CN100394456C - Fire identification method and fire alarm implementing the method - Google Patents

Abstract

When, in addition to infrared light, blue light is additionally irradiated into the measurement volume, and the scattering occurring on the particles in the backscattering and forward scattering regions in the infrared and blue light ranges is measured and evaluated separately from each other, The sensitivity of the scattered light fire alarm to small particles can be significantly improved. This can be achieved by means of a fire alarm comprising two emitting LEDs (2.1a, 2.1b) and two photoelectric receivers (2.2a, 2.2b), where the four components are arranged so that the photoelectric receiving The detectors receive longer and shorter wavelengths of forward-scattered light and backscattered light separately from each other. A corresponding multichannel evaluation circuit is connected downstream of the optoelectronic receiver.

Description

Fire identification method and fire alarm implementing the method

technical field

The invention relates to the introduction of light rays with a first wavelength (Strahlung) along a first emission axis and light rays with a second wavelength shorter than the first wavelength along a second emission axis into the measuring volume according to the principle of scattered light, and A method of identifying a fire/flame by measuring the light scattered on particles located within the measurement volume at a forward scatter angle greater than 90° and a backward scatter angle less than 90°.

The invention also relates to a scattered light fire alarm for carrying out the method.

Background technique

Known from WO 01/07161 A1 is a diffuse light alarm with two LEDs (Light Emitting Diodes). In order to draw conclusions about the type of soot, the difference in the wavelengths of the light emitted by the two LEDs is chosen to be as large as possible.

Known from WO 01/59737 A1 is a kind of diffused light alarm that is specially used to be contained in ventilation channel and air-conditioning channel, it works according to the method mentioned above, and a first LED (light-emitting diode) combines infrared light and a The second LED injects blue light into its measurement chamber. The two LEDs are alternately pulsed. The light generated by "infrared" LEDs makes it possible to identify large particles that are common in dark (stuffy) fires (flameless fires). The scattering produced by the "blue" LEDs makes it possible to identify small particles that are common in fires with open flames. This can be explained by Reyleigh's law, according to which, for particles smaller than the wavelength, the intensity of scattered light decreases with the fourth power of the wavelength. While this is true, it is not practical when it comes to identifying fires on the basis of scattered light. This known fire alarm contains only a photoelectric receiver, which can only provide two kinds of information about the scattered light intensity, that is, depending on the implementation, the intensity of the forward scatter in the infrared or blue wavelength range Either the corresponding intensity of the backscattered light, or the intensity of the forwardscattered light in the infrared wavelength range and the backscattered light in the blue wavelength range. However, the respective arrangement geometries can lead to different measurement volumes from which the respective scattering emanates.

)。例如，高浓度的水蒸汽产生强的向前散射的信号，该信号按较早的现有技术就会触发在此情况下则为假警报的警报。不过，从向前散射的强度和向后散射的强度形成的商对水蒸汽得出一特征值，该值基本上与浓度无关。通过确定此商并在进一步的信号处理中考虑它，就可以由此抑制否则会产生的假警报。不过，该已知的方法和按照它工作的报警器与用红外线工作的散射光火灾报警器的所有其它结构有着共同的缺点，即对小的和非常小的颗粒的灵敏度不够。这首先是使之难于及时识别明火，尤其是其烟的特征是非常小的颗粒尺寸的木材火。因此，在相应的危险情况下，必须仍然采用对小的颗粒很好地起反应的离子火灾报警器，该报警器用弱放射性制剂工作。由于这种放射性制剂，离子化火灾报警器的制造花费大且其使用不受欢迎，在一些国家甚至一般是被禁止的。A fire warning method is known from DE 19902319 A1 in which the decision to warn is made on the basis of the ratio of the intensity of infrared forward scattered light to the intensity of infrared backscattered light. A corresponding fire alarm operates selectively with two infrared LEDs and a photoelectric receiver or vice versa with an infrared LED and two photoelectric receivers. The angle used to measure forward scattered light is preferably 140°, and the angle used to measure backward scattered light is preferably 70°. The formation of the ratio of the intensity of the forward scattered light to the intensity of the backward scattered light makes it possible to distinguish between bright and dark smoke types, since bright smoke provides a strong forward scattered signal and a weaker backward scattered signal. signal, whereas dark smoke provides a weaker forward-scattered signal compared to a stronger backscattered signal. Treating absolute intensities or signal levels and simultaneously processing these signal levels, taking into account that the intensity in the backward scattering region is substantially lower compared to the intensity produced by the same particle at the same concentration in the forward scattering region The ratio or quotient also makes it possible to distinguish certain spurious values of smoke (

