The instrument should be hung in a shaded position with a steady flow of air around the bulbs.
Fill the cistern with distilled or clean rain water to avoid the wick becoming clogged with impurities.The wick should be changed regularly to ensure a constant flow of moisture to the wet bulb.
To take a reading.
1.Fan the bulbs gently until the columns remain steady.
2.Take the reading of the wet and dry bulb columns.
3.Subtract the reading of the wet bulb column from that of the dry,thus arriving at the depression value.
Use of tables.
The following examples illustrate the method of using the tables:
Dry bulb... ... ... ...30℃
Wet bulb... ... ... ...25℃
Hence,the depression of the wet bulb=30℃-25℃=5℃.Reading directly from the tables,the relative humidity is 65%.