fig1 is a highly simplified schematic view of a device 1 for supplying a medical fluid and in particular dialysis fluid , and of a blood treatment apparatus 2 and an apparatus 3 for filling the device for supplying dialysis fluid . the blood treatment apparatus 2 may be an extra - corporeal dialysis apparatus or an apparatus for peritoneal dialysis . in the present exemplary embodiment , the blood treatment apparatus 2 is a dialysis apparatus which has a dialyser 4 which is divided into a blood chamber 6 and a dialysis - fluid chamber 7 by a semi - permeable membrane 5 . a blood infeed line 8 runs from the patient to the blood chamber 6 of the dialyser 4 while a blood return line 9 , into which a blood pump 10 is connected , runs from the blood chamber 6 to the patient . together with the blood chamber 6 , the blood infeed and return lines 8 , 9 form the extra - corporeal blood circuit i of the dialysis apparatus 2 . fresh dialysis fluid is conveyed from a dialysis - fluid reservoir 11 , through a dialysis - fluid infeed line 12 into which a dialysis - fluid pump 13 is connected , to the dialysis - fluid chamber 7 of the dialyser 4 , while used dialysis fluid flows out of the dialysis fluid - chamber through a dialysis - fluid outfeed line 14 . a dialysis - fluid reservoir is not essential however . the dialysis fluid supplied may equally well be fed direct to the dialysis - fluid chamber . the device 1 , which has two bags or canisters 15 a and 15 b in the present exemplary embodiment , is used to supply fresh dialysis fluid . the two bags or canisters 15 a , 15 b form a unit 15 , the bag 15 a being filled with fresh dialysis fluid before the dialysis treatment and the bag 15 b being empty . from the dialysis - fluid bag 15 a , an infeed line 16 runs to one connection 17 a of a plug unit a , while an outfeed line 18 runs from the other connection 17 b of the plug unit a to the empty bag 15 b . to supply dialysis fluid , the plug unit a is connected before the treatment to a socket unit b which is provided on the blood treatment apparatus 2 , thus enabling fresh dialysis fluid to be fed through the infeed line 16 to the dialysis - fluid reservoir 10 and used dialysis fluid to be feed out through the outfeed line 18 . the device 1 for supplying dialysis fluid is filled with fresh dialysis fluid at apparatus 3 . the device 2 for supplying dialysis fluid can also be emptied by the filling apparatus 3 . the fresh dialysis fluid can be produced on - line in the filling apparatus 3 from water and dialysis fluid concentrate . fresh dialysis fluid may however also be fed in from an internal or external source of dialysis fluid . used dialysis fluid can be collected in an internal or external reservoir or discarded at a discharge . fig1 shows an exemplary embodiment in which a tank 20 a is used to hold fresh dialysis fluid and a tank 20 b to receive used dialysis fluid . the lines and pumps required are not shown in the highly schematic representation . as well as this , the device 1 for supplying dialysis fluid also has a control unit ( 80 ) by which all the components of the device 1 are controlled to connect the device for supplying a dialysis fluid to the filling or emptying apparatus and to fill and empty the device for supplying dialysis fluid , as will be described in detail below . the apparatus 3 for filling and emptying the device 1 for supplying fresh dialysis fluid and for receiving used dialysis fluid has a socket unit b ′ to which the plug unit a of the device 1 for supplying dialysis fluid is connected . the socket unit b of the blood treatment apparatus 2 and the socket unit b ′ of the filling or emptying apparatus may be of identical or different forms . in the present exemplary embodiment the socket units b and b ′ are of different forms . however , both the socket units b and b ′ are so designed that fluid - tight flow - permitting connections in both directions for fresh and used dialysis fluid can be made to the two items of apparatus 2 and 3 with the plug unit a of the device 1 for supplying dialysis fluid . as well as this , the filling or emptying apparatus 3 has a control unit 80 for controlling the filling and emptying processes and a reader 81 for reading a machine - readable code from a planar information carrier which is provided at the plug unit a of the device 1 for supplying dialysis fluid . the filling or emptying apparatus 3 also has an analysing unit 82 by which the information read from the information carrier can be analysed . the information may for example be information on the composition or amount of the medical fluid supplied . the analysing unit also enables errors to be detected by which the faulty read - out of a code is detected . in the event of the read - out of the code being faulty following destruction of the information carrier , which includes the event of a code not being able to be read out at all , the analysing unit 82 generates a control signal which the control unit 80 receives . if the control unit 80 receives the control