Working of Hygrometer & Stevenson’s Screen

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A hygrometer is a an easy to use tool for measuring the amount of humidity in the atmosphere and dew point temperature of a parcel of air. The most common type of hygrometer found on board ships is a dry and wet bulb type hygrometer.

It comprises of a pair of basic thermometers fixed on a stand.

One of thermometer has its bulb covered with a muslin wick. One end of the wick is dipped in a small container containing distilled water. The other thermometer’s bulb is left as it is. Hence the name Wet & dry bulb type hygrometer.

Evaporation from the wick of the wet bulb will cause its temperature reading to drop as latent escapes with the water vapour.

Drier the air more will be the evaporation hence more will be the drop in the temperature of the wet bulb. In other words, the difference between the dry and the wet bulb readings will be more for a drier air than a relatively moist air.

The difference between the dry and wet bulb temperature is known as depression of the wet bulb

To find relative Humidity and Dew Point:

Meteorological tables, entered with the dry bulb reading on one vertical axis and the depression on the horizontal axis, give the relative humidity in percentage or dew point temperature of the air.

Separate tables are provided for relative humidity and dew point.

Apart from being meteorological parameters, humidity and dew point temperature are also important from cargo work and seamanship point of view.

Ship sweat and cargo sweat, which can cause damage to cargo inside ship;s holds can be avoided by appropriate ventilation  procedures which involve the knowledge of humidity and dew point of the air inside and outside of the cargo hold.

Painting of ship’s structure requires low relative humidity so that the drying of the paint occurs fast.

Stevenson screen:

This is a wooden box specially constructed to house a hygrometer.

The wooden cupboard has a hinged door.

All three sides and the door fitted with louvers or slots or holes which let air circulate freely without letting in direct solar radiation or reradiated heat from the ship’s structures.

The Stevenson screen ensures that direct heat radiation do not fall on the thermometers thereby raising their temperature.

I f this happens then the thermometers will show their own temperature rather than the air temperature.

Precautions while handling a hygrometer:

  1. The hygrometer should be placed on the windward side, in open air, away from artificial source of heat and draught of air. On board ships one set each is provided on either bridge wings.
  2. It should be fitted at a height convenient to the observer.
  3. It should be fitted as far as possible away from the metal bulkheads to avoid heat radiations from them.
  4. Any foreign material on the muslin will retard the rate of evaporation. Hence it should be clean, free of dust and salt. Never touch the muslin as oil from the finger will contaminate it.
  5. In any case the winch should be replaced once a week.
  6. The muslin should only be just damp. Too much or too little water will cause the wet bulb reading to show higher than correct. This can easily be rectified by adjusting the number of strands of the wick dipped in the water.
  7. Readings of a wet & dry bulb thermometer inside a Stevenson screen when the wind speed is less than 7 knots, are not accurate as sufficient air circulation does not take place.

Can wet bulb reading be more than the Dry bulb reading?

Under some circumstances, as listed below, the dry bulb reading can be less than the wet bulb reading.

  1. Insufficient evaporation taking place from the wet bulb due to dust, salt or other impurities on the muslin.
  2. Insufficient time interval allowed after shifting of Stevenson screen to windward, addition of distilled water, renewal of wick or water etc.

What about readings in sub zero temperatures?

Evaporation occurs as freely from ice as from water, hence as lone there is a thin layer of ice on the wick the hygrometer will function normally.

However, when it dries up no water will be drawn up from the container as the water there will also freeze.

In such case distilled water must be dropped on the muslin and after this has frozen, reading may be taken.

In rare cases, drops of water may be seen on the wet bulb even though the temperature of the dry bulb is well below 0°C.

Such drops of water are known as super cooled water and great error in reading would result.

This can be removed by touching the water droplet with a snow crystal or other cold object and it would immediately freeze.

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