Abstract:
A method and a device for determining an operating parameter of a device for extracorporeal blood treatment as a function of absolute pressure include setting the absolute ambient pressure in a closed container filled partially with air and having an essentially constant container volume by equalization of pressure with respect to the surroundings, and the pressure is maintained by isolating the container. With delivery means, a predetermined sequence of strokes of a liquid is delivered into or out of the container, and the change in the relative container pressure is measured after each delivery stroke. The total volume delivered and the relative pressure are assigned to a value pair, and the absolute pressure and the initial air volume is determined based on the Boyle-Mariotte law by using the value pairs for at least two delivery strokes. The operating parameter is calculated and adjusted as a function of the absolute pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional No. 61/495,060, and the priority of German number 10 2011 106 111.1, filed Jun. 9, 2011, hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a method and a device for determining at least operating parameter of a device for extracorporeal blood treatment as a function of the absolute pressure. For the sake of simplicity, the term blood treatment device is used hereinafter for all devices for extracorporeal treatment of blood. 
     2. Description of the Prior Art 
     Various types of blood treatment devices are known. The known blood treatment devices include, for example, devices for hemodialysis, hemofiltration and hemodiaflltration. During extracorporeal blood treatment, the blood flows through a blood treatment unit in an extracorporeal blood circulation. Of the devices for hemodialysis, hemofiltration and hemodiafiltration, the blood treatment unit is a dialyzer or filter which is separated by a semipermeable membrane into a blood chamber and a dialysis fluid chamber, when considered schematically. During the blood treatment by means of hemodialysis or hemodiafiltration, blood flows through the blood chamber while a dialysis fluid flows through the dialysis fluid chamber. 
     Fresh dialysis fluid may be supplied through a dialysis fluid system which is integrated into the device for extracorporeal blood treatment. Clean water, e.g., from reverse osmosis may be supplied to the dialysis fluid system after first being degassed and then mixed with liquid concentrates to prepare fresh dialysis fluid. Mixing may be accomplished, for example, by adding liquid concentrates to the clean water line at separate addition points and then mixing them thoroughly in a mixing chamber or by adding the liquid concentrates through separate feed points directly to a mixing chamber. The fresh dialysis fluid flows first through a balancing system and is then directed through the dialysis chamber of the dialyzer. The fresh dialysis fluid is then loaded with water and ingredients from the blood and thereby becomes spent dialysis fluid. After leaving the dialyzer, the spent dialysis fluid passes through the balancing system, where any difference between the volume of the fresh dialysis fluid and the spent dialysis fluid is determined. 
     The mixing chamber has incoming fluid lines and outgoing fluid lines. The mixing chamber may receive incompletely premixed mixture from degassed clean water and fluid concentrates. The complete mixing takes place in the mixing chamber. Fresh dialysis fluid is removed from the mixing chamber through a dialysis fluid line. Clean water, liquid concentrates and dialysis fluid are delivered in the lines by pumps. The clean water is degassed by creating a vacuum by means of a degassing pump in the clean water line upstream from the mixing chamber. The liquid concentrates are delivered by metering pumps upstream from the mixing chamber. The dialysis fluid is delivered through a dialysis fluid pump in the dialysis fluid line. With the dialysis fluid line upstream from dialyzer and with the dialysis fluid line downstream from the dialyzer, additional pumps may be in fluid connection, such as, for example, a flow pump in the dialysis fluid line and an ultrafiltration pump. 
     With the generic devices for extracorporeal blood treatment, inexpensive relative pressure sensors are usually used to measure the pressures. The pressures inside the dialysis fluid system are therefore usually set at ambient pressure. The generic devices for extracorporeal blood treatment therefore usually do not have an integrated absolute pressure gauge. 
     The operating parameters may be set as a function of the absolute pressure on known devices for extracorporeal blood treatment, which may be done either by a service technician using an external absolute pressure gauge or by storing approximate values in the central control unit. 
     The operating parameters are set by a service technician, who may carry an absolute pressure gauge with him for this purpose, as a function of the absolute pressure in setting up a blood treatment device in a selected geographic region, for example. 
     For example, according to the state of the art in setting up a blood treatment device, the elevation of the setup site is set through the choice of the value ranges and average values for the operating parameters which depend on the absolute pressure are preselected for each value range. For example, average values of the operating parameters depending on the absolute pressure are preselected for the following value ranges as a function of the elevation of the setup site above sea level:
         setup height less than 800 meters above sea level (N.N.=normal zero of sea level),   setup height between 800 meters above sea level and 1400 meters above sea level,   setup height 1400 meters above sea level to 2000 meters above sea level,   setup height 2000 meters above sea level.       

