GB2284049A

GB2284049A - Gaseous suspension particle size measurement

Info

Publication number: GB2284049A
Application number: GB9501130A
Authority: GB
Inventors: John Albert Pye; Philip Edward Stephens; Paul Steven Derbyshire
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-02-05
Filing date: 1991-03-19
Publication date: 1995-05-24
Anticipated expiration: 2011-03-19
Also published as: GB9501131D0; GB2284050B; GB9501130D0; GB2284050A; GB2284049B

Abstract

A radiation source 5 sequentially projects a beam of radiation at different wavelengths into a test space 7 containing the gaseous suspension to be measured. A sensor 6 arranged at an angle to the intersection of the beam with the test space 7 responds to radiation scattered by the particles. The particle size is determined from the respective amounts of radiation detected by the detector at the different wavelengths. The test space may be defined by the interior of a transparent tube through which the suspension is passed, the source and sensor being disposed on the exterior. An auxiliary sensor may compensate for build up of particles on the tube walls. Alternatively the beam may be directed into the exhaust plume of a motor vehicle, no physical containment being employed. <IMAGE>

Description

GASEOUS SUSPENSION PARTICLE SIZE MEASUREMENT This invention relates to a device for the measurement of particle sizes in a gaseous suspension, for example exhaust smoke particles.

It is likely that legal controls on the emission of smoke from diesel engined road vehicles will become increasingly stricter. To enforce these regulations, there will be a need for easy and accurate measurement of smoke density or particle concentration in the exhaust gas.

Devices in accordance with the present invention will permit accurate measurements to be obtained reliably without the need for a high degree of skill by the operator.

According to the invention, a device for measuring" the size of particles in a gaseous suspension, comprising source means for projecting a beam of electromagnetic radiation at a test space containing said gaseous suspension, and detector means for receiving radiation from the beam scattered by the particles, wherein the source means is arranged to project radiation at each of a plurality of wavelengths sequentially, and the device comprises processing means connected to the source means and the detector means for determining the size of the particles, or a distribution of particle sizes, from the distribution of the amounts of radiation detected by the detector at the different wavelengths.

The electromagnetic radiation is preferably light or infrared radiation.

The scattering of light depends upon a number of factors, including the size of particles causing the scattering. By measuring the intensity of scattered light at various angles, it is possible to determine the sizes of the particles causing the scattering. This may be done by having a number of detectors arranged to collect the light from the particular scattering angles. Such arrangements are the subject of our Patent Application No. (our case reference P/8620/APD/D2) and do not form part of the present invention. However, the angle at which the light is scattered is actually a complex function of the relationship between particle size and the wavelength of the light.

In accordance with the present invention an alternative to having a number of detectors is to have a single detector monitoring a particular angle and either arrange a transmission filter or prism so that only a narrow range of wavelengths reach the detector, or vary the wavelength emitted by the source in a controlled manner by selecting particular wavelengths from a broadband source. The average particle size so determined may be used to calculate the actual concentration of particles in the exhaust gases.

In accordance with one preferred embodiment of the invention, the beam from an optical source, which may be collimated by a suitable lens, is aimed at the region beyond the exhaust pipe of a motor vehicle, for example. The receiving system may be mounted within the same body as the optical source and is provided with a lens or a mirror to focus light scattered by the particles in the exhaust onto an optical receiver. The greater the concentration of smoke particles in the exhaust gases, the greater the light scattered or reflected by the particles, and therefore the higher the signal output from the optical receiver.

To enhance the rejection of ambient light, the light from the optical source may be modulated in accordance with any suitable wave form and at any of a wide range of frequencies. Typically, the modulation may be at a frequency of the order of tens of kilohertz. The receiving system may thus be arranged to accept only the light that is modulated at that frequency, either by filtering within the system to reject unwanted frequencies or by using synchronous detection, making use of a sample of the modulating signal in the receiving circuits. A combination of both techniques may be employed.- Optical filters could also be used to improve rejection of ambient light.

