Development of the Universal Standard Effective Temperature for evaluating thermal comfort across different metabolic rates

The standard effective temperature (SET) is a widely recognised index in thermal comfort studies worldwide. The definition of the standard environment is crucial to the validity of SET. It allows SET to evaluate thermal comfort levels in various clothing insulations, air velocities, and metabolic rates. The thermal sensation votes (TSV) should be equivalent when the SET remains constant. Previous studies have focused on applying SET to analyse thermal comfort with diverse environmental factors or clothing insulation conditions but have not compared SET with different metabolic rates. In this study, the original standard environment was examined using experimental results with different metabolic rates. The results revealed that the original standard environment failed to guarantee the 'same SET, same TSV' principle with different metabolic rates. The original standard environment was insufficient in accurately reflecting the effect of the actual activity status on the standard convective heat transfer coefficient and standard clothing insulation. Thus, a new standard environment was developed, incorporating two new variables: the base value of the standard clothing insulation and the convective heat transfer coefficient generated from the actual activities. Through validation of the experimental data, the new standard environment ensured a consistent linear relationship between SET and TSV, in both transient and steady-state conditions, even when the metabolic rate was changed from 1.0 met to 3.8 met. Furthermore, a universal SET was proposed. The universal SET could be applicable to thermal comfort analysis with different metabolic rates in steady-state conditions and activity status transients.