Abstract:
An electrochemical gas sensor system ( 100 ) detects the concentration of a harmful gas in a measuring environment ( 70 ). The electrochemical gas sensor system ( 100 ) contains a voltage generator ( 19 ) and an electrochemical gas sensor ( 1 ). The electrochemical gas sensor ( 1 ) has a sensor housing ( 2 ) and a gas inlet ( 18 ). A measuring electrode ( 3 ), an auxiliary electrode ( 5 ), a reference electrode ( 17 ), a first generator electrode ( 13 ) and a second generator electrode ( 14 ) are in an electrolyte liquid ( 11 ) in the sensor housing ( 2 ). A salt ( 28 ) (halide) of a halogen is dissolved in the electrolyte liquid ( 11 ). The first generator electrode ( 13 ) and the second generator electrode ( 104 ) are connected to the voltage generator ( 19 ) to form a galvanic source. The galvanic source causes the salt ( 28 ) (halide) to react to form a halogen ( 28 ′). A defined, largely stable reference voltage potential becomes established on the reference electrode ( 17 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2014 009 365.4 filed Jun. 21, 2014, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention pertains to an electrochemical gas sensor system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrochemical gas sensor systems are used for stationary or mobile monitoring tasks in industry, especially for monitoring harmful gas concentrations in chemical or petrochemical plants. In usual technical implementations, electrochemical gas sensor systems are composed of an electrochemical gas sensor with a sensor housing and a corresponding electronic operating and analyzing unit. Via a gas inlet, for example, a gas-permeable membrane, gas enters the sensor housing from a measuring environment. An electrochemical gas sensor with a measuring electrode, an auxiliary electrode, a reference electrode and a protective electrode for measuring oxygen is known from DE 198 45 318 C2. A measuring electrode, an auxiliary electrode, a reference electrode and a protective electrode are arranged in an electrolyte space filled with sulfuric acid as a liquid electrolyte. Both the measuring electrode and the protective electrode are also often called working electrode. The electrodes are electrically connected with an electronic operating unit, a so-called potentiostat. An equivalent electric potential is set on the measuring electrode and the protective electrode against a reference potential on the reference electrode by means of the potentiostat during the operation of the electrochemical gas sensor. 
         [0004]    An electrochemical gas sensor with a measuring electrode made of carbon nanotubes and with an auxiliary electrode in an aqueous lithium bromide electrolyte for detecting hydrocyanic acid is known from DE 10 2010 021 975 A1. In an electrochemical gas sensor according to DE 10 2010 021 975 A1, the electric potential is set at the measuring electrode such that lithium bromide reacts to form bromine and dissolved bromine is thus present in the electrolyte. With this method for generating dissolved bromine, there is a basic current in the electrochemical gas sensor, which current depends on the electric potential set and has therefore an adverse effect on the sensitivity and the basic current stability of the electrochemical gas sensor. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the present invention is to provide an electrochemical gas sensor system with improved stability of the basic current. 
         [0006]    According to the invention, an electrochemical gas sensor system is provided for detecting the concentration of a harmful gas in a measuring environment. The electrochemical gas sensor system comprises a voltage generator and an electrochemical gas sensor. The electrochemical gas sensor has a sensor housing and a gas inlet. The gas inlet is designed to permit harmful gas to enter the electrochemical gas sensor from the measuring environment. An electrolyte liquid is contained in the sensor housing and an electrode configuration is arranged in the sensor housing. The electrode configuration comprises a measuring electrode, an auxiliary electrode, a reference electrode and a first generator electrode and a second generator electrode. The reference electrode is arranged between the first generator electrode and the second generator electrode. The first generator electrode and the second generator electrode are connected with the voltage generator. The voltage generator, the first generator electrode and the second generator electrode form a galvanic cell with the electrolyte liquid and with the voltage generator. 
         [0007]    A device according to the present invention is formed by an electrochemical gas-measuring system. The electrochemical gas sensor system according to the present invention has an electrochemical gas sensor and a voltage generator. The device according to the present invention is designed to detect the concentration of a harmful gas in a measuring environment. The electrolyte liquid is electrically conductive and may also be, for example, an organic electrolyte, an ionic liquid or a combination of aqueous electrolyte liquid, ionic liquid and/or organic electrolyte. 
