ContextualUnits - qudt/qudt-public-repo GitHub Wiki

Contextual Units

Contextual units constitute a subset of the unit vocabulary that have unusual properties. There are a number of reasons why a unit might be declared an instance of qudt:ContextualUnit (in addition to qudt:Unit).

• Discipline-specific terms. These are units that are specializations of general units, but are the term of practice in some discipline. In these cases, the skos:broader relation points from the contextual unit to the more general unit.

  • unit:BEAT-PER-MIN skos:broader unit:NUM-PER-MIN
  • unit:BREATH-PER-MIN skos:broader unit:NUM-PER-MIN
  • unit:CFU skos:broader unit:NUM. This means "Colony Forming Unit" and is used to measure microbial formation. It used to be part of our units vocabulary but was deprecated before we introduced the idea of a ContextualUnit. We could restore it as a ContextualUnit if there is still a desire from the community. (It still resolves on the web even though it doesn't appear in our vocabulary).

• Units that contain a contextual qualifer, denoted by an underscore (_). These units will typically inlude a skos:broader triple pointing to the "context-free" root unit.

  • The motivation for GM_Carbon specifically is because of its use in climate change metrology.
  • CentiM_H2O. This is a unit of pressure, but is measured as a length of water in a barometer.
  • BTU_60DEG_F and CAL_15DEG_C; these are just some of the conditions used to quantify BTU or CAL.
  • CentiM_H2O_4DEG_C. This combines both the material and the conditions of measurement. Even less general than the above examples.

• Compound units whose components are not general units

  • unit:DEATHS-PER-KiloINDIV-YR is a commonly used measure of mortality rate. However, the component unit:DEATHS is not what one would expect to see defined in QUDT. Both of these units are defined as contextual units in order to consistently validate their properties during the build process such as computing factor units and dimensionality.
  • unit:CYC-PER-SEC. While this is still used in some circles instead of Hertz, few if any people refer to a cycle on its own.
  • unit:DEG_C_GROWING_CEREAL-DAY. Same argument, but the compound unit is used in farming.

• The rule is that any compound unit composed of a qudt:ContextualUnit must itself be declared a qudt:ContextualUnit.