Abstract
Particle measurement apparatus comprises an inlet for receiving a gas sample for analysis, a photoionisation chamber, at least one light source arranged to illuminate an interior of the photoionisation chamber, first and second electrodes coupled to a power source and configured to provide a DC potential difference across at least a portion of the photoionisation chamber, and an outlet, together defining a gas flow path from the inlet, through the photoionisation chamber, and towards the outlet.
Claims
1. Particle measurement apparatus comprising an inlet for receiving a gas sample for analysis, a photoionisation chamber, at least one light source arranged to illuminate an interior of the photoionisation chamber, first and second electrodes coupled to a power source and configured to provide a DC potential difference across at least a portion of the photoionisation chamber, and an outlet, together defining a gas flow path from the inlet, through the photoionisation chamber, and towards the outlet.
2. The apparatus of claim 1 further comprising at least one electrode current sensor configured to measure an electrode current flowing to or from at least one of the first and second electrodes.
3. The apparatus according to claim 2 further comprising a charged particle detector and at least one detector current sensor configured to measure a detector current flowing from the charged particle detector.
4. The apparatus according to claim 3 further comprising a comparator circuit configured to compare the electrode current, measured by the at least one electrode current sensor, and the detector current, measured by the at least one detector current sensor.
5. The apparatus according to claim 3 further comprising a processor configured to process the electrode current, measured by the at least one electrode current sensor, and the detector current, measured by the at least one detector current sensor, to determine a particle concentration and/or size parameter.
6. The apparatus according to claim 1, wherein the potential difference provided by the first and second electrodes is variable between at least first and second different DC voltages.
7. The apparatus according to claim 6, wherein the potential difference provided by the first and second electrodes is maintainable at each of the at least first and second different DC voltages for at least 0.5 seconds.
8. The apparatus according to claim 6, wherein the potential difference provided by the first and second electrodes is variable between the at least first and second different DC voltages at a frequency of less than 2 Hz.
9. A method of measuring particles in a gas sample, the method comprising: directing the gas sample through a photoionisation chamber; illuminating the gas sample within the photoionisation chamber; and concurrently applying a DC potential difference between first and second electrodes across at least a portion of the photoionisation chamber through which sample gas flows.
10. The method according to claim 9 further comprising the step of: measuring an electrode current flowing to or from one of the first and second electrodes.
11. The method according to claim 10 further comprising the steps of: directing sample gas exiting the photoionisation chamber towards a charged particle detector; and measuring a detector current flowing to or from the charged particle detector.
12. The method according to claim 11 further comprising the step of: comparing the measured electrode current and the measured detector current.
13. The method according to claim 11 further comprising processing the measured electrode current and the measured detector current to determine a particle concentration and/or size parameter.
14. The method according to claim 9 further comprising the step of: varying the potential difference between the first and second electrodes across the at least a portion of the photoionisation chamber through which sample gas flows, the potential difference being varied between at least first and second different DC voltages.
15. The method according to claim 14 further comprising: maintaining the potential difference between the first and second electrodes at each of the at least first and second different DC voltages for at least 0.5 seconds.
16. The method according to claim 14, wherein, in the step of varying the potential difference between the first and second electrodes, the potential difference is varied between the at least first and second different DC voltages at a frequency of less than 2 Hz.
17. Particle measurement apparatus comprising an inlet for receiving a gas sample for analysis, a photoionisation chamber, first and second electrodes, at least one light source arranged to illuminate an interior of the photoionisation chamber between the first and second electrodes, at least one electrode current sensor configured to measure an electrode current flowing to or from at least one of the first and second electrodes, and an outlet, together defining a gas flow path from the inlet, through the photoionisation chamber between the first and second electrodes, and towards the outlet.
18. A method of measuring particles in a gas sample, the method comprising: directing the gas sample through a photoionisation chamber between first and second electrodes; illuminating the gas sample within the photoionisation chamber between the first and second electrodes; and measuring an electrode current flowing to or from one of the first and second electrodes.
19. Particle measurement apparatus comprising an inlet for receiving the gas sample for analysis, a photoionisation chamber, at least one light source arranged to illuminate an interior of the photoionisation chamber, a plurality of electrodes coupled to at least one power supply and configured to provide at least two concurrent potential differences across respective portions of the photoionisation chamber, and an outlet, together defining a gas flow path from the inlet, through the photoionisation chamber, towards the outlet.
20. A method of measuring particles in a gas sample, the method comprising: directing the gas sample through a photoionisation chamber; illuminating the gas sample within the photoionisation chamber; concurrently applying two or more potential differences, by way of a plurality of electrodes, across respective portions of the photoionisation chamber through which sample gas flows; and measuring one or more electrode currents flowing to or from at least one electrode from the plurality of electrodes.
