Abstract:
The present invention relates to a gas regulating device ( 2 A-E) for use in calibration of a gas analyzer ( 4 ). The gas regulating device comprises an inlet ( 8 A) and an outlet ( 8 B), a valve arrangement comprising at least one valve ( 11; 11 A,  11 B), and valve regulating means for regulating the at least one valve. The gas regulating device is intended to be connected between a calibration gas supply ( 3 ) and a gas analyzer ( 4 ) that is to be calibrated and the valve regulating means is configured to regulate the at least one valve such that gas is allowed to flow through a gas flow path ( 5 B) between the inlet and outlet only when a gas pressure (P 1 ) in the gas flow path, between the at least one valve and the outlet, falls below a predetermined threshold value. The gas regulating device is advantageously used when calibrating side-stream gas analyzers in which case it reduces calibration gas consumption, prevents discharge of calibration gas into the ambient environment and prevents leakages jeopardizing correct calibration.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a gas regulating device for use in calibration of a gas analyzer, calibration arrangements for use in calibration of a gas analyzer, a method for calibrating a gas analyzer, and use of a gas regulating device for calibration of a gas analyzer. 
         [0003]    2. Description of the Prior Art 
         [0004]    Gas analyzers are well known in the art and often used in various breathing assist devices such as ventilators and anesthetic machines to analyze the composition of breathing gases supplied to a patient and/or the composition of gases exhaled by the patient. 
         [0005]    There are mainly two types of gas analyzers used for this purpose; in-line gas analyzers and side-stream gas analyzers. 
         [0006]    An in-line gas analyzer, sometimes also referred to as a main stream gas analyzer, is positioned in the gas flow path of the breathing assist device, normally by connecting the gas analyzer in line with the gas-conducting tubing of the breathing assist device. Thereby, an in-line gas analyzer is automatically traversed by the gas that is to be analyzed. 
         [0007]    A side-stream analyzer on the other hand is connected to the gas flow path of the breathing assist device through a side connection, for example a side connection in form of a thin hose connecting the side-stream gas analyzer with the Y-piece of the breathing assist device. Normally, a side-stream gas analyzer is hence not automatically traversed by the gas that is to be analyzed. Therefore, a side-stream gas analyzer typically comprises a pump or other flow generating means in order to “suck” the gas that is to be analyzed from the gas flow path of the breathing device to the measurement chamber of the gas analyzer where the gas is analyzed. Typically, a side-stream gas analyzer operates at a working flow in the range of 0 to 500 ml/minute, meaning that the flow generating means of the side-stream gas analyzer is adapted to generate a flow within that range. 
         [0008]    In order to ensure reliable operation of gas analyzers they should be regularly maintained and calibrated. Calibration is typically performed once a year or so by connecting the gas analyzer to a calibration gas supply, typically in form of a gas cylinder comprising calibration gas having a very well-defined chemical composition. The gas analyzer is then calibrated such that the gas composition as determined by the gas analyzer corresponds to the well-defined composition of the calibration gas. 
         [0009]    Known calibration arrangements used for calibrating side-stream gas analyzers suffer from disadvantages, some of which will be described below with reference to  FIGS. 1 and 2 . 
         [0010]      FIG. 1  illustrates a calibration arrangement according to prior art for calibrating a side-stream gas analyzer  100 . The side-stream gas analyzer is simply connected to a calibration gas supply in form of a gas cylinder  200 , via a hose  300  and a T-piece  400 . If the gas flow provided by the calibration gas supply  200  is below the working flow of the side-stream gas analyzer  100 , the gas analyzer will start sucking ambient air through the open end  500  of the T-piece, which air will mix with the calibration gas and lead to inaccurate calibration of the gas analyzer. Of course, this is also what happens if the calibration gas cylinder  200  runs out of gas. If, on the other hand, the gas flow provided by the calibration gas supply  200  is too high, excess calibration gas will be discharged to the ambient environment through the free end  500  of the T-piece, which free end hence serves as an excess calibration gas outlet. This is an undesired scenario since a large volume of expensive calibration gas is wasted, which calibration gas, depending on the composition of the calibration gas, further may be inappropriate to release into the ambient environment. Some of these problems can be mitigated, at least to some extent, by connecting a flow meter  600  between the calibration gas supply  200  and the gas analyzer  100  so as to help a user of the calibration arrangement to adjust the supply of calibration gas to a suitable level. However, there will always be either a leakage of excess calibration gas, or an intake of ambient air, through the free end  500  of the T-piece. 