). For example, a high concentration of water vapor produces a strong forward scattered signal which, according to the earlier prior art, would trigger an alarm which in this case would be a false alarm. However, the quotient formed from the forward-scattered intensity and the backscattered intensity yields a characteristic value for water vapor which is substantially independent of concentration. By determining this quotient and taking it into account in the further signal processing, otherwise false alarms can be suppressed thereby. However, this known method and the alarm device operating according to it have the same disadvantage as all other designs of scattered-light fire alarms operating with infrared radiation, namely the insufficient sensitivity to small and very small particles. This above all makes it difficult to identify an open flame in time, especially a wood fire whose smoke is characterized by very small particle sizes. In correspondingly dangerous situations, therefore, ion fire alarms which react well to small particles and which operate with weakly radioactive agents must still be used. Because of this radioactive agent, ionized fire alarms are expensive to manufacture and their use is frowned upon, and even generally banned in some countries.

Contents of the invention

The object of the present invention is to provide a method that can significantly improve the sensitivity of scattered light fire alarms to small particles with little additional expense, and thereby significantly improve the ability of this alarm to detect hot and very hot objects. Applicability in terms of fire without being at the expense of a higher frequency of false alarms.

In a method of the type mentioned at the outset, the object is achieved in that light rays of the first and second wavelength are incident into the measurement volume from opposite sides along coincident illumination axes, on opposite sides of the measurement chamber at the same main axis Scattered light at the first and second wavelengths is measured online.

This results in an advantageous geometric relationship, since the forward scattered light of the first and second wavelength is measured at the same forward scattering angle and the backscattering light of the first and second wavelength is measured at the same backward scattering angle. This thus limits the outlay for optoelectronic components on the one hand to two LEDs and two optoelectronic receivers, eg photodiodes, and on the other hand allows essentially the same electrical processing of all four measured values.

The point symmetry of the ray path with respect to the center of the measurement volume ensures that the measured scattered light intensities originate from the same measurement volume, which facilitates their comparability. In each measurement cycle, four measured values can be obtained in this way, which can be processed individually or combined with one another in order to enable a reliable alarm decision after comparison with specified reference values. Accordingly, the corresponding static value levels multiplied by a factor ≤ 1 are preferably subtracted from the signal levels corresponding to the four measured scattered light intensities, the resulting values are weighted, and evaluated in the logic (circuit) (Auswertelogik ), compare it with stored values, combine and evaluate the results of the comparison, and generate at least one alarm signal depending on the results (claim 2). Depending on the intelligence (processing) performed in the alarm, eg a pre-alarm signal, a smoke recognition signal, a main alarm signal etc. can be generated from the result.

In particular, the ratio of the weighted value of the forward-scattered light intensity to the weighted value of the back-scattered light intensity at the first wavelength and the weighted value of the forward-scattered light intensity to the back-scattered light intensity at the second wavelength can be formed Weighting the ratio between the values and processing in the evaluation logic, comparing with stored values, combining and evaluating the results of the comparison and generating at least one alarm signal depending on the result (claim 3).

It is further possible to form a weighted ratio of the forward scattered light intensities of the first and second wavelengths and a weighted ratio of the backscattered light intensities of the first and second wavelengths and process the determined ratios in the evaluation logic , comparing with stored values, combining and evaluating the results of the comparison, and generating at least one alarm signal depending on the result (claim 4).

Furthermore, it is again possible to carry out a ratio calculation on the obtained ratio itself, compare this result with the stored value, and take the comparison result into account in the further processing (claim 5).

It is expedient that the first wavelength and the second wavelength are chosen such that they are not in integer ratios to each other (claim 6). Because, when the first wavelength and the second wavelength become, for example, a ratio of 1:2, then particles which generate, for example, a particularly strong forward scattering signal at the first wavelength, also produce a second order when illuminated with the second wavelength. The largest overly strong signal. On the other hand, particles with a circumference equal to longer wavelengths which reflect particularly well absorb strongly at half a wavelength and thus generate hardly any scattered light.

According to the current manufacturing level of LEDs, it is appropriate to select the first wavelength in the range of infrared light and the second wavelength in the range of blue light or ultraviolet light (claim 7).

Preferably the first wavelength is in the range of 880 nm and the second wavelength is in the range of 475 nm or 370 nm (claim 8).