Description
DESCRIPTION OF THE DRAWINGS
[0136] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:
[0137] FIG. 1 is a plan view of a particle sensor of the first and second example embodiments of the invention;
[0138] FIG. 2 is a schematic cross sectional view of the particle sensor of FIG. 1;
[0139] FIG. 3 is a schematic illustration of negatively charged gas ions being trapped by an ion trap of the particle sensor of FIGS. 1 and 2;
[0140] FIG. 4 is a schematic illustration of negatively charged gas ions and positively charged particles being trapped by an ion trap of the particle sensor of FIGS. 1 and 2;
[0141] FIG. 5 is a schematic cross sectional view of a particle sensor of the third example embodiment of the invention;
[0142] FIG. 6 is a schematic cross sectional view of a particle sensor of the fourth example embodiment of the invention;
[0143] FIG. 7 is a schematic cross sectional view of a particle sensor of the fifth example embodiment of the invention;
[0144] FIG. 8 is a graph of collector current and average charge per particle as a function of gas sample flow rate through the photoionisation chamber of an experimental sensor similar to the fourth example embodiment of the invention;
[0145] FIG. 9 is a graph of collector current and electrode current as a function of gas sample flow rate through the photoionisation chamber of the experimental sensor of FIG. 8;
[0146] FIG. 10 is a graph of collector current, normalised with respect to particle diameter, as a function of particle concentration in the experimental sensor of FIG. 8;
[0147] FIG. 11 is a graph of collector current, normalised with respect to particle concentration, as a function of particle diameter in the experimental sensor of FIG. 8;
[0148] FIG. 12 is a graph of the particle diameter exponent β as a function of the applied potential difference between the first and second electrodes in the experimental sensor of FIG. 8;
[0149] FIG. 13 is a graph of the electrode current and the collector current as a function of the applied potential difference between the first and second electrodes in the experimental sensor of FIG. 8, the graph further indicating the electrode current when no particles flow through the photoionisation chamber;
[0150] FIG. 14 is a graph of the collector current, and an electrode current compensated for background signal due to photoionisation chamber housing wall photoemission, as a function of applied potential difference between the first and second electrodes in the experimental sensor of FIG. 8;
[0151] FIG. 15 compares the particle diameters estimated using the experimental sensor of FIG. 8 with the known particle diameters for a range of samples each having different particle diameters;
[0152] FIG. 16 compares the particle concentrations estimated using the experimental sensor of FIG. 8 with the known particle concentrations for a range of samples each having different particle concentrations;
[0153] FIG. 17 compares the particle surface area estimated using the experimental sensor of FIG. 8 with the known particle surface area for a range of samples each having different particle surface areas;
[0154] FIG. 18 is a schematic illustration of negatively charged gas ions being trapped by an ion or particle trap of the particle sensor of FIG. 2 or particle sensor or air filter of FIG. 19; and
[0155] FIG. 19 is a schematic cross sectional view of a particle sensor or air filter of the seventh example embodiment of the invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
First Example Embodiment of the Invention
[0156] In a first example embodiment, as illustrated in FIGS. 1 and 2, a particle sensor 1 comprises a sensor body 2 made of aluminium. A sample gas inlet 3 comprises an inlet passageway 4 which extends into the body towards a first end 5. A pump 6 is coupled to the inlet passageway 4 and is configured to pump sample gas into the sensor body through the inlet passageway. A sample gas outlet 7 comprises an outlet passageway 8 which extends out of the body towards a second end 9 opposite said first end 5. The outlet passageway 8 connects the sensor body to a collector chamber 10. A second outlet passageway 11 extends away from the collector chamber 10.
[0157] The interior of the sensor body 2 is hollow. The interior of the sensor body 2 forms an elongate photoionisation chamber 12 (see FIG. 2). A first planar electrode 13, made of stainless steel, is provided on a first internal surface of the photoionisation chamber 12 extending between the inlet 3 and the outlet 7. A second planar electrode 14, also made of stainless steel, is provided on a second internal surface of the photoionisation chamber 12 extending between the inlet 3 and the outlet 7, opposite the first electrode 13. The first and second electrodes 13,14 are both connected to an earthed DC power supply 15 in series with an ammeter 16. The first electrode 13 is connected to the negative terminal of the power supply 15. The second electrode 14 is connected to the positive terminal of the power supply 15.
[0158] A photoionisation lamp 17 is provided at the first end of the sensor body. The photoionisation lamp 17 is coupled to a power supply (not shown). The photoionisation lamp is configured to emit, in use, ultraviolet light having wavelengths within the range 150 nm-260 nm. The photoionisation lamp is oriented such that emitted ultraviolet light illuminates the entirety of the photoionisation chamber 12.
