PHOTOACOUSTIC MEASURING APPARATUS

Abstract: Photoacoustic measuring apparatus intended for gas and gas mixtures, comprising a pulsed or modulated radiation source (4) adapted to illuminate a gas volume (10) to be detected or be subjected to measurement, and a photoacoustic detector (1). The photoacoustic detector (1) is filled with an amount of gas or gas mixture of the same kind as the one (10) to be detected or measured.

Description: Photoacoustic measuring apparatus This invention relates to a photoacoustic measuring apparatus intended for gases and gas mixtures, comprising a pulsed or modulated radiation source adapted to illuminate a gas volume to be detected or be subjected to measurement, and a photoacoustic detector. Photoacoustic techniques are based on a heating effect being known as the photothermal effect. These techniques utilize the principle that absorbed radiation energy, in particular infrared (IR) radiation, results in pressure variations in a confined gas volume, whereby the pressure variations are proportional to the absorbed amount of energy. These pressure variations can then be detected by means of a sensitive pressure transducer. In the apparatus according to the invention it is an advantage to employ a photoacoustic gas detector of the kind described in the applicant's application No. 95.0505. This particular gas detector comprises a chamber for receiving the gas or gas mixture concerned, a path for pulsed or modulated IR radiation into, through and out of the chamber, and a pressure sensor being adapted to measure pressure changes in the chamber resulting from applied IR radiation. The present invention, however, is not restricted to such type of gas detector or IR radiation. Also visible light or ultraviolet radiation, possibly relatively broad-band illumination can be employed in connection with this invention. At this point there is reason to note that gas detectors made of silicium are transparent to IR radiation, but not to visible light. The photoacoustic techniques can be used in the case of gas molecules having two or more atoms and having absorption lines for vibration/rotation for example in the infrared range between 2 and 25 μ . Common and previously known measuring methods based on the photoacoustic effect require a fixed or confined volume of the gas or gas mixture to be subjected to measurement, which means that there is a requirement for instruments com¬ prising a gas cell for the gas sample concerned. Detection of low gas concentrations necessitates the use of large gas cells and an infrared or laser source of high power. Moreover the gas cell must be filled with the gas or gas mixture con¬ cerned, and accordingly external equipment is required, such as pumps, valves and particle filters. A measurement or sensor apparatus being in this way based directly on the photoacoustic principle, therefor will be expensive and cumbersome in dimensions. Examples of known applications of such photoacoustic techniques are to be found, inter alia, in patent public- ations US-H651 and an article by C. F. Dewey Jr, R. D. Kamm, and C. E. Hackett: Acoustic amplifier for detection of atmos¬ pheric pollutants, Appl. Phys. Lett., Vol. 23, No. 11, Decem¬ ber 1973. It is an object of this invention to provide a novel and improved photoacoustic measuring apparatus not being sub¬ jected to the limitation discussed above and being based on a photoacoustic detector with an inherent or built-in selec¬ tivity. According to the invention this is obtained primarily thereby that the photoacoustic detector is filled with an amount of gas or gas mixture of the same type as the one to be detected or measured. According to a particularly preferred embodiment of the apparatus according to the invention, the detector is filled with a controlled amount of the gas or gas mixture concerned. Moreover according to the invention it is possible to let the gas volume to be detected or measured, be located between the source of radiation and the detector. A further more parti¬ cular feature being comprised by the invention consists in the provision of a reference detector in association with the above and essential photoacoustic detector in the measurement apparatus. In the following description the invention shall be explained more closely with reference to the drawings, where: Fig. 1 schematically and simpified shows a first embodiment of the measuring apparatus according to the in¬ vention, and fig. 2 shows another and modified embodiment of the measuring apparatus. A number of elements are incorporated in both embodiments being shown in the drawings, and these are the following elements: A radiation source 4, for example an IR source being provided with a reflector 4A for directing a radiation beam 2 through a measurement volume 10 of the gas or gas mixture concerned, towards the actual photoacoustic detector. This comprises a closed cavity l having an optical window 6 for the radiation or illumination 2 from the source 4, so that an amount of gas or gas mixture being received in the cavity or receptacle l, can be illuminated. The cavity can be provided in a housing of silicium or other transparent material, where the cavity can be formed by means of common semiconductor planar techniques, so called micromachining. The cavity l must be able to be sealed in a completely tight manner after the introduction og the gas or gas mixture concerned into it. If the housing or a wall thereof is made of silicium, for example the window 6, this will constitute an optical window in the IR range. A mirror device (reflector) 11 advantageously can be arranged at the end of the cavity 1 so as to send "unused" radiation out of the detector. With such a reflector, thermal absorbtion in the walls will be avoided. There are provided two microphones 3A and 3B which can be in the form of sensitive pressure transducers or sensors, so as to detect or measure pressure signals caused by the photo¬ acoustic effect during measurement. A controlled or well defined and constant amount of gas or gas mixture of the same type as the one to be detected or measured, is introduced into the cavity l before it is sealed. A necessary pre- requisite is that the gas shall have a constant pressure, i.e. partial pressure, which is not necessarily equal to the pressure in the gas or measurement volume 10. The output signals from both microphones 3A and 3B are passed through a circuit 8A for summing or other combination of the two signals, to an electronic measurement circuit which can comprise more or less comprehensive signal process¬ ing and besides a display or print-out of the measurement results. The modified embodiment in fig. 2 comprises in particular a reference detector 9 receiving illumination from the source 4 after propagation of the radiation through the cavity l of the detector, this being here provided with another optical window 16 in a wall opposite to the first window 6, so that the radiation path from the source 4 goes directly through the main detector and reaches the reference detector 9. This detector accordingly will measure a reference signal which makes it possible to correct for drift in the radiation source 4 or undesired masking effects being caused for exam¬ ple by dust or smoke in the measurement volume 10. Thus in this case it is a necessary precondition that the illumi¬ nation or radiation is not fully absorbed in the cavity 1 of the main detector, but escapes at the back of it through the window 16 as shown. In fig. 2 there is further shown a mea¬ surement or electronic circuit 18 to which the reference detector 9 is also connected. It is obvious that a referece detector can be located and oriented in other ways in relation to the main detector 1 and the source 4, than what is illustrated in fig. 2. Even though it is an advantage that the reference detector receives its proportion of the illumination after propagation through the main detector, a reference detector may also be useful for the above mentioned correction, when located in a different manner, for example so that it receives its illumination more or less directely from the source 4. A photoacoustic detector in a measuring apparatus as described above, has a manner of operation being in short as follows: The detector generates a signal being proportional to the photoacoustic effect in the cavity 1 of the detector, whereby the signal will be a maximum when there is no gas or gas mixture present in the measurement volume, i.e. between the source 4 and the detector 1. A determination of or a measure of the gas concentration in the measurement volume 10 is derived by taking as a starting point the difference between the maximum signal mentioned and the measurement signal. The concentration will be substantially proportional to this difference, provided that only neglictable saturation effects are present in connection with the measurement. Above the invention has been described and illustrated by way of rather simple embodiment, being characterized among other things by a direct and rectilinear radiation path between the source 4 and the detector cavity 1 as well as a possible reference detector 9 (fig. 2) . It is obvious that more specific and complicated arrangements of the whole measuring apparatus or configuration can be contemplated, which depends upon the situation, the location and other conditions to which the measurements shall be adapted. For example it will be possible by means of mirrors to establish radiation paths between source and detector with various branches or deflections in order to illuminate one or more measurement volumes in the total radiation path between source and detecor. Moreover it is obvious that the actual structure of the detector as such, with its reseptacle or cavity 1, being filled with the known amount of gas or gas mixture as mentioned, can be varied in many ways. This for example applies to the number of microphones, which can be one, two or more. As mentioned in the introduction of this description, it may also be an advantage in many applications to employ a gas detector of the kind described in patent application No. 95.0505 mentioned above.