The transmitter and receiver could be combined in a single unit in which the outgoing beam and the focus of the receiving lens or mirror are directed to coincide in a defined region. Single, dual or stereoscopic sights could be used to aid alignment of the instrument on the plume of smoke emanating from the exhaust pipe.

In an alternative embodiment of the invention, the optical source and detector components are mounted in a housing surrounding an open ended transparent tube which may be connected to the exhaust pipe so that the exhaust gases pass therethrough. This has the advantage over the remote sensor in that it can make measurements in a more controlled manner that will be unaffected either by the skill of the operator in achieving satisfactory alignment or by environmental conditions. The tube will serve to protect the optical components and may be cleaned or replaced when deposits build up in it.

The device according to this alternative embodiment may be operated to measure scatter or reflection from the particles in the exhaust gases. Light from the source, which may be modulated, is directed into the region of the centre of the tube through which the exhaust gases pass. When no smoke is present, the signal from the detector or detectors positioned around the tube will be at a low level, and this will serve as a reference. When smoke particles are present, the light from the source will be scattered and some will fall on the detector or detectors, the signals from which will be processed to provide a suitable electrical output.

This processing may comprise any or all of the following: amplification; filtering; and synchronous or asynchronous detection. Optional modulation of the optical source with a suitable signal will aid detection and enable the effect of ambient light to be reduced or eliminated.

The detector or detectors may be positioned at an angle to the beam from the source so as to provide the optimum signal for the scattering characteristics of the sizes of particles in the smoke. Multiple detectors may be used so that the optimum angle for the particular smoke can be selected. Alternatively, the signals from more than one detector can be summed or the greatest of them selected.

At least one detector may be positioned to receive the unscattered light from the source. The signals from this transmission detector may be used to monitor the light output from the source and/or the attenuation caused by passage through the transparent tube, on which deposits may accumulate. These signals may be used either in a circuit to maintain a constant level of light received at the detector, or to provide compensation for reduced level of signal received by the scatter-measuring detectors.

A detector may be used to monitor transmission of the beam through the test space, i.e. the interior of the tube, by positioning it to receive some or all of the light from the source. When smoke is present in the tube, the signal from the circuit connected to the transmission detector would be reduced. The amount of reduction would be related to the density of the smoke in the tube.

The transmission sensor may be used either as a sensor in its own right, or as a sensor whose output signal is used to provide compensation for changes in optical or electrical characteristics of the scattering sensors as hereinbefore described.

The transmission may be initially calibrated in the absence of smoke.

A typical test for exhaust opacity of a diesel-engined commercial vehicle calls for the engine speed to be increased to the limit imposed by the governor, and then reduced. It is known that the result of the test is dependent on the way in which the test is carried out. The variability in results could be reduced if the engine speed could be simply and reliably measured. One possibility would be for the device in accordance with the invention to incorporate a microphone or pressure sensor to derive a variable signal dependent on engine speed, and to deduce engine speed therefrom.

It has been noted that there are distinct puffs of smoke in the exhaust, each being due to release of combustion products from successive cylinders of the engine. According to another aspect of the invention, the variation in scattering of radiation due to these puffs of smoke is used to deduce, with a knowledge of the member of cylinders in the engine, the engine speed.

The engine speed measurements may also be used to derive the acceleration or declaration of the engine, and thus may be used in turn, to ensure consistency of testing procedures for measuring exhaust gas opacity.

Because the engine speed can only vary from a few hundred to a few thousand r.p.m. and the rate of change is limited, a phase locked loop could be used to track the signals produced by the puffs. This would enable the sensor to operate with a relatively poor signal-to-noise ratio resulting from a relatively modest change in density of the exhaust for each puff.

To enable the opacity monitoring device, and the engine speed monitor, to function effectively, it is desirable to connect the measuring instrument to a vehicle exhaust pipe, for example, in a simple, reliable and consistent manner.

