function AbsHumidity (T, RH)
-- taken from https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/
  local a = 6.112     
  local b,c = 17.67, 243.5
  local d = (b*T)/(T+c)
  e = math.exp (d)
   return (a*e*RH*2.1674)/(273.15+T)
end
 
function DHT22_get (dt,dh)
    local tempSID  = "urn:upnp-org:serviceId:TemperatureSensor1"      
    local humidSID = "urn:micasaverde-com:serviceId:HumiditySensor1"  
    local T = luup.variable_get (tempSID, "CurrentTemperature", dt)
    local H = luup.variable_get (humidSID, "CurrentLevel", dh)
    local D = AbsHumidity (T,H)
    luup.variable_set (humidSID, "AbsHumidity",D,dh)
    return D
end