Suitably, the pulse/pause ratio of the light of the first and second wavelength is preferably in the range of 1:20000 (claim 9), since a high light intensity is required for sufficient sensitivity. The electrical power required for this not only loads the power supply/power supply of the alarm, but also leads to a strong heating of the light-generating chips of the LEDs, so that after each pulse a sufficiently long cooling time is required to avoid overheating.

In order to implement the method according to the invention, and thereby achieve the basic purpose, it is suitable to use a scattered light fire alarm, which has a measuring chamber communicating with the surrounding air and defining a measuring volume, an infrared emitting LED and a LEDs emitting blue light are irradiated into the measuring chamber from different directions, and the light scattered on the particles located in the measuring volume is measured photoelectrically and evaluated therein, wherein, according to the invention, the alarm contains two photoelectric Receivers, which are arranged opposite to the measuring volume and have a common main axis, with which the illumination axes of the two LEDs form an acute angle of less than 90° and intersect at a point on the main axis, which is located at Measure the center of the volume (claim 10).

The LEDs can also be arranged on the same side of the main axis (claim 11). Thus, one photoreceiver measures the forward scattered light of the infrared emitting LED and the back scattered light of the blue emitting LED, while the other photoreceiver measures the forward scattering of the blue emitting LED and the infrared emitting LED in reverse. backscattering.

Alternatively, the LEDs are arranged symmetrically about the main axis (claim 12), so that one photoreceiver measures the two forward scattered rays and the other photoreceiver measures the two backward scattered rays.

However, it is preferred that the LEDs are arranged point-symmetrically to the center of the measurement volume so that their illumination axes coincide (claim 13). Thus, both LEDs and photoreceivers are arranged in exact opposite pairs. This has the advantage that the four measured scattered light intensities each emerge from the same measurement volume. Furthermore, this symmetrical arrangement facilitates a substantially non-reflective configuration of the measuring chamber, makes it possible to arrange substantially symmetrically the circuit board on which the LEDs and photodetectors are arranged, and results in a rotational symmetry of the alarm which is thus Sensitivity that is at least substantially independent of the direction of air entry.

Preferably, the illumination axis of the LED and the main axis each enclose an acute angle of approximately 60° (claim 14). The respective backscattered ray is then measured at this angle, and conversely the corresponding forwardscattered ray is measured at a supplementary angle of 120°. It has been shown that this is an advantageous compromise between the value of 70° which is more favorable for measuring the backscattering and the diameter of the measuring chamber which decisively influences the outer diameter of the alarm.

In order to protect the photodetector from direct illumination of the LEDs and from light reflected onto the walls of the measuring chamber, and to keep the measurement volume lightly illuminated by reflected light, it is advisable to separate each LED and Each photoelectric receiver is placed in its own tube; moreover, outside the measuring volume, between the LED and the photoelectric receiver, a light barrier (Blende) and a light trap (Strahlungsfalle) are arranged (claim 15).

Description of drawings

The method according to the invention will be explained below with reference to the drawing, which shows three embodiments of corresponding scattered light fire alarms. In the picture:

1 shows a plan view cut at the level of the optical axis of a base plate of a fire alarm in a first embodiment, which supports the measuring chamber;

FIG. 2 shows a corresponding view of a second embodiment;

FIG. 3 shows a corresponding illustration of a third embodiment.

Detailed ways

The method according to the invention proceeds as follows.

Depending on the type of material combusted, a broad spectrum of combustion products forms, which for simplicity are called aerosols or also particles. Hot fires generate large amounts of small diameter aerosols. For example, an aerosol structure or cluster containing 100 _CO2 molecules has a diameter of approximately 2.5 nm. Fires with a small energy conversion per unit time, ie in particular so-called dark fires, by contrast, produce aerosols with a diameter of up to 100 μm and also partially macroscopic flotages such as small ash particles. Scattered light fire alarms suitable for detecting all types of fire must also be able to detect aerosols with a diameter of 2.5 nm to 100 μm, that is to say cover an area to the 5th power of 10.