It is preferable that the means of connection does not allow significant amounts of clean, particle free, air to be drawn in to dilute the exhaust gases. Since vehicle exhaust pipes vary in size and shape, the means of connection must accommodate all likely variations. Suitable connections include a variable iris fitting over the exhaust pipe and preferably having resilient contact areas, a flexible diaphragm around the inlet to the device, and a torodial bag to seal the annulus between the device and the exhaust pipe, inflatable by compressed air or carbon dioxide, for example from a suitable cartridge, or by piping some of the exhaust from the device into the bag. The torodial bag could alternatively be arranged around the exterior of a sampling pipe inserted into the exhaust pipe.

Another possibility is to provide a small amount of restriction at the outlet end of the device. This would create a positive pressure in the region where the input aperture fits over the exhaust pipe. If the fitting were poor, there would be an outflow of exhaust, which would not be problem. The restriction could take the form of rigid petals or an elasticated or elastic-ended tube.

Additionally, since virtually all commercial vehicle exhaust pipes are made from steel, which is magnetic, the fit of diaphragms or collars as hereinbefore described could be improved by incorporating magnetic material into the diaphragm or collar, or using external magnets.

Exhaust gases resulting from the combustion of hydrocarbon fuels contain water vapour which, on exposure to cold air, condenses into water droplets which also absorb and scatter radiation. To reduce the effect of the water droplets, it is proposed to use radiation at two different wavelengths, from two sources, or a single source emitting at two wavelengths, with different detectors for the different wavelengths, for example using optical filters to achieve selectivity. The two (or more) wavelengths will be selected such that absorption or reflection/scattering by the water vapour differs from one to the other, but both wavelengths are absorbed or scattered by the smoke particles.

By suitable processing, the effects of absorption or scattering/reflection from the water droplets can be subtracted from the values obtained to give an accurate indication of the concentration of the smoke particles. This could be achieved by analogue summing/subtraction with suitable calibration, or by the use of a look-up table stored in a memory device.

Reference is made to the drawings, in which: Figure 1 is a diagram illustrating operation of a device useful for illustrating the operation of the invention; Figure 2 is a diagram illustrating another device illustrating the operation of an aspect of the invention; and Figures 3 and 4 illustrate successive stages in attachment of a device according to an embodiment of the invention to a vehicle exhaust pipe.

Referring first to Figure 1, which illustrates a device disclosed in patent application GB 2252621, from which the present application has been divided, a device comprises a body which may be dimensioned and arranged to be hand-held.

The body 1 has mounted on a face 2 thereof a pair of lenses 3 and 4, the first of these lenses 3 being arranged to project a preferably parallel beam of light from a light source 5 and the other lens 4 having a narrow field of view and focusing light onto a receiver 6. The lenses are aligned so that the focus of lens 4 intersects the beam from lens 3 at a predetermined distance d in front of the device. It may be desirable to provide for adjustment of the intersection point, and a range finding or other sight may be provided on the device to permit the point of intersection to be adjusted to coincide with the point of emission of exhaust gases from a vehicle exhaust pipe, such as indicated at 7. The amount of light detected by the receiver will be proportional to the opacity of the exhaust gas, and suitable calibration will permit the opacity to be directly read from a suitable indicator such as digital or analogue meter.

An arrangement in accordance with the invention determines particle size rather than opacity and has a single detector monitoring a particular angle. It is arranged that only energy at a particular wavelength reaches the detector.

This may be done using a transmission filter or a prism or by using a source whose output wavelength can be varied in a controlled manner, or by selecting particular wavelengths from a broadband source. The size of the particles, or a distribution of particle sizes, is determined from the distribution of the amounts of radiation detected by the detector at the different wavelengths.

The device illustrated in Figure 2 comprises an optically transparent tube 20 formed, for example, from a plastics or glass material which is preferably heat resistant. A light source 21 is mounted at one side of the tube 20 so as to direct a beam of light through the tube.

A detector 22 is positioned on the opposite side of the tube so as to receive light from the source 21 passing therethrough. A reference detector 23 is mounted adjacent to the source 21 to provide a reference signal having the same modulation as is imposed on the output of the source.

