Title:
ELECTROSTATIC MASS PER UNIT VOLUME DUST MONITOR
United States Patent 3718029


Abstract:
Electrogasdynamic methods and apparatus for measuring the mass of particulate matter entrained in a gaseous flow per unit volume of such flow are disclosed. The gaseous flow is passed down a bounded flow path and the entrained particles are exposed to and charged by a corona discharge. The charged entrained particles are carried downstream to a collection section where the particles are repelled by a charged deposition electrode and driven toward a grounded electrode. A portion of the charged particles are collected on a dielectric surface positioned in front of the grounded electrode. The accumulated charge on the dielectric surface is then measured by an induction electrode and an electrometer to show the particle mass per unit volume of the flow.



Inventors:
Gourdine, Meredith C. (Fort Lee, NJ)
Law, Edward S. (Athens, GA)
Application Number:
05/109145
Publication Date:
02/27/1973
Filing Date:
01/25/1971
Assignee:
GOURDINE SYST INC,US
Primary Class:
Other Classes:
96/26, 324/71.4, 324/464
International Classes:
G01N27/60; (IPC1-7): G01N15/00; B01D53/30; B03C3/00
Field of Search:
73/23,28,194F,432PS,421,421.5 324
View Patent Images:



Primary Examiner:
Queisser, Richard C.
Assistant Examiner:
Snee III, C. E.
Claims:
We claim

1. A method for determining the mass per unit volume of particles entrained in an ionizable gas, comprising the steps of

2. A method in accordance with claim 1 wherein the first corona discharge field is established by a constant current power supply.

3. A method in accordance with claim 1 comprising the additional step of discharging the particles by means of a second corona discharge field positioned upstream of said first corona discharge field.

4. A method in accordance with claim 1 comprising the additional step of discharging the charged particles by means of a third corona discharge field positioned downstream of said deposition electric field.

5. A method in accordance with claim 1 comprising the additional step of separating out from the gas substantially all particles above a certain size, the separation taking place upstream of said first corona discharge field.

6. A method in accordance with claim 1 wherein the accumulated charge on the dielectric surface is determined by measurement by an induction electrode and the measurement of the accumulated charge is linearly related to the mass of the particulate matter per unit volume of the gas and is independent of the size of the particulate matter.

7. A method in accordance with claim 1 wherein the maximum voltage V permissible for said deposition electrical field is given by the relation

8. A method for determining the mass per unit volume of particles entrained in an ionizable gas, comprising the steps of

9. A method in accordance with claim 8 comprising the additional step of converting electrically the accumulated charge determined by induction to a measurement of the particulate mass per unit volume of the gas.

10. An apparatus for determining the mass per unit volume of particles entrained in an ionizable gas comprising

11. An apparatus as defined in claim 10 wherein the means for establishing the first electrical discharge in the upstream end of the flow path comprise a corona discharge electrode and a constant current power supply.

12. An apparatus as defined in claim 10 further comprising means for establishing a second electrical discharge in the flow path, the second electrical discharge means positioned upstream of the first electrical discharge means.

13. An apparatus as defined in claim 12 wherein the second electrical discharge means comprises at least one corona discharge electrode and an electrical power supply.

14. An apparatus as defined in claim 10 further comprising means for establishing a third electrical discharge in the flow path, the third electrical discharge means positioned downstream of the deposition electric field means.

15. An apparatus as defined in claim 14 wherein the third electrical discharge means comprises at least one corona discharge electrode and an electrical power supply.

16. An apparatus as defined in claim 10 further comprising means for separating out from the gas substantially all particles above a certain size, the separation means positioned at the upstream end of the flow path.

17. An apparatus as defined in claim 10 wherein the induction electrode is positioned below the ground electrode and the ground electrode is adapted to be adjustably removed from adjacent the dielectric surface.

18. An apparatus as defined in claim 10 wherein the accumulated charge measured by the induction electrode is converted electrically to a measurement of the mass of the particulate matter by an electrometer circuit, the accumulated charge and the mass of the particulate matter per unit volume of the gas measurements being linearly related and the accumulated charge measured being independent of the size of the particulate matter.