     The elevation of the setup site can usually be estimated roughly without any additional assistance. The operating parameters predetermined for the value range are set according to the selection of a value range. 
     The known methods for setting operating parameters depending on the absolute pressure on blood treatment devices are associated with disadvantages because they are either complex and difficult to automate or they are inaccurate and also depend on the reliability of a user input that is susceptible to errors. 
     Examples of such operating parameters, which depend on the absolute pressure include the degassing pressure and the boiling point. The absolute pressure may also itself be an operating parameter of a device for extracorporeal treatment of blood. 
     The local ambient pressure may be between 700 hPa and 1060 hPa, for example, depending on the geographic location and the weather situation. An automatic adjustment in the operating parameters depending on the absolute pressure would also be desirable in the case of weather-related changes in the absolute pressure. 
     The degassing pressure must be set for a safe and reliable degassing of the clean water. The dissolving behavior of gases in liquids depends on the absolute pressure. The saturation concentration decreases with a decline in the absolute pressure. In generic blood treatment devices an absolute degassing pressure of approximately 150 hPa based on the vacuum is the goal. The degassing pressure is generated by a degassing pump and a throttle in the clean water line of the dialysis fluid system. Without an absolute pressure measurement, the degassing pressure can only be determined approximately. For a safe and reliable degassing, the most accurate possible setting of the degassing pressure is desirable. 
     The boiling point is needed to prevent steam from forming in hot cleaning of the dialysis fluid system because when steam forms it changes the flow properties in the dialysis fluid system and the steam can escape from the dialysis fluid system and enter the interior of the housing of the blood treatment device, where the steam can condense and lead to damage. 
     The boiling point drops with a drop in pressure. The change in the boiling point with the absolute pressure is known as a qualitative measure and can be obtained from the professional literature, for example, from tables for water and stored in the central control unit of the blood treatment device. 
     In heat disinfection of the dialysate system, the entire dialysis fluid system must be heated to more than 80° C. in a known device. The entire dialysis fluid system is especially advantageously purified by rinsing with hot water at approximately 85° C. In hot purification, the entire dialysis fluid system is rinsed with heated clean water at a predetermined flow rate. The clean water is heated with an electric heating rod. To keep the heating time of the clean water as short as possible, the clean water temperature on the heating rod should be as high as possible. The clean water temperature on the heating rod in the dialysis fluid system is always regulated at a temperature below the boiling point. 
     The absolute pressure may be appropriate for determining additional operating parameters. For example, in another patent application by the present applicant with the title “Method and Device for Testing the Delivery Power of at Least One Delivery Agent of a Device for Extracorporeal Blood Treatment”: (internal application Ser. No. 10/57-d01 DE) with the same filing date as the present application, a method and a device for which knowledge of the absolute pressure is advantageous are described. Reference is thus herewith made to the full extent to the aforementioned other patent application. 
     For determining and providing many operating parameters, however, the existence of a current measured value of the absolute ambient pressure, also referred to simply as absolute pressure, would be advantageous. If there is a measured value for the absolute pressure, such operating parameters can be determined and adjusted with a particularly high precision. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to further improve a generic blood treatment device without its own absolute pressure gauge such that at least one operating parameter which depends on the absolute pressure can be set automatically and without requiring an absolute pressure sensor or the use of a qualified service technician or a user. 
     Another object of the present invention is to determine at least one operating parameter depending on the absolute pressure with a high precision without requiring an absolute pressure gauge for this purpose. 
     Another object of the present invention is to automatically adjust at least one operating parameter, which depends on the absolute pressure when the absolute pressure in the surroundings changes significantly. 
     Another object of the present invention is to improve the user friendliness so that the user, for example, the treating physician or the dialysis nurse is not burdened with setting the operating parameters, which depend on the absolute pressure and therefore there are also no downtimes of the blood treatment machine for service by a qualified service technician. 
     Another object of the present invention is to increase the reliability of the blood treatment device. The more accurately the operating parameters which depend on the absolute pressure can be set, the more reliable the dialysis fluid system and thus the blood treatment device can operate reliably. 
     Another object of the present invention is to increase the safety of the blood treatment device. The more accurately the operating parameters which depend on the absolute pressure can be set, the more reliably the dialysis fluid system can operate. 
     These objects are achieved according to the invention with the features described herein. Advantageous embodiments of the invention are also described herein. 
     According to the teaching of the present invention, these objects are achieved by first setting the absolute ambient pressure in a closed container, which is filled at least partially with air and has an essentially constant container volume, and to do so through equalization of pressure with respect to the surroundings, and then this pressure is kept constant by cutting off the container from the surroundings and next a predetermined sequence of at least two delivery strokes of a fluid is delivered into the container using a delivery means and after each delivery stroke the increased relative pressure in the container is measured and after each delivery stroke the total volume delivered and the relative pressure are assigned to a value pair and the absolute pressure and/or the initial air volume is/are calculated using the value pairs for the at least two delivery strokes executed. The calculation may be performed on the basis of the Boyle-Mariotte law. According to the teaching of the invention these objects are additionally achieved by calculating the operating parameters which depend on the absolute pressure in an additional step. 
     The adjustment of the at least one operating parameter may in many embodiments include calculation and/or storage in a data memory in a control and computation unit. In many embodiments the adjustment of the at least one operating parameter may include execution of a control intervention on a function unit of the device for extracorporeal blood treatment. An example of a function unit is a pump with a controllable pump drive. 
     All the advantages that can be achieved with the method according to the invention can also be achieved in certain inventive embodiments in undiminished form with the device according to the invention and/or with the device for extracorporeal blood treatment. In some inventive embodiments this is also true of the inventive computer program product and the inventive computer program. 
     The method according to the invention may run automatically without requiring user intervention. The method according to the invention may be executed by a control and computation unit, which may be part of the device according to the invention. The method according to the invention may be started automatically by the control and computation unit at regular intervals, for example. A message that there is an automatic update of the operating parameters, which depend on the absolute pressure, may be displayed on the display screen of the blood treatment device while the method is being performed. During the update, the start of the blood treatment may be suppressed by the control and computation unit. 
     The control and computation unit may provide a means for limiting the relative pressure to a maximum level, so that unacceptably high pressures cannot occur. There may be an error message on the display of the blood treatment device indicating a failure of the update of the operating parameters which depend on the absolute pressure. On reaching an inadmissibly high relative pressure, the method according to the invention is terminated and may be restarted at a later point in time. 
     According to the teaching of the invention, these objects are further achieved by setting at least one calculated operating parameter on the blood treatment machine. 
     The absolute pressure is calculated merely on the basis of the measured values of the relative pressure in the closed container. The at least one operating parameter which depends on the absolute pressure may be calculated by the central control and computation unit of the blood treatment device and stored in a data memory and/or adjusted by control and/or regulating interventions. It is not necessary to use an absolute pressure gauge to do so. 
     The device according to the invention provides a closed container with an essential constant internal volume. An essentially constant container volume is understood to mean that the internal volume changes only negligibly or not at all with an increase in the internal pressure in the relevant pressure range. Closed is to be understood here to mean that there is no free opening with the surroundings while performing the method according to the invention and any inlet lines or outlet lines opening into the container but not needed for performing the method are cut off. Inlet lines or outlet lines may include, for example, tubing or pipelines. The inlet lines or outlet lines may be closed by valves. The inlet lines or outlet lines may be closed by pumps that are shut down. The inlet lines or outlet lines not needed for performing the method according to the invention are cut off by intervention measures by the control and computation unit before performing the method according to the invention. 