         [0008]    The reference electrode, the measuring electrode and the auxiliary electrode are connected with an electronic operating and analyzing unit, the so-called potentiostat, during the operation of the gas sensor and of the gas sensor system. An equivalent electric potential is set or regulated by means of this potentiostat on the measuring electrode and the auxiliary electrode against a reference potential on the reference electrode. The voltage generator delivers a voltage, and an electrical field is formed between the first generator electrode and the second generator electrode. Due to the electric field, a defined reference potential becomes established on the reference electrode arranged between the first generator electrode and the second generator electrode. The electric field is largely independent from the electric potential on the measuring electrode and auxiliary electrode, which potential is set or regulated by the potentiostat. The basic current and the stability of the basic current of the electrochemical gas sensor system are thus advantageously affected only very slightly by the generator of the reference potential due to the present invention. The dependence of the setting of the reference potential is rather largely uncoupled from the setting of the electric potential on the measuring electrode. The basic current stability, improved by the present invention, has an advantageous effect on the sensitivity and/or the measuring accuracy of the electrochemical gas sensor system during the measurement of the harmful gas concentration. The defined reference potential on the reference electrode is determined essentially by the voltage of the voltage generator, the composition of the electrolyte liquid, as well as by the respective relative position in space of the reference electrode relative to the two generator electrodes. The current flow in the electrolyte liquid between the two generator electrodes depends on the available number of free charge carriers in the electrolyte liquid, the distance of the first generator electrode from the second generator electrode, the surfaces of the two generator electrodes and the properties of the voltage generator, such as the idle voltage and the internal resistance. 
         [0009]    In a preferred embodiment, a halide is dissolved in the electrolyte liquid. Halides are various different compounds of a halogen, for example, iodine, chlorine, bromine, fluorine with metals, metalloids or nonmetals, in the form of iodides, chlorides, fluorides or bromides. Metal halides, for example, sodium chloride, potassium bromide, lithium bromide, silver bromide, silver fluoride, are also known as salts. 
         [0010]    A bromide is dissolved in the electrolyte liquid in an especially preferred embodiment. The bromide is dissolved as lithium bromide in the electrolyte liquid in an especially preferred embodiment. 
         [0011]    The voltage generator is preferably designed as a battery. 
         [0012]    In a preferred variant, the voltage generator is designed such that the voltage or current can be set. 
         [0013]    The voltage generator is designed as a settable voltage source in another preferred variant. 
         [0014]    The voltage or current of the voltage generator is stabilized and/or regulated in another preferred variant. 
         [0015]    In another preferred embodiment, the halide contained in the electrolyte liquid reacts by means of the galvanic cell to form a halogen, while a defined, largely stable reference potential becomes established on the reference electrode. The largely stable reference potential can be detected by measurement by means of an additional voltage measurement on the gas sensor, in which an additional reference electrode is inserted into the sensor housing and into the electrolyte liquid from the outside. The readiness of the generator electrodes to operate can thus also be checked indirectly in combination with the state of the voltage generator (battery capacity, battery voltage). 
         [0016]    A reference electrode suitable for this purpose shall always be adapted to the electrochemical system used. A silver/silver bromide reference electrode (Ag/AgBr) is suitable for use as a reference electrode for an electrochemical system with an electrolyte in which a bromide, for example, lithium bromide, is dissolved. 
         [0017]    In another preferred embodiment, a bromide contained in the electrolyte liquid reacts by means of the galvanic cell to form bromine, while a defined, largely stable reference potential becomes established on the reference electrode. This largely stable reference potential is found to be more positive by (935 mV±25 mV) relative to the reference electrode (Ag/AgBr). To operate the gas sensor, the potentiostat is set in this other preferred embodiment such that a potential difference in the range of 0 mV to −100 mV, preferably −40 mV to −60 mV, becomes established on the measuring electrode relative to the reference electrode. 
