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Constants for interoperation of units.
§Operating principle
Here we set out to interoperate US Customary and SI units of length/displacement, mass, and temperature, while preserving invertibility, avoiding uncontrolled precision loss, and replacing structured unit types ― which are tricky to store or compute efficiently ― with plain integers.
This is accomplished by defining common base units for length, mass, and temperature, which can be used to express US customary units and SI units in the same scale for each domain.
§length
For length, there is the iota, defined as 1⁄9 nm.
This allows common fractions of an inch (ten-thousandths, desktop publishing points, and sixty-fourths) and multiples of the nanometer to be represented as natural numbers.
§mass
For mass, there is the whit, defined as 1⁄3200 µg.
The whit is chosen to express practically measurable weights (down to the 0.1 µg range) as well as all common fractional denominations of the international pound (ounces, dram, thousandths of an ounce, grains) and units related to the pound by grains (such as the troy ounce).
§temperature
u128 | i128 |
u64 | i64 |
u32 | i32 |
f64
For temperature, there is the smidge, defined as 1⁄90 mK.
The smidge represents temperatures down to the 100 µK/0.0001 °R range, which is sufficient for almost all practical thermometry. This also allows you to exactly represent temperatures used in industrial metrology standards such as ITS-90 for fixed points and common derived constants, and allows exact interchange between common absolute (Kelvin/Rankine) and relative (Celsius/Fahrenheit) temperature scales.
The temperature module deliberately avoids defining incremental units of temperature by
their relative scale names, to remind users to account for the difference in origin/zero point
between Fahrenheit and Celsius.
Modules§
- length
- Per-target-type modules for units of length denominated in ‘iota’ ― 1⁄9 nm increments.
- mass
- Per-target-type modules for units of mass denominated in ‘whit’ ― 1⁄3200 µg increments.
- temperature
- Per-target-type modules for units of temperature denominated in ‘smidge’ ― 1⁄90 mK increments.