Indices of thermal comfort

Thermal comfort refers to the subjective feeling of temperature in an environment. Optimum levels of thermal comfort helps in maximizing productivity. Measurement of thermal comfort levels are complex and many indices have been proposed over the years. They are:

  • Air temperature
    • Initially the air temperature as measured from a dry bulb thermometer was taken as the indicator of thermal comfort
    • But it was found to be a unsatisfactory measure as comfort levels depended on other factors too
  • Air temperature and humidity
    • Later air temperature and humidity levels were considered to convey the thermal comfort levels
    • This was also unsatisfactory
  • Cooling power
    • This takes into consideration the following factors
      • Air temperature
      • Humidity
      • Air movements
    • A device called Kata thermometer was devised by Hill to measure cooling power
    • A dry Kata reading of 6 or above and an wet Kata above 20 indicates thermal comfort
  • Effective temperature
    • The different factors determining thermal comfort – air temperature, humidity and air movements are combined together into a single index – Effective temperature
    • Effective temperature is the temperature in an environment with 100% humidity and no air movements which will induce the same level of thermal comfort as in the present situation
    • For example, if the effective temperature is said to be 30°C, it means that the thermal comfort is equivalent to one is an environment with temperature 30°C, 100% humidity and no air movements
    • But effective temperature does not take into consideration, the effect or radiant heat energy
  • Corrected effective temperature
    • The effective temperature is adjusted by considering the loss or gain of heat by radiation to arrive at a corrected effective temperature (CET)
    • Thus CET is determined by 4 factors
      • air temperature
      • humidity
      • air movements
      • radiant heat
    • Corrected effective temperature is measured using a combination of
      • globe thermometer – to measure air temperature adjusted for radiant heat
      • wet bulb thermometer – to measure humidity
      • air speed measurement

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