WO1999013303A1

WO1999013303A1 - Gas sensor

Info

Publication number: WO1999013303A1
Application number: PCT/NL1998/000521
Authority: WO
Inventors: Arie Draaijer
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 1997-09-09
Filing date: 1998-09-09
Publication date: 1999-03-18

Abstract

Described is a sensor (1) operating according to the light absorption principle, for detecting CO2. The sensor (1) comprises a housing (2) having therein a measuring chamber (3). On both sides of the measuring chamber (3) are arranged a source (20) and a detector (30) for measuring light (23). The source (20) is a bulk piece of dysprosium-doped lanthanum sulfide glass, which receives pump light (12) generated by an LED (10). The LED (10) can be fed by means of a solar cell (40). The detector (30) generates an electric signal which is representative of the amount of measuring light received (23), which amount is indicative of the amount of CO2 in the measuring chamber (3). The sensor (1) according to the present invention is very simple, compact, inexpensive and energy-saving.

Description

Title: Gas sensor

The invention generally relates to a gas sensor, the operation of which is based on light absorption. The invention particularly but not exclusively relates to a sensor for accurately detecting C0₂ and will therefore be described hereinbelow for such a practical example.

A gas sensor the operation of which is based on light absorption generally comprises a light source, a light detector sensitive to the light from that light source, and a measuring chamber arranged between the light source and the detector. Introduced into that measuring chamber is a specific amount of a gas sample to be examined. This gas absorbs part of the light emitted by the light source before it reaches the detector, the degree of absorption being indicative of the amount of gas in the chamber. In general, it is desired that the concentration of a specific gas component in a mixture can be measured, e.g. the concentration of C0₂ in air. A sensor designed specifically for that component therefore uses light the wavelength of which is in a predetermined wavelength range, that component showing an absorption in that wavelength range, while the other components of the gas mixture in that range do not show absorption. A first problem when designing such a sensor is therefore to find a wavelength that complies with the requirements imposed. A known wavelength suitable for measuring the concentration of C0₂ in air is infrared light having a wavelength of 4.3 μm. When carrying out a measurement, this wavelength is known to be free from disturbing absorption through other gases normally present in the atmosphere, such as nitrogen, oxygen, and water vapor. A second problem when designing such a sensor is, however, to realize a light source capable of generating the light having the desired wavelength. In conventional sensors, there is used for that purpose a thermal radiator, such as a filament, the emitted light of which is then filtered with a suitable filter. Such a technique has various drawbacks. In the first place, the filament must become very hot, so that the spectrum of the generated light contains the desired infrared light to a sufficient degree. Moreover, this method for generating measuring light is very inefficient, because a thermal radiator, such as a filament, "wastes" very much energy through radiation at other wavelengths. As for its - structure, this technique has the further drawback that a filament is rather susceptible to failure, and that an "ordinary" incandescent lamp cannot be used, because the glass bulb surrounding the filament is not transparent to the desired infrared light.

Recently, the use of a combination of a laser and a glass fiber has been proposed. In this connection, reference is made to the article "Spectroscopic date of the 1.8-, 2.9-, and 4.3-μm transitions in dysprosium-doped gallium lanthanum sulfide glass" by T. Schweizer et al . in Optics Letters, Vol. 21, No. 19, October 1996, pages 1594-1596. The glass fiber is made herein of doped chalcogenide glass, which has the intrinsic property of being capable of converting light having a wavelength of 800 nm to light having wavelengths of 1.3 μm and 4.3 μm. The required pump light of 800 nm is supplied by a laser. However, the use of a laser and a glass fiber has the drawback, inter alia, that such a sensor is relatively large and expensive. Also, the manufacture of a glass fiber requires a relatively complicated manufacturing process and relatively expensive apparatus for providing a melt and drawing a fiber from the melt.

There is, however, a need for a C0₂ sensor that is small, inexpensive and energy-saving . An example of a use for which such a sensor is very effective is an installation for automatically regulating the ventilation in a building or an individual space in a building. In order not to be dependent on mains supply and batteries, it is very desirable that such a sensor can be fed by means of a solar cell with a buffer accumulator. An important object of the present invention is to eliminate the drawbacks of the prior art and to provide a sensor that realizes the above desires.

According to an important aspect of the present invention, an LED is used as light source, and the converter used is a small bulk piece of doped chalcogenide glass.

These and other aspects, characteristics and advantages of the present invention will be explained by the following description of a preferred embodiment of a C0₂ sensor according to the invention, with reference to the accompanying drawing in which the single figure is a diagrammatic representation of the structure of a gas sensor according to the present invention.

