This module defines how physical units are attached to dimensionful H5MD elements.

The name of this module is `units`

. The module version is 1.0.0.

The `units`

group possesses, in addition to the `version`

attribute,
a `system`

attribute that defines the unit system in use. `system`

is of scalar dataspace and fixed-length string datatype.

The datasets of any H5MD element that have a physical dimension may
carry an attribute `unit`

to indicate the physical unit of the
respective data. In general, this refers to the dataset itself for
time-independent elements, or to the datasets `value`

and `time`

in
the time-dependent case:

```
<element>
\-- step: Integer[variable]
\-- time: Float[variable]
| +-- (unit: String[])
\-- value: <type>[variable][...]
+-- (unit: String[])
```

The attribute `unit`

is of scalar dataspace and fixed-length
`String`

datatype using the ASCII character set.

The `unit`

string consists of a sequence of unit factors separated by
a space. A unit factor is either a number (an integer or a decimal
fraction) or a unit symbol optionally followed by a non-zero, signed
integer indicating the power to which this factor is raised. Each unit
symbol may occur only once. There may also be at most one numeric
factor, which must be the first one.

Examples:

- “nm+3” stands for cubic nanometers
- “um+2 s-1” stands for micrometers squared per second
- “60 s” stands for a minute
- “10+3 m” stands for a kilometer

The “SI” unit system (“The International System of Units (SI)” 2006) defines SI base units (§2.1), SI derived units (§2.2), and SI prefixes (§3.1).

dimension | symbol | unit name |
---|---|---|

length | m | meter |

mass | kg | kilogram |

time | s | second |

electric current | A | ampere |

temperature | K | kelvin |

amount of substance | mol | mole |

luminous intensity | cd | candela |

Table: SI base unit symbols and names.

dimension | symbol | unit name | conversion |
---|---|---|---|

plane angle | rad | radian | 1 rad = 1 m m⁻¹ |

solid angle | sr | steradian | 1 sr = 1 m² m⁻² |

frequency | Hz | hertz | 1 Hz = 1 s⁻¹ |

force | N | newton | 1 N = 1 m kg s⁻² |

pressure/stress | Pa | pascal | 1 Pa = 1 N m⁻² |

energy/work | J | joule | 1 J = 1 N m |

power | W | watt | 1 W = 1 J s⁻¹ |

electric charge | C | coulomb | 1 C = 1 A s |

voltage | V | volt | 1 V = W A⁻¹ |

capacitance | F | farad | 1 F = C V⁻¹ |

electric resistance | ohm | ohm | 1 Ω = V A⁻¹ |

electric conductance | S | siemens | 1 S = 1 A V⁻¹ |

magnetic flux | Wb | weber | 1 Wb = 1 V s |

magnetic flux density | T | tesla | 1 T = 1 Wb m⁻² |

inductance | H | henry | 1 H = 1 Wb A⁻¹ |

Celsius temperature | degC | degree Celsius | 0 °C = 273.15 K |

luminous flux | lm | lumen | 1 lm = 1 cd sr |

illuminance | lx | lux | 1 lx = 1 lm m⁻² |

radioactivity | Bq | becquerel | 1 Bq = 1 s⁻¹ |

absorbed dose | Gy | gray | 1 Gy = 1 J kg⁻¹ |

dose equivalent | Sv | sievert | 1 Sv = 1 J kg⁻¹ |

catalytic activity | kat | katal | 1 kat = 1 mol s⁻¹ |

Table: SI derived unit symbols, names and conversion rules.

prefix | symbol | factor |
---|---|---|

exa- | E | 10¹⁸ |

peta- | P | 10¹⁵ |

tera- | T | 10¹² |

giga- | G | 10⁹ |

mega- | M | 10⁶ |

kilo- | k | 10³ |

hecto- | h | 10² |

deca- | da | 10¹ |

deci- | d | 10⁻¹ |

centi- | c | 10⁻² |

milli- | m | 10⁻³ |

micro- | u | 10⁻⁶ |

nano- | n | 10⁻⁹ |

pico- | p | 10⁻¹² |

femto- | f | 10⁻¹⁵ |

atto- | a | 10⁻¹⁸ |

Table: SI prefixes.

“The International System of Units (SI).” 2006. 8th ed. Paris: Bureau International des Poids et Mesures. http://www.bipm.org/en/publications/si-brochure/.