# UOME

Need some units of measurements? Here is a long list of units. Plus, we are happy to extend it. But we also provide two different ways to define your own units: via controlled vocabulary, or by deriving units from other units.

Units of Measurement Expressions (**UOME** - pronounced "You Ohm") is a proposal by the Pathways and Models Workgroup for a BioPAX extension to express units of measurements. It consists of two parts: UOME core provides the means define almost any conceivable unit both by controlled vocabulary (such as the OBO Unit Ontology) and by constructing expressions to derive new units from others by multiplication, division, exponentiation, offsetting and rescaling. UOME list features a long list of thusly predefined units, intended to include any unit in common use. This includes commonly used base and derived units and their multiples via standard prefixes (kilo-, Mega, micro- etc). Yes, we do have micromole per milligram minute!

## Contents

### UOME's concept of unit

A unit is an entity that, multiplied with a number, gives a measure of the magnitude of a measurable quantity that is independent of the system that is being measured (e.g. if I tell you I have an object that has a mass of five kilogram, then that gives you a good idea of its mass without knowing anything else about the object, which is why we consider kilogram to be a unit). Two units are considered equivalent, if substituting one for another would express the same magnitude of the same quantity. Dimensionless measures such as fraction, ratio or count are considered equivalent. Logarithmic units such as decibel or byte, however, are not considered equivalent to other dimensionless units, since they can in general not be substituted with other dimensionless units. Percent is dimensionless scaled by a factor of 0.01.

### UOME List

The list is intended to include every unit in common use, including multiples (e.g. microgram, millilitre, Gigabyte, etc.). Please notify the Pathways and Models Workgroup if any unit is missing. Send us a simple reference defining the unit and documenting that it is in fact being used (e.g. a Wikipedia article).

We included all SI units and most of the units mentioned in the Unit Ontology (UO). Among the few units in UO which we did not include are "mass fraction of a system" and "content of a disk", because these describe a quantity in terms dependent on the system that is being measured, and we define a unit a measure that is independent of the system that is being measured.

### UOME Core - Controlled Vocabulary

Units can be defined by using controlled vocabulary, such as the Unit Ontology (UO).

### UOME Core - Unit expressions

Expressions signify that a unit could be substituted by an expression of other units. Typically, units are derived from more basic or more commonly known units. Expressions need not derive units in exactly the same way as their official derivation does (e.g. a cgs unit can be derived from SI units rather than other cgs units). It is permitted to provide more than one way to derive a unit, even if this leads to redundancy and circularity. We recommend to derive all units from SI base units, if possible, although this is not required.

Most dimensionless units (e.g. fraction, ratio, count) are considered equivalent to dimensionless. A few are scaled from dimensionless (e.g. percent is 0.01 times dimensionless). The criterion is whether they can be safely substituted.

Derivation of logarithmic units (e.g. byte and decibel) from non-logarithmic units is currently not supported in UOME, because such derivations would be complex and are not often useful. They are not equivalent to other non-logarithmic dimensionless units. The only possible derivation is from other closely related logarithmic units (e.g. one byte is eight bits, and a decibel is 0.1 bel); other derivations should be avoided.

### Automatic evaluation of UOME unit expressions

UOME is designed to allow automatic evaluation of of nested unit expressions. At the same time, UOME is designed to make it easy for data providers to use and introduce units. This produces challenges for automatic UOME evaluators, which are discussed on that page.