This instrument in its present design was developed by Frankenberger and provides continuous measurement of the "dry bulb" and "wet bulb" temperatures with high accuracy. From these two values the desired relative humidity and dew point temperature may be gained by using psychrometric tables, diagrams or computer systems. The design of the instrument essentially eliminates the effects of wind and radiation.
Inside the downward inclined radiation shield tubes 2 platinum resistance thermometers (Pt 100 W) are mounted. One sensor is continuously moistened with water through a wick, hereby measuring the wet bulb temperature. The wet bulb sensor can be recognised by a groove in the base of the radiation shield.
The sensors are using either a 3-wire or 4-wire supply system. This way measurement over long distance can be carried out without the line resistance falsifying the measurement. The resistance thermometers are continuously aspirated by a ventilating system providing an air flow of about 3.5 m/s. The direction of the air flow opposes the direction of water supply to the wet bulb sensor (counter flow process) - so that a previous evaporation is avoided.
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