Hygrometer (also called psychrometer)
The hygrometer is an instrument for obtaining the relative humidity and/or dew point temperature of air. The type in use at sea on merchant ships is called the Mason’s hygrometer or Wet bulb hygrometer or psychrometer.
Description
The hygrometer consists of 2 identical Celsius thermometers, one called the dry bulb thermometer and the other the wet bulb thermometer. The wet bulb thermometer has a thin, single layer of muslin or cotton (starch free) tied around the bulb by a few strands of cotton wick. The extra length of the strands of wick is immersed in a bottle of distilled water. Both the thermometers are enclosed in a special, ventilated, wooden box called the Stevenson screen.
Principle
Because of Capillary action, the muslin always remains damp. Water is drawn upwards, from the bottle through the strands of wick. If the atmosphere is dry, rapid evaporation takes place from the muslin. Since evaporation causes cooling the wet bulb thermometer will show a much lower reading than the dry bulb thermometer. If the atmosphere is humid, evaporation from the muslin will be slow, and less cooling of the wet bulb will take place. The reading of the wet bulb thermometer will then be not much lower than that of the dry bulb thermometer. In other words, the difference between the readings of the wet bulb and the dry bulb thermometers (called the depression of the wet bulb), gives an indication of the relative humidity of the air.
The greater the difference, the lower the relative humidity and vice versa.
To find relative humidity and dew point, Meteorological tables, entered with dry bulb reading on one axis and the depression of the wet bulb on the other axis, gives the relative humidity or the dew point of the air. Separate tables are provided for relative humidity and for dew point. Separate tables are provided for use with the hygrometer and with the whirling psychrometer.
Precautions
i) The Stevenson’s screen should be on the windward side, in open air, away from artificial sources of heat (heaters or blowers) and artificially heated draughts of air (from accommodation, funnel hold ventilators,skylights, etc..)
ii) It should be about 1.5 m above the deck for the convenience of the observer.
iii) Sunlight falling on the Stevenson’s screen is permitted but not directly on the thermometers
iv) It should be far away from metal bulkheads, etc., which will cause heat radiations that can affect the readings
v) The muslin should be clean free of dust or salt particles carried by spray. If not, the wetbulb thermometer will give higher than correct reading.
vi) In any case, the muslin and strands of wick must be changed once a week. This is because solid particles are left behind by the evaporating water. These particles subsequently prevent free evaporation and the wet bulb reading will be higher than the correct reading. That is why distilled water is used even then the distilled water available is rarely as pure as we would like it to be. An entry should be made in the weather logbook every time the muslin is changed.
vii) The muslin should be only just damp.Too much water on it, or too little, will cause the wet bulb reading to show higher than correct. This can easily be rectified by adjusting the number of strands of wick leading into the water bottle.
viii) The water bottle should be washed and the distilled water in it renewed once a week.
ix) Whenever distilled water has been added or changed, or the muslin has been renewed, or the Stevenson’s screen has been shifted to the Windward side, at least half an hour must elapse before reading the wet bulb thermometer, so as to allow sufficient time for evaporation.
x) The dry bulb should be clean and clear of drops of condensed water.
xi) Reading of a wet bulb thermometer inside a Stevenson’s screen, when the wind speed is less than 7 knots, are not accurate enough.
Ref: Marine Meteorology book by Capt H Subramanian.
