WO1998012542A1 - Procede et dispositif pour determiner numeriquement la concentration d'un gaz cible au moyen d'un systeme capteur optique de gaz - Google Patents

Procede et dispositif pour determiner numeriquement la concentration d'un gaz cible au moyen d'un systeme capteur optique de gaz Download PDF

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Publication number
WO1998012542A1
WO1998012542A1 PCT/US1997/016846 US9716846W WO9812542A1 WO 1998012542 A1 WO1998012542 A1 WO 1998012542A1 US 9716846 W US9716846 W US 9716846W WO 9812542 A1 WO9812542 A1 WO 9812542A1
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WIPO (PCT)
Prior art keywords
optical
sensor
concentration
gas
sensors
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PCT/US1997/016846
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English (en)
Inventor
Lucian E. Scripca
Mark K. Goldstein
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Quantum Group
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Priority to AU44918/97A priority Critical patent/AU4491897A/en
Publication of WO1998012542A1 publication Critical patent/WO1998012542A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
    • G01N21/78—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator producing a change of colour
    • G01N21/783—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator producing a change of colour for analysing gases

Definitions

  • optical gas sensors for detecting the presence of hazardous gases, especially carbon monoxide ("CO"), are known.
  • Exemplary optical gas sensors are described in U.S. Patents Numbers 5,063,164; 5,302,350; 5,346,671; and, 5,405,583 the contents of which are hereby incorporated by reference.
  • An improved optical gas sensor system has been made by optically combining gas sensors having a response over a wide range of humidity and temperature conditions as disclosed in U.S. Patent Number 5,618,493. the contents of which are also hereby incorporated by reference.
  • optical gas sensors include a self-regenerating, chemical sensor reagent impregnated into or coated onto a semi-transparent substrate.
  • the substrate is typically a porous monolithic material, such as silicon dioxide, aluminum oxide, aluminosilicates, etc.
  • the optical characteristics of the sensor change, either darkening or lightening depending on the chemistry of the sensor.
  • Battery powered, target gas detection devices utilizing optical gas sensors are commercially available and have met with great market success.
  • Such devices include at least one sensor placed in a light path between a light emitting means and a light detecting means.
  • the light detecting means monitors the optical characteristics of the sensor by measuring the level of light transmitted through the sensor.
  • Electronic components of the device are devised so that when the detected level of transmitted light falls below a predetermined fixed level, an alarm or other warning means is activated. Beyond the activation of a warning or alarm when hazardous conditions exist, users of such hazardous gas detection devices often have a need or desire to know other relevant information such as concentration, time weighted average (TWA), total dose received in a given time period, and rate of change of the concentration of the target gas.
  • TWA time weighted average
  • the concentration of a target gas is digitally determined using an optical gas sensor system having an array of optical gas sensors. Each of the sensors has a different concentration sensitivity range for the target gas.
  • the sensors in the array are exposed to an environment which may contain the target gas and an active sensor is selected from the array such that the active sensor sensitivity range corresponds to the target gas concentration.
  • a plurality of optical density values of the active sensor are determined and differentiated with respect to time between a first time and a earlier second time to determine the rate of change of the optical density between the two times.
  • the concentration of the target gas is calculated using the value of the rate of change and the value of the optical transmittance at the first time. If desired, a tangible output event can be provided when the concentration of the target gas exceeds a threshold representative of a hazardous condition.
  • FIG. 1 graphically illustrates the optical response of a three different optical gas sensors upon exposure to a target gas.
  • FIG. 2 schematically shows the preferred method of the present invention.
  • FIG. 3 is a block diagram of the basic components within an embodiment of a digital gas detection device utilizing the method of the present invention.
  • FIG. 4 is a elevational view of an optical gas sensor assembly used in an embodiment of the present invention.
  • FIG. 5 is a detailed circuit diagram of an embodiment of the present invention.
  • the present invention is drawn to a method and apparatus for digitally determining the concentration of a target gas using an optical gas sensor system. Accurate measurements made over a broad range of target gas concentrations are achieved by measuring the optical characteristics of a plurality of optical gas sensors contained within an optical gas sensor assembly.
  • Optical gas sensors can be made to detect a wide variety of hazardous gases.
  • carbon monoxide (CO) is used as an exemplary target gas to illustrate the principles of the present invention.
  • optical sensors for other hazardous gases such as ethylene oxide, mercury vapor, hydrogen sulfide, among others, may be substituted for or used in addition to the carbon monoxide sensor.
  • the response of an optical gas sensor to hazardous gas is determined by the formulation of the impregnating chemical sensing reagent and the characteristics of the substrate.
  • the amount of the light transmitted by the sensor which in the present case is a carbon monoxide (CO) sensor, has been found to have a linear static component and an exponential dynamic component.
