Dewpoint.html

An information file concerning the relative humidity measurements using the Kroneis dew-point sensor

Precise humidity measurements are not easy to do: most commercial available electronic hygrometers are frequently not very precise and not stable with time. It is especially difficult to make long-time humidity measurements in a near saturated atmosphere with only very small wind movements; this has been done during a research project in underground climatology (project PHYMOES, CRP-CU; project-leader francis.massen@ci.educ.lu) using the well established dew point measurement method. This method has been found to be the only reliable technique in such a difficult environment. A Kroneis dew point sensor which was used well over two years in the extremely humid maze cave of Moestroff (Luxembourg) has been recalibrated and added to the other sensors of the weatherstation to give a more accurate relative humidity measurement. This instruments works by maintainig a small mirror at dew point temperature (using a pulsed Peltier element); its outputs are two voltage signals:

  • Kai is the output (in mV) of the air-temperature NTC sensor
  • Kde is the output (in mV) of the dew-point NTC sensor

    Several formulas must be used to get the temperatures, than the relative humidity of the air from these two voltages. The NTC sensors need a reference voltage: this is Vref = 2.513 V in our case.

    1. Temperature in degrees Celsius from NTC and Vref voltages: [eq.1]

    2. Vapour pressure in [HPa] from temperature in [Celsius]: [eq.2]

    3. Relative humidity in percent from the 2 vapour pressures. [eq.3]



    An example:

    Kai = 1400 mV and Kde = 1300 mV

    T_air = 26.8 degrees Celsius, T_dewpoint = 19.7 degrees Celsius, rel. humidity = 65%

    Attention:

    One should not be surprised if the Kai temperature is higher than the temperature recorded by the normal air temperature sensor (T_air); the Kroneis NTC sensors are well protected from most direct wind gusts, and somewhat exposed to radiation reflected by the roofs silver rain-proof painting; so there certainly will be a noticeable difference between air temperatures measured during sunny hours. As both Kroneis sensors are located at the same position, this should not affect the true relative humidity of ambient air.

    Technical details:

    The Kroneis dew-point sensors are used by the Austrian TAWES meteorological system (Zentralanstalt für Meteorologie und Geodynamik, Hohe Warte 38, A-1190 WIEN)

    Sensor type: dew-point sensor no. 620

    Manufacturer: Dipl.Ing. A. und W. KRONEIS, Iglaseegasse 30-32, A-1191 WIEN

    francis.massen@ci.educ.lu