signal , it prevents the filling process from being initiated . the initiation of the filling process is preferably prevented by the fact that a flow - permitting connection is not automatically made between the plug unit a of the device 1 for supplying dialysis fluid and the socket unit of the filling or emptying apparatus 3 . it is however also possible for the pumps ( not shown ) involved in the filling process not to be started or the valves ( not shown ) not to be opened . in what follows , the plug unit a of the device 1 for supplying dialysis fluid will be described in detail by reference to fig2 to 6 , together with the socket unit b of the blood treatment apparatus 2 . fig2 is a perspective view of the plug unit a and the socket unit b , while fig3 to 6 show the plug and socket units a , b in section . the socket unit b of the blood treatment apparatus 2 is preferably part of a treatment cassette ( not shown ) which is interchangeable . the socket unit b may however also be part of a non - interchangeable unit . the socket unit b has an outer flange portion 21 which is screwed to the housing wall 22 of the treatment cassette or blood treatment apparatus or it may be integral with the wall . the outer flange portion 21 may for example be screwed to the wall by screws ( not shown ) or it may be one common injection moulded part with the wall . projecting from the outer flange portion 21 are two cylindrical connecting parts 23 , 24 which are arranged in a common plane on the two sides of the central axis 58 of the socket unit . the cylindrical connecting parts 23 , 24 concentrically surround respective connecting pieces 25 and 26 , the connecting piece 25 being used to feed in fresh dialysis fluid and the connecting piece 26 to feed out used dialysis fluid . the plug unit a of the device 1 for supplying fresh dialysis fluid and receiving used dialysis fluid has corresponding connectors 27 , 28 which will be connected to the connecting pieces 25 , 26 with a fluid - tight seal . the plug unit a has a plug body 29 which connects the two connectors 27 , 28 . the plug body 29 has an infeed passage 30 which is connected to one connector 27 and an outfeed passage 31 which is connected to the other connector 28 . the infeed line 16 of the device 1 for supplying fresh dialysis fluid and receiving used dialysis fluid is connected to the connection 17 a to the infeed passage 30 and its outfeed line 18 is connected to the connection 17 b to the outfeed passage 31 . situated between the two connectors 27 , 28 is a projecting piece 32 with which a connection which at first is only loose can be made between the plug unit a and the socket unit b . the projecting piece 32 has a plurality of latching members 33 which are arranged to be circumferentially distributed and which are integrally formed on one end of the plug body 29 . formed on the outer sides of the free ends of the latching members 33 are latching noses 34 . to protect them against physical contact , the connectors 27 and 28 have sleeves 76 and 77 which are fitted onto and latch into the connectors 27 , 28 on the plug body 29 . the connectors 27 , 28 are closed off by respective septums , which are preferably slit , or membranes 35 , 36 , which will be pierced by the connecting pieces 25 , 26 of the socket unit . between the two connectors 27 and 28 , the plug body 29 has a portion 83 in whose outer side there is a groove 85 which extends from the front end of the portion 83 to its rear end , parallel to the axis which defines the direction in which the plug unit a is plugged into the socket unit b , which means that the groove 85 is open at both ends ( fig3 ). applied to the portion 83 is a planar information carrier 86 which hides the groove 85 . the information carrier 86 is a thin substrate , such as a film or paper for example , on which a matrix code is printed . the socket unit b of the blood treatment apparatus 2 has an outward pointing portion 87 of its main body which is provided on the inside with a salient projection 88 . the longitudinally extending groove 85 in the portion 83 and the salient projection 88 on the portion 87 of the main body are so arranged that the salient projection 88 engages in the groove 85 , which is open at the front end , when the plug unit a is plugged into the socket unit b . the information carrier 86 is thus destroyed when the plug unit is connected to the socket unit . because the salient projection 88 also extends parallel to the axis which defines the direction in which the plug unit a is plugged into the socket unit b , the projection 88 also acts a guiding piece for the plug unit when it is plugged into the socket unit . if the plug unit a of the device 1 for supplying fresh dialysis fluid and receiving used dialysis fluid , which latter is intended for once - only use , is connected into the socket unit b ′ of the filling or emptying apparatus 3 , the device 1 can only be emptied , but not refilled , because the information carrier 86 has been destroyed , as has been described