Tube arrangement, including means (11, 12) for connection to a water source (15), a powder cartridge (9) or means (22'') for connection of a powder cartridge and means (7) for connection to a monitor for preparation of a medical solution, for example dialysis fluid, replacement fluid for haemofiltration or fluid for peritoneal dialysis. The tube set according to the invention is characterized by means (10; 10', 8a', 8b'; 10'', 10''', 8a, 8b) for lowering the water pressure to substantially atmospheric pressure. The invention also relates to a process for supplying a powder concentrate to a monitor (1) for preparation of a medical solution for example a dialysis solution, replacement fluid for haemofiltration or dialysis fluid for peritoneal dialysis, whereby the powder concentrate is supplied instead of a concentrate in liquid form, which the monitor is arranged to normally take from a vessel at atmospheric pressure. The process according to the invention is characterized in that the powder is dissolved by the through-flow of water from a water source whereby the dissolving occurs substantially at atmospheric pressure, which pressure is also maintained during the subsequent transportation to the monitors normal intake of concentrate.