Swift how to represent standard atmosphere pressure for unit conversion

Question

I am making a unit converter app. When dealing with pressure, I found that there are already a few units predefined in Foundation:

      UnitPressure.newtonsPerMetersSquared
      UnitPressure.bars
      UnitPressure.poundsForcePerSquareInch

But there's no standard atmosphere pressure. (Note that bar is not the same as standard atmosphere pressure, despite that they are close).

I am wondering how do i handle this case?

Answer 1

You can define your own units very easily:

extension UnitPressure {
    static let standardAtmospheres = UnitPressure(
        symbol: "atm", 
        converter: UnitConverterLinear(coefficient: 101325)
    )
}

Measurement(value: 1, unit: UnitPressure.bars)
    .converted(to: .standardAtmospheres) // 0.9869232667160128 atm
Measurement(value: 1, unit: UnitPressure.standardAtmospheres)
    .converted(to: .bars) // 1.01325 bar

The symbol for standard atmospheres is "atm" and 1 atm = 101325 N/m^2 as suggested by WolframAlpha, hence the magic number used in the code.

Note that N/m^2 is the base unit of UnitPressure.

The UnitPressure class defines its base unit as newtonsPerMetersSquared and provides the following units, which UnitConverterLinear converters initialize with the given coefficients: [...]