by reference to fig1 . mounted in the centre of the flange portion 21 of the socket unit b is a cylindrical guiding piece 37 which extends through a hole 38 in the housing wall 22 . guided to be longitudinally displaceable in the guiding piece 37 is a tubular receiving piece 40 which has a front portion 41 and a rear portion 42 . a drive unit 43 is provided to displace the tubular receiving piece 40 in the cylindrical guiding piece 37 . in the present exemplary embodiment , the drive unit 43 is an electric motor driven spindle drive which has a linear motor 44 and a spindle 45 , which latter is connected to the rear portion 42 of the receiving piece 40 . the receiving piece 40 is thrust out of the guiding piece 37 and retracted into the guiding piece by the extension and withdrawal of the spindle 45 . provided in the interior of the front end of the front portion 41 of the receiving piece 40 are a plurality of recesses 46 which are arranged to be circumferentially distributed and which are of a form such that the latching noses 34 on the latching members 33 of the projecting piece 32 on the plug unit a latch into the recesses 46 when the projecting piece 32 is inserted in the receiving piece 40 , a process which will be described in detail below . the socket unit b has means by which it is detected that the projecting piece 32 is inserted in the receiving piece 40 . these means have a sensing member 47 , in the form of a tubular body , which is guided to be longitudinally displaceable in the tubular receiving piece 40 . the tubular sensing member 47 is pre - loaded by a spring 48 which is mounted in the rear portion 42 of the receiving piece 40 . a limit to the movement of the sensing member 47 in the longitudinal direction is set by a stop member 49 which is merely indicated and which is guided in a slot ( not visible in the present plane of section ) which is provided in the receiving piece 40 . to detect the projecting piece 32 in the receiving piece 40 , the socket unit b has means by which it is detected whether the front end of the sensing member 47 is flush with the front end of the receiving piece 40 ( fig3 ) or whether it has been pushed back into the receiving piece in opposition to the pre - loading from the spring 48 . this is the case when the projecting piece 32 is inserted in the receiving piece 40 . the socket unit b also has a body 50 in pin form which is arranged in an immovable position , inside the tubular sensing member 47 . the body 50 in pin form may for example be locked by a pin 39 which extends through slots ( not shown in the plane of section ) in the sensing member 47 and the receiving piece 40 and into the guiding piece 37 . in what follows , it will be described in detail how the plug unit a is connected to the socket unit b . fig3 shows the starting position , in which the socket unit b is locked . in this position , the spindle 45 of the linear motor 43 is withdrawn , as a result of which the receiving piece 40 and the sensing member 47 are retracted and the body 50 in pin form thus projects from the receiving piece . the body 50 in pin form thus prevents the projecting piece 32 from being inserted in the receiving piece 40 . fig4 shows the position of the receiving piece 40 and the sensing member 47 in which the spindle 45 of the linear motor 44 is extended and the receiving piece 40 and the sensing member 47 are advanced outwards out of the guiding piece 37 . in this position the body 50 in pin form , which is connected to the guiding piece 37 , is retracted sufficiently far for the projecting piece 32 on the plug unit a to be able to be inserted in the receiving piece 40 . fig5 shows the projecting piece 32 on the plug unit a when fitted into the receiving piece 40 of the socket unit b . when the projecting piece 32 is inserted in the receiving piece 40 , the latching noses 34 on the latching members 33 latch into the recesses 46 in the receiving piece 40 by a snap - in action . the plug unit a is held loosely to the socket unit by this means . however , the flow - permitting connection has not been made yet in this case , because the connecting pieces 25 , 26 of the socket unit b have not yet been connected to the connectors 27 , 28 of the plug unit a . in this position the infeed and outfeed passages 30 , 31 in the plug unit are still closed off with a fluid - tight seal by the membranes 35 , 36 . when the projecting piece 32 is inserted in the receiving piece 40 , the sensing member 47 , which is guided to be longitudinally displaceable in the receiving piece , is pushed back into the receiving piece 40 by the latching members 33 of the projecting piece 32 in opposition to the resilient force of the spring 48 . the withdrawn position of the sensing member 47 , which is shown in fig5 , is sensed by means which are not shown , such for example as electrical contacts which are closed or a light barrier , as a result of which the drive unit 43 is started . the linear motor 44 now withdraws the spindle 45 , and the receiving piece 40 and