Overview

The humidity sensor is located inside the sensor unit (Ceilling SU or External SU. Its output is the "relative humidity" (%RH). This is computed as the amount of water in the air, expressed as a percentage out of the total amount of water the air can "hold" at that temperature. It depends on the air temperature, which is taken from the temperature sensor located in the same sensor unit.

More information about the RH can be found here.



Hardware


The humidity sensor used is Honeywell HIH-4000-002, but any HIH-4000 version works.


Pin (sensor facing towards you,
pins pointing down)
General
Left pin 0V
Central pin Output
Right pin 5Vdc


Output

%RH Vdc
0.825 0
2 40
3.2 75
3.8 10

There is a linear relationship between the two variables, as seen in the plot below.

Linear relation between the variables


General rules for placing the humidity sensor

  • Remotely located humidity sensors must be installed with the sensor element facing downwards to be protected from dust or other contamination that could set on the sensor element (E.g of remotely located sensors: under a bath/shower, in the attic or in the attic eaves).
  • Do not place in a hole or tight corner - sensors must have free air movement to allow accurate readings
  • Do not place in direct sunlight