Database Management
Reference Manual
Attributes
:
Real Attributes of Physical Quantities
: Values of Physical Quantities
Values of Physical Quantities
A value of a real physical quantity has a numerical value (the number) and defines units of measure which are the units in which the quantity is measured. For example a distance can be 25.4mm or 1inch but regardless of the number it is the same distance because there is a known conversion factor between millimetres and inches.
Value may be entered into the system with a unit qualifier which determines the dimension of the physical quantity and sets its value in those units.
The system will automatically convert from the input unit to the database unit, and from database unit to a presentation unit (for example the 'current' unit) using internally defined conversion factors. For compound units (e.g. from 'pound per square inch' to 'pascal') the conversion factors of the component units are combined together.
The internal conversion units used are sourced as accurately as possible. Some conversions are arithmetically exact for example, inches:mm, and SI conversions as listed by Bureau International des Poids et Mesures:
www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
Some conversions are experimentally determined (e.g. conversion from Kg force to Newton) and precise published values are used.
All the values are consistent with those published, for example, by US National Institute of Standards and Technology and listed at
http://physics.nist.gov/Pubs/SP811/appenB8.html
When more accurate values greater than 7 significant figures are obtainable from other sources such as
http://en.wikipedia.org/wiki/Template:Conver
t
and elsewhere these have been used in preference.
There is one exception to this rule. The British Thermal Unit (BTU) which is defined by AVEVA E3D™ to be a 'typical' value of 1055.16kJ and not that defined by Fifth International Conference value of 1055.05585262kJ.
1974 to current year.
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