The signals from the detector 22 and the reference detector 23 are passed through separate, but identical, amplifiers 24 and 25 and filters 26 and 27. A signal from the reference detector 23 is also passed through a variable gain inverting amplifier 28 and the resulting signal is summed with the amplified and filtered signal from the detector 22 at a summing amplifier 29. It will be appreciated that the variable gain inverting amplifier may alternatively operate on the output from the detector 22.

With no smoke present, the gain of the inverting amplifier 28 would be adjusted so that the two signals present at the summing amplifier 29 are exactly equal and opposite, thus cancelling out, and hence the output of the summing amplifier 29 would be zero. When smoke is present, the signal level output from the detector 22 would be reduced and this would result in a residual signal output from the summing amplifier 29. This signal would be proportional to the residual signal and hence to the attenuation due to the smoke.

Modulation of the optical source is advantageous so that signals produced by ambient light could easily be eliminated by filtering. The summing amplifier 29 produces an output which would be at the source modulation frequency, when the output from the monitoring detector 22 is reduced by smoke.

The alternating signal from the summing amplifier 29 is converted to a d.c. level using, for example, an rms to dc converter 30. The d.c. output may be displayed by a meter 31. As an alternative to the rms to d.c. converter 30, an A.M. detector may be used.

The advantage of this arrangement is that there is a means of zeroing the instrument to compensate for changes in the optical path, and the reference and signal parts use identical circuits, so that any changes in characteristics should occur in both, and hence have no overall effect. In addition, most of the circuit is processing an a.c. signal, thus avoiding any d.c. drift problems.

Figures 3 and 4 illustrate one way of sealing a device in accordance with the invention on to an exhaust pipe. The device 40 has a collector portion 41 which is of greater diameter than the exhaust pipe 42. The annular space between them is sealed by a flexible diaphragm 43 between the rim of the portion 41 and a deformable ring 44 which fits tightly on to the pipe 42. A plurality of wires 45 are attached to the ring 44, and in Figure 4 are shown pulled outwardly to dilate the ring 44, permitting it to be placed over the exhaust pipe. Figure 5 shows the condition after the wires 45 are released, with the ring 44 firmly engaging the pipe 42 and the diaphragm 43 closing the annular gap. The diaphragm 43 is made sufficiently loose that, if there is a reduced pressure within the device, it can lie along the pipe to improve the seal.

Claims

1. A device for measuring the size of particles in a gaseous suspension, comprising source means for projecting a beam of electromagnetic radiation at a test space containing said gaseous suspension, and detector means for receiving radiation from the beam scattered by the particles, wherein the source means is arranged to project radiation at each of a plurality of wavelengths sequentially, and the device comprises processing means connected to the source means and the detector means for determining the size of the particles, or a distribution of particle sizes, from the distribution of the amounts of radiation detected by the detector at the different wavelengths.

2. A device according to claim 1, wherein the source means is arranged to modulate the radiation projected thereby in accordance with a modulating signal, and the processing means is arranged to select only a signal representing the modulated radiation.

3. A device according to claim 2, wherein the processing means comprises means for filtering unwanted frequencies.

4. A device according to claim 2 or claim 3, wherein the processing means is arranged to receive from the source means a sample of the modulating signal and to apply the sample to detect the received modulated signal synchronously.

5. A device according to any preceding claim, wherein the electromagnetic radiation is visible light.

6. A device according to any of claims 1 to 4, wherein the electromagnetic radiation is infra red radiation.

7. A device according to claim 5 or claim 6, wherein the detector means comprises an optical filter to filter out at least some of the unwanted ambient light.

8. A device according to any preceding claim, wherein the test space is defined by a tube, at least part of which is transparent, and the source means and detector means are mounted externally of the tube.

9. A device according to any preceding claim, wherein the processing means is also arranged to determine from the rate of variation in the radiation received by the detector, and from the number of pistons in the engine, the speed of rotation of an internal combustion piston engine.

10. A device for measuring the size or distribution of sizes of particles, substantially as described.