Charakterisierung und Kalibrierung von Blattfeuchtesensoren in einer Taukammer
Lisa von der Heyden (08/2013-10/2013)
Support: Andreas Held
BSc-Arbeit, Universität Bayreuth, 66 S.
In this Bachelor thesis wetness sensors are calibrated and characterized in their measurement. Furthermore, the sensors are verified whether they are able to measure deliquescence of a salt. The sensors are developed for the detection of wetness films on leaf surfaces. They measure electrical resistance between two clamp contacts. Measurements with empty sensors and with defined resistors clipped between the contacts are performed in a dew chamber under controlled up- and downturns of relative humidity from 50 % to 90 %. Silica filters which are steeped in dilutions of potassium chloride are clipped between the clamps. The sensors rarely show deviations in their measuring characteristics among each other and they are able to provide stable data over periods of several days. Measured voltages are slightly influenced by high relative humidity of the ambient air. The resistors are used to calibrate the sensors, thus measurements can be expressed as a conductance instead of a voltage. A formula is fitted for conductance G [S], the reciprocal value of electrical resistance. Depending on the dimensions of the conducting medium, in this case the filters, a calibration curve for the electrical conductivity [S cm-1] can be estimated. The electrical conductivity of the filters treated with potassium chloride depends on relative humidity and on the solution’s concentration. Hysteresis between the conductivity of increasing and decreasing humidity occurs at high concentrations. This is explained by deliquescence and recrystallization processes of the potassium chloride particles on the filters. There are some uncertainties remaining concerning the measured points of deliquescence and recrystallization and concerning the measured amounts of conductivity. The latter is supposed to be caused by the silica filters as an unsuitable medium.