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 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 also considered basic 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 basic SI-units.
Common derived SI-units:
Joule Coulomb Voltage Ohm Siemens Farad Tesla Newton Watt Pascal
Programming rules
Primary rules:
- Use basic SI-units (without prefixes) 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.