Particle-measuring system and method of determining particle-mass concentration in an aerosol

Abstract

A particle-measuring system for determining particle mass concentrations in aerosols has a laser diode serving as a radiation source and projecting a beam of laser light through a flowing stream of the aerosol. A receiver for receiving the light from the diode after passing through the stream and converting the received light into a measurement. A frequency radiation output of the laser diode is modulated such that the frequency is substantially greater than a cutoff frequency of the receiver so that a specifiable radiation output of the laser diode is achieved on average over a duration of a measurement signal of the receiver.

Claims

1. A particle-measuring system for determining particle mass concentrations in aerosols, the system comprising: a laser diode serving as a radiation source and projecting a beam of laser light through a flowing stream of the aerosol; a receiver for receiving the light from the diode after passing through the stream and converting the received light into a measurement; a driver for modulating a frequency radiation output of the laser diode such that the frequency is substantially greater than a cutoff frequency of the receiver so that a specifiable radiation output of the laser diode is achieved on average over a duration of a measurement signal of the receiver; and an evaluation device for selecting a modulation depth of the modulation of the radiation output is selected such that a high number of operating modes passed through and that a single particle-measurement signal is generated by the receiver.

2. The particle-measuring system defined in claim 1, wherein an operating current of the laser diode is modulated by the driver in order to modulate the radiation output thereof.

3. The particle-measuring system defined in claim 1, wherein the frequency of the modulation of the radiation output of the laser diode exceeds the cutoff frequency of the receiver by a factor of at least ten.

4. A method of determining particle mass concentrations in aerosols, the method comprising the steps of: projecting light through an aerosol from a laser diode; receiving the projected light after projection through the aerosol in a receiver and converted the received light into a measurement; modulating a radiation output of the laser diode at a frequency that is substantially higher than a cutoff frequency of the receiver so that a specifiable radiation output of the laser diode is achieved over a duration of a measurement signal of the receiver; and passing the radiation output of the laser diode through a high number of operating modes while the receiver generates a single particle-measurement signal.

5. The method defined in claim 4, wherein the radiation output is modulated by modulating an operating current of the laser diode.

6. The method defined in claim 4, wherein the frequency of the modulation of the radiation output of the laser diode exceeds the cutoff frequency of the receiver by a factor of at least ten.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

(2) FIG. 1 is a schematic view of a first embodiment of the invention; and

(3) FIG. 2 is a similar schematic view of a second embodiment of the invention.

SPECIFIC DESCRIPTION OF THE INVENTION

(4) As seen in FIG. 1, a first embodiment of an optical particle-measuring system 1 according to the invention is embodied as an aerosol photometer (APM) and serves to determine the particle mass concentration in an aerosol.

(5) The aerosol photometer 1 has a laser diode serving as monochromatic light source 2. The light radiation emitted by the laser diode 2 of the aerosol photometer 1 is concentrated in an optical lens 3. The light beam leaving the optical lens 3 traverses a gas stream 4 that entrains the aerosol to be measured. Light is reflected from the particles of the aerosol contained in the gas stream 4 toward an additional optical lens 5. The two optical lenses 3 and 5 determine the measurement volume 6 shown schematically in FIG. 1. The light radiation directed at a photodetector 7 serving as receiver due to the particles present in the measurement volume or in the measuring chamber 6 and focused by the optical lens 5 is detected by the photodetector 7, and a photometer measurement corresponding to the detected light radiation is forwarded to an evaluation unit 17 of the optical particle-measuring system 1 that is not shown in the drawing.

(6) The photometer measurement forwarded by the photodetector 7 of the aerosol photometer 1 to the evaluation unit 17 corresponds to the particle load present or detected in the measurement volume 6.

(7) In the case of the aerosol photometer 1 of the embodiment described with reference to FIG. 1, a great advantage resides in the fact that the measurement detected in the evaluation unit 17 is independent of the flow rate of the gas stream 4 carrying the aerosol to be measured. In the case of the aerosol photometer 1, the measurement volume is determined by the optical measurement volume.

(8) An embodiment of the optical particle-measuring system shown in FIG. 2 has a single-particle counting photometer 10. The single-particle counting photometer 10 also has a laser diode serving as monochromatic light source 11. The laser diode 11 emits light radiation that is focused by an optical lens 12. The light beam that is focused in the optical lens 12 traverses a gas stream that carries the aerosol to be measured. A measurement volume 14 of the single-particle counting photometer 10 is substantially smaller than the measurement volume of the aerosol photometer 1. In the embodiment of the single-particle counting photometer 10 of FIG. 2, this is achieved in that the light emitted by the laser diode 11 is focused much more intensely by the optical lens 12 than is achieved by the optical lens 3 of the aerosol photometer 12. The measurement volume or measuring chamber 14 of the single-particle counting photometer 10 is dimensioned in consideration of the expected values of aerosols to be measured, so that only a single particle of the aerosol is present therein. The light radiation reflected in the measurement volume 14 of the single-particle counting photometer 10 is directed through an optical lens 15 at photodetector serving as a receiver of the single-particle counting photometer 10 that is located in the radiation path behind the optical lens 15. For each individual particle of the aerosol that travels through the measurement volume or measuring chamber 14 of the single-particle counting photometer 10 with the gas stream 13, a single measurement corresponding to a single particle is thus forwarded at the photodetector 16 of the single-particle counting photometer 10 to an evaluation unit 17 of the single-particle counting photometer 10. Each individual measurement corresponds to the light reflected by a single particle of the aerosol to be measured and directed through the optical lens 15 at the photodetector 16 of the single-particle counting photometer 10.

(9) In contrast to the aerosol photometer 1 described above in connection with FIG. 1, the single-particle counting photometer 10 detects individual particles. Such single-particle counting photometers 10 are used to measure comparatively low particle concentrations, for example in interior spaces. Within the scope of their area of application, i.e. at comparatively low to medium particle concentrations usually between 1000 and 20,000 particles/cm.sup.3, high-quality information can be obtained about the particle count and the particle size distribution in the aerosol.

(10) The laser diode 2 or 11 of the two above-described particle-measuring systems 1, 10 is not operated or driven at a single, predetermined operating point in a stable operation. Rather, in the case of the laser diodes 2, 11 of the particle-measuring systems 1, 10, the operating current of the laser diodes 2, 11 is modulated by a driver 18 such that the laser diodes 2, 11 pass through a wide range of different operating conditions in a very short time.

(11) The modulation depth is selected such that a very large number of operating states or operating modes are passed through, for example 100 operating modes. Due to the high modulation frequency, the time spent in a single operating mode is much shorter than the time required for the generation of a single particle-measurement signal of the receiver 7, 16.

(12) For example, the modulation frequency can be set up as follows:

(13) The receivers 7, 16 have a cutoff frequency of about 100 kHz. In order to ensure that no appreciable artifacts of the modulation of the laser-diode operation appear in the particle-measurement signal, it is specified that the frequency of the modulation of the operation of the laser diodes 2, 11 is at least ten times the amplifier cutoff frequency of the photodetectors 7, 16. If ten operating modes are passed through during one modulation period due to the modulation depths of the laser diodes 2, 11, the shortest time interval of the jumps between the operating modes is 100 kHz10102=20 MHZ. The duration of an operating mode is thus 0.05 s, and a single operating mode no longer occurs as an artifact behind the photodetectors 7, 16.