Physical units¶

All variables in scipp have a physical unit. Variables are used for coordinates, labels, data, as well as attributes, i.e., all of these have a unit.

When not specified explicitly the unit of a variable defaults to scipp.units.dimensionless (scipp.units.one is a shorter alias for this), i.e., the variable is considered dimensionless.

scipp.units provides a number of pre-defined elementary units as well as operations between units. This can be used to create units that do not have a pre-defined identifier:

[1]:

import scipp as sc

length = sc.units.m
area = length * length
area

[1]:

m^2

[2]:

volume = length * length * length
volume

[2]:

m^3

[3]:

speed = length / sc.units.s
speed

[3]:

m/s

You can get a list of all predefined units (accessible through sc.units) with

[4]:

[unit for unit in dir(sc.units) if not unit.startswith('_')]

[4]:

['K',
 'angstrom',
 'counts',
 'deg',
 'dimensionless',
 'kg',
 'm',
 'meV',
 'mm',
 'ns',
 'one',
 'rad',
 's',
 'us']

Units can also be created directly from strings:

[5]:

sc.Unit('m/s')

[5]:

m/s

[6]:

sc.Unit('counts')

[6]:

counts

To convert data between compatible units, such as m to mm, see sc.to_unit.

[7]:

sc.to_unit(1 * sc.Unit('m'), 'mm')

[7]:

scipp.Variable (8 Bytes)

- ()
  int64
  mm
  1000
```
Values:
array(1000)
```

Supported Units¶

Base Units¶

All SI base units are supported with the following names:

Name	Unit
‘m’	meter
‘s’	second
‘kg’	kilogram
‘K’	kelvin
‘A’	ampere
‘mol’	mole
‘cd’	candela

[8]:

sc.Unit('K')

[8]:

In addition, these base units are supported for cases not covered by SI:

name	Unit
‘rad’	radian
‘count’	single object counting

Derived units¶

A great number of derived units can also be specified as arguments to sc.Unit. Some examples are

Name	Unit
‘Hz’	hertz
‘J’	joule
‘V’	volt
‘W’	watt
‘angstrom’ / ‘Å’	ångström
‘eV’	electron volt
‘L’	liter
‘min’	minute
‘D’ / ‘day’	day

Units can be modified with SI prefixes, for instance

[9]:

print(sc.Unit('mm'), sc.Unit('microsecond'), sc.Unit('micro s'), sc.Unit('MJ'))

mm µs µs MJ

You can also specify exponents for units or exponentiate them explicitly:

[10]:

print(sc.Unit('m^2'), sc.Unit('m**2'), sc.Unit('m')**2)

m^2 m^2 m^2

The Scipp units library is built on top of LLNL Units. You can take a look at the page on Defined Units for a more complete list of supported units. However, this should be considered an implementation detail and it is best not to rely on the more specialized unit systems.