     In a preferred embodiment, the container is a mixing chamber in a dialysis fluid system of a blood treatment device for preparing fresh dialysis fluid. 
     The device according to the invention has means for measuring the relative pressure in the container. The means for measuring the relative pressure may be a pressure sensor in the container. In particular the means may be a pressure sensor in the air volume in the interior of the container but it may also be a pressure sensor which is functionally connected to the interior of the container for measuring the pressure. The pressure sensor provides an electrical pressure signal as a function of the relative pressure in the container, this signal being relayed over a data line to the control and computation unit. However, it is also possible for the pressure sensor to transmit the pressure signal wirelessly to the control and computation unit. The pressure sensor may be an RFID sensor. The means for measuring the relative pressure may be a relative pressure sensor which is present anyway in the mixing chamber of a dialysis fluid system. 
     The device according to the invention provides means for setting the absolute pressure (absolute ambient pressure) in the container. The absolute pressure level is initially unknown but is being sought. The means for initial setting of the absolute pressure may be a cutoff valve with which an opening in the otherwise closed container with respect to the surroundings can be opened or cut off for performing the additional steps of the method according to the invention. The mixing chamber may have as an opening a line to the surroundings, the end of which is open to the surroundings. The line has a cutoff valve, with which the line can be closed in an airtight manner. The ambient pressure can be adjusted in the container easily by briefly opening the cutoff valve to the surroundings, so the pressure is equalized with the surroundings, and the absolute ambient pressure is automatically established in the container. After the pressure equalization, the cutoff valve is closed. The pressure thereby set remains upheld when the cutoff valve in the container is closed. The cutoff valve always remains closed to the surroundings during the following steps of the method according to the invention, so there cannot be a renewed equalization of pressure with respect to the surroundings. Only after the method according to the invention has been performed completely is a renewed pressure equalization with the surroundings ordered to depressurize the excess pressure that has been built up. 
     The valve may be automatically operable or operated. The valve can be opened and closed by the control and computation unit of the device according to the invention. However, the valve may also be operated by a manual control intervention by the user. For example, it may be a solenoid valve which is controllable electrically. However, it may also be a pneumatically controllable valve. 
     After setting the ambient pressure, an air volume which is referred to below as the initial air volume, is established in the container. The initial air volume fills the container at least partially. The remaining container volume may be filled with an initial liquid volume, which forms a liquid level. It is also possible for the complete internal volume of the container to be filled with air and to form the initial air volume. Then there is no liquid in the container. 
     Means for measuring the liquid volume may be present in the container. The means for measuring the liquid volume may be a filling level measurement by means of a filling level sensor. The filling level sensor delivers an electric filling level sensor as a function of the filling level in the container and this signal is forwarded over a data line to the control line computation unit. The liquid volume can be calculated by the control and computation unit from the measured filling level and the known geometry of the container. The initial air volume can be calculated as the difference between the container volume and the initial fluid volume. For performing the method according to the invention, means for measuring the liquid volume are not necessary because the initial air volume can be calculated by the method according to the invention but a filling level measurement may still be provided in the mixing chamber of a dialysis fluid system, so that it can advantageously be used for a plausibility check of the results of the calculation of the initial air volume. It is of course also possible to determine the initial air volume only by means of a filling level measurement. 
     It is also possible to analyze a measured filling level change as a criterion for a pressure change in the container by means of a filling level measurement in the container with known initial conditions of the pressure and air volume in the container as an alternative to the method according to the invention and from this to conclude the absolute pressure. 
     At least one liquid line having a fluid-carrying connection to a liquid source opens into the container; at least one delivery means for delivering the fluid in a first direction of delivery into the container is arranged in this liquid line. The delivery means is characterized in that there is no return flow through the delivery means in the opposite direction to the first direction of delivery when the delivery means is at a standstill. The delivery means functions like a non-return valve. 
     The delivery performance of the delivery means, in particular the delivery rate of the delivery means per delivery stroke and/or per unit of time is known and is stored in the control and computation unit. In a preferred embodiment, the delivery stroke may be a pump stroke. The liquid is incompressible or is assumed to be incompressible. 
     The at least one delivery means may be controlled by the control and computation unit for starting and stopping and/or for executing at least one delivery stroke. The at least one delivery means may be controlled by the control and computation unit for executing a sequence of delivery strokes. The delivery strokes may each have the same stroke volume but it is not necessary for the implementation of the invention for all the delivery strokes to have the same stroke volume. In a preferred embodiment, all the delivery strokes have the same stroke volume. 
     In a preferred embodiment, the liquid source is from the group of supplying clean water, supplying sodium bicarbonate concentrate and supplying acid concentrate from the dialysis liquid system of a blood treatment device. 
     In a preferred embodiment, the delivery stroke is the pump stroke of a diaphragm pump. The delivery behavior of a diaphragm pump is discontinuous from one delivery stroke to the next. The delivery volume of a complete pump stroke of a diaphragm pump depends only on the geometry of the diaphragm pump. With each complete pump stroke, the same volume of liquid can always be delivered. 
     Another preferred embodiment concerns the pump stroke of a piston pump. The delivery performance of a piston pump is discontinuous from one delivery stroke to the next. The pump stroke of a piston pump depends only on the geometry of the piston pump. The same volume of liquid can always be delivered with each complete pump stroke. 
     In another preferred embodiment, the pump stroke involves a predetermined delivery time of a delivery means having a continuous delivery performance. The delivery means may be, for example, a gear pump or a hose pump. 
     In a preferred embodiment, the delivery means may be a metering pump. The metering pump may be a high precision pump. The metering pump may be a diaphragm pump. 
     In an especially preferred embodiment, the pump stroke may be predefined by specifying a number of steps or a step angle on a stepping motor, which drives the delivery means. The stepping motor may be electrically operated. The step angle and/or the number of steps can be predetermined by electrical pulses of a control and computation unit. 
     The pressure in the container after adjusting the ambient pressure should change only through the supply of fluid through the pump strokes of the delivery means in the method according to the invention. 
     The delivery means can be controlled by a control and computation unit. The control and computation unit may be configured to induce a sequence of delivery strokes of the delivery means. The sequence of delivery strokes has a predetermined number of delivery strokes. 
     The control and computation unit may be configured to detect and/or save a measured pressure value in the container after each delivery stroke is completed. 
     The control and computation unit may additionally be configured to calculate the total liquid volume delivered after each delivery stroke is performed. 
     The total liquid volume delivered is understood to be the sum of the liquid volumes of all delivery strokes already performed while performing the method according to the invention. 
     In a preferred embodiment, the volume of each delivery stroke always has the same amount, namely a constant stroke volume. In this embodiment, the total liquid volume delivered is calculated especially easily as the product of the amount of the stroke volume and the number of delivery strokes performed. 
     The control and computation unit may additionally be configured to assign the amounts of the total liquid volume delivered and the relative pressure in the container to a value pair after each delivery stroke is made and to store them. 
     The control and computation unit may additionally be configured to automatically start the calculation of the absolute pressure and/or of the initial air volume on the basis of all value pairs as the next step after the conclusion and analysis of the last delivery stroke of a predetermined sequence of delivery strokes. 
     The control and computation unit may additionally be configured to calculate the absolute pressure and initial air volume parameters that are sought by way of a fitted calculation on the basis of all value pairs using the following approach, which is obtained by applying the Boyle-Mariotte law to the changes in state in the container such that a best possible approximation of the function described by this approach to the measured values of the relative pressure is obtained as follows: 
     
