The usual method for evaluating the performance and efficiency of the air conditioning system in vehicles is to measure the air temperature with small thermocouples at the feet and head level. The main purpose of such a measurement is to see how quickly the temperature will rise inside the cabin and reach the level where thermal comfort can be established. By measuring the air temperature only, any influence of air velocity and radiation (cold or hot), is neglected and the measurements might lead to false conclusions as only one of the three main climatic parameters that influences the degree of thermal comfort is measured.

Using the operative temperature, measured with an unheated ellipsoid shaped sensor, for the performance evaluation, the influence of the air velocity is neglected. As the air conditioning system in vehicles is based on heating/cooling by air flow, very high local air velocities are often seen. The cooling effect of these air velocities will therefore be neglected, and as demonstrated in the measurement this might also lead to false conclusions.

The combined effect of the air velocity, air temperature and mean radiant temperature can be expressed by the equivalent temperature. The difference between the operative temperature and equivalent temperature was carefully studied by Madsen et. al. [3] and was found to be the preferred parameter for the evaluation of thermal comfort, if high air velocities are present.

Measuring each parameter requires a lot of instrumentation and it is difficult to measure all the parameters in the exact  same location and then later calculate the combined influence. Using a transducer that measures the combined effect of all climatic parameters, the equivalent temperature, makes the evaluation much easier.