Precautions when using a whirling psychrometer:
i) The muslin should be clean and free of dust or deposit left behind by evaporation. If not, the dirt or deposit will not allow free evaporation and the wet bulb reading will be higher than the correct reading. To ensure this, the muslin should be changed at least once a week
ii) Only one drop of water should be dropped on the muslin and this should soak into it. If it does not readily soak, it means that the muslin needs changing.
iii) Never touch the muslin with a finger. If touched, body oils present on the finger would get transferred to the muslin and make it water-resistant
iv) When the muslin does not soak water readily, officers have been known to coax the water in by rubbing the muslin with a wet finger, instead of renewing the muslin. This is wrong and only makes the muslin more and more water-resistant and results in erroneous readings.
v) Always swing the instrument in the open air on the windward side of the ship, away from direct sunlight.
vi) While swinging the instrument care should be taken to ensure that it does not strike against any obstruction, resulting in broken thermometers.
vii) The instrument should be swung for at least 2 minutes. The readings of the thermometers should be made soon thereafter, while out in the open air, using a torch if necessary. If was brought under the chart room light for reading, the reading would have altered by then.
Ref: Marine Meteorology by Capt H Subramanian