the sensing member 47 are thus retracted . fig6 shows the position of the receiving piece 40 when the spindle 45 of the linear motor 44 is completely withdrawn and the receiving piece 40 and the sensing member 37 are fully retracted . as the receiving piece 40 is retracted , the body 50 in pin form is advanced into the projecting piece 32 , as a result of which the latching noses 34 on the latching members 33 of the projecting piece 32 are secured in the recesses 46 in the receiving piece 40 . the projecting piece 32 is thus locked in the receiving piece 40 . the locking of the projecting piece 32 in the receiving piece 40 takes place simultaneously with the relative movement of the connecting pieces 25 , 26 and the connectors 27 , 28 . in the position shown in fig6 , in which the spindle 45 of the linear motor 44 is fully withdrawn , the two connecting pieces 25 , 26 and the two connectors 27 , 28 are connected together with a fluid - tight seal . the locking of the projecting piece 32 in the receiving piece 40 on the one hand ensures that the plug unit a , which at first was only loosely inserted in the socket unit b , can be drawn onto the socket unit in opposition to the forces which arise , and on the other hand prevents the plug unit from being able to detach from the socket unit once the flow - permitting connections have been made . the fluid - tight and non - releasable connection between the plug unit a and socket unit b is thus made automatically once the plug unit has been loosely inserted in the socket unit . the unlocking of the plug unit a from the socket unit b takes place in the reverse order from the locking of the plug unit to the socket unit . for this purpose , the drive unit 43 is started again . this may for example be done by pressing a button or the like . when the spindle 45 of the linear motor 44 is extended again , the receiving piece 40 and the sensing member 47 slide forward again over the body 50 in pin form , as a result of which the locking of the latched connection between the projecting piece 32 and receiving piece 40 is released . at the same time , the connecting pieces 25 , 26 are disconnected from the connectors 27 , 28 . the plug unit a is thus situated in the starting position ( fig5 ) again , in which the plug unit is still held loosely on the socket unit . this prevents the plug unit from being able to drop easily off the socket unit . fig7 - 11 show an alternative exemplary embodiment b ′ of the socket unit which is provided on the apparatus 3 for filling the device 1 for supplying the dialysis fluid . basically , this alternative exemplary embodiment may equally well be provided on the blood treatment apparatus 2 . it is however also possible for the exemplary embodiment which was described by reference to fig2 to 6 to be provided on the filling apparatus 3 . the alternative exemplary embodiment b ′ of the socket unit will be described in detail in what follows . fig7 is a perspective view of the alternative exemplary embodiment b ′ of the socket unit together with the plug unit a . both the exemplary embodiments b and b ′ of the socket unit can be connected to the same plug unit a , thus enabling the device 1 for supplying dialysis fluid to be connected on the one hand to the apparatus 3 for filling and emptying and on the other hand to the blood treatment apparatus 2 . the two exemplary embodiments of the socket unit differ from one another in particular in that , when the plug unit a is being connected to the socket unit b to make the flow - permitting connections , the connectors 27 , 28 of the plug unit a are drawn automatically onto the connecting pieces 25 , 26 of the socket unit b ( fig2 to 6 ) by moving the plug unit a , whereas in the alternative exemplary embodiment b ′ of the socket unit the connecting pieces of the socket unit are moved into the connectors 27 , 28 of the plug unit a , in which case the plug unit a is not moved . what is more , the alternative exemplary embodiment b ′ of the socket unit envisages the closing - off of the two connecting pieces or the making of a flow - permitting connection between the connecting pieces for a flushing process without the plug unit a and socket unit b ′ being connected together . the socket unit b ′ has a housing body 51 which is inserted in a wall 52 of the housing of the filling apparatus 3 ( fig7 ). the housing body 51 has a central opening 53 in which the receiving piece 57 for the projecting piece 32 of the plug unit a is arranged ( fig9 ). in contrast to the exemplary embodiment which was described by reference to fig2 - 6 , the receiving piece 57 of the socket unit b ′ is not guided to be displaceable in the direction defined by the longitudinal axis 58 of the socket unit b ′ but is mounted to be rotatable on the longitudinal axis 58 by means of a bearing 59 which is inserted in the central opening 53 in the housing body 51 . the receiving piece 57 is rotated by a drive unit which is not shown . the receiving piece 57 has a front portion 60 which extends out of