Device for detection of a partial pressure and method for its operation

Abstract

In order to provide a device for detection of a partial pressure of a measurement gas contained in a surrounding gas, comprising a measurement space with at least one inlet opening to admit surrounding gas into the measurement space and at least one outlet opening to discharge surrounding gas from the measurement space, a quantity that characterizes the partial pressure of the measurement gas in the measurement gas sensor sensitive to the measurement gas to measure a quantity that characterizes the partial pressure of the measurement gas in the measurement space, as well as readout device to read out the measurement gas sensor as well as a method for its operation, which permit metrological detection of the partial pressure of a measurement gas contained in a surrounding gas with improved chronological and spatial resolution, it is proposed that means to determine a response characteristic of device be provided.

Claims

1. A method of operation of a measurement space and a measurement gas sensor sensitive to a measurement gas contained in a surrounding gas for measurement of a quantity that characterizes the partial pressure of the measurement gas in the measurement space, comprising: detecting a partial pressure of the measurement gas, and determining the response characteristic of a device including a. traversing the measurement space with a reference gas, wherein the measurement gas sensor and/or a correction gas sensor sensitive to a correction gas is not sensitive to said reference gas, observing an output signal of the measurement gas sensor and/or an output signal of the correction gas sensor until it reaches an essentially constant minimal value; b. traversing the measurement space with a surrounding gas after the output signal has reached the essentially constant minimal value, and recording the output signal of the measurement gas sensor and/or the output signal of the correction gas sensor in a measurement series with time resolution during a calibration time interval; and c. evaluating the measurement series to determine the response characteristic.

2. The method of claim 1, evaluating the measurement series initially a correction response characteristic is determined based on the output signal of the correction gas sensor and the response characteristic of the device is determined from the correction response characteristic.

3. The method of claim 1, wherein the output signal of the measurement gas sensor is corrected by the response characteristic in order to obtain a chronological assignment of the output signal through the partial pressure of the measurement gas at the time of recording of the output signal.

4. The method of claim 1, wherein the calibration time interval is chosen so that the partial pressure of the measurement gas and/or the partial pressure of the correction gas is essentially constant during the calibration time interval.

5. The method of claim 1, wherein the determining of the response characteristic a first section of the measurement series which was recorded during a mixing phase with the surrounding gas with the reference gas in the measurement space is not considered.

6. The method of claim 5, wherein the first section of the measurement series extends to a transition point from an essentially linear to an essentially exponential curve trend.

7. The method of claim 1, wherein the response characteristic is determined by a regression analysis of the measurement series, using the following model: $p_{\left(t\right)}=p_{\mathrm{water}}+\left(p_{\left(t_0\right)}-p_{\mathrm{water}}\right)e^{-\frac{t}{}}$ in which P(t) denotes the output signal at time t, p.sub.water denotes the partial pressure of the measurement gas within the surrounding gas, p(t.sub.o) denotes the output signal at time t.sub.o and denotes a time constant.

8. The method of claim 1, wherein the output signal of the measurement gas sensor is corrected by means of the response characteristic in which the output signal p(t) at time t is assigned to the partial pressure p.sub.water of the measurement gas within the surrounding gas at the time of recording of the output signal according to the following formula: $p_{\mathrm{water}}=\frac{p\left(t\right)-p\left(t_0\right)e^{-\frac{t}{}}}{1-e^{-\frac{t}{}}}$ in which p(t.sub.o) is the output signal at time t.sub.o.