Due to their high efficiency, only infrared-emitting gallium arsenide (GaAs) LEDs, which generate a wavelength λ of 880 nm, have been used as light sources for scattered light fire alarms in practice. The intensity of scattered light produced by a particle preferably depends on the ratio of the diameter of the particle, assumed to be spherical for simplicity, to the wavelength of the incoming light. Furthermore, although particle shape and absorption coefficient also play a role, these parameters apparently have no influence in this paper. For a particle diameter smaller than 0.1λ, the so-called Rayleigh scattering decreases proportionally to ^λ4 . It follows that for particle diameters of less than about 90 nm, fire alarms operating with infrared-emitting LEDs have a sharply reduced sensitivity. In addition, Rayleigh scattering is not omnidirectional but has obvious maximum values at 0° and 180°, and obvious minimum values at 90° and 270°. For particles with a diameter of 0.1 λ to 3 λ, that is to say in the case of infrared-emitting LEDs from about 90 nm to about 2.5 μm, it is the Mie scattering that is decisive, which is stronger than the Rayleigh scattering Depends on the direction and exhibits destructive and constructive interference by the interaction of the incident light and the light reflected on the particle. For (particle diameter) greater than 3λ, the scattered light intensity is substantially independent of the wavelength, but primarily depends on the type and shape of the particle.

It follows that the low sensitivity of scattered light fire alarms to hot fires, such as open wood fires, depends on the wavelength of the infrared light which is large relative to the diameter of the particles to be detected. This can neither be solved by increasing the amplification of the signal provided by the photoreceiver nor by increasing the intensity of the incident light, since, in both cases, for large and macroscopic particles, such as dust, from industrial The sensitivity of the alarm will be too high due to the steam of the process and the smoke of the cigarette.

As is known in principle from the aforementioned WO 01/59737, although it is possible to greatly improve the alarm by alternately illuminating the measuring volume with infrared light and blue light and by separately processing the signal proportional to the received scattered light Sensitivity to small diameter particles - especially particles for which Rayleigh scattering is dominant. It can be shown by calculation that the sensitivity can be increased by a factor of 10 or more. However, in order to obtain a reliable alarm decision, that is to say, to avoid false alarms or fraudulent alarms, it is not enough to merely increase the sensitivity of the alarm to small diameter particles. The assumption made in WO 01/59737 that illuminating the measurement volume with blue light would give scattered light with about the same intensity for large as for small particles is particularly untrue. Experiments in this area have instead shown that small particles provide very similar intensities of scattered light in the infrared and in blue light both in the forward illuminated region and - at low levels - in the rearward illuminated region. As has been further shown, only by adding the angular dependence of the scattered light intensity can a reliable criterion be obtained which makes it possible to distinguish fraudulent quantities from fire products substantially independently of the type of combustion substance.

According to the invention, therefore, four scattered intensities are measured in each measurement cycle, that is to say the intensities of forward scattered light and backward scattered light in the infrared range and the same in the blue range. Subtracting the corresponding static value level preferably with a safety discount (Sicherheitsabschlag) from the signal level proportional to the measured intensity (correspondingly multiplying the static value level by a factor < 1) in order to increase the measurement dynamics ( Messdynamik) and simplify further processing. The resulting value thus obtained is then compared in an evaluation logic with a stored value, in particular a threshold value. Additional information is obtained by forming a quotient of the resulting value and comparing again with a stored reference value. The results of this calculation can in turn be adapted, combined and evaluated, for example according to the respective environment in which the warning device operates. In this way, a series of informative intermediate results, for example for different early warnings, and finally also warning signals are obtained.

A preferred embodiment of a warning device suitable for carrying out the method is shown in FIG. 1 . A spherical measuring volume with a center 1.5 is defined on the base plate 1.7, indicated by a thin circle. An infrared-emitting LED 1.1a illuminates the measurement volume along a first illumination axis. Opposite it is a blue-emitting LED 1.1b which illuminates the measurement volume along a second illumination axis. The first and second illumination axes coincide. A main axis also runs through the center 1.5 of the measurement volume at an angle α=120° to the common illumination axis. First photodiodes 1.2a and 1.2b are arranged opposite each other on this main axis. Thus, the main axis of the respective receiving axes of the two photodiodes on which they are placed encloses an acute angle β=60° with the illumination axis of the "infrared" LED 1.1a. Correspondingly, the main axis encloses the same acute angle with the (second) illumination axis of the "blue" LED 1.1b. Thus, the photodiode 1.2a measures the infrared forward scattered light produced by the "infrared" LED 1.1a on the particles in the measurement volume at an angle of 120°, and at a backscattered angle of 60° from the "blue" Blue scattered light produced by LED 1.1b. Conversely, the photodiode 1.2b measures the blue forward scattered light produced by the "blue" LED 1.1b at an angle α of 120° and the infrared backward scattered light produced by the "infrared" LED 1.1a at a backscattered angle of 60°. Scatter light. To avoid interfering reflections, both the LED and the photodiode are located in a tube like 1.6. For the same reason, suitable shaped shutters 1.3a, 1.3b and 1.4a and 1.4b are arranged between the LED and the photodiode.