  • the static component is expressed by the equation: wherein k j is a constant value dependent upon the sensor formulation; k2 is a constant value corresponding to the predetermined threshold of the sensor; I Q is the optical transmittance in the absence of CO; and I(t n ) is the optical transmittance at time t n .
  • the optical transmittance of the sensor becomes a constant value described by the above equation.
  • This equation reflects the establishment of an equilibrium between the chemical sensor reagent of the sensor and the CO present in the surrounding air.
  • concentration of CO can be readily determined given a value of I(t n ) and knowledge of k j , I Q and k 2 for that particular sensor formulation.
  • I(t n ) is the optical transmittance at time t n
  • I(t n . j ) is the optical transmittance at time t n.j previous to t n
  • [dl(t)/dt] is the first derivative of the optical transmittance over the time period t.
  • [CO] ⁇ k,[I 0 - I(t n )] + k 2 ⁇ exp ⁇ k 3 [dl(t)/dt] ⁇
  • [CO] is the concentration of CO
  • k j is a constant value dependent upon the sensor formulation
  • k 2 is a constant value corresponding to the predetermined threshold of the CO sensor
  • I 0 is the optical transmittance in the absence of CO
  • I(t n ) is the optical transmitt-ance at time t n
  • k 3 is a constant dependent upon the formulation of the chemical sensor reagent
  • [dl(t)/dt] is the first derivative of the optical transmittance over the time period t, also known as the rate of change in the optical transmittance.
  • [C0] ⁇ k,[i 0 - i(g] + k 2 ⁇ ⁇ i + k 3 [di(tydt] ⁇
  • concentration derived values such as time weighted average (TWA), theoretical dose, total exposure over a predetermined time period, etc.
  • TWA time weighted average
  • Other expansions for solving exponential equations may be used, but the Taylor series appears to be most efficient in terms of the computational power required for precision results.
  • the Taylor series converges rapidly and by solving for just the first three terms of the Taylor series, a precision of about 3 ppm can be obtained for gas concentration.
  • a broad range e.g. 10 ppm to 1000 ppm CO
  • a suitable impregnating chemical reagent is a mixture of chemical compounds containing at least one compound from each of the following groups: Group 1 comprising palladium compounds and their hydrates; Group 2 comprising molybdenum compounds and their hydrates; Group 3 comprising copper compounds and their hydrates; Group 4 comprising cyclodextrin molecular encapsulants; Group 5 comprising soluble chloride and bromide salts and their hydrates; Group 6 comprising halogenated acetic acid, and alkali metal and alkaline-earth metal salts of halogenated acetic acids; Group 7 comprising a soluble metal trifluoroacetylacetonate; .and Group 8 comprising heteropolymetallic acids, their salts and hydrates.
  • Preferred Group 1 materials include those selected from the group consisting of palladium sulfate; palladium sulfite; palladium pyrosulfite; palladium chloride; palladium bromide; calcium, sodium and potassium salts of the tetrachloropallidate, bromotrichloropallidate, dibromodichloropallidate, tribromochloropallidate and tetrabromopallidate ions; and mixtures thereof.
  • Preferred Group 2 materials include those selected from the group consisting of silicomolybdic acid; salts of silicomolybdic acid; molybdenum trioxide; -ammonium molybdate; alkali or alkaline earth salts of molybdate anion; and mixtures thereof.
  • Preferred Group 3 materials include those selected from the group consisting of copper sulfate; copper bromide; copper chloride; copper fluoride; copper iodide; copper trifluoroacetate; copper perchlorate; and mixtures thereof.
  • Preferred Group 4 materials include those selected from the group consisting of ⁇ -cyclodextrin; ⁇ -cyclodextrin; modified ⁇ -cyclodextrin; ⁇ -cyclodextrin; cyclodextrins
  • ⁇ _ -10 having an internal cavity of at least 5 A (5 x 10 m); and mixtures thereof.
  • Preferred Group 5 materials include those selected from the group consisting of sodium, lithium, platinum, magnesium, calcium, strontium, beryllium, barium, zinc and mixtures thereof.
  • Preferred Group 6 materials include those selected from the group consisting of trichloroacetic acid; tribromoacetic acid; the sodium, potassium, calcium salts of trichloroacetate; the sodium, potassium, calcium salts of tribromoacetate; and mixtures thereof.
  • Preferred Group 7 materials include those selected from the group consisting of copper, calcium, magnesium, sodium, potassium, lithium and mixtures thereof.
  • Preferred Group 8 materials include those selected from the group consisting of compounds having the formula H 4 SiM ⁇ j 2 . ⁇ E ⁇ ⁇ Q , where E is selected from the group consisting: chromium, vanadium, cobalt, manganese, iron, niobium, tantalum and tungsten, and x has value between 2 and 6; silicotungstic acid; and mixtures thereof.
  • the response threshold of a CO sensor can be controlled.
  • heteropoly acids, their salts and hydrates selected from compounds having the general formula H 4 SiM ⁇ j 2 . ⁇ E ⁇ O 40 , where E is selected from the group consisting: chromium, vanadium, cobalt, manganese, iron, niobium, tantalum and tungsten and x has value between 2 and 6; silicomolybdic acid; silicotungstic acid; and mixtures thereof.
  • the silicomolybdic acid retards the reduction of the molybdenum containing species by the palladium catalyst.
  • the sensor does not darken as readily upon exposure to low level concentrations of CO. Therefore by carefully adjusting the amount and the redox properties of the molybdenum containing compounds, the response threshold of the sensors can be controlled.
  • sensors can be formulated so as to respond (i.e. darken) upon exposure to CO at or above a predetermined response threshold. Using these principles, one skilled in the art can make sensors having a controlled response threshold in excess of 500 ppm CO.
