We basically made a study of two similar flat plate collectors, in which the main difference was the gas filling of their chambers. One of them was supposed to have lower heat losses and so a higher efficiency due to the heavy gas filling of the collector. For that purpose a complete solar system was built in SERC, in which both type of collectors where connected in series and a valve system was used to run the system in different senses. Firstly the main part of the thesis has consisted in building a complete real installation. Secondly we wanted to be able to make a trustworthy performance test that could give us feasible reference values of how each collector worked, how they gained energy and how they lost heat. And of course a performance test that could give as data to plot an efficiency diagram of each collector. The performance of the whole system has also been tested in both flow senses. And finally a third part has been included, in which, we tried to demonstrate how due to the sealing of the gas filled chamber, the pressure changes in the chamber affected in the movement and effort that the cover and absorber had to suffer. Distance measurements have been taken between the glass and the absorber at different temperatures. This report consists in a theoretical explanation of how the installation has to be built and how we have done it. A description of what are the theoretical requirements needed to do the performance test, which were the steps that had to be followed and how we did it. In the last part of the report results of the testing and the distance measurements are shown with their conclusions and discussions. Nevertheless our practical work has shown us that theory is often far from real life, and lower efficiency values than we expected have been seen during all our work. On the other hand, some interesting results have been obtained related to the distance measurements as they have clearly shown us how the absorber suffers while exposed to different conditions.