       
         
           
             
               
                 
                   P 
                   i 
                 
                 = 
                 
                   
                     P 
                     
                       abs 
                       , 
                       amb 
                     
                   
                   · 
                   
                     
                       i 
                       · 
                       
                         V 
                         pump 
                       
                     
                     
                       
                         V 
                         
                           vessel 
                           , 
                           air 
                           , 
                           0 
                         
                       
                       - 
                       
                         i 
                         · 
                         
                           V 
                           pump 
                         
                       
                     
                   
                 
               
               ; 
               
                 i 
                 = 
                 1 
               
             
             , 
             … 
             ⁢ 
             
                 
             
             , 
             n 
           
         
       
     
     In equation (1), P abs,amb  denotes the absolute pressure being sought in the surroundings, V vessel,air,0  is the initial air volume being sought in the mixing chamber, V pump  is the stroke volume of the diaphragm pump used here, n denotes the total number of delivery strokes of the diaphragm pump, i is the running index for the number of pump strokes and P is the relative pressure in the mixing chamber after performing a pump stroke. 
     The control and computation unit may additionally be configured to calculate the at least one operating parameter as a function of the absolute pressure. 
     The control and computation unit may additionally be configured to adjust the at least one calculated operating parameter in the blood treatment device as a function of the absolute pressure by means of control interventions. The at least one operating parameter can be adjusted through control intervention measures and/or regulating intervention measures on a control and computation unit. 
     The control and computation unit may additionally be configured to regulate the heating of clean water, which can be performed by means of an electric heater, so that the temperature of the clean water is always kept below a temperature limit which is below the boiling point. To this end, the control and computation unit can regulate the heating power and/or the on time of the electric heater. The electric heating may also be a heating rod. 
     The control and computation unit may additionally be configured to adjust the desired degassing pressure as a vacuum with respect to the absolute pressure by means of a control intervention on the degassing pump of the dialysis fluid system. The degassing pressure may be adjusted as a pressure drop on a degassing throttle in the clean water line by means of the rpm-regulated degassing pump. The degassing throttle may be a throttle valve, for example. 
     The device according to the invention may form an independent unit but may also be part of a blood treatment device. It may be a device for retrofitting a blood treatment device which does not have its own inventive container. An especially preferred embodiment provides that the device according to the invention is part of the blood treatment device because the known blood treatment devices already have components which can be utilized by the device according to the invention. 
     In another particularly preferred embodiment, equipping a blood treatment device with the device according to the invention may be limited to updating the software of the control and computation unit because all the required components are present in the blood treatment device through the software. For such embodiments and other embodiments, an inventive computer program with program code and an inventive computer program product with a program code stored on a machine-readable carrier are made available. 
     The device and the method according to the invention may be used with the known blood treatment devices without requiring any great renovation measures. A mixing chamber in a dialysis fluid system is present anyway with the known blood treatment devices. The means for measuring the relative pressure are present anyway on the known blood treatment devices. Retrofitting of a suitable chamber is possible without any great effort on blood treatment devices which do not have a mixing chamber in the dialysis fluid system. Suitable delivery means for delivering a fluid are always present in the dialysis fluid system of the known blood treatment devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is explained in greater detail below with reference to the figures. On the basis of the exemplary embodiment shown in the figures, additional details and advantages of the invention will be described in greater detail. The method and the device according to the invention are described on the example of a blood treatment device designed as a hemodialysis device. However, the method according to the invention may also be used in the same way with other blood treatment devices, for example, a hemodiafiltration device. 
         FIG. 1  shows a flow chart of the dialysis system of a blood treatment device having a mixing chamber, designed as a hemodialysis device; 
         FIG. 2  shows a graphic plot of the pressure conditions of the relative pressure in the mixing chamber of the dialysis fluid system from  FIG. 1  in performing the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       FIG. 1  shows in a simplified schematic diagram the essential components of the dialysis fluid system  1  of a blood treatment device designed as a hemodialysis device. In the present exemplary embodiment the blood treatment device which is designed as a hemodialysis device is referred to further in simplified terms as a “blood treatment device” having a dialyzer  2  which is separated schematically by a semipermeable membrane  3  into a blood chamber  4  and a dialysis fluid chamber  5 . The blood chamber is part of the extracorporeal blood circulation (not shown) and the dialysis fluid chamber  5  is part of the dialysis fluid system  1 . The central control and computation unit  100  operates and monitors the blood treatment device and the dialysis fluid system. The control and computation unit  110  of the device according to the invention is part of the central control and computation unit  100  of the blood treatment device in the exemplary embodiment. However, the control and computation unit  110  may also be separate from the central control and computation unit  100  and connected to the latter by data lines. 