         [0011]    To solve the problems of undesired mixing of ambient air and calibration gas, and discharge of calibration gas into the ambient environment, a calibration arrangement as shown in  FIG. 2  has been suggested. In this arrangement, a gas collection bag  700  is slipped onto the free end  500  of the 1-piece. However, this is not a satisfactory solution since the calibration gas collected in the bag  700  can normally not in practice be returned to the calibration gas cylinder  200 , and should not be returned since there will inevitably be a change in gas composition when disconnecting the bag from the T-piece. That the composition of the calibration gas collected in the bag is changed when disconnecting the bag from the 1-piece may also induce errors in the calibration of gas analyzers if the collection bag  700  is reused between different calibration procedures. 
         [0012]    Another problem with this calibration arrangement may arise if the calibration gas cylinder  200  runs completely out of gas. When the calibration gas cylinder  200  is run empty of calibration gas, the gas analyzer  100  will start sucking calibration gas out of the collection bag  700  until the bag too is empty. Since the gas analyzer will continue to suck in gas even when the bag  700  is empty, a small negative pressure will occur in the hose  300  and 1-piece  400 . This negative pressure may cause the calibration gas cylinder to discharge some more gas, thus creating a small negative pressure also within the gas cylinder. When disconnecting the gas analyzer  100  from the port of the 1-piece to which it is connected during calibration, the negative pressure in the hose  300 , 1-piece  400  and calibration gas cylinder  200  will cause ambient air to be sucked in through this 1-piece port and leak into the calibration gas cylinder  200 . If the empty calibration gas cylinder  200  is erroneously used in a subsequent calibration procedure, a small amount of the air that leaked into the gas cylinder  200  may be discharged and ruin the calibration result. 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of the invention to provide a solution solving or at least mitigating at least one of the above mentioned problems associated with calibration of side-stream gas analyzers. 
         [0014]    This object is achieved by a gas regulating device for use in calibration of a gas analyzer. The gas regulating device has an inlet for receiving a flow of calibration gas from a calibration gas supply, an outlet for passing on said flow of calibration gas to the gas analyzer to be calibrated, a valve arrangement comprising at least a one valve disposed in a gas flow path through which said inlet and outlet can be brought in gaseous connection, and a valve regulator for regulating the at least one valve. The valve regulator is configured, when the gas regulating device is operated in a first operational mode, to regulate the at least one valve such that gas can flow between the inlet and outlet only when a gas pressure in the gas flow path, between the at least one valve and the outlet, falls below a predetermined threshold value. 
         [0015]    When connecting a side-stream gas analyzer to a calibration gas supply via the gas regulating device according to the invention, the flow of calibration gas delivered by the calibration gas supply will always correspond to the working flow of the currently calibrated side-stream gas analyzer since the gas regulating device ensures that gas is only delivered to the gas analyzer “upon demand”. 
         [0016]    The gas regulating device hence serves as a demand regulator. If the flow generator of the side-stream gas analyzer does not generate a negative pressure falling under the predetermined threshold value downstream of the valve arrangement, no calibration gas is requested for and will therefore not be delivered to the gas analyzer. 
         [0017]    Thus, by connecting the side-stream gas analyzer to the calibration gas supply via the gas regulating device according to the invention, the consumption of expensive calibration gas will be minimized. Furthermore, the gas regulating device according to the invention eliminates the need for any excess calibration gas outlet through which excess calibration gas can be discharged, thus eliminating the risk for exposing people in the immediate surroundings of the calibration arrangement to the sometimes injurious calibration gas. Another advantage of the gas regulating device according to the invention is that it eliminates the need for using 1-pieces and calibration gas collection bags during calibration procedures, thus facilitating calibration of gas analyzers. Yet another advantage is that the gas regulating device prevents suction of air into an empty gas cylinder upon disconnection of the gas analyzer, which otherwise may occur due to the small negative pressure sometimes arising inside a gas cylinder which is run completely out of gas. This is due to the fact that no negative pressure will occur downstream of the valve arrangement once the gas analyzer is removed and, therefore, the valve will be regulated to prevent any gas to flow through the gas regulating device and into the gas cylinder. 
         [0018]    Preferably, the gas regulating device further includes a flush actuator that when actuated by a user, is configured to set the gas regulating device in a second operational mode in which the valve regulator regulates the at least one valve such that gas can flow between the inlet and outlet of the gas regulating device no matter the pressure between the valve arrangement and the outlet. 
         [0019]    By activating the flush actuator the “on-demand functionality” provided by the gas regulating device can hence be switched off. When the flush actuator is activated, the calibration gas from the calibration gas supply will pass straight through the gas regulating device and on to the gas analyzer, even though no negative pressure exists downstream of the valve arrangement. This functionality is advantageous in that a calibration arrangement comprising the gas regulating device according to the invention can be used for both side-stream gas analyzers generating such a negative pressure by means of their integrated flow generation means, and in-line gas analyzers which normally does not include any flow generation means and, therefore, are unable to generate such a negative pressure. 
