Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
  • G01N27/68—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas
  • G01N27/70—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas and measuring current or voltage
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/06—Investigating concentration of particle suspensions
  • G01N15/0656—Investigating concentration of particle suspensions using electric, e.g. electrostatic methods or magnetic methods

Definitions

• the present invention relates to a method for detecting particles according to the preamble of claim 1 and specifically to a method where the measurement duty cycle is less than 50%.
• the present invention further relates to a particle sensor for detecting particles according to the preamble of claim 9.
• a major problem in any particle measurement device where particles are charged is the deposition of the charged particles on the inner surfaces of the measurement sensor causing particle accumulation or buildup. This is caused by the repulsive Coulombic force of the charged particles on each other. This repulsive force is especially effective in the space where the particles are charged by an electric discharge, because the strength of the electrostatic field affecting the particle accumulation is further increased by free ions and potentially also by the high-voltage electrode required for the electric discharge.
• Accumulation of the charged particles on the inner surfaces of the sensor may block the aerosol flow channel (pathway) or they may reduce the electrical insulation capacity of the electrical insulation
• Patent application US 2006/0156791, 20.7.2006, Dekati Oy describes a method and a sensor device for determining particle emissions from exhaust gases of a combustion engine substantially during the use in an exhaust pipe system or a corresponding exhaust gas duct, in which method emitted particles contained in the exhaust gases are charged and the particle emissions are determined by measuring the electric charge carried by the emitted particles in the exhaust gas duct.
• the emitted particles are charged by varying the way of charging or the charging power with respect to time in such a manner that as a result of the charging, emitted particles brought into at least two different electrical charge states are present, wherein the charge of the emitted particles is further determined as a difference value/values measured from the emitted particles brought into said at least two different electrical charge states.
• the application states that the relative lengths in time of the charging cycles generated by the charger C (charger on/off cycles) may vary freely as required by each application. Said lengths in time may also be equal, wherein the charger operates at a pulse ratio of 1 : 1. Thus the application does not teach any advantages of certain duty cycles (charger on/off cycles).
• Patent application US 2005/0083633, Riebel et al., 21.4.2005 relates to a device for charging or adjusting the charge of gas-borne particles into a defined charge distribution under utilization of corona discharge in the aerosol space.
• the voltage waveform and the voltage regulation are of great significance for the result.
• the application further relates to a method for operating the device. The application does not teach the advantage of small duty cycle.
• Patent application WO 2010/049870 Koninkl Philips Electronics NV, 6.5.2010, presents a device that is capable of recording the evolution over time of the characteristics of a size distribution of electrically-charged airborne particles in an airflow.
• the device comprises an air inlet, a particle charging unit, a concentration variation section, a particle sensing section and a data evaluation unit.
• the particle sensing section of the device generates at least two serially obtained measurement signals II and 12 from which the data evaluation unit can infer values for both the average particle diameter dp,and the number concentration N of the size distribution of electrically-charged airborne particles.
• this application fails to teach the advantage of small duty cycle.
• the particle sensors of the prior art possess the technical problem of particle accumulation.
• a sensor which can measure or monitor particle concentration in real-time and with long measurement periods without frequent sensor cleaning or maintenance. Such need exists particularly with car combustion engine exhaust emission monitoring.
• the inventor has surprisingly found a method which will solve the prior art electrical impactor problems described above.
• the problems are solved by implementing a process where at least one of the parameters: (1) particle charging or (2) free ion and small particle collection or (3) volumetric flow through the particle sensor is switched, preferably on and off, or modulated, over time, so that the duty cycle, i.e. the ratio of sensor ON time (ION) to the cycle time (sensor ON time combined with sensor OFF time, toN+toFF),———— is less than tON+OFF
• All the parameters (1), (2), and (3) are such parameters that their switching affects particle accumulation on the particle sensor and thus their switching with small duty cycle represents preferred embodiments of the present invention.
• Charged particles tend to accumulate in the sensor due to Coulombic force, small particles have high diffusion coefficients and thus they accumulate in the sensor due to diffusion and it is obvious that particles may accumulate in the sensor only if and when they are flowing through the sensor.
• the generalization of the current invention from the embodiments is that switching any parameter which affects particle accumulation in the sensor with a low duty cycle, decreases or even removes the problem of the prior art.
• the duty cycle is less than 50%, preferably less than 25% and even more preferably less than or equal to 10%.
