Units

RAPID-N has a powerful and generic unit conversion library, which can determine whether a user entered unit is valid, test whether two units are convertible into each other, and convert a value given in one unit into its corresponding value in another unit if they are convertible. The conversion library uses unit records as the data source.

The data fields of the unit records are listed in Table 4.1.

Table 4.1. Unit data fields

Each unit is identified by a unique, case-sensitive code. Derived units can be defined by combining unit codes with appropriate operators, which are slash (/), dot (.), and numbers (1-9). Numbers following unit codes are evaluated as power values, whereas dot and slash denote multiplication and division, respectively. For example, kg.m/s2 represents kg.m/s2 (which is Newton). The unit conversion library recognizes International System of Units (SI) prefixes. Therefore, SI prefixed units are not needed to be stored as separate records. While displaying units, multilingual symbols and names of the units are used instead of the codes if they are available. The position of the unit with respect to the data value can be specified as left or right. The spacing between unit and value can also be indicated. Units without spacing are displayed as attached to the value (e.g. 25°C) (Figure 1).

There are three types of units supported by the system:

• Base units are nominally dimensionally independents units. SI defines seven base units, which are meter (m), kilogram (kg), seconds (s), ampere (A), kelvin (K), mole (mol), and candela (cd). All other units are derived from these base units. Internally, RAPID-N uses gram (g) instead of kg as the base unit for weight, since kg can be derived from g by using a SI prefix.
• Units with conversion factors are units that can be converted to other units by simple multiplication with numeric conversion constants. For example, 1 Pascal (Pa) is 1 N/m2 (factor is 1) and 1 atmosphere (atm) is 101325 Pa (factor is 101325).
• Units with special conversion functions are units that can be converted to other units only by using custom conversion functions. Conversion of a temperature value from degrees Fahrenheit (⁰F) to degrees Celcius (⁰C) is an example.

The system can determine whether two units are convertible into each other. If they are, a value in the source unit is converted to its corresponding value in the destination unit using appropriate combinations of conversion factors and conversion functions, which are determined automatically. Conversion information is used in two-ways, both for source to destination and destination to source conversion. Hence, if a unit record containing conversion information of unit A to unit B exists in the database, another record to express conversion information of unit B to unit A is not required. Inverse conversion factors are automatically calculated by the library. User-defined inverse functions are utilized for units with conversion functions. The system is capable of converting derived units as well. For example, lbs/cm2.min can be converted to kg/L.s.

For SI prefix enabled unit, the unit conversion system generates SI prefixed units with their corresponding conversion factors and adds them to the list of available units. For example, MPa (mega Pascal) and kPa (kilo Pascal) are generated from Pa (Pascal) with conversion factors of 106 and 103 to N/m2, respectively. The codes given in Table 4.2 are used as prefixes to the unit codes while generating new units. They are standard SI symbols, except for micro. Since μ is not a standard ASCII character it is replaced with u for data input, but it is properly displayed as μ.

Table 4.2. SI prefixes and symbols used by the system

REMARK: Units are accessible to everyone, but can be created or modified only by the administrators.