         [0018]    In a preferred embodiment of the voltage generator, a set voltage is applied to a terminal of one of the two generator electrodes via a middle tap, the so-called slide, by means of a setting element, preferably a resistor potentiometer, in a parallel circuit with the voltage source, which potentiometer is connected to the plus pole and the minus pole of the voltage source, while the other terminal of the other of the two generator electrodes is connected with the plus pole or the minus pole of the voltage source. 
         [0019]    In a preferred variant, the setting element, preferably designed as a settable resistor with a preferably low ohmic resistance, is arranged in a series connection with one pole of the voltage source towards the terminals of the two generator electrodes in order to form a settable voltage source. 
         [0020]    In another preferred variant, a settable resistor with a preferably high ohmic resistance, is arranged as a setting element in a parallel connection with one pole of the voltage source towards the terminals of the two generator electrodes in order to form a settable voltage source. 
         [0021]    The voltage generator is designed as a settable power source in another preferred variant. 
         [0022]    Further possibilities or combinations of possibilities of setting the voltage or current are also covered in the sense of the present invention. This covers circuitry embodiments of voltage sources, voltage dividers, power sources by means of resistive and/or electronic resistors, as well as circuitry embodiments with relays or semiconductor components, for example, transistors, field-effect transistors, bipolar transistors, diodes, operational amplifiers, and ASICs. 
         [0023]    In another preferred variant, the voltage generator, the voltage source or the settable power source is designed as part of the electronic operating and analyzing unit, is connected or coupled with the electronic operating and analyzing unit or is integrated in the electronic operating and analyzing unit. 
         [0024]    Permeable nonwovens or nonwoven elements are arranged at the first and/or the second generator electrode in an especially preferred embodiment. These nonwovens or nonwoven elements are permeable to the electrolyte liquid and guide and/or transport the electrolyte liquid in the sensor housing to the electrodes. 
         [0025]    In another preferred embodiment, the nonwovens permeable to the electrolyte liquid or the nonwoven elements permeable to the electrolyte liquid are arranged at the first and/or second generator electrode such that an installed position with predetermined distances of the generator electrodes from the reference electrode and with predetermined distances between the generator electrodes is obtained. 
         [0026]    In another preferred embodiment, the first generator electrode and the second generator electrode as well as the reference electrode and the nonwovens permeable to the electrolyte liquid are enclosed by an inner housing impermeable to the electrolyte liquid. 
         [0027]    The inner housing impermeable to the electrolyte liquid makes it possible to connect the generator electrodes and the reference electrode electrochemically directly to the measuring electrode and to connect the generator electrodes and the reference electrode electrochemically less directly to the auxiliary electrode. 
         [0028]    In another preferred embodiment, at least one of the nonwovens permeable to the electrolyte liquid or the nonwoven element permeable to the electrolyte liquid is designed to transport the electrolyte liquid from the sensor housing to the area located between the first and second generator electrodes into the inner housing to the first and second generator electrodes. 