Further sensors are arranged on the base plate 1.7, for example a temperature sensor 1.8 and a gas sensor 1.9.

As usual, there is a circuit board under the substrate 1.7 for generating current pulses for the LEDs 1.1a and 1.1b and for processing the electrical signals given by the photodiodes 1.2a and 1.2b. Also as usual, the base plate 1.7 is housed in an alarm box (not shown) which allows an exchange between the ambient air and the air in the measurement chamber, but prevents extraneous light from entering the measurement chamber.

FIG. 2 shows a second embodiment of the warning device, which has the same components as in FIG. 1 , but with a different geometrical arrangement. To illustrate this point, the first digit of each reference number is replaced with "1" by "2" herein.

In contrast to FIG. 1 , only the radiation axes of the infrared-emitting LED 2.1a passing through the measuring center 2.5 coincide with the blue-emitting LED 2.1b. The receiving axis of the photodiode 2.2a forms an angle of α1=120° with the irradiation axis of the infrared-emitting LED 2.1a, and forms an angle of β2=60° with the irradiation axis of the blue-emitting LED 2.1b. Conversely, the receiving axis of the photodiode 2.2b forms an angle of β1=60° with the irradiation axis of the infrared-emitting LED 2.1a, and forms an angle of α2=120° with the irradiation axis of the blue-emitting LED 2.1b. Correspondingly, the first photodiode 2.2a measures the forward scattered light of the "infrared" LED 2.1a and the backscattered light of the "blue" LED 2.1b. In contrast, the second photodiode 2.2b measures the forward scattered light produced by the "blue" LED 2.1b and the backscattered light produced by the "infrared" LED 2.1a.

The photodiodes 2.2a and 2.2b can exchange their positions with the LEDs 2.1a and 2.1b, so that the two photodiodes can be precisely oppositely placed with respect to the measurement center 2.5.

The geometrical arrangement of the four components, ie two LEDs and two photodiodes, is somewhat disadvantageous compared to that according to FIG. 1 , since only 75% of the four measured scattered rays each emanate from the same measurement volume. This can be illustrated in terms of the intersecting surfaces between the individual ray bundles, which are shown greatly simplified, that is to say, omitting the angle dependence of the emitted light intensity and the sensitivity of the photodiode and at the inevitable edge Diffraction effect on. As in the embodiment, in an alarm comprising other sensors such as 2.8 and 2.9, the measurement center 2.5 is also placed very eccentrically with respect to the midpoint of the base plate 2.7. This has the consequence that the sensitivity of the alarm is not omnidirectional as in the first embodiment, but depends on the direction in which fire products enter the alarm and into its measuring volume.

FIG. 3 shows a third embodiment of the warning device, which has the same components as in FIG. 2 but with a different geometrical arrangement. To illustrate this point, the first digit of each reference number is replaced with "2" by "3" herein.

In contrast to FIG. 1 , only the reception axis of the photodiode 3.2a passing through the measurement center 3.5 coincides with the reception axis of 3.2b. This receiving axis forms the main axis. The "infrared" LED 3.1a encloses an acute angle β1 = 60° and an obtuse angle α1 = 120° with the main axis. The "blue" LED 3.1b and the "infrared" LED 3.1a are arranged opposite to the main axis, the "blue" LED correspondingly enclosing an acute angle of β2 = 60° and an obtuse angle of α2 = 120° with the main axis. Thus, photodiode 3.2a receives both infrared forward scattered light and blue forward scattered light, while photodiode 3.2b receives both infrared backscattered light and blue backscattered light.

In contrast to the situation in FIG. 2 , in this embodiment the two LEDs and the two photodiodes cannot be arranged interchangeably, since in this case both photodiodes simultaneously measure the forward scattered light of an LED and then The backscattered light of the other LED is measured, ie four measured values are provided, of which at least two are always approximately the same in pairs.

As in the case of FIG. 2 , in the embodiment according to FIG. 3 , only 75% of the four measured scattered rays each emanate from the same measurement volume. It is superior to the situation in Figure 2 in that when the alarm contains other sensors such as 3.8 and 3.9, the measurement volume is placed closer to the midpoint of the base plate 3.7 so that the sensitivity of the alarm is less strongly dependent on the entry of fire products into the alarm direction. Compared to FIG. 2 , it is also advantageous that, with the geometry of FIG. 3 , all baffles 3.3a, 3.3b and 3.4a, 3.4b are arranged close to the measurement volume and substantially symmetrically around it. However, other things being equal, the positioning of the "blue" LED 3.1b necessarily results in a larger diameter of the substrate 3.7 compared to FIG. 1 .

