Two integrable 2D relativistic field theory models are studied by the thermodynamic Bethe ansatz method. One of them describes the scaling limit T → T c of the 3-state Potts model and the other corresponds to the scaling region near the Lee-Yang singularity of the 2D Ising model. The finite volume ground state energy of these two theories is calculated numerically using the integral equations of the temperature Bethe ansatz approach. Numerical results are compared with the perturbations near the corresponding conformal theories. This allows us to relate the mass scales of the theories to their dimensional coupling constants.