         [0029]    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 preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In the drawings: 
           [0031]      FIG. 1  is a schematic view showing a first gas sensor system with an electrochemical gas sensor with two generator electrodes in a first variant of a cylindrical design; and 
           [0032]      FIG. 2  is a schematic view showing a second gas sensor system with an electrochemical gas sensor with two generator electrodes in a second variant of a planar design. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Referring to the drawings,  FIG. 1  schematically shows in a side view a first electrochemical gas-measuring system  100  in a first variant  1  of an electrochemical gas sensor with a cylindrical sensor housing  2  and with a voltage generator  19 . In the cylindrical sensor housing  2 , a measuring electrode  3  is arranged on a first diffusion membrane  4  and an auxiliary electrode  5  on a second diffusion membrane  6 . The sensor housing  2  is filled with an aqueous electrolyte liquid  11 . Gas to be measured enters the cylindrical sensor housing  2  from a measuring environment  70  by means of a gas inlet  18  designed in the form of an opening in the cylindrical sensor housing  2  through the first diffusion membrane  4 . The measuring electrode  3  and the auxiliary electrode  5  are kept at a fixed distance from one another by the arrangement of two nonwovens  8 ,  9  and by means of a wick  7 . The pressure in the electrochemical gas sensor  1  is equalized with the environment  70  and the cylindrical sensor housing  2  is ventilated by means of a pressure-equalizing and ventilating opening  10  and through the second diffusion membrane  6 . The measuring electrode  3  and the auxiliary electrode  5  are electrolytically connected with one another through the aqueous electrolyte liquid  11 . An anode  13  as a first generator electrode and a cathode  14  acting as a second generator electrode are arranged laterally from two further nonwovens  15 ,  16  in the wick  7  in an inner housing  12  sealed for liquids. A reference electrode  17  is located between the nonwovens  15 ,  16 . The nonwovens  15 ,  16  maintain the generator electrodes  13 ,  14  and the reference electrode  17  at a fixed distance from one another. The anode  13 , the cathode  14  and the reference electrode  17  are arranged concentrically. The nonwovens  8 ,  9 ,  15 ,  16  and the wick  7  are designed such that they are permeable to the aqueous electrolyte liquid  11 . The aqueous electrolyte liquid  11  is transported through the nonwoven  9  as well as at least one of the nonwovens  15 ,  16  into the inner housing  12  to the generator electrodes  13 ,  14  and to the reference electrode  17 . A bromide  28  is dissolved in the aqueous electrolyte liquid  11 . The anode  13  and the cathode  14  are connected to the voltage generator  19  designed as an electric voltage source. The measuring electrode  3 , the auxiliary electrode  5  and the reference electrode  17  are connected electrically to a potentiostat  80 , not shown in detail in this  FIG. 1 , or to an electronic operating unit  81  with integrated potentiostat  80 , which electronic unit is not shown in detail in this  FIG. 1 . The anode  13  and the cathode  14  are connected with the electric voltage source  19  and the electrolyte liquid  11  to form a galvanic source. The galvanic source forms during the operation an electric circuit  20 , in which an electric current  21  becomes established. An optional resistor  22  is preferably arranged connected in series with the generator electrodes  13 ,  14 . The flow of the electric current  21  in the electric circuit  20  causes the bromide  28  contained in the aqueous electrolyte liquid  11  to react to form bromine  28 ′. The intensity of the electric current  21  is determined essentially by the preferred resistor  22  as well as by the resistance of feed lines and the internal resistance R i  of the voltage source  19 , which is not shown in this  FIG. 1 . The bromine  28 ′ dissolved in the aqueous electrolyte liquid causes a largely stable electric potential to become established on the reference electrode. 
         [0034]    In a special embodiment of the electrochemical gas-measuring system  100 , the electric voltage source  19  is equipped with an optional setting element  75 . This optional setting element  75  is designed to set or change the voltage of the voltage source  19 . An optional control unit  82  is present in another optional variant of the electrochemical gas-measuring system  100 . The optional control unit  82  is connected in this further optional variant of the electrochemical gas-measuring system  100  with the setting element  75  via a control line  820 , which is drawn by a broken line in this  FIG. 1 . It is thus possible via the control line  820  to control the setting element  75  and, moreover, to set or change the voltage of the voltage source  19 . The optional control unit  82  may be designed as a separate module or as a component of the electronic operating unit  81 . A first data and control line  821 , which is drawn by a broken line