referring to the figures , in which like reference numerals refer to like portions thereof , fig1 - 3 show by way of a block diagram a preferred embodiment of the system according to the present invention in its three different configurations . fig1 is intended to show how a recirculation circuit 20 , and a separate powder cartridge 5 are , respectively , filled with water . the water is taken from an inlet 1 , through a heating vessel 2 , and through conduits 3 and 4 to powder cartridge 5 , which is located between two connection pieces 6 and 7 . when the cartridge 5 is filled , the water is further passed as shown in fig1 through a conduit 8 and a valve 9 to a detector 10 , which detects the water and / or concentrate dissolved therein . it is possible to carry this out with a time delay so that the fluid will have time to fill conduit 11 , valve 12 , conduit 13 , valve 14 , and conduit 15 . at the same time , water is conveyed from a branch point 16 through conduit 17 , valve 18 and conduit 19 , to a recirculation circuit , which in its entirety is denoted by reference numeral 20 . recirculation circuit 20 includes a mixing vessel 21 , a recirculation pump 22 , a concentration measuring device 23 , such as a conductivity meter , and between two valve units 24 and 25 , there are connected in parallel a number of cartridges or other vessels 27 , 28 and 29 for one or more powder concentrates or liquid - based concentrate . when a sufficient quantity of fluid has been supplied to the mixing vessel 21 , valve 18 is closed . this can occur with the assistance of a level indicator , which is schematically shown in fig1 by the dashed line 30 . thereafter , the recirculation pump 22 is initiated , along with valve units 24 and 25 , which are so positioned that the water is passed through one of the vessels or cartridges 27 - 29 . in fig1 it is vessel 29 which is shown as being thus connected . when the desired conductivity is obtained , which is measured by means of meter 23 , the next cartridge 28 can then be connected , and finally cartridge 27 . should the prepared fluid in mixing vessel 21 include another liquid - based or easily dissolvable concentrate , then dissolution can occur during the filling of mixing vessel 21 , with the vessel containing this concentrate being connected to conduit 17 . such a vessel 31 is shown in fig1 by means of dashed lines . when the recirculation circuit 20 is filled , valve 18 is closed , as shown in fig2 . in the same manner , valve 9 is actuated when vessel 5 is filled . this valve position is also shown in fig2 . at the same time , valve 12 is switched from the position shown in fig1 to that shown in fig2 so that water from conduit 4 can pass to another vessel or powder cartridge 32 , which is located between two connection pieces 33 and 34 . when cartridge 32 has been filled , the water with dissolved concentrate therein can be conducted through conduit 35 , valve 12 , conduit 13 , valve 14 and conduit 15 to a main line or conduit 36 , which includes a pump 37 and a detector 38 , such as a conductivity measuring device . detector 38 controls valve 14 , so that it is then switched to the position shown in fig3 . the fluid from vessel 32 is now conveyed from valve 14 through a conduit 39 , which includes a dosage pump 40 , to the main line 36 at a point 41 upstream of a restrictor 42 . this restrictor 42 , together with pump 37 and a gas trap , arranged further downstream ( not shown in fig3 ), is used for degasing the prepared fluid . by switching valve 14 to the position shown in fig3 the system is ready to provide a solution for use in dialysis . an inlet valve 43 in the recirculation circuit 20 is thus actuated so the pump 22 can pump prepared fluid from the mixing vessel 21 to a mixing point 44 , to which concentrate from the vessel 5 is conducted by means of valve 9 and conduit 45 . the appropriate concentration of the dissolved substance is measured in this conduit upstream and downstream of the mixing point 44 . this can be achieved , for example , by means of a differential conductivity meter , whose measuring points are denoted by reference numerals 46 and 47 , respectively . the prepared solution can then be conducted further through conduit 45 by means of pump 48 , which preferably consists of an accurately metering dosage pump . the prepared solution can thus be conducted to the main line 36 , in which the concentration is once more checked by means of a meter 49 , such as another conductivity measuring device . this meter 49 then controls pump 48 , as shown by the dotted line 50 in fig3 . additional concentrate is then added to the prepared solution at mixing point 41 by means of a pump 40 , which