Problem get UOM to work in the srm_color method

UnitOfMeasure.fs

//ounce
//gallon
[<Measure>] type gal
//lovibond
[<Measure>] type L
//pounds
[<Measure>] type lbs

//gc = grain color
//weight = grain weight in lbs
//vol = batch volume in gal
//(gc:float) (weight:float)
let mcu (arr:(float<L>*float<lbs>)[]) (vol:float<gal>) =
    let total =
        arr |> 
            Seq.map(fun x-> 
                        let gc, weight = x
                        gc*weight) |>
                        Seq.sum
    total / vol

        
let srm_color mcu:float<L lbs/gal> =
    1.4922 * (mcu**0.6859)