         [0020]    The gas regulating device may hence be configured to be operated in either a first operational mode, or side-stream operational mode, in which it acts as a demand regulator, or a second operational mode, or in-line operational mode, in which the on-demand functionality is switched off. 
         [0021]    Preferably, the valve regulating means of the gas regulating device is further configured to regulate the at least one valve of the valve arrangement such that gas can flow between the inlet and outlet only when a gas pressure in the gas flow path, between the inlet and the at least one valve, exceeds a predetermined threshold value. 
         [0022]    This feature is advantageous in that it prevents a calibration gas supply, such as a calibration gas cylinder, connected to the gas regulating device from running completely out of gas. Since no gas can flow through the gas regulation device unless there is a certain positive pressure upstream of the valve arrangement, a certain amount of calibration gas will always remain in the calibration gas supply, thus preventing a negative pressure to occur within the calibration gas supply. Thereby, the risk that ambient air is sucked into an empty calibration gas supply due to such a negative pressure is eliminated, hence eliminating the risk that a calibration gas supply containing a non-well-defined gas mixture is used in subsequent calibration procedures. 
         [0023]    According to one aspect of the invention, the gas regulating device further includes an on-off switching unit that when actuated by a user, is configured to set the gas regulating device in a non-operational mode in which the valve regulator regulates the at least one valve of said valve arrangement such that no gas can ever flow between said inlet and outlet. 
         [0024]    This feature, which hence serves to set the gas regulating device in either of an ‘ON’ or ‘OFF’ mode, is particularly advantageous if the calibration gas supply does not comprise any on/off switch for the calibration gas flow. Then the gas regulating device can be put in a non-operational mode, or ‘OFF’ mode, before connecting it to the calibration gas supply and the gas analyzer. Thereby, assembling of the calibration arrangement is facilitated and the risk that calibration gas is undesirably discharged into the ambient environment is reduced. 
         [0025]    Preferably, the gas regulating device according to the invention has a housing making it a self-contained unit which can be provided as an off-the-shelf product for interconnection between a calibration gas supply and a gas analyzer that is to be calibrated. In this case, the gas regulating device has an inlet connection configured to detachably connect the inlet of the gas regulating device to the calibration gas supply, and an outlet connection configured to detachably connect the outlet of the gas regulating device to the gas analyzer. 
         [0026]    Even more preferably, the inlet connection of the gas regulating device is configured to be detachably connected directly to matching outlet connection of the calibration gas supply. Thus, according to an aspect of the invention, there is provided a calibration arrangement for use in calibration of a gas analyzer, which calibration arrangement has a calibration gas supply and a gas regulating device as described above. The calibration gas supply has an outlet connection configured to be detachably connected directly to the inlet connection of the gas regulating device. 
         [0027]    According to an aspect of the invention the gas regulating device is realized in the form of an electronic device having a single valve controlled by a control unit based on pressure measurements obtained from one or several sensors disposed in the gas flow path. In this case, the control unit may be connected to a user input actuator of the gas regulating device, such as an on-off button and/or a flush button, and configured to set the gas regulating device in either of the first and second operational mode, or the non-operational mode, based on user input. 
         [0028]    According to another aspect of the invention the gas regulating device is realized in form of an entirely mechanical device having one or several mechanical valves for providing the above described functionality. In this case, the gas regulating device may likewise include a user input actuator that when activated by a user, mechanically acts on the valve(s) to set the gas regulating device in any of the above modes. 
         [0029]    The object of the invention is also achieved by a calibration arrangement for use in calibration of a gas analyzer, having a calibration gas supply having an outlet, a gas analyzer having an inlet, and a gas regulating device as described above. The inlet of the gas regulating device is connected to the outlet of the calibration gas supply and the outlet of the gas regulating device is connected to the inlet of the gas analyzer such that calibration gas can flow through a gas flow path from the outlet of the calibration gas supply to the inlet of the gas analyze, through the gas regulating device. 
         [0030]    The object of the invention is also achieved by a method for calibrating a gas analyzer. The method includes the step of supplying a calibration gas from a calibration gas supply to the gas analyzer via a gas flow path in which a valve arrangement comprising at least one valve is disposed. The method further includes the step of regulating the at least one valve to allow gas to flow through said gas flow path only when a gas pressure in the gas flow path, downstream of the at least one valve, falls below a predetermined threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    A more complete appreciation of the invention disclosed herein will be obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying figures briefly described below, in which drawings the same reference numerals are used to represent the same functional elements. 
