All number tag values can be annotated with an optional unit. In addition, it is required to annotate each numeric point with the unit tag. In both cases, the unit must be an identifier defined by the standard unit database.
As a general principle, all the data associated with a given site should exclusively use either the SI metric system or the US customary system. Mixing unit systems within one site will cause serious headaches.
The unit database used by Project Haystack is managed by the Fantom open source community as part of the sys::Unit API. This database was originally based on the oBIX specification, but has since been expanded to allow multiple aliases to be used for each unit.
Each unit of measurement has a full name and zero or more symbols which are used as aliases for that unit. For example "square_meter" is the full name and the symbol alias is "m²". Some units might have multiple symbols, for example "hour" has the symbols "hr" and "h". Some units like "day" have no symbols.
All unit identifiers are limited to the following characters:
a
- z
and A
- Z
_
/
%
$
By convention the symbol is the preferred notation. If there are multiple symbols, then the last symbol defined by the database is the preferred one.
Below are some commonly used units. You can download the full unit database from project-haystack.org or from the fantom.org website.
--- Misc --- percent, % --- Area --- square_meter, m² square_foot, ft² -- Currency --- australian_dollar, AUD british_pound, GBP, £ canadian_dollar, CAD chinese_yuan, CNY, 元 euro, EUR, € us_dollar, USD, $ --- Energy --- kilowatt_hour, kWh --- Power --- kilowatt, kW --- Pressure --- kilopascal, kPa pounds_per_square_inch, psi inches_of_water, inH₂O inches_of_mercury, inHg --- Temperature --- fahrenheit, °F celsius, °C --- Temperature Differential --- fahrenheit_degrees, Δ°F celsius_degrees, Δ°C --- Time --- millisecond, ms second, sec minute, min hour, hr, h day week, wk julian_month, mo year, yr --- Volumetric Flow --- liters_per_second, L/s cubic_feet_per_minute, cfm