For over 150 years humidity has been measured using the principle of "wet" and "dry" bulb temperature measurement (using a psychrometer). This reliable and above all accurate system is widely used in greenhouse horticulture. The system works using one "dry" and one "wet" temperature sensor. By measuring both temperatures a computer can accurately calculate the humidity in the air.

The wet and dry bulb humidity measurement has many advantages:

  • Accurate
  • Large measurement range (up to 100% RH)
  • Correct operation can be easily verified
  • Reliable
  • Low purchase price


Diagram showing the principle of dry and wet bulb measurement:

Aquanex  NDB principe

Continue on next page...


Look into the wikipedia for more information about hygrometers.

Aquanex handpsychrometer


The airflow is obtained by swinging the psychrometer around.
After swinging, the dry and wet bulb temperatures can be read from the thermometers.


Aquanex Psy-chart


The temperatures are used in the chart to look up the relative humidity, vapour pressure, dew point, or the moisture deficit.