Claims: C l a i m s 1. Photoacoustic measuring apparatus intended for gases and gas mixtures, comprising a pulsed or modulated radiation source (4) adapted to illuminate a gas volume (10) to be detected or be subjected to measurement, and a photoacoustic detector (1) , c h a r a c t e r i z e d in that the photoacoustic detector (1) is filled with an amount of gas or gas mixture of the same kind as the one (10) to be detected or measured. 2. Apparatus according to claim 1, c h a r a c t e r i z e d in that the photoacoustic detector (1) is filled with a controlled amount of said gas or gas mixture, preferably determined by the partial pressure of the gas or gas mixture. 3. Apparatus according to claim l or 2, c h a r a c t e r i z e d in that the photoacoustic detector (1) is adapted to be so located and oriented in relation to the radiation source (4), that said gas volume (10) can be located in the radiation path (2) between the radiation source (4) and the detector (1) . 4. Apparatus according to claim 1, 2 or 3, c h a r a c t e r i z e d in that it is provided with at least one reference detector (9) which is also located and oriented so as to receive illumination (2) from the radiation source (4) . (Fig. 2) 5. Apparatus according to claim 4, c h a r a c t e r i z e d in that the photoacoustic detector (1) is adapted to let applied illumination (2) pass through it (16), in order to illuminate the reference detector (9). 6. Apparatus according to claim 5, c h a r a c t e r i z e d in that the photoacoustic detector (1) is provided with a first window (6) for applied illumin¬ ation (2) and in direct continuation of the radiation path, a second window (16) for alowing said illumination to escape towards the reference detector (9) . 7. Apparatus according to any one of claims 1-4, c h a r a c t e r i z e d in that in the photoacoustic detector (l) there is provided a reflecting surface (11) adapted to redirect a substantial portion of the incident illumination back and out of the detector (1) .

Inventors: BERNSTEIN RALPH W, FERBER ALAIN, BERNSTEIN, RALPH, W., FERBER, ALAIN
Applicants: BERNSTEIN RALPH W, FERBER ALAIN, NYFOTEK AS, BERNSTEIN, RALPH, W., FERBER, ALAIN, NYFOTEK A/S
Application year: 1996
Main classification: G01N21
IPC codes: G01N21