[0159] A Faraday cup electrometer 18 as is generally known in the field of particle detectors is mounted in the collector chamber 10. The Faraday cup electrometer 18 typically consists of an electrically conductive filter (not shown) mounted within a wire mesh (not shown). The Faraday cup electrometer 18 is positioned within the collector chamber 10 such that flow of gas from the photoionisation chamber 12 and into the collector chamber 10 passes through the Faraday cup electrometer 18. In use, any charged particles exiting the photoionisation chamber 12 through the outlet 7 are incident on the Faraday cup electrometer 18. An electrical outlet of the Faraday cup electrometer is connected to earth 19 in series with an ammeter 20.
[0160] In use, a gas sample for analysis is pumped into the sensor through the inlet by the pump. Sample gas consequently flows sequentially through the inlet passageway, into and through the photoionisation chamber from the first end towards the second end, and through the outlet passageway to the outlet. The pump is configured to pump the gas sample through the sensor at a known, generally constant gas flow rate.
[0161] The power supply connected to the photoionisation lamp 17 is switched on such that the lamp illuminates the photoionisation chamber 12 with ultraviolet light. The gas sample generally comprises a mixture of gaseous atoms and/or molecules and particles suspended in the gas. The gas molecules and/or atoms, as well as the particles, are typically uncharged (i.e. electrically neutral). As the gas sample flows through the photoionisation chamber, particles in the gas sample absorb one or more photons of ultraviolet light emitted by the photoionisation lamp. The wavelength of the ultraviolet light is selected such that absorption of a photon by a particle causes that particle to emit a photoelectron. The wavelength of ultraviolet light emitted by the photoionisation lamp is therefore below a threshold wavelength for particle photoionisation (that is to say, each photon emitted by the photoionisation lamp has an energy above a threshold energy required for particle photoionisation). Absorption of a photon and consequent emission of an electron causes a particle to acquire a net positive charge. Each electron which is emitted by a particle on photoionisation generally proceeds to ionise a nearby gas atom or molecule, thereby forming a negatively charged gas ion. Photoionisation of the particles in the gas sample takes place between the first and second electrodes 13,14.
[0162] When the power supply 15 connected to the electrodes 13,14 is switched off, after a characteristic period of time (dependent on, inter alia, the concentration of particles in the gas), negatively charged gas ions and positively charged particles in the gas sample in the photoionisation chamber 12 are attracted to one another and tend to come into contact with one another, resulting in the transfer of electrons back from the ions to the particles, thereby neutralising both charges in a process called recombination. Consequently, only a portion of the particles in the gas sample exiting the photoionisation chamber 12 at the second end 9 and flowing over the Faraday cup electrometer 18 in the collector chamber 10 are electrically charged. While a collector signal measured by the ammeter 20 is detectable, the strength of the signal is typically only partially correlated to the number and size of the particles in the gas sample. Therefore, the sensitivity of the particle detector 1, when the power supply 15 is switched off, is relatively poor.
[0163] However, when the power supply 15 connected to the electrodes 13,14 is switched on, a substantially uniform DC potential difference V is provided across the photoionisation chamber between the first and second electrodes 13,14. The applied potential difference V typically generates an electric field of between around 1 V/cm to around 100 V/cm across the photoionisation chamber. This mode is illustrated schematically in FIG. 3. Photoionisation of the particles in the gas sample takes place in the electric field between the first and second electrodes 13,14. In this mode, negatively charged gas ions 21 formed on photoionisation of particles in the gas sample are less likely to recombine with positively charged particles 22. The electric field provided between the first and second electrodes exerts a force on the negatively charged ions 21, causing these ions to drift away from the particles towards the second electrode 14 (which functions as an anode) as they flow through the photoionisation chamber 12. In addition, any free photoelectrons which have not combined with gas atoms or molecules to form gas ions will also drift toward the second electrode 14 under the action of the electric field. The electric field also exerts a force on the positively charged particles 22, causing these particles to drift towards the first electrode 13 (which functions as the cathode) as they flow though the photoionisation chamber 12. Nevertheless, because the negatively charged gas ions 21 and any free electrons are relatively more mobile, only the ions and electrons tend to reach the second electrode 14, thereby generating an electrode current which is measured by the ammeter 16. The ions and the free electrons are thus at least partially removed from the flow of gas through the photoionisation chamber 12. The first and second electrodes 13,14 function as an ion trap embedded within the photoionisation chamber.