     The dialysis fluid system has a mixing chamber  6  for mixing fresh dialysis fluid of clean water and liquid concentrates. The dialysis fluid system has a line  7  for delivering clean water to which a passive membrane  8 ′ is connected, forming a delivery means having a metering function only in cooperation with a gear pump  19  (degassing pump), which is situated upstream and which is referred to hereinafter as delivery means  8 . The line  7  opens into the mixing chamber  6 . The delivery means  8  comprise delivery means for clean water. 
     A line  9  opens into the line  7  downstream from the delivery means for clean water  8 . A delivery means  10  is connected to the line  9 . The delivery means  10  in the exemplary embodiment is a metering pump for sodium bicarbonate concentrate. The metering pump  10  is embodied as a diaphragm pump. 
     Another line  11  also opens downstream from the delivery means for clean water  8  into the first line  7 . Delivery means  12  is connected to the line  11 . In this exemplary embodiment, the delivery means  11  is a metering pump for acid concentrate. The metering pump  12  is designed as a diaphragm pump. 
     A line  21  leads downstream from the mixing chamber  6  to the dialysis fluid chamber  5  of the dialyzer  2 . One delivery means  22  is connected to line  21 . In this exemplary embodiment, this delivery means is a gear pump  22 , which is part of a balancing device  25 . A bypass line  23  having a bypass valve  24  is provided in the parallel connection to the line  21 . The bypass valve  21  is closed during operation of the gear pump  22 . 
     In principle in the exemplary embodiment, each of the delivery means  8 ,  10 ,  12  or  22  may be selected for delivering a fluid into the container  6  or for delivering the fluid out of the container  6  for performing the method according to the invention. For delivering fluid into the container  6  the delivery means  8 ,  10  and  12  must be operated in the normal direction of delivery, while the delivery means  22  would have to be operated in the opposite direction of delivery. For delivering fluid out of the container  6 , the delivery means  8 ,  10  and  12  would have to be operated in the reverse direction of delivery, while the delivery means  22  would have to be operated in the normal direction of delivery. In the present exemplary embodiment, the performance of the method according to the invention is explained as an example using the delivery means  10  with which fluid is delivered into the container  6  in the normal direction of delivery. 
     The control and computation unit  110  may have means for selecting one of the delivery means ( 8 ,  10 ,  12 , and  22 ) for performing the method according to the invention. The choice may also be set fixedly in the control and computation unit  110  or may be made by the user through user intervention, for example, via the touchscreen of the blood treatment device (not shown in  FIG. 1 ). 
     In the exemplary embodiment the diaphragm pump  10  for liquid concentrate is selected by the control and computation unit  110  as the delivery means. A delivery stroke of the diaphragm pump  10  in the present example corresponds exactly to the stroke volume which is delivered in the case of a complete pump stroke. This stroke volume of a complete pump stroke is known and constant for the selected pump. However, as an alternative the delivery stroke could also include part of a complete pump stroke, for example, if the pump drive is a stepping motor. 
     The control and computation unit  110  has means for ordering a predetermined number of delivery strokes. The delivery strokes are ordered by control intervention measures  10   a.    
     The device according to the invention has a pressure sensor  13  which measures the relative pressure in the mixing chamber  6 . The measured values of the pressure sensor  13  are transmitted to the control and computation unit  110  where they are stored in the data memory  120  for analysis. 
     In addition, the control and computation unit  110  has means for assigning the delivery volume delivered after each delivery stroke to the measured relative pressure in the mixing chamber as a value pair for the completed delivery stroke. The value pairs of all delivery strokes are stored in the control and computation unit  110 . 
     In addition, the control and computation unit  110  has means for calculating the absolute ambient pressure and/or the initial air volume using the stored value pairs for all delivery strokes. 
     The calculations of the parameters that are sought are performed with the help of a computer program using program code to order the machine steps of the method and to analyze the measurement results. The computation equations are implemented in the program code. The program code is stored in the control and computation unit  110 . 
     The computer program is stored as computer program product with the program code stored on a machine-readable carrier for ordering the machine steps of the method. The computer program runs in the control and computation unit  110 . The control and computation unit  110  has a data memory  120 . 
     The computer program with program code for ordering the machine steps of the method and for analyzing the measurement results starts the calculations as soon as the entire predetermined number of delivery strokes is concluded. 
     The total internal volume of the mixing chamber  6  is known and amounts to 350 mL in this exemplary embodiment. The initial liquid volume  16  in the mixing chamber  6  is calculated from the measured value of the filling level measurement device  15 . The initial air volume  14  in the mixing chamber is calculated by the control and computation unit  110  as the difference in the total internal volume and the liquid volume and amounts to 242 mL in the exemplary embodiment. The temperature in the mixing chamber is 37° C. and is assumed to be constant while the method according to the invention is being performed. 
     The number of delivery strokes to be performed is advantageously more than 50 and especially advantageously up to 120. However, more than 120 delivery strokes are not necessary in the exemplary embodiment and do not yield a more accurate result. The number n of the delivery strokes to be performed is predefined as n=110, for example, in the central control of the blood treatment device in t his exemplary embodiment. The stroke volume of the diaphragm pump  10  in the exemplary embodiment is 1.0 mL (one milliliter) and corresponds to the delivery volume of a single delivery stroke in the diaphragm pump  10 . The control and computation unit  110  starts and stops the delivery strokes of the diaphragm pump through control intervention measures  10   a . With each delivery stroke one milliliter of liquid is pumped into the mixing chamber. There is no return flow of fluid through the diaphragm pump  10 . The pressure in the mixing chamber increases with each delivery stroke in accordance with the Boyle-Mariotte law because with each delivery stroke the liquid volume increases by the amount of one delivery stroke, and on the other hand, the air volume decreases with each delivery stroke to the same extent by the amount of the delivery stroke. The air is therefore compressed by the same amount with each delivery stroke. This relationship is described by equation (1). The thermodynamic basis of equation (1) is the Boyle-Mariotte law applied to the changes in state of the air volume in the mixing chamber caused by the delivery strokes:
 