           [0032]      FIGS. 1 and 2  illustrate prior-art calibration arrangements for calibrating gas analyzers. 
           [0033]      FIG. 3  illustrates a ventilator and the way a side-stream analyzer may be connected thereto. 
           [0034]      FIG. 4  illustrates a calibration arrangement and a gas regulating device according to an exemplary embodiment of the invention. 
           [0035]      FIG. 5  illustrates another exemplary embodiment of a gas regulating device according to the invention. 
           [0036]      FIG. 6  illustrates yet another exemplary embodiment of a gas regulating device according to the invention. 
           [0037]      FIG. 7A  illustrates a calibration arrangement and a gas regulator according to yet another exemplary embodiment of the invention. 
           [0038]      FIG. 7B  shows a detailed view of the gas regulating device shown in  FIG. 7A  and an exemplary embodiment of an interface between a gas regulating device and a calibration gas supply according to the invention. 
           [0039]      FIG. 8  is a functional view of a gas regulating device according to an embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]      FIG. 3  illustrates an exemplary field of application for a gas analyzer that can be calibrated using a gas regulating device according to the invention. 
         [0041]    In  FIG. 3 , a ventilator  50  for mechanically ventilating a patient  60  to which the ventilator is connected is shown. A side-stream gas analyzer  70  is arranged in gaseous connection with a Y-piece  80  of the ventilator through a side connection  90 . The side-stream gas analyzer includes a flow generator (not shown), such as a pump, for generating a flow of gas to be analyzed from the Y-piece, through the side connection, to the gas analyzer. The gas analyzer  70  may have a display unit for displaying the result of the gas analysis to an operator, or it may be electrically connected to an intensive care monitoring system or the like which in turn may be adapted to display information received from the gas analyzer  70  to an operator. The side-stream gas analyzer  70  may be configured to analyze the composition of the breathing gases supplied to the patient and/or the composition of the gases exhaled by the patient. Although not common, the side-stream gas analyzer  70  may also be electrically connected to a control unit  110  of the ventilator which may be configured to control a gas mixing and flow generator  120  of the ventilator based on the result of the gas analysis such that a desired breathing gas composition always is delivered to the patient  60 . 
         [0042]      FIG. 4  illustrates a calibration arrangement  1  comprising a gas regulating device  2 A according to an exemplary embodiment of the invention. The calibration arrangement  1  is further seen to have a calibration gas supply  3  in form of a calibration gas cylinder, a side-stream gas analyzer  4  that is to be calibrated, and a tube or hose arrangement  5 A,  5 C which, together with an interior gas flow path  5 B of the gas regulating device  2 A, form a gas flow path connecting an outlet  6  of the calibration gas supply  3  to an inlet  7  of the gas analyzer  4 , via the gas regulating device  2 A. 
         [0043]    The gas regulating device  2 A has an inlet  8 A for receiving a flow of calibration gas from the calibration gas supply  3 , and an outlet  8 B for forwarding the calibration gas received from the calibration gas supply  3  to the side-stream gas analyzer  4 . The inlet  8 A and outlet  8 B form openings in a housing  9  of the gas regulating device  2 A, which housing makes the gas regulating device  2 A a self-contained unit which can be easily connectable between existing gas calibration devices and gas analyzers. The gas regulating device  2 A is equipped with an inlet connection  10 A and an outlet connection  10 B configured to detachably connect the inlet  8 A and outlet  8 B of the gas regulating device  2 A to the calibration gas supply  3  and the gas analyzer  4 , respectively, via the intermediate tubes or hoses  5 A,  5 C. The inlet connection means  10 A and the outlet connection means  10 B may be adapted for connection with standardized tube or hose connectors, and/or separate connector adapters (not shown) may be provided for connecting the connection means  10 A,  10 B of the gas regulating device  2 A to different types of tubes or hoses  5 A,  5 C. The calibration gas supply  3  and the gas analyzer  4  are equipped with a similar outlet connection  6 A and inlet connection  7 A, respectively. 
