Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air – evaporation is slower when the air is already full of water vapour. For this reason, the difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity.
Wetbulb and Dry bulb temperature are recorded on live export vessels every 4 hours to monitor environmental conditions. If there has been a water leak or rain come onto an upper deck, the increase in wetbulb will be noted which reflects the increased humidity. This information is used to make management decisions and monitor certain areas of the vessel more closely.
The industry is trailing the use of more automated real time monitoring as it moves forward to provide even more detailed information about micro environments on certain decks.