9. The method of claim 1, wherein determining a response characteristic is conducted several times, especially after each significant change of one or more surrounding parameters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The figures of the drawing show in particular:

(2) FIG. 1: schematic depiction of a preferred variant for underwater use of a detection device according to the invention;

(3) FIG. 2: schematic depiction to show the preferred embodiment of the method according to the invention for determination of response characteristics;

(4) FIG. 3: example of a measurement series which was determined during performance of the method for determination of the response characteristic in order to show a preferred embodiment of the method for determination of the response characteristics;

(5) FIG. 4: schematic depiction of another preferred variant for underwater use of a detection device according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) FIG. 1 shows in a schematic sectional view a partial pressure sensor 1, which is immersed in water 2. The partial pressure sensor 1 has a watertight housing 3. The watertight housing 3 is open on one side to the surrounding water 2 via a membrane equilibrator 4. The membrane equilibrator 4 is permeable to the surrounding gas contained in the surrounding water 2 but not to water. A measurement gas sensor 5 is arranged as core piece in the interior of the partial pressure sensor 1 delimited by housing 3 and the membrane equilibrator 4 from the surrounding water 2. This is formed as a non-dispersive infrared detector in the practical example depicted here. The measurement gas sensor 5 has an inlet opening 6 and an outlet opening 7. A measurement space not explicitly shown is arranged within the measurement gas sensor 5 in the schematic depiction according to FIG. 1.

(7) In addition, an inlet line 8 for the surrounding gas connected to the surroundings via the membrane equilibrator 4 permeable to the surrounding gas contained in water 2 and in turn containing carbon dioxide as measurement gas is arranged in interior housing 3.

(8) The inlet line 8 discharges in inlet opening 6 of the measurement gas sensor 5. A pump 9 and a heating device 10 downstream of pump 9 to heat the surrounding gas that enters inlet line 8 are arranged in the inlet line 8. Downstream of the heating device 10 a sensor 11 for temperature and moisture as well as a pressure sensor 12 are arranged in succession in the inlet line 8. The outlet line 7 of the measurement gas sensor 5 discharges in an outlet line 13, which communicates with the surrounding water 2 via the membrane equilibrator 4. A pressure sensor 14 and a temperature and moisture sensor 15 are also arranged in the outlet line 13.

(9) Upstream of pump 9 a valve 16 is arranged in inlet line 8. The values 16 is configured is that it produces in the first switching state a connection of the suction side of pump 9 with a section of inlet line 8 facing the membrane equilibrator 4. This switching state is sketched in FIG. 1. In the second possible switching state of the valve, which valve 16 can assume, the suction side of pump 9 of inlet line 8 is connected to a reference gas source 17 so that the section of the inlet line 8 between membrane equilibrator 4 and valve 16 does not communicate with the suction side of pump 9. In this section of the inlet line 8 a sensor 18 for temperature and moisture as well as a pressure sensor 19 are also provided.

(10) A valve 20 is arranged in the outlet line 13 upstream of sensor 15 for temperature and moisture as well as pressure sensor 14. Valve 20 in the outlet line 13 is designed similar to the valve 16 and inlet line 8 and can also assume two possible switching states. The first possible switching state of valve 20 and outlet line 13 is the one depicted in the figure, in which the outlet opening 7 of the measurement gas sensor 5 communicates via valve 20 and membrane equilibrator 4 with the surrounding water 2. In the second possible switching state of valve 20 and outlet line 30, on the other hand, the outlet opening 7 of the measurement gas sensor 5 is connected to an input 21 of the reference gas source 17, in which the outlet opening 7 of the measurement gas sensor 5 in this switching state is not connected to the surrounding water 2 via the membrane equilibrator 4. In the second possible switching state of valve 20 in the outlet line 13 with corresponding position of valve 16 in the inlet line 8 a bypass line that connects the inlet opening 8 with the outlet opening 7 of the measurement gas sensor 5 is switched, in which case the bypass line in this switching state of valves 20, 16 connects the outlet opening 7 with the inlet opening 6 of the measurement gas sensor 5 via the valve 20 and the outlet line 13, the reference gas source 17, the valve 16 and the inlet line 8, pump 9 and heating device 10. In this switching state gas circulates through the measurement gas sensor 5 within housing 3 of the partial pressure sensor 1 without gas exchange occurring with the surrounding water 2 via membrane equilibrator 4.