  • FIG. 1 shows the typical response of three different sensors to 200 ppm CO over several hours.
  • Sensor 1 having a low response threshold (e.g. 10 ppm) crosses the alarm point (I re
  • Sensor 2 having a moderate response threshold (e.g.
  • Sensor 3 having a high response threshold (e.g. 150 ppm) darkens slightly over the course of two or more hours but not sufficiently to trigger an alarm response.
  • the present invention is directed to a method and apparatus in which an array of optical gas sensors are used to accurately determine the concentrations of target gas in the surrounding environment.
  • the method of the present invention includes selecting the active sensor from the optical gas sensor array, measuring a plurality of optical transmittance values of the active sensor, differentiating the plurality of measurements over time to calculate the rate of change in the optical transmittance of the active sensor between a first time and an earlier second time (i.e. dl/dt) and the value of optical transmittance of the active sensor at the first time (i.e. I(t)), and calculating the concentration of target gas at time t n as a function of both I(t) and dl dt.
  • the target gas concentration information is used to determine if a hazardous condition exists in the surrounding environment, thus triggering an alarm.
  • the calculated concentration value may also be displayed for the user or used to calculate derived values such as time weighted average (TWA), theoretical dose, exposure over a given time period, etc.
  • TWA time weighted average
  • U.S. Patent No. 5,573,953 discloses a method for determining carbon monoxide concentration in a self regenerating optical sensor by determining the rate of change of light transmission through the sensor. Transmitted light falls on a photodetector and the photodetector current charges a capacitor. The capacitor charge is sampled as a digital reading to determine the darkening of the sensor. Successive readings, in essence, differentiate over time to determine the rate of change of optical transmission of the sensor. The rate of change information is used to determine the concentration of carbon monoxide and whether an alarm should be sounded. This approach is employed to avoid measuring the absolute value of optical transmission of the sensor which may be influenced by variables in addition to carbon monoxide concentration.
  • An additional feature of the present method is the automatic selection of the active sensor from the optical gas sensor array. During this selection process, the optical transmittance is determined for each sensor. If the optical transmittance of the first sensor reflects 0% or 100% optical transmittance, that sensor is excluded and another reading is made on the next sensor in the array. If the first sensor has an optical transmittance between, but not equal to 0% or 100%, the first sensor is selected as the active sensor and a second measurement of optical transmittance I(t n ) is made after a predetermined time. The optical transmittance values of the active sensor are differentiated over time and the rate of change in optical transmittance is calculated. If at any time the optical transmittance of the active sensor becomes 0% or 100%, the search for an active sensor is initiated.
  • FIG. 3 An exemplary apparatus useful in the implementation of the earlier disclosed principles and the above method is schematically illustrated in FIG. 3. As shown, the apparatus comprises several functional "blocks", each block carrying out one or more tasks as generally described below.
  • the target gas sensing block 38 comprises a plurality of optical gas sensors 10, 12, 14 and 16 having different response thresholds which are optically aligned between a plurality of light emitting means 18, such as an array of infrared light emitting diodes (IR-LEDs), and a plurality of corresponding light detecting means 20, such as an array of photodiodes.
  • a plurality of corresponding light detecting means 20 such as an array of photodiodes.
  • Each target gas sensor is located so that the light generated by the matching IR-LED passes through the sensor.
  • the attenuated light transmitted by the sensor is measured by a corresponding photodiode.
  • the sensors are aligned so that the light passes axially through the sensor.
  • Each IR-LED-Sensor- Photodiode triad is assigned to a different measurement range.
  • the first set measures 30-100 ppm; the second set measures 100-200 ppm; the third set measures 200-400 ppm; and, the fourth set measures 400-1000 ppm. If desired the sensitivity ranges may overlap at the ends.
  • the optical gas sensors 10, 12, 14, and 16 are part of an optical gas sensor assembly illustrated in FIG. 4, and generally designated by arrow 22.
  • the optical gas sensor assembly 22 includes a housing 24 having a lower end 26 that holds the sensors in alignment between the LED and the photodiode arrays.
  • the housing 24 has an upper end 28, which is in fluid communication with the lower end, .and may contain filtering or gettering materials (not shown). Such filtering or gettering materials prevent dust, excess moisture, and other harmful agents from reaching the optical gas sensor 10, 12, 14, and 16.
  • the gettering materials and sensors are held in place by a perforated cap 30 which is secured by way of a retaining ring 32. Power is delivered to each infrared light emitting diode by a standard LED power circuit collectively designated as the LED power block 40.
  • a central microprocessor 42 controls and provides the necessary signals to the LED power block so that only the LED for the active sensor is powered. During the active sensor search mode, the central microprocessor ensures that at only one IR-LED has power at any particular moment.
