A semiempirical model for the heat exchanger design to find out a minimum length to dampen temperature fluctuations is presented. Stationary heat transport processes through the tube wall are analyzed by means of Nusselt numbers. The conservative treatment of the problem reveals that the tube length should be of the order of 1000 times the inner diameter of the tube to neglect the corrections due to sensible heat flux in eddy covariance measurements of trace gases. The model is analytical and easy to use regarding the choice of tube and flow characteristics. Teflon tube is found to be practically as good a heat conductor as metals. The minimum length required for the damping is not affected by the possible heating of the tube outer surface.¿ 1997 American Geophysical Union