(11) Finally, the partial pressure sensor 1 according to FIG. 1 includes an only schematically depicted control and evaluation unit 22. The control and evaluation unit 22 is connected via a control line 23 to valve 16, 20 and outlet line 13 and in inlet line 8 in order to switch the valve 16, 20 between the described possible switching states. In addition, the control and evaluation unit 22 is connected via a signal line 24 to readout devices 25 for readout of the measurement gas sensor 5. Finally, the control and evaluation unit 22 is connected via additional signal lines not shown in the figure to sensors 18, 19, 11, 12, 14, 15 for temperature, moisture and pressure in order to determine the partial pressure by means of the absorption signal detected by the optical measurement gas sensor 5.

(12) The control and evaluation unit 22 has means to store the measured values read out via the readout device 25 from the measurement gas sensor 5. In addition, means for analysis and processing of the measured data are provided in the control and evaluation unit.

(13) FIG. 2 schematically depicts a preferred variant of the method according to the invention for operation of a partial pressure sensor 1 according to FIG. 1. In part a) of FIG. 2 an instrument support 26 is shown, which can be an AUV (autonomous underwater vehicle), ROV (remotely operated vehicle) a ship or the like. The partial pressure sensor 1 according to FIG. 1 is fastened to the instrument support 26. As indicated by an arrow in FIG. 2A, the instrument support 26 with the partial pressure sensor 1 according to the invention fastened to it is moved over the sea bottom 27 through the surrounding water in the direction of the arrow. In the forward movement direction of instrument support 26 and area 28 with increased measurement gas concentration is schematically shown. For example, this is carbon dioxide, which emerges into the water tube from a source in sea bottom 27. The spatial position and extent as well as partial pressure of the area 28 with increased measurement gas concentration is now to be measured as precisely as possible with the partial pressure sensor 1 arranged on the instrument support 26, while the instrument support 26 is moved through area 28 with a specific speed.

(14) In order to counteract the problems occurring during this measurement by the moving instrument support 26 in conjunction with a finite response time of the partial pressure sensor 1 to changes in the measured signal, i.e., changes in the partial pressure of the measurement gas, according to the invention determination of the response characteristic of the partial pressure sensor 1 is initially conducted as follows while the instrument support 26 with the partial pressure sensor 1 is moved in an area of sufficient distance from the area 28 with increased measurement gas concentration in which a constant partial pressure of the measurement gas prevails.

(15) As shown in FIG. 1, a bypass line between outlet opening 7 and inlet opening 6 of the measurement gas sensor 5 is initially created via the control and evaluation unit 22 by switching valve 20 in the outlet line 13 and valve 16 in the inlet line 8, which is passed through the reference gas source 17. The surrounding gas passed through the reference gas source 17 is then converted to reference gas in the reference gas source. In the example considered here, in which the measurement gas is carbon dioxide, conversion occurs by passing the surrounding gas through a column of soda lime. In one variant of the invention, in which methane is the measurement gas, conversion can occur by oxidation with the oxygen contained in the surrounding gas in order to generate CO.sub.2 and H.sub.2O on a thermocatalytic element, which can be present in the form of a thermocatalytic sensor as catalytic oxidation sensor.

(16) The measurement gas sensor 5 according to the practical example is only sensitive to CO.sub.2. After passing through reference gas source 17 reference gas, i.e., surrounding gas which contains no CO.sub.2 therefore enters valve 16 in inlet line 8 and is introduced via pump 9 and heating device 10 through the inlet line 6 into the measurement space of the measurement gas sensor 5. A time series 29 of recorded measurement data read out from the measurement gas sensor 5 via the readout device 25 of the control and evaluation unit 22 is shown in FIG. 2B. The time series 29 is arranged in FIG. 2B beneath the three-dimensional sketch according to FIG. 2A so that it is apparent which measurement location, i.e., which position of the partial pressure sensor 1 fastened to the instrument support 26 corresponds to a measurement point of the time series 29.

