Process for the calibration of a flow sensor in a respiratory system
FIELD OF THE INVENTION 
     The present invention pertains to a process for calibrating a flow sensor in a respiratory system, and more particularly to a respiratory system wherein control elements are provided for influencing the respiration gas stream in a drive unit for feeding the respiration gas stream. 
     BACKGROUND OF THE INVENTION 
     A respiratory system with a flow sensor has become known from DE-A1-34,34,908. The respiratory system contains nonreturn valves as final control elements and direction valves which control the flow of the respiration gas stream. Fresh gas is fed into the respiratory system via a fresh gas connection and a first nonreturn valve. On the inspiration side, the respiration gas flows through a carbon dioxide absorber, a first directional valve and a pressure sensor and to the lung of a patient; on the expiration side, there are a flow sensor and a second directional valve, via which the respiration gas is returned into the inspiration arm. A second nonreturn valve for releasing excess respiration gas and a drive unit with variable volume are provided in the respiratory system, and the drive unit feeds respiration gas to the patient&#39;s lung in a rhythmic sequence. The course of respiration over time is determined by a control unit. 
     Different forms of respiration can be realized by correspondingly actuating the nonreturn valves. A so-called closed respiratory system is obtained when only as much fresh gas is metered into the respiratory system as is consumed by the patient, while in a semi-closed respiratory system, one operates with excess fresh gas and the excess respiration gas is released from the respiratory system after each breath. 
     Respiration is monitored with a pressure sensor for the respiration pressure and a flow sensor which measures the volume expired by the patient. One widely used embodiment of a flow sensor is based on constant-temperature hot wire anemometry. This measurement method depends on the type of the gas, on the one hand, and, on the other hand, a reference value for &#34;zero&#34; flow must be available in the measuring device. Concerning correction for the effect of the type of the gas, compensation methods are known which are able to take binary gas mixtures containing known components, e.g., laughing gas and oxygen, adequately into account. However, these compensation methods are unable to evaluate gas mixtures containing more than two components. Three components are present, e.g., when anesthetic is also metered into the respiratory system in addition to oxygen and laughing gas. The influence of the anesthetic on flow measurement is negligible if anesthetics to be metered at low doses are used, at concentrations of up to ca. 4%. 
     In contrast, the accuracy of measurement of the flow sensor can be expected to be reduced in the case of anesthetics to be metered at high doses (up to 20%). 
     The reference value for &#34;zero&#34; flow is usually obtained by turning off the drive unit and measuring the reference value with the respiration gas stream stopped. However, it is also possible to determine this reference value during the reversal pause between inspiration and expiration. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to improve the method for calibrating a flow sensor in a respiratory system. 
     The task is accomplished in that the final control element is switched over to a closed respiratory system with directed respiration gas stream for the calibration, the drive unit feeds the respiration gas present in the respiratory system via the flow sensor according to a predetermined time-volume relationship, and a calibration value is formed for the flow sensor by comparing the time-volume relationship with the measured signal of the flow sensor. 
     The advantage of the present invention is essentially the fact that the respiration gas is fed from the closed respiratory system with the drive unit via the flow sensor according to a predetermined time-volume relationship, and the calibration value for the flow sensor is formed by comparing the volume fed per unit time with the measured signal of the flow sensor. Thus, to calibrate the flow sensor, the respiration gas is used in the composition in which it happens to be present in the respiratory system. Because of the closed respiratory system, no fresh gas is able to flow into the respiratory system and no respiration gas is able to escape. The time-volume relationship is established in the control unit and is sent to the drive unit. Thus, the respiration gas volume per unit time which flows through the flow sensor is known. The reference value for the &#34;zero&#34; flow state is determined in the known manner, e.g., by measuring the reference value with the flow sensor in stagnant respiration gas flow prior to turning on the drive unit. 
     It is advantageous to use a piston-cylinder assembly unit with a drive, which has a displacement-measuring device for the piston stroke, as the drive unit. An accurate displacement-volume relationship can be realized with this embodiment. The time can be taken into account by withdrawing the piston at a defined velocity, preferably a constant velocity. With the known displacement-volume relationship and known time (piston withdrawn at constant velocity over displacement range), the aforesaid time-volume relationship is established. The respiration gas stream is routed through the directional valves in the respiratory system so that the respiration gas is fed through the flow sensor during the expiration phase. 
     It is advantageous to perform the calibration of the flow sensor periodically. Continuous adjustment of flow measurement to the qualitative composition of the respiration gas is thus possible. However, it is also possible to initiate a calibration cycle by the control unit at the time of transition from the induction phase to the maintenance phase of anesthesia. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     The only FIGURE is a schematic representation of the respiratory system of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The only FIGURE shows a respiratory system 1, in which the respiration gas circulates in the direction of the direction arrow 2. At a fresh gas connection 3, fresh gas is fed into the respiratory system 1 via a first final control element 4, which is preceded by a breathing bag 5 serving as a buffer volume. A first directional valve 6 and a second directional valve 7 determine the direction of flow of the respiration gas to the patient 8 and back via a flow sensor 9, which determines the amount of respiration gas expired. The carbon dioxide expired by the patient 8 is removed from the respiration gas with a carbon dioxide absorber 10. A second final control element 11, with which excess respiration gas can be released from the respiratory circulation 1 via a nonreturn valve 12, is located behind the second directional valve 7 in the direction of flow. 
     The respiration gas is delivered with a drive unit 13, which consists of a piston-cylinder unit 14 and a drive 15. 
     The displacement of the piston of the piston-cylinder unit 14 is measured with a displacement-measuring device 16. The final control elements 4, 11 and the drive 15 are controlled via a control unit 17, which receives the current displacement value of the piston-cylinder unit 14 via the displacement-measuring device 16. The flow sensor 9 is also connected to the control unit 17, and the measured flow values are displayed on a display unit 18. 
     The calibration of the flow sensor 9 is performed as follows: 
     At the end of the inspiration stroke, the piston of the piston-cylinder unit 14 is located at its left-hand stop, i.e., the respiration gas is fed completely into the respiratory system 1. The final control elements 4, 11 are subsequently closed, so that no fresh gas is able to flow into the respiratory system 1, and no respiration gas escapes via the nonreturn valve 12. The &#34;zero&#34; flow value is first measured as a reference value with stagnant respiration gas flow. The piston of the piston-cylinder unit 14 is then withdrawn at constant velocity according to a displacement-volume relationship preset by the control unit 17, and the respiration gas is delivered via the flow sensor 9. The calibration value for the flow sensor 9 is formed by comparison of the measured value sent by the flow sensor 9 to the control unit 17 with the displacement-volume relationship. The time can be taken into account by withdrawing the piston at a defined velocity, preferably a constant velocity, to provide a time volume relationship. 
     To adjust the calibration to a qualitatively different gas composition, it is necessary to periodically repeat the calibration. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.