the housing body 51 and a rear portion 61 which extends into the housing body 51 , the front portion 60 being of a larger inside and outside diameter than the rear portion 61 . provided on the inside of the front end of the front portion 60 of the receiving piece 57 are the recesses 62 which are arranged to be circumferentially distributed and into which the latching noses 34 on the latching members 33 of the projecting piece 32 latch when the plug unit a is fitted loosely onto the socket unit b ′. guided to be longitudinally displaceable in the tubular receiving piece 57 is the sensing member 63 , which takes the form of a tubular body and which is pre - loaded by a spring ( not shown ) so that the sensing member 63 is pushed back in opposition to the loading from the spring when the projecting piece 32 is inserted in the receiving piece 57 . guided in the tubular sensing member 63 is the body 64 in pin form for locking the projecting piece 32 in the receiving piece 57 . the body 64 in pin form can be advanced in the longitudinal direction of the axis 58 and retracted again , to respectively lock and release the projecting piece 32 in the receiving piece 57 , by a drive unit ( not shown ). in the alternative exemplary embodiment b ′ of the socket unit , the connecting pieces 65 , 66 are mounted in cylindrical openings 67 , 68 in a connecting part 69 which is guided to be longitudinally displaceable in the housing body 51 , thus enabling the connecting pieces 65 , 66 to be advanced out of and retracted into the housing body 51 . the drive unit for advancing and retracting the connecting part 69 having the connecting pieces 65 , 66 is not shown in the drawings . fig9 shows the socket unit b ′ in the position in which the plug unit a is fitted loosely onto the socket unit b ′. the body 64 in pin form is retracted into the receiving piece 57 and the latching members 33 of the projecting piece 32 which have the latching noses 34 are thus able to latch into the receiving piece 57 which has the recesses 62 . fig1 shows the position in which the plug unit a is fitted loosely onto the socket unit b ′, with the projecting piece 32 latched into the receiving piece 57 . the plug unit a is held only loosely in this case without the flow - permitting connections being made . the position of the sensing member 63 is once again monitored . because the sensing member 63 has been pushed back by the projecting piece 32 , it is detected that the plug unit a has been loosely fitted . when the plug unit is loosely fitted , the drive unit ( not shown ) is started , as a result of which the body 64 in pin form is advanced in the receiving piece 57 . as a result , the connection between the projecting piece 32 and receiving piece 57 , which at first was only loose , is now locked . at the same time , the connecting part 69 having the two connecting pieces 65 , 66 is advanced out of the housing body 51 . it is also possible for the body 64 in pin form and the connecting part 69 to be connected together and to be moved in unison by a drive unit . the displacement of the connecting part 69 and the two connecting pieces 65 , 66 causes the connecting pieces 65 , 66 to pierce the membranes 35 , 36 of the plug unit a , as a result of which the fluid - tight connections are made between the connecting pieces and the connectors . it is true that there is , once again , a relative movement between the connecting pieces 65 , 66 and the connectors 27 , 28 . however , in this exemplary embodiment it is not the plug unit a itself which is moved . because the plug unit a is firmly seated on the socket unit b ′ once the projecting piece has been locked to the receiving piece , the forces which occur when the plug and socket units are connected together can be absorbed . the connection of the plug unit to the socket unit thus once again takes place automatically . the release of the plug unit a from the socket unit b ′ takes place in the reverse order . for this purpose , the body 64 in pin form is retracted in the receiving piece 57 , and the connecting part 69 and the connecting pieces 65 , 66 are retracted in the housing body 51 , as a result of which the connection between the projecting piece 32 and receiving piece 57 is unlocked and the connecting pieces 65 , 66 are drawn out of the connectors 27 , 28 . the unlocking may take place simultaneously with the drawing back of the connecting pieces or prior to the said drawing back of the connecting pieces . the alternative exemplary embodiment b ′ of the socket unit has a means 70 for closing off the two connecting pieces 65 , 66 , or for making a flow - permitting connection between the two connecting pieces 65 , 66 to enable a flushing process to be carried out with a flushing solution ( fig7 ; fig8 a and fig8 b ). the means 70 has two connectors 71 , 72 which are arranged at the same distance from one another as the connectors 27 , 28 of the plug unit a and which are of the same form as the connectors of the plug unit . the two connectors 71 , 72 are closed