Effective temperature (ET) is defined as the temperature of still or stagnant saturated air (that is air with 100% relative humidity (RH)) in the absence of radiation effects of surroundings so that the subject (person, equipment etc) would experience the same feeling of comfort as experienced in the actual unsaturated environment (that is RH<100%). It thus combines DBT and RH into a single index.
There is another ET called New Effective Temperature (ET*) which is defined as the temperature of the air at 50% relative humidity so that a subject would experience the same feeling of comfort as experienced in the actual environment. Here, the effect of radiation and convection is considered.
Constant ET lines in a Psychrometric chart or in a vapor pressure vs DBT diagram is an inclined line towards the right like constant enthalpy lines or constant WBT lines. In this regard, at lower humidities, the DBT of the air can be higher for the same ET and for the same feeling of comfort. So at higher temperatures, the body will loose more heat in the form of latent heat (evaporation or perspiration).
Important points to be noted for Effective Temperature
- It is the best comfort index for still air condition with 30% to 70% of relative humidity.
- It is not possible to generate a universal ET chart because it depends on activity and clothing.
- Effective temperature is not a true comfort index as it does not consider the radiation effect of surrounding surfaces as well as air velocity.
- To include the air velocity criterion, another index named Corrected Effective Temperature (CET) is introduced. CET will consider the effects of air velocity, radiation effects, DBT and RH.
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