• the maintenance or cleaning period of the sensor may be doubled, if the Coulombic force is the main effect, which has an influence on particle accumulation.
• Coulombic force In real life Coulombic force seldom is the only mechanism affecting particle accumulation, e.g. with fine particles diffusionphoresis is a vital particle accumulation mechanism.
• the duty cycle is preferably less than 0,25 (25%) or most preferably 0,1 (10%) or less.
• the duty cycle does not have to stay constant during particle monitoring, but a beneficial advantage of the present invention is that the duty cycle can be dynamic, i.e. it's value may be optimized during the measurement as long as it stays below 50%. Not only the duty cycle may be varied, but also the length of the t 0 N may be varied as well. The variation and optimization may be based e.g. on the sensor measurement result (particle concentration, time-wise derivative of particle concentration) or on external factors (like combustion engine revolution speed or torque).
• the length of toN is typically dependent on the ratio of the volumetric flow Q through the sensor to the sensor volume V. When the time- wise resolution is important, the ⁇ -ratio should be high, e.g. 10 1/s or higher, and in such occasions the length of toN can be fractions of a second. In situations, where the particle concentration varies slowly, toN can be tens of seconds or higher.
• sensor's sensing element output typically does not follow the signal-level variation during the separate cycle, yielding average values over the cycle, instead.
• sensor's sensing element can be realized so that its output gives essentially average values over a separate cycle, rather than follows momentary sensor's input signal.
• the duty cycle is varied so that the measurement signal of the sensor's sensing element, which provides data from which particle concentration is determined, is kept essentially constant, i.e. within small variation.
• the sensing element providing the data may be operated at its optimal measurement area and information in the duty cycle is then used to determine particle concentration.. In this case also the accumulation tendency is reduced extremely effectively with increasing particle concentrations to be measured.
• Such embodiment could obviously be used also with duty cycles higher than or equal to 50%
• the measurement range of the particle sensor it is also possible to widen the measurement range of the particle sensor to be wider than the sensor's sensing element by adapting the duty cycle according to the sensor's signal level so that at higher signal level of the sensor lower duty cycles are used.
• the duty cycles can be adapted continuously with changing sensor's signal level, or there can be discrete duty- cycle steps.
• the final measurement value is formed using both the sensor's signal level and used duty-cycle value.
• Such embodiment of the present invention yields, in addition to the wider dynamic measurement range, also contamination-tendency reduction at higher concentrations. .
• the particle sensor comprises an electrical discharging unit, typically a corona charger, which charges the particles flowing into the sensor, and the charge carried by the charged particles is then detected.
• the means for current measurement lays usually an ion trap which removes free ions, ultrafine particles or fine particles from the sample flow, depending on the electrical field strength of the ion trap.
• the sensor may also comprise a neutralizer which neutralizes the electrical charge of the particles before they enter the sensor, and the sensor is in this case used in neutralizer (NE) mode.
• the neutralizer may be a separate unit or the neutralization may be carried out by the electrical discharging unit, e.g. by such a way that the corona charger comprises two charging units with opposite electrical potential or by using a single corona charger in AC (alternating current) mode which produces ions with opposite charges.
• the electrical discharging unit of the present invention has a unique feature that it can be switched or modulated between different charging modes.
• the electrical discharging unit is a corona charger which is switched periodically between the ON-mode and OFF-mode, i.e. the corona voltage is periodically switched ON and OFF.
• Essentially continuous comparison of the measurement results between the ON and OFF modes with less than 50% duty cycle reduces particle accumulation in the sensor.
• the term "periodical switching" means that the switching may have a fixed frequency or the frequency may vary and also the length of the ON and OFF modes may vary. The length of the ON mode may be less than 100 seconds, less than 1 second or even less than 0,1 seconds.
• An advantageous feature is that in high-concentration conditions, where the soiling rate is high, extremely low duty cycles can be used without practical degrading the noise properties of the signal. The reason for this is the fact that during even extremely short ON times the high signal levels can yield enough noise-free measurement data.
• it is advantageous to use switching between ON and neutralizer (NE) modes to minimize the particle accumulation, as the NE-mode decreases the Coulombic accumulation of the naturally charged particles. Then the duty is obviously the ratio of to N to ⁇ + ⁇ , i.e. duty cycle toN , and duty cycle is less than 50%.
