F# conversion of Larry O'Brien Android Barometer post: http://www.knowing.net/index.php/2013/04/06/accessing-the-android-barometer-using-monotouch-android/

barometer.fs

[<Activity (Label = "HelloBarometer", MainLauncher = true)>]
type MainActivity () =
  inherit Activity ()
  
  let calculateAltitudeInFeet hPAs =
      let pstd = 1013.25
      (1.0 - Math.Pow((hPAs/pstd), 0.190284)) * 145366.45
      
  let mainLabel = ref Unchecked.defaultof<_>
  
  override x.OnCreate(bundle) =
      base.OnCreate(bundle)
      //Set our view from the "main" layout resource
      x.SetContentView(Resource_Layout.Main)
      
      //Detect the barometer
      let sm = x.GetSystemService(Context.SensorService) :?> SensorManager
      let pressure = sm.GetDefaultSensor(SensorType.Pressure)
                           
      //Subscribe to it
      let eventBind = sm.RegisterListener(x, pressure, SensorDelay.Normal)                                                  
             
      // find the main label and assign it
      mainLabel := x.FindViewById<TextView>(Resource_Id.mainLabel)
         
  interface ISensorEventListener with
      member x.OnSensorChanged(pressureEvent) =
          Console.WriteLine("Under pressure {0}",pressureEvent.ToString())
          let hPAs = pressureEvent.Values.[0]
          let msg = sprintf "Current pressure: %f hPA!" hPAs
          (!mainLabel).Text <- msg
          let calcedAltitude = calculateAltitudeInFeet(float hPAs)
          x.FindViewById<TextView>(Resource_Id.barometer).Text <- String.Format("Calculated altitude is {0} ft", calcedAltitude)
      
      member x.OnAccuracyChanged(sensor,  accuracy) =
          Console.WriteLine("Things have changed")