Capacitive air humidity sensors have emerged in increasing numbers, makes and qualities. Of all hygrometers available at present, it is estimated that 70-75% employ a capacitive type of sensor. In this paper the results of outdoor experiments over a period of more than one year are summarized for most of the commercially available types.
In hygrometry the dew-point meter and the psychrometer are considered as secondary references. The value of the psychrometer coefficient of reference psychrometers has been subject to much discussion during the past decade. The present work gives the results of comparisons between a standard mirror dew-point meter and several psychrometers of different types. amongst which the WMO reference and the Assmann psychrometer. Both instruments have been accepted by the World Meteorological Organisation as references, both with a defined psychrometer coefficient. The results found here demonstrate an inconsistency in the values of the accepted psychrometer coefficients. A recapitulation of the most important parameters affecting the good performance of a psychrometer is given.
A commercially available small humidity sensor with a very fast response was used in an unventilated diffusion porometer to measure leaf epidermal resistance. Measurements over calibrated dummy resistances showed that such a porometer had definite advantages in transpiration measurements in the field. In particular the waiting time before each measurement, necessary to obtain reproducible results with the commonly used LiCl sensor, could be shortened from 2 min to 15 s or even less. (Abstract retrieved from CAB Abstracts by CABI’s permission)
It is shown that a new calibration method that appeared to be valid for a ventilated dynamic diffusion porometer can be used for the unventilated types as well. A discussion on the method and the results obtained in the determination of dummy epidermal resistance (nickel multipore membranes) is given. (Abstract retrieved from CAB Abstracts by CABI’s permission)
A non-destructive thermo-electric method is described for the measurement of water flow in the stems of plants such as wheat and potatoes. The 2 temp. sensors are 10 or 20 mm apart. The miniature sensing is made by evaporation techniques. The sensor is suitable for laboratory as well as for field work. Flow rate in the stem can be monitored for several wk at relatively low cost. (Abstract retrieved from CAB Abstracts by CABI’s permission)
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