(17) In the time series 29 according to FIG. 2B it is now apparent that the described flow through the measurement space is the reference gas generated in the reference gas source 17 leads to an abrupt drop 30 in the measurement signal in time series 29. It is further apparent that the measurement signal during flow through the measurement space of the measurement gas sensor 5 assumes a reference plateau 31 with the reference gas. After the reference plateau 31 is reached, the time series 29 of the measurement signal corresponds to the actual partial pressure in the measurement space of measurement gas sensor 5, since the stationary state is reached at which any effects caused by a delayed response behavior no longer play a role. The reference plateau 31, as is known, can also be used for calibration of the measurement gas sensor 5 in order to correct for any reference point shift of the measurement gas sensor.

(18) The flank-like rise 32 of the measurement signal connected to the reference plateau 31 is also apparent in the time series 29 of the recorded measured data according to FIG. 2B, which is recorded when the valve 16 in the inlet line 8 and the valve 20 in the outline line 13 are switched so that the reference gas source 17 is no longer connected to the measurement space of the measurement gas sensor 5 and flow through the measurement gas sensor 5 instead occurs with surrounding gas drawn into inlet line 8 via the membrane equilibrator 4.

(19) Since, as comparison with FIG. 2A shows, in which the spatial segment of the partial pressure of the measurement gas, for example, carbon dioxide, corresponding to the flank-like rise 52 of the time series 29, does not change, an abrupt rise at the reference plateau 31 should be connected instead of the flank-like rise 32 during an ideal measurement. The fact that the flank-like rise 32 is recorded instead is attributed to the response characteristic of the partial pressure sensor 1, which is influenced by the all the components of the partial pressure sensor 1. This includes the membrane equilibrator 4, the valve 16 and inlet line 8, the pump 9, the heating device 10, the measurement gas sensor 5, the valve 20 and outlet line 13. Since the real measured quantity rises abruptly during flow of reference gas through the measurement gas sensor 5 in situ according to the invention with essentially constant partial pressure of the measurement gas, for example, carbon dioxide, the response characteristic of the measurement gas sensor 5 can be determined by evaluating the flank-like rise 32.

(20) Determination of the response characteristic by evaluation of the flank-like rise 32 is further explained in FIG. 3. FIG. 3 shows an enlargement of area 3 of the time series 29 according to FIG. 2B in order to show the method according to the invention for determination of the response characteristic of the measurement gas sensor 5. As is apparent in FIG. 3, the measurement series initially shows the initial partial pressure 33 of the measurement gas in water 2, which prevails at the position of the instrument support 26 and therefore the partial pressure sensor 1 depicted in FIG. 2A. The measured and actual partial pressures disappear roughly in the area of reference plateau 31.

(21) To evaluate the response characteristic by means of the flank-like rise 32 of the time series 29 the procedure according to the invention is as follows according to the practical example described here. The evaluation of the flank-like rise 32 is based on an evaluation series within a calibration interval 34, during which the partial pressure of the measurement gas in the surroundings is constant, as is apparent in FIG. 2A. Initially the area of the flank-like rise 32 was divided analytically into a first section 35, which was recorded during a mixing phase of the measurement gas with the reference gas in the measurement space of the measurement gas sensor 5, and a second section 36. Values directly connected to the reference plateau 31 up to a transition point 37 then correspond to the first section 35 of the measurement series 29. The second section 36 of the measurement series 29 accordingly extends over measurement points from the transition point 37 to the last measurement point within the calibration time interval 34.

(22) To determine the transition point 37 a regression line 38 is determined according to the invention starting from the first measured values in the measurement series 29 which are connected to the reference plateau 31. The transition point 37 is then obtained by determining the point at which the measured values leave the regression line 38. Analytically this is the point at which the residuum between the actual measured values and the corresponding values on the regression 38 exceeds the specific value.

(23) In the next step with consideration of only the second section 36 of the measurement series 29 a regression analysis is performed in which the measurement values within the second section of the measurement series 36 are adjusted to the following model:

(24) $p_{\left(t\right)}=p_{\mathrm{water}}+\left(p_{\left(t_0\right)}-p_{\mathrm{water}}\right)e^{-\frac{t}{}}$
in which p.sub.(t) corresponds to measured values of the time series 29 in the second section 36, p.sub.water corresponds to the initial partial pressure 33 of the measurement gas and p.sub.(t.sub.o.sub.) corresponds to the output signal of the measured partial pressure at time t.sub.0, i.e., at transition point 37 and in which T denotes a time constant to be determined in the context of a regression analysis.

