Patent Application
20140083167
Instruments for sampling and measuring particulate matter in air are useful for a variety of purposes. They can be used for scientific research to study the nature of a particulate air pollutant and its transport and dispersion in the ambient atmosphere. They are also useful for studying the effect of a particulate air pollutant on human health. In addition, such instruments can also be used for sampling and measuring airborne particulate matter for regulatory compliance purposes to determine if the ambient level is within safe limits prescribed by law.
The present disclosure describes a method and an apparatus for sampling and measuring airborne particulate matter in the ambient atmosphere. The method and apparatus are particularly useful for compliance measurement purposes where ease of use and accuracy of measurement are most important.
The apparatus of this disclosure includes an inlet for particulate containing gas to enter. A mechanism then removes coarse particles larger than a selected size while permitting particles of less than the selected size to pass through. A chamber containing a quartz crystal sensor permits the particles that have passed through to deposit to create an output signal in response to the deposited particle mass.
The present disclosure also includes a method for measuring the concentration of particles in a gas using an apparatus in which the particulate containing gas enters into the chamber. The chamber contains a quartz crystal sensor on which particles deposit to create an output signal in response to the deposited particle mass. The chamber is maintained at a temperature sufficient to prevent vapor condensation on the sensor. The method includes removing coarse particles larger than about 10 μm in equivalent aerodynamic diameter permitting particles smaller than about 10 μm in equivalent aerodynamic diameter to pass through. Charging the particles of less than about 10 μm in equivalent aerodynamic diameter with ions generated in a corona discharge. Depositing the passed through particles on the quartz crystal sensor and measuring the output signal of the quartz crystal sensor.
One part of the air flowing along path 120 enters a sampling chamber 130 containing a filter 134 for collecting airborne particles on a filter for analysis, the rate of air flow along this path being maintained by a small electric pump 140. The flow rate of air is measured by a flow sensor and electronic controller 150. With the help of the electronic controller 150, which has feedback control, the flow rate can be maintained at a specific set-point value. The flow rate is typically less than about 30 liters per minute. Higher flow rates can be used. However a small amount of material is capable of being accurately analyzed for mass and/or chemical composition analysis using the apparatus and method described herein. It is unnecessary to use a high sampling flow rate to collect a large amount of material for analysis. A small and compact instrument as described herein is easy to setup and use in the laboratory or in the field, while providing the needed accuracy for the measurement. In comparison, traditional air samplers for environmental monitoring operate at a flow rate of approximately 40 cubic feet per minute, i.e. 1120 liters per minute. The 30 liter per minute flow rate described here is only approximately 2.5% of the traditional flow rate used for this purpose.
Another part of the airflow along path 160 enters a chamber 170 containing a particle mass sensing transducer 174 to monitor the mass of particles deposited on the sensing surface of the transducer 174. The airflow along this path is also maintained by a small pump 180, the flow rate being measured by a flow sensor and controller 190 to maintain the air flow at a specific set-point value.
A third part of the air flows along path 200 and enters another coarse particle separator 210 to remove coarse particles larger than approximately 2.5 or approximately 1.0 μm. After the coarse particle separator the air is divided into two downstream paths 220 and 230. Air path 220 leads to chamber 240 which contains a filter sampler collect airborne particle samples for analysis. The other air path 230 leads to chamber 250 which contains a transducer 254 for monitoring the mass of particles deposited on the transducer sensing surface for monitoring the mass concentration of airborne particles smaller than approximately 2.5 or approximately 1.0 μm in diameter.
Downstream of each of the chambers 240 and 250 are small electric pumps, 260 and 270 and flow sensors and controllers, 280 and 290, for controlling the rate of gas flow through the chambers 240 and 250 to their respective set-point values. The schematic diagram of the system located generally at 100 is therefore capable of collecting airborne particle samples in the PM10 and PM2.5 or PM1.0 size ranges for mass and/or chemical analyses, as well as providing airborne mass concentration values for PM10 and PM2.5, or PM10 and PM 1.0 determination. The terms PM10, PM2.5 and PM1.0 particles are defined as atmospheric particulate matters from which particles larger than 10 μm, 2.5 μm or 1.0 μm in aerodynamic equivalent diameter have been removed. In the context of the present disclosure concerning method and apparatus for sampling and measuring atmospheric particulate matter, the term refers to a sample stream from which particles larger than the 10 μm, 2.5 μm or 1.0 μm in aerodynamic equivalent diameter have been removed.
The aerodynamic equivalent diameter of a particle is the diameter of a unit density sphere having the same settling speed as the particle in question. The concept of aerodynamic equivalent diameter and size separation by inertial impaction are well known to those skilled in the art in designing inertial particle separation devices and therefore will not be further discussed.
The filter sampling apparatus of
The upper chamber 510 is constructed of a conducting material, such as stainless steel. Air, carrying particles smaller than about 10 μm in diameter, enter the chamber through inlet 530. Inside the chamber there is a needle 545 with a fine tip 555. The needle is embedded in an insulator 540 and connected to a source 550 of high DC voltage to create a corona discharge from the needle tip 555 to an inside conical surface 560 of the chamber. Most of the corona current is collected on the conical surface 560 inside the chamber. Surfaces that are farther away, such as 575 and 570, will have very little of the current collected there because of the much weaker electric field there. The high voltage source 550 is provided with electronic control circuitry in order to provide a stable voltage and/or current output to insure a high charging efficiency and repeatable performance characteristics.
The walls 610 of the lower chamber 520 are constructed of an insulating material, such as plastic or a ceramic. A metal electrode 590 is placed above the quartz crystal mass sensing transducer 600. The source of high voltage 550 is connected to electrode 590 while the quartz crystal transducer 600 is grounded. The high voltage source 550 is also provided with control circuitry (not shown) in order to vary the voltage to achieve optimal performance, while providing a repeatable voltage output to insure stable operation of the precipitator.
To prevent water vapor in air to condense on the transducer 600 both the upper and lower chambers of
a and 5b are schematic diagrams of a quartz crystal micro-balance for sensing the deposited particle mass on the quartz crystal.
The mass sensing area 610 is on the front side of the transducer. Particles to be sensed are deposited in this area. The central area for mass sensing is surrounded by an annular electrode area 620 which is usually coated with a thin layer of gold. There is also an edge exclusive zone 630 where uncoated quartz is present. One suitable quartz is an AT cut crystal.
On the backside of the quartz crystal transducer, there is also a gold coated electrode 640 which is approximately the same diameter as the sensing zone on the front side. The uncoated quartz area is shown at 650. The transducer is set to vibrate at its resonant frequency by applying an excitation voltage to the electrodes. With the conventional AT cut quartz crystal used for mass sending, the vibration is in the transverse mode, i.e. in the direction parallel to the surface on which the particles are deposited. With deposited particle mass on the crystal surface, the resonant frequency of the crystal will become smaller compared to the resonant frequency when the crystal is clean. The change in frequency is thus proportional to the deposited particle mass, which can be used for particle mass determination.
The above description of the instrument has been described in terms of its use for airborne particle measurement. The same apparatus can also be used to measure particulate matter suspended in a gas media other than air. Particulate matter suspended in nitrogen, argon, and other inert gases may also contain suspended particles that need to be measured. The method and apparatus described herein are also suitable for such applications as well.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in faun and detail without departing from the spirit and scope of the invention.
The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 61/704,148, filed Sep. 21, 2012, the content of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61704148 | Sep 2012 | US |