[0164] In contrast, because the positively charged particles 22 are significantly less mobile than the gas ions, the charged particles 22 tend to flow through the photoionisation chamber 12 and through the outlet without being captured by the ion trap. If the photoionisation of particles in the gas sample within the photoionisation chamber 12 is sufficiently high for detectability, the flux of charged particles onto the Faraday cup electrometer 18, and thus the collector signal measured by the ammeter 20, is proportional to a combined parameter N.sup.αD.sup.β, in some instances proportional to a measurement of the total effective particle surface area in the gas sample, where N is the concentration of particles in the gas and D is the average particle diameter. The sensor may therefore be calibrated in order to provide a measurement of this total effective particle surface area. The total effective particle surface area measured by the sensor in this mode is more accurate than that detected when no potential difference is applied between the first and second electrodes 13,14.
[0165] In this first example embodiment, the measured anode current and the measured collector current are both typically dependent on the total effective particle surface area. The gas sensor may therefore be calibrated to output a total effective particle surface area based on measurement of either the electrode current or the collector current.
Second Example Embodiment of the Invention
[0166] In a second embodiment of the invention, the output of the DC power supply 15 in FIG. 2 is varied in use so as to vary the DC potential difference provided between the first and second electrodes 13,14 between a first voltage V.sub.1 and a second, larger voltage V.sub.2 having the same polarity as V.sub.1.
[0167] When the first voltage V.sub.1 is applied between the first and second electrodes 13,14, the sensor operates in essentially the same mode as outlined above for the first embodiment of the invention and as illustrated in FIG. 3. Negatively charged gas ions 21 and free photoelectrons generated by photoionisation of sample gas in the photoionisation chamber 12 drift towards the second electrode 14 and are captured by the ion trap. Positively charged particles 22 are not captured by the ion trap and instead exit the photoionisation chamber through the outlet 7, being collected by the Faraday cup electrometer 18. Both an electrode current measured by the ammeter 16 connected between the first and second electrodes and a collector current measured by the ammeter 20 connected to the Faraday cup electrometer are indicative of a total effective surface area of particles in the gas sample.
[0168] When the potential difference between the first and second electrodes 13,14 is increased to voltage V.sub.2, however, the force exerted on the negatively charged gas ions 21, the free electrons and the positively charged particles 22 in the gas sample flowing through the photoionisation chamber 12 is increased. This typically results in a greater proportion of the negatively charged gas ions 21 and free electrons being trapped by the second electrode 14, resulting in both an increased electrode current. Additionally, as illustrated schematically in FIG. 4, some of the positively charged particles 22 may also now drift towards and reach the first electrode 13, thereby further contributing to the electrode current. Smaller, less massive negatively charged particles 23 are most likely to be captured by the ion trap and to contribute to an increased electrode current. Consequently, fewer charged particles 22 exit the photoionisation chamber 12 and are incident on the Faraday cup electrometer 18, resulting in a decreased collector current measured by the ammeter 20. The electrode and detector currents depend sensitively and non-linearly on the magnitude of the applied potential difference.
[0169] For a given applied voltage between the first and second electrodes 13,14 and a given flow rate of sample gas through the photoionisation chamber 12, the dependence of the electrode current on the particle concentration N and the mean particle diameter D can be modelled according to
I.sub.e=α.sub.eN.sup.α.sup.eD.sup.β.sup.e. (1)
[0170] The inventors have found that dimensionless parameters α.sub.e and β.sub.e are functions of the magnitude of the applied potential difference and typically take values between 0.5 and 3.
[0171] Similarly, the dependence of the collector current on the particle concentration N and the mean particle diameter D can be modelled according to
I.sub.c=α.sub.cN.sup.α.sup.cD.sup.β.sup.c. (2)
[0172] Again, the inventors have found that dimensionless parameters α.sub.c and β.sub.c are functions of the magnitude of the applied potential difference and typically take values between 0.5 and 3.
[0173] Accordingly, by measuring both the electrode current and the collector current at a first applied voltage V.sub.1 and at a second applied voltage V.sub.2, equations (1) and (2) may be solved simultaneously to determine both N and D. Consequently, both the particle concentration and the mean particle diameter may both be determined independent of one another from one set of measurements, providing more detailed information about the particles in the gas sample than can be determined using only the first embodiment of the invention.
[0174] In a variation of the second example embodiment of the invention, the potential difference applied between the first and second electrodes is varied between a plurality of different voltages (for example, the potential difference may be varied between 10 different voltages). For each additional voltage, the electrode and the collector currents change, thereby providing two more equations (1) and (2) which may be solved to determine N and D. The inventors have therefore found that by increasing the number of different voltages applied sequentially across the photoionisation chamber, the accuracy to which the particle concentration and mean particle diameter may be determined is increased.