         [0044]    The inlet  8 A and outlet  8 B are arranged in gaseous connection with each other through a gas flow path  5 B within the gas regulating device  2 A. A valve arrangement, here in form of a single valve  11 , is disposed in the gas flow path  5 B. The valve  11  is connected to a valve regulating means, here in form of an electric control unit  12 , for regulating the valve  11  so as to adjust the flow of calibration gas flowing from the inlet  8 A towards the outlet  8 B. The control unit  12  is further connected to a pressure sensor  13  disposed in the gas flow path  5 B, downstream of the valve  11  (i.e. after the valve  11  from the gas flow&#39;s point of view), for measuring a gas pressure P 1 . The control unit  12  is configured to receive pressure measurements from the pressure sensor  13  and to open the valve  11  only when a negative pressure (i.e. a pressure below ambient pressure) is measured. Thus, when the gas regulating device  2 A is operated in a first operation mode, or normal operation mode, the control unit  12  regulates the valve  11  such that gas can flow between the inlet  8 A and outlet  8 B only when a negative pressure arises in the gas flow path  5 B, between the valve  11  and the outlet  8 B. A negative pressure arising at this location indicates that the flow generating means of the side-stream gas analyzer  4  is active and strives to suck in gas to be analyzed. Thus, the gas regulating device  2 A is configured to ensure that the valve  11  remains closed unless the side-stream gas analyzer  4  “asks” for gas and hence functions as an on-demand regulator. Typically, the control unit  12  is adapted to open the valve when the pressure P 1  measured by the pressure sensor  13  falls below a certain threshold value, for example a small negative pressure relative to ambient pressure, such as 10-20 mbar below ambient pressure. This threshold value will hereinafter be referred to as the negative pressure threshold value. The criterion that the valve  11  should open only if the gas pressure P 1  in the gas flow path  5 B between the valve  11  and the outlet  8 B falls below the negative pressure threshold value will hereinafter be referred to as the on-demand criterion. 
         [0045]      FIG. 5  illustrates another embodiment of the regulator device according to the invention. In this embodiment, the gas regulating device  2 B comprises a first  13  and a second  13 A pressure sensor. The first pressure sensor  13  is the pressure sensor of the embodiment described above with reference to  FIG. 4 , and the control unit  12  is configured to regulate the valve  11  based on the pressure measurements obtained by this pressure sensor  13  in accordance with the principle described above. The second pressure sensor  13 A is disposed in the gas flow path  5 B, upstream of the valve  11  (i.e. before the valve  11  from the gas flow&#39;s point of view), for measuring a gas pressure P 2 . The control unit  12  is configured to receive pressure measurements also from this second pressure sensor  13 A, and to open the valve  11  only when a positive pressure (relative to ambient pressure) exceeding a certain threshold value is measured, indicating that the calibration gas supply  3  (see  FIG. 3 ) still contains at least a minimum amount of calibration gas. Thus, according to this embodiment, the control unit  12  regulates the valve  11  such that gas can flow between the inlet  8 A and outlet  8 B only when a negative pressure below the negative pressure threshold value arises in the gas flow path  5 B, downstream of the valve  11 , and a positive pressure above a certain threshold value, hereinafter referred to as the positive pressure threshold value, arises in the gas flow path  5 B, upstream of the valve  11 . A suitable positive pressure threshold value may be approximately 50 mbar, which is sufficient to ensure that the valve  11  will be closed before the calibration gas supply  3  runs completely out of calibration gas. The criterion that the valve  11  should open only if the gas pressure P 2  in the gas flow path  5 B between the inlet  8 A and the valve  11  exceeds the positive pressure threshold value will hereinafter be referred to as the pressure-guard criterion. 
         [0046]    The gas regulating device  2 B further includes a manometer  14 , here in form of a digital manometer. The digital manometer  14  is connected to the second pressure sensor  13 A and configured to indicate the measured pressure P 2  on a display of the manometer  14 , which pressure is indicative of the amount of calibration gas left in the calibration gas supply  3 . Alternatively, the control unit  12  can be connected to a digital display (not shown) of the gas regulating device  2 B and configured to display the pressure P 2  measured by the pressure sensor  13 A on the digital display. The feature of indicating the pressure P 2  measured by the second pressure sensor  13 A to the user of the gas regulating device  2 B is particularly advantageous when the calibration gas supply itself lacks means for indicating the amount of calibration gas left. 
         [0047]    Furthermore, the gas regulating device  2 B has a flush actuator  15 , for example in form of a press button or an on/off switch. The flush actuator  15 , when actuated by a user, serves to set the gas regulating device  2 B in a second operational mode in which the control unit  12  keeps the valve  11  open such that gas can flow between the inlet  8 A and outlet  8 B no matter the gas pressure P 1  measured by the pressure sensor  13 . Activation of the flush actuator  15  hence sets the gas regulating device  2 B in a flush mode in which the on-demand criterion is overruled such that the valve  11  is kept open no matter whether the pressure P 1  in the gas flow path  5 B between the valve  11  and the outlet  8 B is below the negative pressure threshold value or not. This feature is advantageous when the gas regulating device  2 B is used for calibrating in-line gas analyzers which differ from side-stream gas analyzers in that they do not generate any flow by them selves. Since in-line gas analyzers do not cause any negative pressure downstream of the valve  11 , the valve  11  would not open if the gas regulating device would be operated in normal operational mode. Preferably, the control unit  12  is configured such that activation of the flush actuator means  15  makes it disregard the on-demand criterion but not the pressure-guard criterion, meaning that the gas pressure P 2  measured by the pressure sensor  13 A still must exceed the positive pressure threshold value in order for the control unit  12  to open the valve  11 , even when the flush actuator  15  has been activated. 
