Hey,
I am using fuel in heating system and it’s properties varies depending on the supplier.
To calculate how much energy I am using for heating I need to input value of the fuel’s density from certificate of quality. It is usually denoted in kg/m³ at 15°C.
HA only supports mg/m³ under ‘concentration’ but not kg/m³ or other density units.
I personally do not need ‘special’ units but adding lb/ft³ at the same time would be make it more complete.