The first step in the theoretical modeling of photoacoustic experiments is the calculation of the Temperature within the sample and/or its environment. For solids it is usually calculated by solving the heat diffusion equation. The same equation is used often for liquid samples, as well as for the gas filling the PA chamber, neglecting the role played by hydrodynamic effects. In the present work we will briefly describe how the Theory of Similarity shows that the heat diffusion equation, in its usual formulation, can be only applied for fluids when the product of Grashof (Gr) and Prandtl (Pr) numbers, the so-called Rayleigh, Ra, number, is lower than 1000
