Articles
CONTINUOUS RECORDING OF HUMIDITY IN GLASSHOUSE EXPERIMENTS
There are a number of basic methods which can be used, each one capable of some variation and having its own associated problems.
The four best known types of hygrometers are:
- Wet and dry bulb hygrometers (sling hygrometer; Assmann hygrometer).
- Dew-point hygrometers (silver thimble or disc hygrometer; Alnor hygrometer).
- Mechanical hygrometers (hair hygrometer; goldbeaters skin hygrometer).
- Electrical hygrometers (Gregory hygrometer; dewcell hygrometer).
Methods 1 and 3 are the ones which have been most employed at the N.I.A.E. for measurements in glasshouses, and the instruments described in the following therefore come mainly from these two classes.
Perhaps the best known type of wet and dry bulb hygrometer is the sling or whirling hygrometer (Figure 1). This consists simply of two thermometers held in a frame, one of which is fitted with a wick which dips into a reservoir.
When a reading is desired the frame is whirled in the air for a period of time and the two thermometers are then read.
From this simplest form numerous variations have evolved, using thermocouples, resistance thermometers or thermistors in place of thermometers, and employing fans to provide aspiration.
Some of these are manually operated and others completely automatic.