is controlled by measuring device 38 , as indicated by the dotted line 51 . by means of measuring device 38 , which preferably consists of a conductivity meter , it is possible to regulate the final concentration of the concentrate solution obtained from the system according to this invention . referring next to fig4 there is shown a diagram of how the conductivity varies in recirculation circuit 20 . it is assumed in this case that meter 23 consists of a conductivity measuring device , and that vessels 27 , 28 and 29 contain three different salts . thus , between time t0 and t1 one of these vessels is connected , until conductivity value c1 is obtained . thereafter , the next vessel is connected until conductivity value c2 is reached . then the third or final vessel is connected until the conductivity obtains the desired value c3 . at this point , valve 43 is switched over so that the thus - prepared solution can be conducted to mixing point 44 . at this mixing point , concentrate from vessel 5 is added . the mixture thus obtained is then fed to mixing point 52 in main line 36 , where it is mixed with water from heating vessel 2 . at the next mixing point 41 , concentrate from vessel 32 is then added . after a final check in the measuring device 38 , the thus - prepared solution can then be conducted to its place of consumption , as symbolized by arrow 53 . finally , fig5 a and 5b show a modification of the system according to the present invention . since this modification still corresponds in principal to the above - described system , the same reference numerals have been used , but with the addition of a . in this system , pure water enters through inlet 1a and is pumped by means of pump 22a through recirculation circuit 20a , which includes a heater 61a , a control device , such as a conductivity meter 23a , a sterile filter 58a and a mixing vessel 21a . utilizing this arrangement , mixing vessel 21a should contain the desired quantity of salt from the outset . this salt can possibly be in a dry form , or it can be a concentrated liquid obtained by adding a small quantity of liquid thereto . such a concentrated liquid can be complemented by the addition of any necessary acid in liquid form . such acid may , however , also be added in different forms . certain acids can , for example , be obtained in the form of a dry powder . acid in liquid form can be added through a branch conduit 55a with a dosage pump 56a , as is shown in fig5 b . the acid can thus be drawn from a vessel 57a , and introduced into the recirculation circuit by means of valve 60a . the mixing vessel 21a is preferably connected to the recirculation circuit 20a by means of quick - release connectors 59a , so that it can be readily uncoupled and connected , for example , to a dialysis machine , instead of the traditionally employed dialysis concentrate container . utilizing the system described in fig5 a and 5b for preparation of a dialysis liquid solution , the following mixture was present in vessel 21a : all of these salts were present in the form of a powder . to this was added approximately 2 dl of fluid , essentially comprising water , but to which approximately 32 g of acetic acid had been added . in this case a flexible plastic bag was used for the mixing vessel 21a . in this manner , it was possible to check that a lump - free concentrated liquid was obtained . it should be appreciated , however , that a somewhat smaller quantity of liquid can be used . alternatively , an excess of liquid can be added , but for practical reasons it is advisable to keep the weight of the mixing vessel 21a and its contents down . naturally the invention is not limited simply to the embodiment described above , but may be varied within the scope of the following claims . the parts included in the system may thus be varied within wide limits with regard to their form as well as their function . for example , the containers 27 - 29 and 31 can be combined together , such as in the form of a flexible bag or the like , with a desired number of compartments therein .
apparatus for the preparation of fluids intended for medical use is disclosed , including a source of pure water and at least one cartridge or other vessel containing a powder which is to be dissolved in the water for preparation of the desired fluid . the apparatus includes a container such that the water and a powder or powder concentrate can be mixed and a concentrate provided in the container , and a recirculation circuit for recirculation of the water or concentrate solution into the container for further mixing of the water and powder or powder concentrate to prepare a concentrate having a predetermined concentration .