A third heat load index is called the effective temperature index (ETI) and has been described in an MLA publication on ventilation in export vessels (SBMR.002, 2001). ETI incorporates dry bulb temperature, relative humidity and wind speed. It does not incorporate solar radiation. It is suggested that ETI is potentially useful for shipboard conditions where solar radiation is not a major issue and where mechanical ventilation provides air flows below decks (LIVE.0211, 2002; SBMR.002, 2001).
The ETI is highly correlated with wet bulb temperature, whereas the THI may be considered as closer to an arithmetic average of dry bulb temperature and wet bulb temperature.
ETI = 27.88 - 0.456*T + 0.010754*T2 - 0.4905*RH + 0.0088*RH2 + 1.1507*V - 0.126447*V2 + 0.019876*T*RH - 0.046313*T*V.
ETI = Effective temperature index; T = dry bulb temperature; RH = relative humidity; V = air speed (m/second).
ETI is considered to be more appropriate as a measure of heat stress for export vessel conditions than THI or the HLI. Observations of measurements made on export vessels suggest that ETI is closely correlated to wet bulb temperature particularly across the range of values for the component measurements that are experienced during export voyages.
While ETI is a potentially useful measure of heat stress, the relative complexity of the index against the simple measurement of wet bulb temperature alone, and the close agreement between ETI and wet bulb temperature for the regions of interest during export voyages, mean that wet bulb temperature alone is preferred as a single, practical measure of heat stress potential on board export vessels (LIVE.0211, 2002).