 P   abs,amb   ·V   vessel,air,0 =( P   abs,amb   +P   i )·( V   vessel,air,0   −i·V   pump );  i= 1, . . . , n   (1)
 
     In equation (1), P abs,amb  denotes the absolute ambient pressure being sought in the surroundings, V vessel,air,0  denotes the initial air volume being sought in the mixing chamber, V pump  denotes the stroke volume of the diaphragm pump, n denotes the total number of delivery strokes of the diaphragm pump, i denotes the running index for the number of pump strokes and P i  denotes the relative pressure in the mixing chamber after the i-th pump stroke. 
     Equation (2) is obtained by rearranging equation (1) and is used in the program code for a fitting calculation to calculate the parameters P abs,amb  and V vessel,air,0  being sought, wherein no filling level sensor is needed: 
     
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         i 
                       
                       = 
                       
                         
                           P 
                           
                             abs 
                             , 
                             amb 
                           
                         
                         · 
                         
                           
                             i 
                             · 
                             
                               V 
                               pump 
                             
                           
                           
                             
                               V 
                               
                                 vessel 
                                 , 
                                 air 
                                 , 
                                 0 
                               
                             
                             - 
                             
                               i 
                               · 
                               
                                 V 
                                 pump 
                               
                             
                           
                         
                       
                     
                     ; 
                     
                       i 
                       = 
                       1 
                     
                   
                   , 
                   … 
                   ⁢ 
                   
                       
                   
                   , 
                   n 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The calculation is performed under the assumption of a constant absolute pressure during the performance of the method according to the invention. 
     With the substitutions x i =i·V pump  and y i =P i  the following equation is stored in the program code: 
                     f   ⁡     (     x   i     )       =       P     abs   ,   amb       ·       x   i         V     vessel   ,   air   ,   0       -     x   i                   (   3   )               
where n pump strokes are performed and n value pairs (x i , y i ), where i=1, . . . , n are determined. In these equations i=1, . . . , n denotes the running index for the sequence of pump strokes beginning with the first delivery stroke (i=1), the second delivery stroke (i=2) up to the last delivery stroke (i=n), so that a total of n delivery strokes are executed. The value pairs are stored in data memory  120 .
 
     Using the known fitting equation of “minimization of the sum of the distance squared,” also known by the English term “method of least squares” applied to equation (2) together with equation (3), the parameters being sought are calculated on the basis of equation (4). 
     
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               y 
                               i 
                             
                             - 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   x 
                                   i 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               y 
                               i 
                             
                             - 
                             
                               
                                 P 
                                 
                                   abs 
                                   , 
                                   amb 
                                 
                               
                               · 
                               
                                 
                                   x 
                                   i 
                                 
                                 
                                   
                                     V 
                                     
                                       vessel 
                                       , 
                                       air 
                                       , 
                                       0 
                                     
                                   
                                   - 
                                   
                                     x 
                                     i 
                                   
                                 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The parameters P abs,amb  and V vessel,air,0  which are being sought are calculated according to the method of least squares so that the error total S in equation (4) assumes a minimum. The first of two necessary conditions for this is obtained from equation (5): 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       S 
                     
                     
                       ∂ 
                       
                         P 
                         
                           abs 
                           , 
                           amb 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         ∂ 
                         
                           ∂ 
                           
                             P 
                             
                               abs 
                               , 
                               amb 
                             
                           
                         
                       
                       ⁢ 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           n 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 y 
                                 i 
                               
                               - 
                               
                                 
                                   P 
                                   
                                     abs 
                                     , 
                                     amb 
                                   
                                 
                                 · 
                                 
                                   
                                     x 
                                     i 
                                   
                                   
                                     
                                       V 
                                       
                                         vessel 
                                         , 
                                         air 
                                         , 
                                         0 
                                       
                                     
                                     - 
                                     
                                       x 
                                       i 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                     = 
                     
                       
                         2 
                         · 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             n 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   y 
                                   i 
                                 
                                 - 
                                 
                                   
                                     P 
                                     
                                       abs 
                                       , 
                                       amb 
                                     
                                   
                                   · 
                                   
                                     
                                       x 
                                       i 
                                     
                                     
                                       
                                         V 
                                         
                                           vessel 
                                           , 
                                           air 
                                           , 
                                           0 
                                         
                                       
                                       - 
                                       
                                         x 
                                         i 
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 - 
                                 
                                   
                                     x 
                                     i 
                                   
                                   
                                     
                                       V 
                                       
                                         vessel 
                                         , 
                                         air 
                                         , 
                                         0 
                                       
                                     
                                     - 
                                     
                                       x 
                                       i 
                                     
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                       = 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     It follows from equation (5) by rearranging: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             y 
                             i 
                           
                           · 
                           
                             
                               x 
                               i 
                             
                             
                               
                                 V 
                                 
                                   vessel 
                                   , 
                                   air 
                                   , 
                                   0 
                                 
                               
                               - 
                               
                                 x 
                                 i 
                               
                             
                           
                         
                         ) 
                       
                     
                     - 
                     
                       
                         P 
                         
                           abs 
                           , 
                           amb 
                         
                       
                       · 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           n 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 x 
                                 i 
                               
                               
                                 
                                   V 
                                   
                                     vessel 
                                     , 
                                     air 
                                     , 
                                     0 
                                   
                                 
                                 - 
                                 
                                   x 
                                   i 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     From equation (6) we obtain by rearranging a first determination equation for the two parameters being sought. The determination equation (7) is implemented in the program code. 
     