However, it applies to all embodiments, ie the scattered light is measured at an angle of 120° or at an angle of 60°. However, compliance with these perspectives is not a necessary condition for carrying out the method proposed by the present invention. It is only important that the angles be chosen so that on the one hand a sufficiently high intensity can be measured in the forward scattering direction and in the backward scattering direction and on the other hand that the Sufficiently different intensities are measured in the forward and backward scattering regions of the particles concerned.

Claims (14)

1. be used for according to the scattered light principle by in measurement volumes, injecting pulsedly along the light with first wavelength of the first emission axis with along the light of having of the second emission axis second wavelength shorter than first wavelength, and by to measure the method for discerning fire at the light that is positioned at scattering on the particle of measurement volumes dividually greater than 90 ° forescatering angle with less than 90 ° back scattering angle, it is characterized by, the light of first and second wavelength is incident to the measurement volumes along the irradiation axis that overlaps from opposite side, opposite side at measuring chamber is measured the scattered beam of first and second wavelength on same main axis, and dividually it is assessed.

2. method as claimed in claim 1 is characterized by: the quiescent value level that deducts the factor of corresponding multiply by≤1 from the signal level corresponding with four measured scattered beam intensity; Resulting value is weighted; And processing is compared with the value of storing through the value of weighting in evaluate logic, and compared result is carried out combination and assessed; And, produce at least one alarm signal according to the result.

3. method as claimed in claim 2 is characterized by: the weighted value and the weighted value and the ratio between the weighted value of scattered beam intensity backward of the forescatering light intensity of the ratio between the weighted value of scattered beam intensity and second wavelength backward that form the forescatering light intensity of first wavelength; And in evaluate logic, handle resulting ratio, and comparing with the value of storing, compared result is carried out combination and assessment; And produce at least one alarm signal according to the result.

4. method as claimed in claim 2 is characterized by: the ratio of weighted value that forms the intensity of scattered beam backward of the ratio of weighted value of forescatering light intensity of first and second wavelength and first and second wavelength; And in evaluate logic, handle resulting ratio, and comparing with the value of storing, compared result is carried out combination and assessment; And produce at least one alarm signal according to the result.

5. as the method for claim 3 or 4, it is characterized by: resulting ratio itself is carried out scale operation; The value of operation result and storage is compared; And when further handling, consider the result of the value comparison of described operation result and storage.

6. method as claimed in claim 1 is characterized by, and first wavelength and second wavelength are chosen to make them not become the ratio of integer each other.

7. as the method for claim 1 or 6, it is characterized by, in the scope of Infrared, choose first wavelength, in the scope of blue light or ultraviolet light, choose second wavelength.

8. a scattered light fire-alarm, the measuring chamber that has the qualification that is communicated with surrounding air one measurement volumes, one the emission ultrared LED and one the emission blue light LED from different directions to this measuring chamber internal radiation, and assess with the light of scattering on the photoelectric measurement particle in measurement volumes on the throne and to it therein, it is characterized by: alarm comprises two photelectric receivers, and they relatively are placed on the common main axis with respect to measurement volumes; The irradiation axis of two LED and this main shaft wire clamp one intersect less than 90 ° acute angle and on a point that is positioned on the main axis, and this point is positioned at the center of measurement volumes.

9. alarm as claimed in claim 8 is characterized by, and LED is arranged on the same side of main axis.

10. alarm as claimed in claim 8 is characterized by, and LED is symmetrical in main axis and arranges.

11. alarm as claimed in claim 8 is characterized by, LED is arranged to the center of point symmetry in measurement volumes, so that its irradiation dead in line.

12. alarm as claimed in claim 8 is characterized by, the irradiation axis of LED is about 60 ° acute angle respectively with main shaft wire clamp one.

13. alarm as claimed in claim 8 is characterized by, and each LED and each photelectric receiver are placed in its own pipe; In addition, in measuring chamber,, between LED and photelectric receiver, arrange shadow shield and light trap in the outside of measurement volumes.

14. alarm as claimed in claim 8, it is characterized by, first photelectric receiver receives the forescatering light of launching ultrared LED and the scattered beam backward of launching the LED of blue light, and second photelectric receiver then receives the scattered beam backward of the ultrared LED of emission and launches the forescatering light of the LED of blue light.

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