         [0048]    The gas regulating device  2 B also has an on-off switching actuator  16 , for example in form of a press button or an on/off switch. The on-off switching actuator  16  serves to set the gas regulating device  2 B in either of an ‘ON’ or ‘OFF’ mode. In the ‘ON’ mode, the gas regulating device  2 B works in normal operational mode or whatever mode the gas regulating device  2 B is put in by activation of other user input actuator. When put in the ‘OFF’ mode, or non-operational mode, the gas regulating device  2 B is configured to close the valve  11  and maintain it in a closed state. This functionality may e.g. be implemented by designing the gas regulating device  2 B such that the current to an electric valve actuator (not shown) which must be powered to keep the valve  11  open is cut by shifting the on-off switching means  16  from its ‘ON’ state to its ‘OFF’ state. It may also be implemented by configuring the control unit  12  to actively close the valve  11  when the on-off switching means  16  is shifted to its ‘OFF’ state. 
         [0049]      FIG. 6  illustrates another embodiment of a gas regulating device  2 C according to the invention. Whereas  FIGS. 4 and 5  show embodiments according to which the gas regulating device  2 A,  2 B is realized as an electric device,  FIG. 6  illustrates a gas regulating device  2 C realized in form of a mechanical device. 
         [0050]    The flow of calibration gas from the calibration gas supply (see  FIG. 3 ) enters the gas regulating device  2 C through the inlet  8 A and is guided towards the outlet  8 B via the interior gas flow path  5 B of the gas regulating device. The gas flow path  5 B is indicated by hatched lines and the direction of calibration gas flow is indicated by white arrows. 
         [0051]    The gas regulating device  2 C has a valve arrangement including a first valve  11 A and a second valve  11 B. In this embodiment, the first valve  11 A is a combined pressure-guard valve and shut-off valve, whereas the second valve  11 B is a combined on-demand valve and flush valve. 
         [0052]    The first valve  11 A has a spring-loaded membrane or diaphragm  17  which, when no calibration gas flows through the gas flow path  5 B, closes the first valve  11 A by being pressed tight against a valve seat  18  by a compressed helical spring  19 . By forcing the membrane  17  into sealing-tight engagement with the valve seat  18 , the helical spring  19  ensures that the first valve  11 A remains closed as long as no pressure build-up occurs in the gas flow path  5 B, upstream of the first valve  11 A. To distribute the force applied by the spring  19  over a larger area of the membrane  17 , a valve plate  20 , e.g. in form of a washer or a spring washer, is arranged between the membrane-facing end of the spring  19  and the membrane  17 . 
         [0053]    When a flow of calibration gas enters the inlet  8 A, the calibration gas will exert a force on the membrane  17 , which force, when exceeding the spring force applied to the membrane  17  by the helical spring  19 , will force the membrane  17  away from the valve seat  18  such that the calibration gas can pass through the first valve  11 A. The first valve  11 A hence serves as a pressure-guard valve which will remain closed unless the gas pressure P 2  in the gas flow path upstream of the valve exceeds a certain positive pressure threshold value, which threshold value can be chosen by choosing a suitable spring  19 . The spring  19  and membrane  17  thus act as valve regulator allowing gas to flow between the inlet  8 A and the outlet  8 B only when the gas pressure P 2  in the gas flow path  5 B between the inlet  8 A and the first valve  11 B exceeds this positive pressure threshold value. 
         [0054]    The first valve  11 A further has a mechanical on-off switching actuator  16 . The on-off actuator  16  is shaped as a nail, the tip portion of which protrudes from the housing  9  to form a press button, and the head portion of which faces the valve plate  20 . The on-off actuator is freely movable in the vertical direction, inside the helical spring  18 . In the ‘ON’ state, the head of the nail-shaped on-off actuator  16  rests at a distance from the valve plate  20 , allowing the calibration gas to press the membrane  17  upwards and flow through the first valve  11 A. The on-off actuator is set in ‘OFF’ state by pressing the tip portion of the on-off actuator  16 . By doing so the on-off actuator  16  is moved downwards towards the valve plate  20  and maintained in a position where the head portion of the on-off actuator  16  blocks any upward movement of the membrane  17 . In the ‘OFF’ state, the first valve  11 A is hence closed and the gas regulating device  2 A is put in an ‘OFF’ mode in which no gas can ever flow between the inlet  8 A and the outlet  8 B. 
