For the accurate measurement of diffusion coefficients, the shear cell method has been newly developed to overcome the disadvantages of the conventional long-capillary method for diffusion coefficient measurements in the large isothermal furnace of the MSL-l (First Microgravity Science Laboratory) and the multipurpose furnace of TR-IA (sounding rocket)  To achieve good accuracy, the thickness of the cell segments and the rotation rate of the cell (shear process) are crucial. For estimating the depth of convection, a new concept of “isotope trace method in hydrodynamical simulation” is proposed by applying the idea of an isotope tracer experiment, and the mathematical model is appended to the Navier-Stokes equation and continuity equation. We suppose that the liquid used is a mixture of the sample and a kind of "isotope liquid," which has the same physical properties as the original in terms of density, viscosity, specific heat, etc. The redistribution of isotopes is caused by shear convection only, but the isotope has no effect on the flow field. By tracing the “isotope,” we can evaluate the convection depth that is due to the shearing.