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The absolute and relative air humidity is determined using the August stationary psychrometer (see fig. 6.2-а).
The reservoir of the psychrometer is filled with water. One of the device’s thermometers is wrapped with the fabric. The fabric is put down into the water so that the reservoir is located about 3 cm above the water surface. After this the psychrometer is hanged onto the support at the determination point. The wet and dry thermometer data are taken 8-10 minutes later.
Fig. 6.2. The devices for the air humidity determination
(a - August psychrometer; b – Assmann psychrometer; c – hygrometer)
The absolute humidity is calculated using the Regnault formula:
А = f – a · (t - t1) · B,
where, А – the air absolute humidity at the current temperature in Hg mm;
f – maximum pressure of water vapour at the wet thermometer’s temperature (see the table of saturated water vapours, table 3);
а – psychrometric coefficient is 0.0011 for enclosed spaces;
t – temperature of the dry thermometer;
t1 – temperature of the wet thermometer;
В – barometric pressure during the humidity determination, Hg mm.
The relative humidity is calculated using the following formula:
P = ,
where, Р –the value of relative humidity to be found, %;
А – absolute humidity, Hg mm;
F – maximum pressure of water vapour at the dry thermometer temperature, Hg mm (see the table of saturated water vapours, table 3).
Table 3
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