Examples

Pressure of an ideal gas

>>> from si_units import *
>>> temperature = 298.15 * KELVIN
>>> volume = 1.5 * METER**3
>>> moles = 75.0 * MOL
>>> pressure = moles * RGAS * temperature / volume
>>> print(pressure)
123.94785148011941 kPa

You can use division to perform unit conversions.

>>> from si_units import *
>>> temperature = 298.15 * KELVIN
>>> volume = 1.5 * METER**3
>>> moles = 75.0 * MOL
>>> pressure = moles * RGAS * temperature / volume
>>> print(pressure / BAR)
1.2394785148011942
>>> print(pressure / (MILLI * NEWTON / ANGSTROM**2))
1.239478514801194e-12
>>> print(volume / LITER)
1500.0

Gravitational pull of the moon on the earth

>>> from si_units import *
>>> mass_earth = 5.9724e24 * KILOGRAM
>>> mass_moon = 7.346e22 * KILOGRAM
>>> distance = 383.398 * KILO * METER
>>> force = G * mass_earth * mass_moon / distance**2
>>> print(force)
1.992075748302325e26  N

Pressure distribution in the atmosphere

Using the barometric formula. This example demonstrates how dimensioned arrays can be constructed using numpy.ndarray’s.

>>> from si_units import *
>>> import numpy as np

>>> z = np.linspace(1.0, 70.0e3, 10) * METER
>>> g = 9.81 * METER / SECOND**2
>>> m = 28.949 * GRAM / MOL
>>> t = 283.15 * KELVIN
>>> p0 = BAR
>>> pressure = p0 * np.exp((-z * m * g) / (RGAS * t))
>>>
>>> # dividing with the unit of an SIArray returns a numpy.ndarray
>>> # iteration is currently not implemented.
>>> for zi, pi in zip(z / METER, pressure / (KILO * PASCAL)):
>>>     print(f'z = {zi:16.10f}   p = {pi:16.10f}')

z =     1.0000000000   p =    99.9879378249
z =  7778.6666666667   p =    39.1279560236
z = 15556.3333333333   p =    15.3118163640
z = 23334.0000000000   p =     5.9919235296
z = 31111.6666666667   p =     2.3448000375
z = 38889.3333333333   p =     0.9175830080
z = 46667.0000000000   p =     0.3590747881
z = 54444.6666666667   p =     0.1405155744
z = 62222.3333333333   p =     0.0549875048
z = 70000.0000000000   p =     0.0215180823

Using numpy Functions

Functions such as exp, sqrt and cbrt work with methods or the equivalent numpy functions.

>>> from si_units import *
>>> import numpy as np
>>> sqm = METER**2
>>> np.sqrt(sqm)
1  m
>>> sqm.sqrt()   # this is equivalent
1  m

This also works with numpy.ndarray’s.

>>> from si_units import *
>>> import numpy as np
>>> ms = np.array([2.0, 3.0, 4.0]) * METER
>>> sqms = ms**2
>>> sqms
[4, 9, 16] m²
>>> sqms.sqrt()
[2, 3, 4] m