Third Example Embodiment of the Invention
[0175] In a third embodiment of the invention, as illustrated in FIG. 5, first, second and third cathodes 24,25,26 are provided along a first side of the photoionisation chamber 12 of the particle detector 1. First, second and third anodes 27,28,29 are provided along a second side of the photoionisation chamber 12, opposite said first, second and third cathodes respectively. Each cathode-anode pair is electrically connected to one another via a respective variable DC power supply 30,31,32 in series with a corresponding ammeter 33,34,35. The variable power supplies 30,31,32 are configured to provide DC potential difference V.sub.3 between the first cathode 24 and the first anode 27, DC potential difference V.sub.4 between the second cathode 25 and the second anode 28, and DC potential difference V.sub.5 between the third cathode 26 and the third anode 29. V.sub.3, V.sub.4 and V.sub.5 each have the same polarity; V.sub.4 is larger than V.sub.3, and V.sub.5 is larger than V.sub.4.
[0176] In use, a gas sample is pumped by the pump through the inlet 6, into and through the photoionisation chamber 12, and out through the outlet 7. Particles in the gas sample are photoionised by ultraviolet light emitted by the photoionisation lamp 17, thereby forming positively charged particles and negatively charged gas ions. As the particles and ions flow through the photoionisation chamber 12, they are deflected away from a general flow direction by the applied potential differences. As the gas flows between the first cathode-anode pair, all of the gas ions are trapped by the first anode 27. In this region, the potential difference V.sub.3 is however not strong enough to trap any of the positively charged particles. As the positively charged particles subsequently flow through the stronger potential difference V.sub.4 provided between the second cathode-anode pair, some of the smaller charged particles are trapped by the second cathode 25. Additionally, as the remaining positively charged particles subsequently flow through the even stronger potential difference V.sub.5 provided between the third cathode-cathode pair, even more of the smaller particles and some of the larger particles are trapped by the third cathode 26. Any remaining positively charged particles in the gas flow not trapped by any of the three electrode pairs flow out of the photoionisation chamber 12 and are detected by the Faraday cup electrometer 18.
[0177] Accordingly, four currents are measured: three electrode currents corresponding to the current flowing between each of the three anode-cathode pairs and one detector current. Each of these four currents may be related to the particle concentration and mean particle diameter in the gas sample according to the following equations:
I.sub.e1=α.sub.e1N.sup.a.sup.e1D.sup.β.sup.e1, (3)
I.sub.e1=α.sub.e2N.sup.α.sup.e2D.sup.β.sup.e2, (4)
I.sub.e3=α.sub.e3N.sup.α.sup.e3D.sup.β.sup.e3, (5)
and
I.sub.c=α.sub.cN.sup.α.sup.cD.sup.β.sup.c, (6)
where the subscripts e1, e2 and e3 indicate each of the first, second and third electrode pairs respectively. By measuring the three electrode currents and one detector current, the concentration and mean diameter of particles trapped by each electrode pair or escaping from the photoionisation chamber to the detector can be determined simultaneously. A particle size distribution of particles in the gas sample may therefore be determined.
Fourth Example Embodiment of the Invention
[0178] In a fourth example embodiment of the invention, as illustrated in FIG. 6, the sensor body 2 of the first and second example embodiments of the invention is replaced by a substantially cylindrical sensor body 38. The cylindrical sensor body 38 is positioned with respect to the inlet 3 and the outlet 7 such that gas flows substantially parallel to the longitudinal axis of the cylindrical body as it travels from the inlet to the outlet. A substantially cylindrical first electrode 39 is provided on an internal surface of the cylindrical sensory body 38. The cylindrical first electrode 39 extends between the inlet and the outlet, around the internal circumference of the cylindrical sensor body 38. A second, axial electrode 40 is provided within the cylindrical sensor body, aligned along the longitudinal axis, spaced apart from the first electrode 39. The axial electrode is mounted to the sensor body at the second end 9. The first and second electrodes 39 and 40 are connected to a variable DC power supply 41 in series with an ammeter 42. The first electrode 39 is connected to the positive terminal of the power supply 41, the second electrode is connected to the negative terminal of the power supply 41. In use, a variable DC potential difference may be applied across the photoionisation chamber between the first cylindrical electrode 39 and the second axial electrode 40. The sensor of this fourth example embodiment of the invention may be used in any of the modes described with regard to the first and second example embodiments of the invention.