       
         
           
             
               
                 
                   
                     P 
                     
                       abs 
                       , 
                       amb 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             y 
                             i 
                           
                           · 
                           
                             
                               x 
                               i 
                             
                             
                               
                                 V 
                                 
                                   vessel 
                                   , 
                                   air 
                                   , 
                                   0 
                                 
                               
                               - 
                               
                                 x 
                                 i 
                               
                             
                           
                         
                         ) 
                       
                     
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               x 
                               i 
                             
                             
                               
                                 V 
                                 
                                   vessel 
                                   , 
                                   air 
                                   , 
                                   0 
                                 
                               
                               - 
                               
                                 x 
                                 i 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The first of two required terms is obtained according to equation (8): 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       S 
                     
                     
                       ∂ 
                       
                         V 
                         
                           vessel 
                           , 
                           air 
                           , 
                           0 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         ∂ 
                         
                           ∂ 
                           
                             V 
                             
                               vessel 
                               , 
                               air 
                               , 
                               0 
                             
                           
                         
                       
                       ⁢ 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           n 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 y 
                                 i 
                               
                               - 
                               
                                 
                                   P 
                                   
                                     abs 
                                     , 
                                     amb 
                                   
                                 
                                 · 
                                 
                                   
                                     x 
                                     i 
                                   
                                   
                                     
                                       V 
                                       
                                         vessel 
                                         , 
                                         air 
                                         , 
                                         0 
                                       
                                     
                                     - 
                                     
                                       x 
                                       i 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                     = 
                     
                       
                         2 
                         · 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             n 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   y 
                                   i 
                                 
                                 - 
                                 
                                   
                                     P 
                                     
                                       abs 
                                       , 
                                       amb 
                                     
                                   
                                   · 
                                   
                                     
                                       x 
                                       i 
                                     
                                     
                                       
                                         V 
                                         
                                           vessel 
                                           , 
                                           air 
                                           , 
                                           0 
                                         
                                       
                                       - 
                                       
                                         x 
                                         i 
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 + 
                                 
                                   
                                     
                                       P 
                                       
                                         abs 
                                         , 
                                         amb 
                                       
                                     
                                     · 
                                     
                                       x 
                                       i 
                                     
                                   
                                   
                                     
                                       ( 
                                       
                                         
                                           V 
                                           
                                             vessel 
                                             , 
                                             air 
                                             , 
                                             0 
                                           
                                         
                                         - 
                                         
                                           x 
                                           i 
                                         
                                       
                                       ) 
                                     
                                     2 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                       = 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Equation (8) yields a second determination equation (9) for the two parameters being sought. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             y 
                             i 
                           
                           · 
                           
                             
                               x 
                               i 
                             
                             
                               
                                 ( 
                                 
                                   
                                     V 
                                     
                                       vessel 
                                       , 
                                       air 
                                       , 
                                       0 
                                     
                                   
                                   - 
                                   
                                     x 
                                     i 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                         ) 
                       
                     
                     - 
                     
                       
                         P 
                         
                           abs 
                           , 
                           amb 
                         
                       
                       · 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           n 
                         
                         ⁢ 
                         
                           
                             x 
                             i 
                             2 
                           
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     vessel 
                                     , 
                                     air 
                                     , 
                                     0 
                                   
                                 
                                 - 
                                 
                                   x 
                                   i 
                                 
                               
                               ) 
                             
                             3 
                           
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Inserting the equation for the absolute pressure being sought according to equation (7) into equation (9) yields an implicit determination equation (10) for the parameter V vessel,air,0  being sought. The determination equation (10) is implemented in the program code and is solved by a numerical problem-solving method in the known manner. The known numerical problem-solving method, selected, for example, from the bisection method (e.g., interval halving method) or regula falsi [the false position method] or the Newton-Raphson method is implemented in the program code. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 y 
                                 i 
                               
                               · 
                               
                                 x 
                                 i 
                               
                             
                             
                               
                                 ( 
                                 
                                   
                                     V 
                                     
                                       vessel 
                                       , 
                                       air 
                                       , 
                                       0 
                                     
                                   
                                   - 
                                   
                                     x 
                                     i 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                           ) 
                         
                         · 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             n 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   x 
                                   i 
                                 
                                 
                                   
                                     V 
                                     
                                       vessel 
                                       , 
                                       air 
                                       , 
                                       0 
                                     
                                   
                                   - 
                                   
                                     x 
                                     i 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                     - 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               
                                 y 
                                 i 
                               
                               · 
                               
                                 x 
                                 i 
                               
                             
                             
                               
                                 V 
                                 
                                   vessel 
                                   , 
                                   air 
                                   , 
                                   0 
                                 
                               
                               - 
                               
                                 x 
                                 i 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             n 
                           
                           ⁢ 
                           
                             
                               x 
                               i 
                               2 
                             
                             
                               
                                 ( 
                                 
                                   
                                     V 
                                     
                                       vessel 
                                       , 
                                       air 
                                       , 
                                       0 
                                     
                                   
                                   - 
                                   
                                     x 
                                     i 
                                   
                                 
                                 ) 
                               