• the particle accumulation in the sensor is decreased by switching the operation mode of the ion trap with less than 50% duty cycle.
• an ion trap which removes free ions or ultrafine particles or even fine particles from the sample flow.
• Increasing the ion trap voltage increases the diameter of trapped particles and with high trap voltage the ultrafine and fine particles which would potentially deposit in the sensor are removed from the sample flow.
• the modulated/switched parameter affecting the particle accumulation is the volumetric flow Q through the sensor.
• Volumetric flow switching is beneficial because it reduces both particle deposition due to Coulombic force as well as particle deposition due to diffusionphoresis.
• Volumetric flow can be switched e.g. by switching a valve shutting the sample flow to the sensor inlet or from the sensor outlet or by switching or modulating the pump which creates the volumetric flow through the sensor.
• the term "pump" is here understood as any means of creating the volumetric flow through the sensor and thus pump can be e.g. an ejector pump where either the motive fluid flow or the side fluid flow may be switched or modulated.
• particle charging is switched or modulated in synchronically with the volumetric flow, so that particle charging is switched OFF or NE slightly before setting the volumetric flow OFF. This ensures that the particles which are in the sensor when the volumetric flow is switched OFF do not accumulate in the sensor due to Coulombic force. Even though accumulation by diffusionphoresis is still present, this synchronous embodiment is preferably especially with applications where sensor soiling due to charged particles has a large effect.
• Fig. 1 shows one embodiment of the present invention
• Fig. 2 shows another embodiment of the present invention.
• the present invention describes a process for particle measurement.
• the process comprises modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF).
• Essential for the current invention is that the modulation or switching duty cycle is less than 50% and preferably less than 10 %.
• the invented process comprises electrically charging at least a fraction of the particles under measurement and measuring the electrical current carried by the charged particles. Ions, charged ultrafine particles or charged fine particles (particle diameter smaller than 100 nm) - or even charged particles larger than 100 nm in diameter - can be removed with the aim of an electrical field of the ion trap.
• the modulated or switched parameter which affects particle accumulation or deposition is the electrical charge of the particles, the electrical field strength of the ion trap or the volumetric flow through the measurement apparatus.
• the volumetric flow is switched or modulated, it is beneficial to use synchronous switching or modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode. This essentially reduces particle deposition due to Coulombic force.
• the duty cycle can be adjusted dynamically during the measurement as long as the duty cycle stays below 50%.
• the duty cycle is adjusted in such manner that the measurement signal from the sensing signal of the particle measurement apparatus, averaged over the period toN+toFF, is kept essentially constant.
• the present invention also includes apparatus 1 (numbers refer to figures 1 and 2) for particle measurement, comprising means 11 for modulating or switching a parameter which affects particle accumulation or deposition into the measurement equipment between at least a mode where particle accumulation or deposition occurs (ON) and another mode where particle accumulation and deposition does essentially not occur (OFF) with less than 50% modulation or switching duty cycle and more preferably with less than 10 % duty cycle.
• apparatus 1 comprises means 6 for electrically charging at least a fraction of the particles under measurement and means 10 for measuring the electrical current carried by the charged particles.
• Apparatus 1 also comprises means 8 for removing ions, charged ultrafine particles or charged fine particles with the aim of an electrical field.
• apparatus 1 comprises means 1 lb for the modulation or switching of the electrical charge of the particles, means 1 lc for the modulation or switching of the electrical field removing ions, charged ultrafine particles or charged fine particles or means 11a, 1 Id for the modulation or switching of the volumetric flow through apparatus 1.
• apparatus I comprises means for synchronous switching or modulation of the particle charge and the volumetric flow so that the particle charge is switched or modulated to the OFF mode before the volumetric flow is switched or modulated to OFF mode.
• Figure 1 shows one embodiment of a particle sensor 1 according to the present invention.
• Particle sensor 1 comprises a sample inlet 2 through which a sample from an aerosol, i.e.
• Sensor 1 further comprises an outlet 3, through which the measured sample exits sensor 1.
• Sensor 1 may be particle collecting sensor, such as e.g. a sensor based on measuring the resistivity of collected particles, such as described e.g. in WO 2009/043711, Bosch, or a sensor based on measuring optical attenuation or reflection produced by collected particles.
• Sensor 1 may also be a non-collecting particle sensor, such as e.g. a sensor based on electrically charging particles and measuring the electrical current carried by the particles, such as described in FI 20080182, Navaro 245 Oy.