(25) With the described method the response characteristics of a partial pressure sensor 1 is determined according to the invention in situ. The regression parameters, especially parameter , implicitly allow for with great advantage all factors that influence the response characteristic of the partial pressure sensor 1 at the location of the measurement, like temperature, pressure, properties of the membrane equilibrator 4, etc.

(26) With reference to FIG. 2A the subsequent measurement process is then shown. The instrument support 26 is moved after the response characteristic has been determined with the described method through the area 28 with increased measurement gas concentration. A raw measurement response 39 is then recorded in the time series 29 of the recorded measurement data. This is necessarily distorted by the response characteristic of the partial pressure sensor 1. Based on the determination of the response characteristic described with reference to FIG. 3 in detail, the method according to the invention now permits computer compensation of the measured data of the time series 29. This occurs according to the practical example described here by means of the formula:

(27) $p_{\mathrm{water}}=\frac{p\left(t\right)-p\left(t_0\right)e^{-\frac{t}{}}}{1-e^{-\frac{t}{}}}$

(28) In a typically discretely recorded measurement series 29 evaluation of this formula in practice occurs through the following iterative formula:

(29) $p_{\mathrm{corrected},t_{i+1}}=\frac{p\left(t_{i+1}\right)-p\left(t_i\right)e^{-\frac{t}{}}}{1-e^{-\frac{t}{}}}\mathrm{with}t=t_{i+1}-t_i$

(30) The times t.sub.i+1 and t.sub.i then denote values of the time series 29 occurring in succession, in which it must be kept in mind that t, i.e., the scanning rate, is chosen small enough that the partial pressure of the measurement gas in water 2 can be assumed constant within this time interval. This is therefore evaluated according to the partial pressure gradient in water 2.

(31) If one now applies the mentioned correction to the time series 29 of the measurement data recorded via the readout device 25 of the measurement gas sensor 5, the corrected measurement series 40 depicted in FIG. 2C is obtained, which reflects a correct chronological and therefore spatial assignment of the measured values to the measurement conditions explained in FIG. 2A with respect to partial pressure of the measurement gas. In particular, it is apparent that a rectangular jump function is shown in the initial area, which corresponds to the above described zeroing process. It is also apparent how a spatially correct assignment of the measured data to the area 29 with increased measurement gas concentration occurs, especially the raw measurement response 39.

(32) One skilled in the art will recognize that the data analysis, especially the regression analysis requires appropriate generally common data filtering.

(33) By means of the figures a device according to the invention as well as a method according to the invention for its operation are therefore described, which permits by means of in situ zeroing, i.e., in situ determination of the response behavior of the partial pressure sensor 1, improved measurement of the measurement gas in the surrounding gas. The improvement pertains in particular to an improved spatial and chronological assignment of the measured values to the real partial pressure conditions.

(34) The described flow through the measurement gas sensor 5 by corresponding switching of valve 16, 20 and subsequent determination of the response behavior can be conducted repeated in situ according to the invention, especially after surrounding parameters have changed. A conversion of the time series 29 by means of the response characteristic to the actual values 40 according to FIG. 2C then occurs based on the last determined response characteristic.

(35) Between such determinations of the response characteristic a computer correction of the response characteristic can also be conducted in the context of the invention by means of the values for temperature, moisture and pressure measured in the surrounding water 2, i.e., outside the partial pressure sensor 1, if a corresponding model on the dependence of the response characteristic on these parameters is used as a basis.

(36) The method described with reference to FIGS. 2 and 3 is implicitly based on the fact that the surrounding gas at the location and performance of the zeroing method according to the invention contains measurement gas with a non-negligible initial partial pressure 33. Since the partial pressure sensor 1 is a partial pressure sensor provided with a measurement gas sensor 5 for CO.sub.2, which is designed for detection of CO.sub.2 in the surrounding gas, this condition is generally the case during use of the partial pressure sensor 1 in water 2 described in the figures. A surrounding air dissolved in water normally has a partial pressure for CO.sub.2 lying above the detection limit of the measurement gas sensor 5.

