hum   = 0.8
  diffusefrac <- function(humidity = 0.8){
    df18O = 14.2
    df2H  = 12.5
    eps18 = df18O*(1-humidity)
    eps2H = df2H *(1-humidity)
    epsdf = data.frame(humidity, eps18, eps2H)
    return(epsdf)
  }
  epsdf <- diffusefrac(hum)