• the particle sensor is based on charging at least a fraction of the particles in the aerosol sample under measurement and measuring the current escaping from the sensor with the charged particles.
• the embodiment of Figure 1 comprises particle charging by dielectric barrier discharge (DBD), where particle charging takes place between two electrodes, 6a and 6c. Between the electrodes is also a dielectric layer 6b. Electric discharge occurs on the dielectric surface and at least a fraction of particles passing the gap between electrodes 6a and 6c are charged. Electric discharge is initialized by an alternating electric potential between electrodes 6a and 6c.
• the electrical potential may be in the order of few kilovolts or few tens of kilovolts and the frequency of the alternating voltage may be e.g. 500 Hz - 500 kHz.
• the required voltage is applied to electrodes 6a and 6c from the high voltage source 7.
• Generated ions which are not attached to the particles are collected with a direct electrical field between electrodes 8a and 8b, the direct field being generated by power unit 9.
• the electrical current carried by the charged particles is measured with a sensitive electrometer 10.
• Means 11 for parameter switching consists of e.g. 1 la and 1 Id, which modulate power supply to valves 4 and 5, where valve 4 is a valve opening and closing volumetric entry flow to sensor 1 and valve 5 is a valve opening and closing exit flow from sensor 1.
• means 1 la and l i d may provide a tool for modulating the flow through sensor 1.
• Means l i b controls power supply to the high voltage generator 7, which is the power supply unit of the electrical discharge unit 6.
• the high voltage generator 7 which is the power supply unit of the electrical discharge unit 6.
• Means 11 c controls power supply to the voltage generator 9, which is the power supply unit of the ion trap 8.
• the voltage generator 9 which is the power supply unit of the ion trap 8.
• Figure 2 shows a schematic drawing of another embodiment of the present invention.
• a sample flow from the aerosol under measurement enters into sensor 1 through coupling 2 and the sample flow exits sensor 1 through coupling 3.
• Sensor 1 may be particle collecting sensor, such as e.g. a sensor based on measuring the resistivity of collected particles, such as described e.g. in WO 2009/043711, Bosch, or a sensor based on measuring optical attenuation or reflection produced by collected particles.
• Sensor 1 may also be a non- collecting particle sensor, such as e.g. a sensor based on electrically charging particles and measuring the electrical current carried by the particles, such as described in FI 20080182, Navaro 245 Oy.
• the particle sensor is based on charging at least a fraction of the particles in the aerosol sample under measurement and measuring the current escaping from the sensor with the charged particles.
• the embodiment of Figure 1 comprises particle charging by corona charger 6. Essentially clean air is fed through coupling 14 and valve 12 into the sensor 1. This essentially clean air is ionized by the corona charger 6 and the ionized air is mixed with the particle-containing sample flow which enters sensor 1 through coupling 2 and valve 4.
• Such sensor is in detail explained in applicant's patent application WO
• particle sensor 1 comprises means 11 for switching or modulating a parameter which affects particle deposition (which is here used as a synonym for particle accumulation) in the sensor with a duty cycle lower than 0,5 (50%). Modulation or switching is done between two parameter modes, the first one (ON) being a mode where particles may deposit in the sensor and the second one (OFF) being a mode where particle deposition is significantly reduced.
• Means 11 for parameter switching consists of e.g. 1 la and 1 Id, which modulate power supply to valves 4 and 5, where valve 4 is a valve opening and closing volumetric entry flow to sensor 1 and valve 5 is a valve opening and closing exit flow from sensor 1.
• means 1 la and l i d may provide a tool for modulating the flow through sensor 1.
• With means 1 le it is possible to switch or modulate flow of the essentially clean air passing through valve 12 to an ejector-based particle sensor as the one described in applicant's patent application WO 2009/0109688, and by this way
• Means l i b controls power supply to the high voltage generator 7, which is the power supply unit of the electrical discharge unit 6.
• the high voltage generator 7 which is the power supply unit of the electrical discharge unit 6.
• Means 11 c controls power supply to the voltage generator 9, which is the power supply unit of the ion trap 8.
• the voltage generator 9 which is the power supply unit of the ion trap 8.
• Control unit 13 analyses the measurement signal of sensor 1 and thus is able to provide information on particle concentration as well as the time- wise derivative of the particle concentration. This information can be used to optimize the duty cycle s long as it is below 50%.

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