Fifth Example Embodiment of the Invention
[0179] In a fifth example embodiment of the invention, as illustrated in FIG. 7, the sensor body 2 of the third example embodiment of the invention is replaced by a substantially cylindrical sensor body 47. The cylindrical sensor body 47 is positioned with respect to the inlet 3 and the outlet 7 such that gas flows substantially parallel to the longitudinal axis of the cylindrical body as it travels from the inlet to the outlet. Three substantially cylindrical first electrodes 44,45,46 are provided along an internal surface of the cylindrical sensory body 38, spaced apart from one another. Each cylindrical first electrode 44,45,46 extends around the internal circumference of the cylindrical sensor body 38. A second, axial electrode 47 is provided within the cylindrical sensor body, aligned along the longitudinal axis, spaced apart from the first electrodes 44,45,46. The axial electrode 47 is mounted to the sensor body at the second end 9. The first and second electrodes 44,45,46,47 are connected to a variable DC power supply 48 in series with an ammeter 49. The first electrodes 44,45,46 are connected to the positive terminal of the power supply 48, while the second electrode 47 is connected to the negative terminal of the power supply 48. In use, a variable DC potential difference may be applied across the photoionisation chamber between the first cylindrical electrodes 44, 45,46 and the second axial electrode 47. The sensor of this fourth example embodiment of the invention may be used as is described with regard to the third example embodiments of the invention. Additionally, a voltage regulation circuit (not shown) may be provided between the power supply 48 and the first electrodes 44,45,46, in order to individually select the potential difference between each electrode pair 44,47 and 45,47 and 46,47. Alternatively, each first electrode 44,45,46 may be connected to the second electrode 47 by way of an individual power supply in series with an individual ammeter, such that each potential difference may be varied individually.
[0180] Further variations and modifications may be made within the scope of the invention herein disclosed.
[0181] The pump need not be coupled to the inlet passageway as is shown in FIG. 1. The pump may instead be positioned at any suitable location in the device in order to generate flow of sample gas through the photoionisation chamber. For example, an extraction pump may be provided towards the second end of the sensor body. Alternatively, the pump may be replaced by any other component capable of driving flow of sample gas through the photoionisation chamber, such as a fan.
Sixth Example Embodiment of the Invention
[0182] In a sixth example embodiment of the invention, when the output of the DC power supply 15 in FIG. 2 connected to the electrodes 13,14 is switched on, a substantially uniform DC potential difference V is provided across the photoionisation chamber between the first and second electrodes 13,14. The applied potential difference V typically generates an electric field of between around 100 V/cm to around 10000 V/cm across the photoionisation chamber. This mode is illustrated schematically in FIG. 18. Photoionisation of the particles in the gas sample takes place in the electric field between the first and second electrodes 13,14. In this mode, negatively charged gas ions 21 formed on photoionisation of particles in the gas sample are less likely to recombine with positively charged particles 22, 23. The electric field provided between the first and second electrodes exerts a force on the negatively charged ions 21, causing these ions to drift away from the particles towards the second electrode 14 (which functions as an anode) as they flow through the photoionisation chamber 12. In addition, any free photoelectrons which have not combined with gas atoms or molecules to form gas ions will also drift toward the second electrode 14 under the action of the electric field. The electric field also exerts a force on the positively charged particles 22, 23, causing these particles to drift towards the first electrode 13 (which functions as the cathode) as they flow though the photoionisation chamber 12. Because the electric field strength is high enough, the negatively charged gas ions 21 and any free electrons tend to reach the second electrode 14 and the positively charged particles 22, 23 tend to reach the first electrode 13 to be captured, thereby both generating an electrode current which is measured by the ammeter 16, as well as acting as an airborne particle filter by removing particles from the gas flow. The ions, free electrons, and particles are thus at least partially removed and more likely substantially or fully removed from the flow of gas through the photoionisation chamber 12. The first and second electrodes 13,14 function as an ion and particle trap embedded within the photoionisation chamber.
Seventh Example Embodiment of the Invention
[0183] In a seventh example embodiment of the invention, when the output of the DC power supply 15 in FIG. 19 connected to the electrodes 13,14 is switched on, a substantially uniform DC potential difference V is provided across the photoionisation chamber between the first and second electrodes 13,14. The applied potential difference V typically generates an electric field of between around 1 V/cm to around 100 V/cm across the photoionisation chamber. This mode is illustrated schematically in FIG. 3. Photoionisation of the particles in the gas sample takes place in the electric field between the first and second electrodes 13,14. In this mode, negatively charged gas ions 21 formed on photoionisation of particles in the gas sample are less likely to recombine with positively charged particles 22, 23. The electric field provided between the first and second electrodes exerts a force on the negatively charged ions 21, causing these ions to drift away from the particles towards the second electrode 14 (which functions as an anode) as they flow through the photoionisation chamber 12. In addition, any free photoelectrons which have not combined with gas atoms or molecules to form gas ions will also drift toward the second electrode 14 under the action of the electric field. The electric field also exerts a force on the positively charged particles 22, 23, causing these particles to drift towards the first electrode 13 (which functions as the cathode) as they flow though the photoionisation chamber 12. Nevertheless, because the negatively charged gas ions 21 and any free electrons are relatively more mobile, only the ions and electrons tend to reach the second electrode 14, thereby generating an electrode current which is measured by the ammeter 16. The ions and the free electrons are thus at least partially removed from the flow of gas through the photoionisation chamber 12 reducing the likelihood of recombination between charged particles 22, 23 and charged gas ions 21, and thereby increasing the extrinsic charging efficiency of particles. Because the positively charged particles 22, 23 are significantly less mobile than the gas ions, the charged particles 22, 23 tend to flow through the photoionisation chamber 12 and through a tube 50 to a capture chamber 51 in which a high DC potential difference is applied.
