The calculation engine
Units: getting your quantity onto the factor's basis
You enter litres; the publisher prices per tonne, or per gigajoule, or per gallon. Slate converts exactly, refuses conversions that don't make physical sense, and never invents a density.
On this page you'll learn
- How a quantity is converted onto a factor's unit
- Where density and heat content come from
- Which conversions are refused, and why
- Function
normalize_activity_to_basis- Density / heat content
- Your entry, then the publisher's, then Slate's fuel table
- Constants
- Exact where a definition exists
How a conversion works
The quantity is reduced to whichever physical measures can be known or derived (mass, volume, energy, distance), then expressed in the one the factor needs. Density bridges mass and volume; net calorific value bridges mass and energy.
- Mass: 500 L × density 0.84 kg/L
- 420 kg
- Energy: 420 kg × NCV 43.0 MJ/kg
- 18,060 MJ
- On the factor's basis
- 18.06 GJ
- Multiplied by the factor
- per GJ
Illustrative density and NCV. In a real calculation the values used, and where they came from, are recorded on the row.
Where density and heat content come from
- 1
Your activity record
If you entered a density or calorific value, it wins: it's specific to your fuel.
- 2
The publisher
If the factor's publisher states one, it's used, so a DEFRA factor uses DEFRA's net values and an EPA factor uses EPA's gross ones.
- 3
Slate's fuel table
The last resort, needed because publishers rarely give densities for gases and liquids. CNG and LPG are normally sold by mass.
Withheld on purpose
Seven fuels have no default property. Ethane and ethylene had gas-phase densities while EPA prices them per liquid gallon, about 400 times out. Bagasse, bamboo, hardwood, softwood and straw had calorific values that appear in no EPA publication, and raw biomass varies with moisture from about 7 to 19 MJ/kg. The plant that burns them supplies its own.
What is refused
Different physical dimensions
A mass can't become a distance. Slate raises an error instead of returning a number.
A gas volume standing in for a liquid volume
scf and gallons are both volumes. Each unit carries a phase so that ethane in scf can't be priced per liquid gallon.
Composite bases
tonne-km, passenger-km, room-night, head-year and FTE-hour are never converted through a plain mass or distance. They're passed through as tallies.
Litres of compressed gas
Ambiguous for CNG, and silently about 1,000 times under. Refused for mobile combustion.
Exact constants
| Unit | Exact value |
|---|---|
| Pound | 0.45359237 kg |
| Mile | 1,609.344 m |
| Short ton | 907.18474 kg |
| International Btu | 1,055.05585262 J |
Correcting truncated constants moved EPA results by at most 4.8 parts per million. A worse error was found the same way: a kg-per-kWh constant labelled per terajoule held the per-gigajoule value, a factor of 1,000.