  • the signal from the photodiode of the active sensor is amplified by an operational amplifier 44 and the amplified signal sent to an analog demultiplexer 46.
  • the central microprocessor sends a two bit binary address to the demultiplexer selecting the output signal for the active sensor.
  • the analog signal from the demultiplexer is converted to a digital signal by an analog to digital converter 48.
  • the central microprocessor also controls and provides the necessary signals to the analog to digital converter, such as enable and clock, and collects the eight bit word in a serial mode.
  • the eight bit byte is fed to the central microprocessor which adds a start bit and a stop bit to the eight bits received from the analog to digital conversion block and sends it to the math-processor and display block 50.
  • the math-processor and display block 50 Upon receipt of the input signal from the central microprocessor, the math-processor and display block 50 calculates a concentration value of CO using the equation previously discussed. The concentration value may be displayed on a digital display or used internally to calculate concentration derived values such as TWA, dose, etc. If the central microprocessor and the microprocessor in the math-processor and display block run independently, they communicate using a synchronization protocol. In this case, the math- processor and display block, spends most of the time driving the digital display, or processing the data into the equation. At the end of a cycle, a signal is sent by the math-processor and display block to the central microprocessor indicating that an information byte is to be sent and the central processor must start a data conversion cycle.
  • the central microprocessor sends a message back to the math-processor and display block, signalling that it is ready to transmit the data.
  • the data is transmitted and the math- processor and display block starts another mathematical processing cycle and display cycle.
  • the display cycles are interrupted, from time to time, for receiving another byte of information, however, the interruption is too short for a user to notice.
  • the calculated value is used to determine if a hazardous condition exists and if an alarm should be triggered. Should an alarm condition exist, the math-processor and display block sends an alarm signal to the central microprocessor. Upon receipt of an alarm signal, the central microprocessor activates the hex logic inverter in a oscillator configuration used in the enunciator block 52 to drive a piezo-electric speaker 54.
  • FIG. 5 A specific example of the above described digital gas alarm is shown in FIG. 5 using two eight bit microprocessors and an analog to digital converter to detect CO.
  • the target gas sensor block contains a set of four CO sensors designed to cover a range of 0 to 1000 ppm CO concentration.
  • the measurement range is divided into four ranges to improve accuracy of the calculated CO concentration while utilizing an inexpensive eight bit A/D conversion microprocessor.
  • An alternative is to use a 16 bit A/D converter and a sixteen bit central microprocessor.
  • the price of these devices are about 20 times higher than the eight bit version, therefore it is easier and more economical to divide the measuring range into four sub-ranges, e.g., 0 to 100 ppm, 100 to 200 ppm, 200 to 400 ppm and 400 to 1000 ppm.
  • the information byte When the information byte is received by the microprocessor in the math-processor and display block, it is first converted into a decimal number, loading the number into three different registers. For example, loading the number 123 so that the number 1 is in a register for hundreds, the number 2 is in a register for tens and the number 3 is in a register for units.
  • Eight registers are employed, four for the first operand and four for the second operand. Another four registers are used as a big work register. Since operations are much easier to implement into the program, the operands and the work register are set to have four registers: thousands, hundreds, tens and units in order to easily process large numbers.
  • the number 5535 must then be divided by 1000. This is easily done by shifting the work register three times to the right. The end result will be 0005, meaning a value of 5 will be found in the units register. In a similar way, the remaining calculations are made and the final result is moved to a set of display registers.
  • microprocessors components and other circuit elements can be used to implement the present invention, including those with built-in analog to digital converters, built-in liquid crystal display drivers and so on.
  • transmittance of light is used herein to illustrate the present invention, one skilled in the art would realize that other means of determining the optical characteristic of the sensor, such as absorption, reflection, refraction, etc. could be used. Therefore, such embodiments are considered to be within the scope of the present invention.
  • the application of the present invention to a wide variety of products is not intended to be limited to residential CO detectors. Other application are possible including, but not limited to: residential detectors, personnel monitors, medical gas monitoring units, breath diagnostic units, industrial HVAC control units, and combustion exhaust sensing devices.
  • the present invention has been described in relation to limited examples and embodiments which are for illustrative purposes and are not intended to limit the scope of the invention. Although a number of specific embodiments, methods, -and compositions have been described and illustrated herein, it will be apparent to one skilled in the art that further variations are possible. Thus, the present invention may be embodied and practiced otherwise than specifically described herein, and therefore the scope of the invention is defined by the following claims.

