Dilution refrigeration techniques have been considerably
improved during the past ten years, but their performances are still limited by the efficiency of the heat exchangers compared to the residual heat leak into the mixing chamber. Various techniques have been tried to improve this exchange. The best results have been obtained with very thin metallic powders providing exchange areas up to several hundred square metres, ~ and also with direct exchange between both liquid phases by double He 3 and He 4 circulation. 2' 3 One year ago, De Waele 4 presented n refrigerator with two successive mixing chambers which reached respectively 13.5 and 5.5 mK. In this case, these very good results are also obtainable by direct exchange in the liquid phase. But this method, in contrast with the He 4 circulation, allows calculation of the performance with well-known parameters, as for the classical case. These calculations pointed out that it was possible to improve the ultimate performances of the double mixing chambers system by precooling with classical powder exchangers.