An improved guarded parallel-plate method for measuring the thermal conductivity of fluids in the critical region

A description is presented of the construction and operating procedure of a n improved guarded parallel-plate instrument for measuring the thermal cond uctivity of fluids in the critical region. With the improved arrangement, i t has been possible to reduce the temperature differences across the fluid layer between the two horizontal plates to values as low as 0.3 mK, thus al lowing the thermal conductivity of fluids to be measured at temperatures as close as 20 mK to the critical temperature. The instrument has been used t o measure the thermal conductivity of methane and of a mixture of methane a nd ethane in the close vicinity of the critical point.