         [0055]    It should be appreciated that this is merely an exemplary embodiment of the first valve  11 A and that there are many ways known in the art to design a mechanical valve achieving the same effects. For example, according to another embodiment (not shown), the first valve  11 A has a screw that is fitted in the valve such that it acts on the helical spring  19 . By tightening or loosening the screw from outside the housing  9 , e.g. by rotating a rotary control knob, the spring  19  can be more or less compressed, thereby allowing the positive pressure threshold value to be adjusted. By tightening the screw to an extent where the spring  19  becomes fully compressed, the membrane  17  is prevented from lifting from the valve seat  18  such that the first valve  11 A will remain closed no matter the gas pressure P 2 . Thus, according to this exemplary embodiment in which the first valve  11 A is realized as a “screw-and-spring valve”, the spring  19  and membrane  17  act as a valve regulator whereas the rotary control knob controlling the position of the screw acts as an on-off switching regulator which, when rotated to a position in which the screw compresses the spring  19  to a certain extent, puts the gas regulating device  2 C in an ‘OFF’ mode in which no gas can ever flow between said inlet  8 A and outlet  8 B. 
         [0056]    The second valve  11 B has a spring-loaded valve body  21  that is movable between a closed position in which it blocks the gas flow path  5 B such that calibration gas is prevented from flowing from the inlet  8 A to the outlet  8 B, and an open position in which it does not block the gas flow path  5 B such that calibration gas is allowed to flow from the inlet  8 A to the outlet  8 B. The valve body  21  is maintained in its closed position by a compressed helical spring  26  which presses the valve body  21  against a valve seat  27 . The valve body  21  is moved into its open position by the movement of a membrane or diaphragm  17 B, which movement in turn is controlled by a difference between the gas pressure in a gas pocket  22  located on one side of the membrane  17 B and the gas pressure P 1  in a portion  23  of the gas flow path  5 B located on the other side of the membrane  17 B, downstream of the valve body  21 . In this embodiment, the gas pocket  22  is in gaseous connection with ambient air via openings  24  in the housing  9  and is thus filled with air at ambient air pressure. If the gas pressure P 1  in the gas flow path  5 B downstream of the valve member  21  becomes negative in relation to the gas pressure in the gas pocket  22 , the pressure difference on the respective sides of the membrane  17 B will make the membrane  17 B bend downwards and apply a pushing force to the valve body  21 , via a valve plate  20 A in form of a washer or spring washer attached to the underside of the membrane  17 B. This force will make the valve body compress the helical spring  26  and move the valve body  21  into its open position. Thus, the helical spring  26 , the membrane  17 B and the valve body  21  serve as a valve regulator that regulates the second valve  17 B such that gas can flow between the inlet  8 A and outlet  8 B only when a pressure below a predetermined threshold value occurs in the gas flow path  5 B, between the second valve  11 B and the outlet  8 B. This threshold value thus corresponds to the previously described negative pressure threshold value and the skilled person will appreciate that the negative threshold value can be adjusted to any suitable value, e.g. by modifying the properties of the valve regulating means. Of course, the exemplary second valve  11 B can be replaced by any other type of mechanical valve adapted to open only when a certain negative gas pressure occurs on one side of the valve to achieve the same effect. 
         [0057]    The second valve  11 B further has a flush actuator  15 , here in form of a press button, which provides the previously described flush functionality allowing the gas regulating device  2 C to be used also when calibrating in-line gas analyzers. When pressing the flush actuator  15  it will force the membrane  17 B to bend downwards and force the valve body  21  into its open position as described above. Preferably, there is also provided a locking means (not shown) configured to maintain the flush actuator  15  in the active position in which the second valve  11 B is kept open no matter the pressure P. 
         [0058]    The gas regulating device  2 C further includes a mechanical manometer  14  which is in gaseous connection with the gas flow path  5 B via a gas channel  25  which is branched out from the gas flow path  5 B at a point located between the inlet  8 A and the first valve  11 A. The manometer  14  will hence, when the inlet  8 A of the gas regulating device  2 C is connected to a calibration gas cylinder, indicate the pressure in the part of the gas flow path  5 B located upstream of the first valve  11 A and thus indicate the amount of calibration gas left in the calibration gas cylinder. 
         [0059]      FIG. 7A  illustrates a calibration arrangement  1 A according to yet another exemplary embodiment of the invention. The calibration arrangement  1 A includes a gas calibration supply  3  in form of a calibration gas cylinder, a gas regulating device  2 D, and a side-stream gas analyzer  4  to which the gas regulating device  2 D is connected via an intermediate hose  5 C. 
