This work presents a study of the transient behavior of a wet bulb and dry bulb psychrometer, as well as the determination of the humidity of air under different environment conditions. The wet bulb and dry bulb psychrometer are widely used in the air conditioning industry, for monitoring the humidity in closed environments, and in meteorology stations for the monitoring the atmosphere humidity and in the prevention of the climatic conditions. For the development of the present work, a wet bulb and dry bulb aspirated psychrometer was built, using two thermometers, which are physically separated by an acrylic plate. Two types of thermometers were used: PT-100 and Thermocouple. In the evaluation of the humidity content, the measurement of temperature was been registered by two thermometers (used as the psychrometer components), where one of them registers the temperature of the air that impacts the psychrometer and serves as reference; and the other one covered by a wick wet with distilled water, registers a lower temperature due to the phenomenon of evaporative cooling, which is a consequence of the mass transfer between the wet wick and the air that is forced to pass through it. In the evaluation of the transient behavior of the psychrometer different models of adjustment to the experimental data have been applied, seeking the best theoretical model that represented the behaviour through the time of the humid bulb thermometer. Among the adjustment models are: the linear one, the exponential one, the addition of two exponential ones, the rational one, polynomial of exponential degree 6 and the linear one (which adds the exponential curve and the straight line). For each of the models the method of the minimum square had been applied, and it was found by means of the of Aikaike´s criterion that the linear exponential model is the best one. The uncertainties of the constants of the different models of adjustment were calculated through the method of the finite differences. The values of the time constants of time for different values of air speed and different degrees of humidity for both thermometers PT-100 and Thermocouple type had been presented. Finally, the results of the application of the multiple linear regression are presented among the time constants, the speed, and the humidity of air for both psychrometer based on the thermometers PT- 100 type and the thermometers Thermocouple type. This MSc research contributes towards the development of the thermal metrology in Brazil.