off at a rear end in a flushing piece 73 or are connected to make a flow - permitting connection in the flushing piece 73 . the flushing piece 73 has semi - circular indentations 74 , 75 on the two opposite sides on which the connectors 71 , 72 are not arranged . the flushing piece 73 having the connectors 71 , 72 is connected to the front portion 60 of the receiving piece 57 of the socket unit b ′. for this purpose , the flushing piece 73 has a central opening 89 through which the front portion 60 of the receiving piece 57 extends ( fig9 ). because the receiving piece 57 is mounted to be rotatable on the longitudinal axis 58 , the flushing piece 73 too and the connectors 71 , 72 can be rotated on the longitudinal axis 58 as a result of the receiving piece 57 being rotated by the drive unit ( not shown ). fig7 shows the flushing piece 73 having the connectors 71 , 72 in the position in which the plug unit a can be fitted onto the socket unit b ′. in this position , the semi - circular indentations 74 , 75 are situated in front of the connecting pieces 65 , 66 of the socket unit b ′, while the connectors 71 , 72 are positioned in a plane which is perpendicular to the plane in which the connecting pieces 65 , 66 are positioned . to initiate the flushing process , the flushing piece 73 having the connectors 71 , 72 is pivoted through 90 ° as a result of the receiving piece 57 being rotated by the drive unit ( not shown ), and the connectors 71 , 72 are thus situated in front of the connecting pieces 65 , 66 . however , this means that the connecting pieces are not yet closed off or that a flow - permitting connection has not yet been made between the connecting pieces ( fig8 a ). the connecting part 69 having the connecting pieces 65 , 66 is then advanced out of the housing body 51 and the connectors 65 , 66 are thus forced into the membranes of the connectors 71 , 72 . this makes a fluid - tight connection between the connecting pieces 65 , 66 of the socket unit b ′ and the connectors 71 , 72 and the two connecting pieces 65 , 66 are thus closed off or short - circuited by means of the flushing piece ( fig8 b ). on completion of the flushing process , the connecting pieces 65 , 66 are retracted again and the flushing piece 73 having the connectors 71 , 72 is rotated back to the starting position ( fig7 ). the flushing piece 73 of the socket unit ( b ′), and the plug body 29 of the plug unit a , are of asymmetrical forms and the plug unit a can thus only be plugged into the socket unit ( b ′) when in the correct position shown in fig7 . for this purpose , the flushing piece 73 has a u - shaped projection 78 on the outer side opposite the plug body , while the plug body 29 has a projection 79 on the outer side opposite the flushing piece . the projection 79 is so arranged that it collides with the projection 78 if the plug unit a is plugged into the socket unit ( b ′) in a position in which it is rotated through 180 °. the particular design of the means 70 for closing off or for making the flow - permitting connection forms part of the socket unit b ′. there is no need for a separate plug or the like . the socket unit b ′ allows fully automatic control both of the connection of the plug unit a to the socket unit b ′ and of the initiation of the flushing process , thus simplifying the handling process as a whole . because the insertion of the plug unit in the socket unit is detected , the filling process or emptying process can be initiated automatically . the filling process may however also be prevented if the information carrier 86 on the plug unit a is destroyed . the control unit 80 of the filling apparatus 3 preferably prevents the connecting pieces 65 , 66 of the socket unit b from being extended to make a flow - permitting connection . after the filling or emptying , the plug unit can be released automatically . the same is true of the flushing process . when the device 1 for supplying dialysis fluid is connected to the dialysis apparatus 2 , the filling of the fluid reservoir 10 can also be started automatically by the insertion of the plug unit a in the socket unit b of the dialysis apparatus .
a medical treatment apparatus , and in particular an extra - corporeal blood treatment apparatus , has a socket unit for the connection of a plug unit belonging to a device for supplying a medical fluid for the medical treatment apparatus . a filling apparatus for filling a device for supplying a medical fluid has a socket unit for the connection of a plug unit of the device for supplying a medical fluid . the device has a plug unit having means for indicating two states of operation , while the socket unit of the treatment apparatus has means for destroying the means for indicating two states of operation . when the plug unit is connected to the socket unit , the means for indicating two states of operation are destroyed , to rule out the possibility of the device being re - used . re - use of the device is ruled out by the fact that the device cannot be re - filled with fluid .