         [0060]      FIG. 7B  illustrates an exterior close-up view of the gas regulating device  2 D and the connection interface between the gas regulating device  2 D and the gas calibration supply  3  illustrated in  FIG. 7A . 
         [0061]    In this embodiment, the gas regulating device  2 D is configured to be detachably connected directly to the gas calibration cylinder  3 , without any interconnecting hoses or tubes. The calibration gas cylinder  3  has an outlet connection  6 A configured to be detachably connected directly to matching inlet connection  10 A of the gas regulating device  3 D. In this embodiment, the calibration gas cylinder  3  is provide with the outlet connection  6 A in form of a protruding portion having an outer thread whereas the inlet  8 A of the gas regulating device  2 D is provided with an inner thread forming the inlet connection  10 A of the gas regulating device  20 . Thus, the outlet  6  of the gas calibration supply  3  is connected to the inlet of the gas regulating device  2 D by means of a threaded engagement. It should be appreciated that many other types of gas connectors can be used to detachably connect the inlet  8 A to the outlet  6 . 
         [0062]    The gas regulating device  2 D may be realized as an electric device as described with reference to e.g.  FIG. 4 , a mechanical device as described with reference to  FIG. 5 , or a combination thereof. When hereinafter referring to internal components of the gas regulating device  2 D or external units to which the gas regulating device  2 D is connectable, reference will, when adequate, simultaneously be made to previous drawings. 
         [0063]    The gas regulating device  2 D has an outlet connection  10 B for connecting the outlet  8 B to an inlet  7  of a side-stream gas analyzer  4  via an intermediate hose  5 C, and a manometer  14  for indicating the gas pressure P 2  in the gas flow path  5 B upstream of a valve arrangement  11 ,  11 A,  11 B of the gas regulating device  2 D. As previously mentioned, when the inlet  8 A is connected to the outlet  6  of the calibration gas cylinder  3 , this pressure P 2  corresponds to the pressure in the calibration gas cylinder  3  and hence serves as an indicator of the amount of calibration gas left. Just like the valve arrangement  11 A,  11 B of the gas regulating device  2 C illustrated in  FIG. 6 , this gas regulating device  2 D has a valve arrangement (not shown) including a first and second valve. These valves are disposed in a gas flow path  5 B between the inlet  8 A and the outlet  8 B and the first valve is located upstream of the second valve (i.e. between the inlet  8 A and the second valve). The first valve is a screw-and-spring valve as previously described and the second valve corresponds to the second valve  11 B in  FIG. 6 . The gas regulating device  2 D has an on-off switching actuator  16  in form of a rotary control knob mounted external to the gas regulating device housing  9 . By rotating the rotary control knob the position of the screw acting on the helical spring of the first valve is adjusted, thereby allowing a user to switch on and off the gas flow through the gas regulating device  2 D and set a positive pressure threshold value which must be exceeded by the pressure P 2  in the gas flow path  5 B upstream of the first valve in order for calibration gas to pass through the gas regulating device  2 D. The gas regulating device is further seen to comprise a flush actuator  15  which corresponds to flush actuator  15  in  FIG. 6  and hence serves to force the second valve into an open position no matter the pressure P 1  in the gas flow path  5 B downstream of the second valve. 
         [0064]      FIG. 8  illustrates a functional view of a gas regulating device  2 E according to an embodiment of the invention. The gas regulator  2 E is connected between a calibration gas supply and a gas analyzer. The gas regulating device  2 E embodies an gas-on-demand functionality denoted ‘Demand Regulator’, which functionality allows calibration gas to be delivered to the gas analyzer only when a gas pressure P 1 , downstream of the Demand Regulator, falls below a certain threshold value which herein is referred to as the negative pressure threshold value. The gas regulating device  2 E may further embody a flush functionality denoted ‘Flush’, which functionality allows the gas-on-demand functionality to be “overruled” such that calibration gas can be delivered to the gas analyzer no matter the pressure P 1 . Furthermore, the gas regulating device  2 E may have a pressure-guard functionality denoted ‘Pressure Guard’, which functionality allows the calibration gas supply to discharge calibration gas only when a gas pressure P 2 , upstream of the Pressure Guard, exceeds a certain threshold value which herein is referred to as the positive pressure threshold value. Finally, the gas regulating device  2 E may have an on-off functionality denoted ‘On/Off’, which functionality serves to set the gas regulating device  2 E in either an ‘ON’ mode in which the other functionalities of the gas regulating device  2 E are unaffected, or an ‘OFF’ mode in which no calibration gas can ever pass through the gas regulating device  2 E. 
         [0065]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.