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Abstract

La concentration d'un gaz cible est déterminée dans un système capteur optique de gaz comprenant un ensemble (38) de capteurs optiques (10, 12, 14, 16) de gaz, chaque capteur ayant une plage de sensibilité différente pour un gaz cible. On sélectionne un capteur actif dans l'ensemble (38) en déterminant quel capteur a une transmittance optique comprise entre 0 et 100 %. La transmittance optique du capteur actif est différenciée par rapport au temps.
PCT/US1997/016846 1996-09-23 1997-09-19 Procede et dispositif pour determiner numeriquement la concentration d'un gaz cible au moyen d'un systeme capteur optique de gaz WO1998012542A1 (fr)

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AU44918/97A AU4491897A (en) 1996-09-23 1997-09-19 Method and apparatus for digitally determining the concentration of a target gas using an optical gas sensor system

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US2653496P 1996-09-23 1996-09-23
US60/026,534 1996-09-23

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017070819A1 (fr) * 2015-10-26 2017-05-04 Shanghai Eagle Safety Equipment Ltd. Systèmes et procédés de diagnostic de dispositif de surveillance de gaz personnel
WO2017139568A1 (fr) * 2016-02-11 2017-08-17 Honeywell International Inc. Film de sondage qui absorbe les gaz et réagit avec ceux-ci, avec lumière transmise pour une plus grande sensibilité au gaz

Citations (4)

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Publication number Priority date Publication date Assignee Title
US4464653A (en) * 1981-12-09 1984-08-07 The Bendix Corporation Combustible gas detection system
US4617277A (en) * 1984-03-23 1986-10-14 The Babcock & Wilcox Company Process and apparatus for monitoring ambient carbon monoxide
US5567622A (en) * 1995-07-05 1996-10-22 The Aerospace Corporation Sensor for detection of nitrogen dioxide and nitrogen tetroxide
US5573953A (en) * 1994-09-09 1996-11-12 Quantum Group, Inc. Method for enhancing the response of a biomimetic sensor

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4464653A (en) * 1981-12-09 1984-08-07 The Bendix Corporation Combustible gas detection system
US4617277A (en) * 1984-03-23 1986-10-14 The Babcock & Wilcox Company Process and apparatus for monitoring ambient carbon monoxide
US5573953A (en) * 1994-09-09 1996-11-12 Quantum Group, Inc. Method for enhancing the response of a biomimetic sensor
US5567622A (en) * 1995-07-05 1996-10-22 The Aerospace Corporation Sensor for detection of nitrogen dioxide and nitrogen tetroxide

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017070819A1 (fr) * 2015-10-26 2017-05-04 Shanghai Eagle Safety Equipment Ltd. Systèmes et procédés de diagnostic de dispositif de surveillance de gaz personnel
WO2017139568A1 (fr) * 2016-02-11 2017-08-17 Honeywell International Inc. Film de sondage qui absorbe les gaz et réagit avec ceux-ci, avec lumière transmise pour une plus grande sensibilité au gaz
EP4043867A1 (fr) * 2016-02-11 2022-08-17 Honeywell International Inc. Procede de detection d'un gaz utilisant un film de sondage qui absorbe les gaz et réagit avec ceux-ci, avec lumière transmise pour une plus grande sensibilité au gaz
US11788970B2 (en) 2016-02-11 2023-10-17 Honeywell International Inc. Probing film that absorbs and reacts with gases, with transmitted light for higher gas sensitivity

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