in this  FIG. 1 , and a second data and control line  822 , which is drawn by a broken line in this  FIG. 1 , are preferably provided for a bidirectional data exchange between the control unit  82  and the potentiostat  80 . This makes it possible to involve the potentiostat  80  in the setting or changing of the voltage of the voltage source  19  or even in the setting or changing of the voltage of the voltage source  19  by means of the electronic operating unit  81 . This setting may be preferably used for presetting or configuration during the putting into operation of the electrochemical gas-measuring system  100  or preferably for fine adjustment during the operation of the electrochemical gas-measuring system  100 . As a further optional expansion of the electrochemical gas-measuring system  100 , an optional current-measuring resistor (shunt)  89  is arranged in the electric circuit  20 . A voltage difference dropping over this optional current-measuring resistor  89  is sent by means of a first measuring line  831  and a second measuring line  832  to an optional measuring unit  83 . The optional measuring unit  83  is designed, for example, in the form of a voltmeter, to measure the voltage difference dropping over the current-measuring resistor  89  and to determine the electric current  21  flowing in the circuit from this. The optional measuring unit  83  may be designed as a separate module or as a component of the electronic operating unit  81 . In an especially preferred variant of this further optional expansion of the electrochemical gas-measuring system  100 , the measuring unit  83  is connected via a third data or control line  833 , drawn by broken line in this  FIG. 1 , with the control unit  82 . There is an interaction of the potentiostat  80 , measuring unit  83 , control unit  82  and setting element  75 , preferably arranged in the electronic operating unit  81 , for setting or changing the voltage of the voltage source  19  in this especially preferred variant of this further optional expansion of the electrochemical gas-measuring system  100 . It is thus possible to detect the electric current  21  actually flowing in the electric circuit  20 , to directly or indirectly change the voltage of the voltage source  19  based on this and thus to set, control or regulate the electric current  21 . An especially accurate setting of the electric potential on the reference electrode  17  is thus made possible with this especially preferred variant of this further optional expansion of the electrochemical gas-measuring system  100 , because the potentiostat  80  can also be included in the setting or it can bring about itself the setting of the electric potential on the reference electrode  17 . 
         [0035]      FIG. 2  schematically shows in a side view a second electrochemical gas-measuring system  200  in an embodiment with an electrochemical gas sensor  1 ′ with a planar sensor housing  2 ′ and with a voltage generator  19 . Identical components in  FIG. 2  and  FIG. 1  are designated in  FIG. 2  by the same reference numbers as in  FIG. 1 . 
         [0036]    The planar sensor housing  2 ′ is filled with an aqueous electrolyte liquid  11 . A measuring electrode  3  is arranged in the planar sensor housing  2 ′ on a first diffusion membrane  4  and an auxiliary electrode  5  is arranged on a second diffusion membrane  6 . The planar sensor housing  2 ′ is vented through a ventilation opening  31  and through the second diffusion membrane  6 . In addition, there is a protective electrode  90 , which is arranged between the auxiliary electrode  5  and the measuring electrode  3 . The protective electrode  90  is used to protect the measuring electrode  3  from substances diffusing from the aqueous electrolyte liquid  11  or from the ventilation opening  31  to the measuring electrode  3 . Gas to be measured enters the planar sensor housing  2 ′ from a measuring environment by means of a gas inlet  18  designed in the form of an opening in the planar sensor housing  2 ′ through the first diffusion membrane  4 . The measuring electrode  3 , the protective electrode  90  and the auxiliary electrode  5  are maintained at a fixed distance from one another by means of two nonwovens  34 ,  35 . The measuring electrode  3 , the protective electrode  90 , the auxiliary electrode  5  and the nonwovens  34 ,  35  are held as a stack in the planar sensor housing  2 ′ between the first diffusion membrane  4  and the second diffusion membrane  6 . The measuring electrode  3 , the protective electrode  90  and the auxiliary electrode are connected with one another electrolytically by the aqueous electrolyte liquid  11 . The pressure in the electrochemical gas sensor  1  is equalized with the atmospheric pressure of the environment  70  via a pressure equalization opening  32  and a third diffusion membrane  23 . An anode is arranged in the planar sensor housing  2 ; as a first generator electrode and a cathode  14  is arranged as a second generator electrode. In addition to the nonwovens  34 ,  35 , an additional nonwoven  15  is arranged in the planar sensor housing  2 ′. A reference electrode  17  is located between the nonwovens  15 ,  35 . The