[0184] When the output of the DC power supply 54 in FIG. 19 connected to the electrodes 52,53 is switched on, a substantially uniform DC potential difference V is provided across the photoionisation chamber between the third and fourth electrodes 52,53. The applied potential difference typically generates an electric field of between around 100 V/cm to around 10000 V/cm across the capture chamber. This mode is illustrated schematically in FIG. 18. Because the electric field strength is high enough, any remaining negatively charged gas ions 21 and any free electrons tend to reach the fourth electrode 53 and the positively charged particles 22, 23 tend to reach the third electrode 52 to be captured, thereby both generating an electrode current which is measured by the ammeter 55, as well as acting as a sensor or as an airborne particle filter by removing particles from the gas flow.
Experimental results
[0185] Experimental results achieved using a sensor of the type illustrated schematically in FIG. 7 are shown in FIGS. 8 to 15. The experimental setup was as follows. A photoionisation chamber 210 mm in length, 50 mm in external diameter, and 30 mm in internal diameter, and made of electrically insulating PTFE, enclosed three adjacent aluminium cylinders (functioning together as the first electrode) of 65 mm in length, 30 mm in external diameter, and 25 mm in internal diameter, separated by PTFE spacers of 5 mm in length, 30 mm in external diameter, and 25 mm in internal diameter. A concentrically located stainless steel rod (functioning as the second electrode) of 210 mm in length and 1.5 mm in diameter was mounted at the end of the photoionisation chamber nearest the outlet, extending co-axially along the entire length of the photoionisation chamber. A 3W UV lamp (Dinies Technologies GmbH, Germany, Model Mini3W52ozon) emitting light of wavelength 185 nm was mounted at the end of the photoionisation chamber nearest the inlet in order to illuminate the photoionisation chamber through a UV-extended fused silica optical window of 25 mm in diameter and 3 mm in thickness. A Keithley electrometer (Keithley Instruments Inc., Cleveland, Ohio, USA, Model 5617B) was connected in series with the first and second electrodes and a variable power supply. The electrometer had an accuracy of ±3 fA. The photoionisation chamber was enclosed in an electrically isolated, aluminium box acting as a Faraday cage, grounded with the electrometer triaxial measurement cable. Flow from the photoionisation chamber was also sampled by an aerosol electrometer (TSI Inc., Shoreview, Minn., USA: Model 3068B) (functioning as the collector) with an accuracy better than 1 fA.
[0186] The particle size and concentration were measured by aerosol characterization instrumentation. The number-weighted particle mobility diameter distribution was measured in parallel with the photoionization chamber for aerosol characterization using a Scanning Mobility Particle Sizer (SMPS; TSI Inc.: 3080 Electrostatic Classier, 3081 Differential Mobility Analyzer [DMA], 3025 Condensation Particle Counter [CPC]). The number-weighted mobility diameter distribution gives a measure of mean particle size and total concentration of aerosol particles.
[0187] A sample flow of carbonaceous soot particles was produced by flows of propane (65-105 std. cm.sup.3/min), air (1.2 std. L/min), and N.sub.2 (3 std. L/min) in a co-flow inverse diffusion flame. An ejector diluter with filtered compressed air was used to provide a vacuum to draw the sample gas into the photoionisation chamber. The concentration of particles in the gas flow was varied by changing a diameter of a critical orifice (for example, 0.635 mm) and the flow rate of dilution air. The particle size was varied by changing the flow rate of propane into the flame (higher flow rates yielding larger particle sizes). Stainless steel and conductive silicon tubing were used to minimize particle losses in the sampling lines. Semivolatile organic carbon was removed from the sample gas using a catalytic stripper operated with an internal gas temperature of 350° C. An electrostatic precipitator operated at 3 kV was used to capture particles having any residual charge from the combustion process before they entered the photoionisation chamber and aerosol characterization equipment. The orifice diameter, the flow rate and the voltage state were held constant after any change in each said parameter until a steady state reading could be measured by both the photoionization instrumentation (Keithley and TSI Aerosol Electrometers) and the aerosol characterization instrumentation, taking up to 5 minutes per measurement.
