dew_point.lua

--[[
%% properties
93 value
95 value
100 value
102 value
%% weather
%% events
%% globals
--]]

-- Calculate the dew point using air temp, humidity and floor temps, to be
-- used in Fibaro Home Center 2 scene, global variables are set to be used
-- in other scenes.

------------------------------------------------------------------------
--
-- Dew point calculation using Magnus formula: http://en.wikipedia.org/wiki/Dew_point
-- 
-- http://forum.micasaverde.com/index.php?topic=37464.0
--
function dewPoint (T, RH)
-- a,b,c taken from a 1980 paper by David Bolton in the Monthly Weather Review
--  local a = 6.112     -- a is not used in this approximation
  local b,c = 17.67, 243.5
  RH = math.max (RH or 0, 1e-3)
  local gamma = math.log (RH/100) + b * T / (c + T)
  return c * gamma / (b - gamma)
end

local startSource = fibaro:getSourceTrigger();

-- Device ids are stored in global variables
local FLOOR_LEFT_AIR_TEMP = fibaro:getGlobal('D_FLOOR_LEFT_AIR_T')
local FLOOR_LEFT_TEMP 	  = fibaro:getGlobal('D_FLOOR_LEFT_TEMP')
local FLOOR_LEFT_HUM      = fibaro:getGlobal('D_FLOOR_LEFT_HUM')
local FLOOR_R_AIR_TEMP 	  = fibaro:getGlobal('D_FLOOR_R_AIR_T')
local FLOOR_R_TEMP        = fibaro:getGlobal('D_FLOOR_R_TEMP')
local FLOOR_R_HUM         = fibaro:getGlobal('D_FLOOR_R_HUM')

local floor_left_air_temp  = tonumber(fibaro:getValue(FLOOR_LEFT_AIR_TEMP, "value"))
local floor_left_hum       = tonumber(fibaro:getValue(FLOOR_LEFT_HUM, "value"))
local floor_left_temp      = tonumber(fibaro:getValue(FLOOR_LEFT_TEMP, "value"))
local floor_right_air_temp = tonumber(fibaro:getValue(FLOOR_R_AIR_TEMP, "value"))
local floor_right_hum      = tonumber(fibaro:getValue(FLOOR_R_HUM, "value"))
local floor_right_temp     = tonumber(fibaro:getValue(FLOOR_R_TEMP, "value"))

-- offset for the air temperture
local air_temp_offset = -1

-- take the maximum temperature and the maximum humidity,
-- assume the "worst" combination
local dew_point	= math.ceil(
  dewPoint(
      math.max(floor_left_air_temp, floor_right_air_temp) + air_temp_offset, 
      math.max(floor_left_hum, floor_right_hum)
  ) * 100
) / 100

-- Store the dew point in a global variable so we can use it in other scenes
fibaro:setGlobal(
  "cellar_dew_point", 
  dew_point
);

fibaro:debug(dew_point)

-- Compute the temperature difference between dew point and floor temperature
fibaro:setGlobal(
  "cellar_dew_diff_l",
  floor_left_temp - dew_point
)

fibaro:setGlobal(
  "cellar_dew_diff_r",
  floor_right_temp - dew_point
)