base unit in each unit system. All units of a quantity are derived from the base base unit of the same unit system.

The conversion between units of one quantity is done by first converting the quantity to its base unit. If the target units is not in the same unit system the quantity is next converted to the base unit of the target unit system. After that the quantity is converted to the target unit.

Example: If a quantity given as 1mm shall be converted to inches it is first converted to meters the quantity in meters is converted to feet and finally the quantity in feet is converted to inches.

Any unit conversion can introduce a small rounding error.