(37) If, on the other hand, the zeroing method according to the invention is to be conducted at a location at which the measurement gas has an unduly low initial partial pressure 33 within the surrounding gas, which is smaller than the detection limit than the measurement gas sensor 5, the evaluation method described with reference to FIG. 3 cannot be conducted or not conducted with satisfactory results for the response characteristic. In such cases a method according to a modification of the invention with the schematically depicted partial pressure sensor 100 can be carried out in the context of the invention. The partial pressure sensor 100 according to FIG. 4 is designed to detect the partial pressure of methane gas as measurement gas in the surrounding gas. Methane gas, on the other hand, in many cases is normally not present above a detection limit in the surrounding gas.

(38) The partial pressure sensor 100 is designed essentially similar to the partial pressure sensor 1 according to FIG. 1. However, instead of measurement gas sensor 5 for CO.sub.2, which is used in the partial pressure sensor according to FIG. 1, the measurement gas sensor 50 is arranged, which is sensitive to methane gases as measurement gas in the surrounding gas. A correction gas sensor 51 is connected in series upstream of the measurement gas sensor 50 sensitive to methane gas in the inlet line 8. The correction gas sensor 51 is in principle a sensor similar to the measurement gas sensors 5, 50. However, the correction gas sensor 51 is sensitive to CO.sub.2 gas as measurement gas in the surrounding gas.

(39) If the zeroing method is now carried out by appropriate switching of valve 16, 20 for flow of the reference gas source 17, this means, as described at length above, that the inlet line 8 and therefore the correction gas sensor 51 and the measurement gas sensor 50 for CH.sub.4 is traversed with a gas from which the CO.sub.2 fraction was removed by passing through reference gas source 17 and which also contains no methane gas (CH.sub.4) from the outset. For the measurement signal of the measuring gas sensor 50, which reacts exclusively to methane gas as measurement gas, this does not lead to a change. Before, during and after zeroing the gas fed to the measurement gas sensor 50 contains no methane gas. The measurement signal which is recorded by the correction gas sensor 51 exhibits the behavior explained at length with reference to FIG. 3, which permits determination of the response behavior. Based on a calibration performed under laboratory conditions, the correction response characteristic of the reference gas sensor 50 determined in the described manner then permits a conclusion concerning the sought response characteristic of the partial pressure sensor 100.

(40) Although two series-connected gas sensors 50, 51 are schematically depicted in FIG. 4, one gas sensor can likewise be used, in which, in addition to the detector for the measurement gas, in this example methane, an additional detector for the reference gas, here CO.sub.2-free gas, can be used, which measures the partial pressure of the reference gas within the same measurement chamber.

LIST OF REFERENCE NUMBERS

(41) 1 Partial pressure sensor 2 Water 3 Housing 4 Membrane equilibrator 5 Measurement gas sensor for CO2 6 Inlet opening 7 Outlet opening 8 Inlet line 9 Pump 10 Heating device 11 Sensor for temperature and moisture 12 Pressure sensor 13 Outlet line 14 Pressure sensor 15 Sensor for temperature and moisture 16 Valve 17 Reference gas source 18 Sensor for temperature and moisture 19 Pressure sensor 20 Valve 21 Input 22 Control and evaluation unit 23 Control line 24 Signal line 25 Readout device 26 Instrument support 27 Sea bottom 28 Area with increased measurement gas concentration 29 Time series of measurement data recorded via the readout device 30 Abrupt drop 31 Reference plateau 32 Flank-like rise 33 Initial partial pressure 34 Calibration time interval 35 First section of the measurement series 36 Second section of the measurement series 37 Transition point 38 Linear regression line 39 Raw measurement response 40 Corrected measurement series 50 Measurement gas sensor for CH4 51 Correction gas sensor for CO2 100 Partial pressure sensor for CH4 525 Readout device 526 Readout device