nonwovens  15 ,  35  hold the two generator electrodes  13 ,  14  and the reference electrode  17  at a fixed distance from one another in the planar sensor housing  2 ′. The anode  13 , the cathode  14  and the reference electrode  17  are planar and are preferably arranged in parallel to one another. The nonwovens  15 ,  34 ,  35  are designed as nonwovens permeable to the aqueous electrolyte liquid  11  and are used to wet the electrodes  3 ,  5 ,  17 ,  90 ,  13 ,  14  with the aqueous electrolyte  11 . The nonwoven  35 , in particular, is used to transport the aqueous electrolyte liquid  11  in the planar sensor housing  2 ′ from the arrangement comprising the measuring electrode  3 , the protective electrode  90  and the auxiliary electrode  5  to the arrangement comprising the generator electrodes  13 ,  14  and the reference electrode  17 . The generator electrodes  13 ,  14 , the measuring electrode  3 , the protective electrode  90  and the auxiliary electrode  5 , as well as the reference electrode  17  in the planar sensor housing  2 ′ are thus connected with one another directly electrolytically by the aqueous electrolyte liquid  11 . A bromide  28  is dissolved in the aqueous electrolyte liquid  11 . The anode  13  and the cathode  14  are connected to the voltage generator  19  designed as an electric voltage source. The measuring electrode  3 , the auxiliary electrode  5 , the protective electrode  90  and the reference electrode  17  are connected electrically to a potentiostat  80 , which is not shown in detail in this  FIG. 2 . The anode  13  and the cathode  14  are connected with the electric voltage source  19  and the electrolyte liquid  11  to form a galvanic cell. The galvanic cell forms an electric circuit  20  during the operation, in which an electric current  21  becomes established. An optional electric resistor  22  is preferably arranged connected in series with the generator electrodes  13 ,  14 . The current flow  21  in the electric circuit  20  causes the bromide  28  contained in the aqueous electrolyte liquid  11  to react to form bromine  28 ′. The intensity of the current flow  21  is determined essentially by the preferred electric resistor  22  as well as by the resistance of the feed lines and the internal resistance R i  of the voltage source  19 , which is not shown in this  FIG. 2 . The bromine  28 ′ dissolved in the aqueous electrolyte liquid causes a largely stable electric potential to become established on the reference electrode  17 . 
         [0037]    The special embodiments of the first electrochemical gas-measuring system  100  in  FIG. 1  with variants and options for the arrangement and the interaction of setting elements  75  ( FIG. 1 ), the electronic operating unit  81  ( FIG. 1 ), the control unit  82  ( FIG. 1 ), the current-measuring resistor  89  ( FIG. 1 ), and the measuring unit  83  ( FIG. 1 ) can also be extrapolated in a similar embodiment to the design of the second electrochemical gas-measuring system  200  and are thus also covered by the description of  FIG. 1  in the sense of the present invention. 
         [0038]    While specific embodiments of the invention have 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. 
       APPENDIX 
     List of Reference Numbers 
       [0000]    
       
           1  Electrochemical gas sensor in a first variant 
           1 ′ Electrochemical gas sensor in a second variant 
           2  Cylindrical sensor housing 
           2 ′ Planar sensor housing 
           3  Measuring electrode, working electrode 
           4  First diffusion membrane 
           5  Auxiliary electrode 
           6  Second diffusion membrane 
           7  Wick 
           8 ,  9  Nonwovens 
           10  Pressure equalization and ventilation opening 
           11  Aqueous electrolyte liquid 
           12  Inner housing 
           13  First generator electrode, anode 
           14  Second generator electrode, cathode 
           15 ,  16  Nonwovens 
           17  Reference electrode, reference electrode 
           18  Gas inlet, opening in the sensor housing 
           19  Voltage generator, electric voltage source 
           20  Electric circuit 
           21  Electric current I 
           22  Electric resistor 
           23  Third diffusion membrane 
           28  Bromide 
           28 ′ Bromine 
           31  Ventilation opening 
           32  Pressure equalization opening 
           34 ,  35  Nonwovens 
           70  Measuring environment 
           75  Setting element (optional) 
           80  Potentiostat 
           81  Electronic operating unit 
           82  Control unit (optional) 
           83  Measuring unit (optional) 
           89  Current-measuring resistor/shunt (optional) 
           90  Protective electrode 
           100  First electrochemical gas-measuring system 
           200  Second electrochemical gas-measuring system 
           820  Control line (optional) 
           821  First data or control line (optional) 
           822  Second data or control line (optional) 
           831  First measuring line (optional) 
           832  Second measuring line (optional) 
           833  Third data or control line (optional)