Units in Matlab

Introduction

There are several ways to use variables with units in a program. Space probes have crashed on Mars due to invalid assumptions about units for some variables. So how can we avoid this type of disasters? We have to make it as simple and explicit as possible!

SI units

We have the primary SI-units of the Franse Système international d'unités:

Meter (m) - Unit of length
Kilogram (kg) - Unit of mass
Second (s) - Unit of time
Ampere (A) - Unit of electric current
Kelvin (K) - Unit of temperature
Mole (mol) - Unit of amount of substance
Candela (cd) - Unit of luminous intensity

Combinations of these units like m/s are considered derived SI-units, and also units that can be expressed in terms of SI-units but have their own name, like: Newton (kg.m/s²) or Pascal (N/m²).

SI-units with prefixes like millisecond or centimeter are not considered primary or derived SI-units.

Primary and derived SI-units will be here defined as basic SI-units. Units with prefixes do not fall under this definition.

Some commonly used derived SI-units:

Joule    (energy)
Coulomb  (electric charge)
Voltage  (electric potential)
Ohm      (electric resistance)
Farad    (electric capacitance)
Tesla    (magnetic field strength)
Newton   (force)
Watt     (power)
Pascal   (pressure)

Programming rules

Primary rules:

• Use basic SI-units as much as possible.
• Use of basic SI-units is implicit: variables like someLength, someTime, someVolume are assumed in SI-units.
• Variables in non-basic SI-units like nanometer, millisecond or liter should have explicit suffixes like someLength_nm, someTime_ms or someVolume_liter.
• Use of radians for angles is implicit. When using degrees use the suffix "_deg".

Secondary rules:

• If an input interface has non-basic SI units, convert them to basic SI-units as soon as possible after you read them from the interface.
• If an output interface needs non-basic SI units, convert them from SI-units as late as possible before outputting them.