[0188] FIG. 8 shows the total current measured, using the above experimental setup, at the collector (indicated by squares), as well as the average charge per particle (indicated by circles) calculated by dividing the total current by the known total concentration of particles and flow rate, as a function of the gas sample flow rate through the photoionisation chamber at zero applied potential difference (solid lines) and at an applied potential difference of 7.6 V between the first and second electrodes (dashed lines). It can be seen that application of the potential difference causes both the total collector current and the average charge per particle to increase. This indicates that the applied potential difference functions as an ion trap, trapping negatively charged gas ions and reducing ion-particle recombination. It can also be seen that the average charge per particle decreases with increasing flow rate. This is because at higher flow rates there is less time for particle photoionisation to occur. Nevertheless, the total current continues to increase with increasing flow rate because more charged particles pass through the photoionisation chamber, more than compensating for the reduced time for photoionisation. The maximum number of charged particles occurs at a flow rate of 1 std L/min, corresponding to a particle residence time within the photoionisation chamber of 6.2 s.
[0189] FIG. 9 compares the same data for collector current as seen in FIG. 8 with the corresponding electrode current (i.e. the ion trap current) measured at the same time. The flow-rate dependence of the electrode current is similar to that of the collector current. However, the electrode current is consistently higher than the collector current. The inventors believe this is due to loss of charged particles on collision with tube walls between the ion trap and the collector, reducing the number of charged particles reaching the collector.
[0190] FIGS. 10 and 11 show how the collector current varies as a function of both particle concentration and particle diameter, and how this behaviour is a function of the potential difference applied between the first and second electrodes. In general, the collector current depends on both the particle concentration N and diameter D according to:
I.sub.c=α.sub.cN.sup.α.sup.cD.sup.β.sup.c. (7)
[0191] Accordingly, FIG. 10 shows the collector current normalised by D.sup.β.sup.c and plotted on a logarithmic scale as a function of particle concentration for two different applied voltages (the upper data set corresponds to an applied voltage of 7.6 V, while the lower data set corresponds to an applied voltage of 85 V). The concentration exponent α.sub.c is determined from each data set by linear regression. Similarly, FIG. 11 shows the collector current normalised by N.sup.α.sup.c and plotted on a logarithmic scale as a function of particle concentration for two different applied voltages (the upper data set corresponds to an applied voltage of 7.6 V, while the lower data set corresponds to an applied voltage of 85 V). The concentration exponent β.sub.c is again determined from each data set by linear regression. The inventors believe that the higher voltage measurements show a decreased collector current because more mobile charged particles (i.e. the smaller charged particles) are captured by the ion trap at these voltages (and potentially also because of increased photoemission from housing walls).
[0192] FIG. 12 shows the calculated values of a normalised particle diameter exponent
[00001]
(that is to say, the exponent is normalised such that the power of N in Equation (7) is 1) as a function of the applied potential difference using both the Keithley electrometer and the aerosol electrometer. The normalised exponent, α, is particularly sensitive to changes in the ion trap voltage. This means that the sensor may be calibrated such that, when the current is measured at two different trap voltages, both the particle concentration and the particle diameter may be independently determined.
[0193] FIG. 13 shows the variation in the electrode current (i.e. the ion trap current) and the collector current (i.e. the outlet current) as a function of the ion trap voltage for a known particle distribution (N=1.64×10.sup.5 cm.sup.−3, mean D=47.9 nm). The collector current peaks at a low ion trap voltage as expected. The electrode current, however, continues to increase in magnitude with increasing ion trap voltage. This behaviour is also observed when no particles are present in the gas flow, indicated as the ‘background current’ in FIG. 13. The inventors believe that photoemission of electrons from photoemission chamber housing walls leading to negative ion formation in the gas is responsible for this observed behaviour. Electrons or gaseous ions, which would normally be likely to return to the same housing wall from which they are emitted, are instead drawn across the photoionisation chamber by the applied electric field and contribute to an increase in the electrode current. As shown in FIG. 14, the inventors have found that by subtracting the background signal from the measured electrode current, an adjusted electrode current which displays qualitatively similar behaviour to the collector current may be determined. The background signal may therefore be measured in an initial sensor calibration step, such that the measured electrode current may be compensated for housing wall photoemission during use of the sensor.
[0194] Nevertheless, despite the large effect of housing wall photoemission which causes a significant non-zero electrode current to flow even at zero particle concentration, the electrode current does remain a function of particle concentration for all applied voltages tested (in a range of between 8.5 V and 88 V). It is therefore also possible to calibrate the sensor to output a measured particle concentration and diameter without explicitly determining the housing wall photoemission contribution to the electrode current or explicitly compensating the electrode current for this background contribution. For example, FIGS. 15, 16 and 17 respectively compare the particle diameters, concentrations and surface areas estimated using the experimental particle sensor with the known diameters, concentrations and surface areas for a range of samples. These initial results indicate that the estimated particle surface areas deviate by only up to ±30% from the surface areas determined by more precise methods. The spread in the independently estimated particle diameters and concentrations is only slightly greater.
