The interpretation of tensiometers measurements requires special knowledge of the mechanics of equilibrium of the tensiometer with its environment. The classical concept of response time τ = (1/K′ S) of the system: porous cup (conductanceK') and manometer (sensitivityS) can be improved by taking into account the elasticity of the tubing. By analogy one can appreciate the role of entrapped air in the circuit. In general, it is possible to theorically observe the response characteristics of the tensiometer by imposing pressure step in the system and by studying the equilibrium response. Such tests showed certain anomalies of behaviour which can be attributed to very small bubbles of air entrapped to the internal wall of the porous cup.