In the figure a gas sensor according to the present invention is generally denoted by reference numeral 1. The sensor 1 comprises a housing 2, which defines a measuring chamber 3. The measuring chamber 3 is suitable for introducing a gas sample, e.g. by means of diffusion or forced supply of gas, as will be clear to a skilled person. The measuring chamber 3 has an inlet and an outlet for the gas to be examined, or an opening for diffusion, which, for simplicity's sake, will not be shown. Since the manner in which the measuring chamber 3 is filled with the gas to be examined is no subject of the present invention and knowledge thereof is not necessary for a skilled person to properly understand the present invention, this manner will not be described in more detail .

In the housing 2 is mounted an LED 10, which has a light-emitting surface 11 suitable for emitting light 12 having a first predetermined wavelength λ_x . Preferably, the LED 10 is a GaAs or GaAlAs LED. In the example to be described, in which the sensor 1 is intended to detect C0₂, the said first wavelength λ_x ranges between 800 and 900 nm. Since the nature and structure of such an LED is no subject of the present invention and knowledge thereof is not necessary for a skilled person to properly understand the present invention, this LED will not be described in more detail. It is only observed that LEDs are known per se, and that a skilled person knows how to design an LED, so that it emits the desired wavelength; an LED emitting light of 890 nm is, e.g., commercially sold by the firm of OPTEK (Texas USA) under type number OP290. The dimensions of this commercially sold LED are about 5 mm thickness and 8 mm length.

An advantage of the use of an LED is that an LED can be controlled by relatively short current pulses, so that the average energy consumption may be very low.

A further advantage of an LED is that it is less expensive than, e.g., a laser or a laser diode.

In the housing 2 is further mounted a piece 20 of a luminescent wavelength-converting glass. The piece of glass 20 has a first main surface 21 directed to the LED 10 to receive the light 12. A second main surface 22 located opposite the first main surface 21 is directed to the measuring chamber 3. The composition of the glass 20 is such that the light 12 is absorbed and is converted to light 23 having a second predetermined wavelength λ₂. In the example to be described, in which the sensor 1 is intended to detect C0₂, the piece 20 is preferably made of dysprosium-doped gallium lanthanum sulfide glass, which converts the light ^'12 of 800 nm to light 23 having a wavelength λ₂ of about 4.3 μm. Preferably, the glass 20 has the following composition: 70% Ga₂S₃ : 30% La₂S₃ + 10,000 ppm Dy₂S₃.

It is observed that glass having such a composition is known per se, e.g. from the said publication. The known glass is provided in the form of a fiber, i.e. produced by melting the glass-forming constituents and drawing a thin fiber from the melt, for which the process parameters are to be selected such that the refractive index over the cross-section of the fiber has a specific desired profile. Such a process, however, requires relatively complicated apparatus, relatively much energy (heating) , and such a fiber is

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drawing, but that it is possible to change or modify the embodiment shown of the sensor according to the invention within the scope of the inventive concept. Thus, for instance, it is possible that the sensor is designed for detecting a gas other than C0₂, e.g. NO_x or H₂0 or alcohol . Although the structure of the sensor then remains the same, the wavelength used will be adapted to the gas to be detected, for which purpose the material of the piece of glass 20 and, if required, the wavelength λ_x of the pump light 12 will have to be adapted, as will be clear to a skilled worker when perusing the specification and using general expert knowledge. By way of example, for detecting H₂0 the wavelength to be used may be about 1.8 μm, and for detecting an alcohol the wavelength to be used may be about 2.9 μm.

Finally, it is observed that although the present invention has been described for the examination of gas mixtures, the principle of the present invention is likewise applicable to liquids.

Claims

1. A sensor (1) for measuring the amount or concentration of a predetermined component in a fluid, comprising: a measuring chamber (3) for receiving a sample of the fluid to be examined; an LED (10) for generating pump light (12) ; a piece of bulk glass (20) arranged near the measuring chamber (3) , for receiving the pump light (12) and for generating luminescence light (23) in the measuring chamber (3); a detector (30) arranged near the measuring chamber (3) , opposite the piece of bulk glass (20) , for receiving measuring light passed through the measuring chamber (3) and for generating an electric measuring signal indicative of the amount of light received.

2. A sensor according to claim 1, wherein the fluid is C0₂, and wherein the piece of bulk glass (20) is designed for converting the pump light (12) to light (23) having a wavelength (╬╗₂) of about 4.3 ╬╝m.

3. A sensor according to claim 1 or 2 , wherein the piece of bulk glass (20) is made of dysprosium-doped chalcogenide glass .

4. A sensor according to any one of the preceding claims, wherein the LED (10) is designed for generating pump light (12) having a wavelength (╬╗-) of about 800 nm.

5. A sensor according to claim 1, wherein the fluid is H₂0, and wherein the piece of bulk glass (20) is designed for converting the pump light (12) to light (23) having a wavelength (╬╗₂) of about 1.8 ╬╝m.