[′läg ¦mēn ′tem·prə·chər ‚dif·rəns] (thermodynamics) The log-mean temperature difference TLM= (T2-T1)/ln T2/ T1, where T2 and T1 are the absolute (K or °R) temperatures of the two extremes being averaged; used in heat transfer calculations in which one fluid is cooled or heated by a second held separate by pipes or process vessel walls.