                               3 
                             
                           
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     By inserting the calculated parameter V vessel,air,0 , equation (7) yields the second parameter P abs,amb . It would of course also be possible to determine P abs,amb  first and then V vessel,air,0  in the opposite order. 
     The absolute pressure being sought is calculated as being 978 hPa in the exemplary embodiment and the initial air volume is calculated as 242 mL. It has surprisingly been found that the accuracy and reproducibility of the absolute pressure determined comply very well with the requirements of accuracy and reproducibility of the calculation of the operating parameters so that no direct measurement of the absolute pressure with an absolute pressure gauge is required. 
       FIG. 2  shows a graphic plot of the pressure conditions of the relative pressure in the mixing chamber of the dialysis fluid system from  FIG. 1  in performing the method according to the invention. The number of  110  pump strokes performed is plotted on the abscissa. The relative pressure in the mixing chamber measured after each delivery stroke is plotted on the ordinate in  FIG. 2  as a function of the number of pump strokes. The individual value pairs of the measured values of the relative pressure are represented by triangles in  FIG. 2 . 
     The absolute pressure parameter 978 hPa and the initial air volume 242 mL in the mixing chamber  6 , which were calculated in performing the fitting calculation, are inserted into equation (2) yielding the fitted curve shown as a solid-line curve in  FIG. 2 . In other words the parameters of absolute pressure and initial air volume that are being sought are determined by the control and computation unit  110  by using the fitting equation and using all value pairs, so that the fitted curve according to equation (2) describes the dependence of the measured relative pressure on the number of delivery strokes in the best possible way. To avoid misunderstanding, it is pointed out that the absolute pressure cannot of course be read directly from the curve in  FIG. 2 . 
     The control and computation unit  110  calculates the at least one operating parameter, which depends on the absolute pressure after the absolute pressure has been determined in another step. 
     In the present exemplary embodiment the control and calculation unit  110  calculates the boiling point being sought on the basis of the data stored in the control and computation unit for the vapor pressure table for water or on the basis of an approximation equation stored in the control and computation unit. The clean water temperature on the heating rod in the dialysis fluid system is regulated in such a way that it is below the boiling point. The clean water temperature during heat disinfection is especially advantageously always regulated at approximately 1.2° C. below the boiling point. The control and computation unit  110  regulates the heating process of the clean water on the heating rod  17  through regulating intervention measures  17   a  in such a way that the temperature of the clean water measured by means of the temperature sensor  18  after passing through the heating rod  17  differs from the calculated boiling point by 1.2° C. and therefore a buildup of steam is reliably prevented. No user intervention is required for this. An absolute pressure gauge is not needed according to the invention. The temperature of the clean water is regulated with a very high precision. The safety and reliability of the dialysis fluid system are thereby improved. 
     To set the desired degassing pressure, the control and computation unit  110  causes regulating intervention measures  19   a  on the degassing pump  19  based on the measured value of the absolute pressure of 150 hPa, such that the degassing pressure required as an example is reached. The degassing pressure is therefore set as the pressure drop on the degassing throttle  20  by means of the rpm-regulated degassing pump  19 . The pressure drop on the degassing throttle  20  is measured (pressure measurement points not shown in  FIG. 1 ) and transmitted to the control and computation unit  110 . The pressure difference between the calculated absolute pressure and the desired degassing pressure (i.e., 978 hPa minus 150 hPa) is calculated by the control and computation unit  110  and preselected as the setpoint value for the regulation. The pressure drop on the degassing throttle  20  is measured with two relative pressure sensors (not shown in  FIG. 1 ) upstream and downstream from the degassing throttle  20  and compared with the setpoint value of the pressure difference. The rotational speed of the degassing pump  19  is regulated by the regulating intervention measures  19   a  so that the required pressure difference of 828 hPa at the degassing throttle  20  drops and the desired degassing pressure is reached. No user intervention measure is necessary for this. An absolute pressure gauge is not required according to the invention. The degassing pressure is set very accurately. The safety and reliability of the dialysis fluid system are thereby improved. 
     The control and computation unit  110  stores all the results in the memory  120 . The memory content of the data memory  120  can be displayed on a display screen of the blood treatment device (not shown in  FIG. 1 ) or can be read out of the memory via a data interface for documentation purposes. 
     The absolute pressure cannot be displayed visibly for the user, for example, the absolute pressure may be displayed as a numerical value on the display screen of the blood treatment device. 
     According to the invention the objects of the present invention are solved with the exemplary embodiment presented here. However, the present invention is not limited to this exemplary embodiment. 
     The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all, such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of reference numerals 
               
             
          
           
               
                 Reference numeral 
                 Explanation 
               
               
                   
               
               
                  1 
                 dialysis fluid system 
               
               
                  2 
                 dialyzer 
               
               
                  3 
                 semipermeable membrane 
               
               
                  4 
                 blood chamber 
               
               
                  5 
                 dialysis fluid chamber 
               
               
                  6 
                 mixing chamber 
               
               
                  7 
                 first line 
               
               
                  8 
                 first delivery means, metering chamber/ 
               
               
                   
                 diaphragm pump 
               
               
                  8a 
                 control intervention 
               
               
                  9 
                 second line 
               
               
                 10 
                 second delivery means, metering 
               
               
                   
                 pump/diaphragm pump 
               
               
                  10a 
                 control intervention 
               
               
                 11 
                 third line 
               
               
                 12 
                 third delivery means, metering pump/ 
               
               
                   
                 diaphragm pump 
               
               
                  12a 
                 control intervention 
               
               
                 13 
                 pressure sensor 
               
               
                 14 
                 initial air volume 
               
               
                 15 
                 filling level measurement device 
               
               
                 16 
                 initial filling volume 
               
               
                 17 
                 heating rod 
               
               
                  17a 
                 regulating measure 
               
               
                 18 
                 temperature sensor 
               
               
                 19 
                 degassing pump 
               
               
                  19a 
                 control intervention 
               
               
                 20 
                 degassing throttle 
               
               
                 21 
                 Line 
               
               
                 22 
                 gear pump 
               
               
                  22a 
                 control intervention 
               
               
                 23 
                 bypass line 
               
               
                 24 
                 cutoff valve 
               
               
                 25 
                 balancing device 
               
               
                 100  
                 central control and computation unit 
               
               
                 110  
                 control and computation unit 
               
               
                 120  
                 data memory