We propose a representation of initial adsorption kinetic constant as a function of convection in a slit flow cell device, averaged over some restricted length of a wall acting as a sensor. The complete domain from transport-control to surface reaction control is included. The intercepts with axes give access to adsorption constant and solute diffusion coefficient. It is shown that, provided the close entrance is avoided, the function for the restricted length is very close to the function for local values.