19. An apparatus as defined in claim 10 further comprising means for inducing a flow of the gas in the flow path.

20. An apparatus as defined in claim 19 wherein the flow inducing means comprises a fan positioned adjacent the downstream end of the flow path.

21. An apparatus for determining the mass per unit volume of particles entrained in an ionizable gas comprising

22. An apparatus as defined in claim 21 wherein the accumulated surface charge density determined is converted electrically to a measurement of the mass of the particulate matter per unit volume of the gas by an electrometer circuit.

23. An apparatus for continuously determining the mass per unit volume of particles entrained in an ionizable gas comprising

24. An apparatus as defined in claim 23 wherein the endless dielectric surface is attached to a rotatable cylinder, the means for moving the endless dielectric surface is connected to the cylinder, and the cylinder is grounded and acts as the ground electrode for the deposition electric field.

25. An apparatus as defined in claim 23 further comprising means for discharging the accumulated charge on the endless dielectric surface.

26. An apparatus as defined in claim 23 wherein the endless dielectric surface comprises an endless belt positioned around at least two rollers and the means for moving the surface is connected to at least one of the rollers.

27. An apparatus for determining the mass per unit volume of particles entrained in an ionizable gas comprising

28. An apparatus as defined in claim 27 wherein the means defining the bounded flow path, the corona discharge electrode, the deposition field electrode and the dielectric surface are contained in a sampling module, the deposition field ground electrode, the induction electrode and the electrometer circuit are contained in an electrometer module, and the corona discharge power supply means and the deposition field power supply means are contained in a supply module, the three modules adapted to be separable.

29. An apparatus as defined in claim 28 wherein the power supply means for the first corona discharge electrode is a constant current power supply means.

30. An apparatus as defined in claim 28 further comprising a fan for inducing a flow of gas in the flow path, the fan positioned in the supply module and in communication with the downstream end of the flow path.

31. An apparatus as defined in claim 27 further comprising a second corona discharge electrode in the flow path to discharge the charged entrained particles, the second corona discharge electrode positioned upstream of the first corona discharge electrode.

32. An apparatus as defined in claim 27 further comprising a third corona discharge electrode in the flow path to discharge the charged entrained particles, the third corona discharge electrode positioned downstream of the deposition electric field.

33. An apparatus as defined in claim 27 further comprising a holding plate and a lance attached to the holding plate, the means defining the bounded flow path, the corona discharge electrode, the deposition field electrode and the dielectric surface contained in a sampling module, and the sampling module positioned on the holding plate whereby the mass per unit volume of particles entrained in the gas in a stack can be determined.

Description:
BACKGROUND OF THE INVENTION

The present invention relates to improved methods and apparatus for measuring the mass of particulate or aerosol matter entrained in a flowing gaseous system. Specifically, the invention deals with improved methods and apparatus of this type employing electrogasdynamic techniques for measuring the mass of the entrained particulate matter per unit volume of the gas sampled.

Pollution of our environment has become an increasingly important problem in recent years. In this respect, most municipalities have standards regulating the amount of particulate emissions allowable from industrial stacks, diesel and reciprocating engines, and the like. The amount of particulate matter in the air in mines also is of much concern.

Many legislative regulations relating to the permissible amount of particulate matter in such emissions and mines, however, are specified in terms of particulate mass per unit volume (mass/volume) of gas flowing from the source. There are a few methods and instruments presently used for such mass/volume measurements, but they generally require much time and are costly and have definite shortcomings.

The most commonly employed method for mass/volume measurement is the gravimetric method. The gravimetric method involves the forced flow of the particulate gas mixture through a filter or filters porous only to the gas. After sampling a known volume of gas, the collected mass on the filters is removed and weighed on a delicate analytical balance. The primary disadvantage of this method is the length of time necessary to ascertain the mass/volume measurement.

Another technique used for mass/volume measurement involves the beta-ray backscatter instrument. The particulate mass collected on a porous filter is measured directly by a beta-ray backscatter. This technique is very expensive, however, and also requires a permanent installation.

Other techniques for air pollution measurements have also been used which operate on the basis of opacity or light scattering theories. These techniques have large measurement errors, however, due to their strong dependence upon particulate size.

The present invention overcomes all of the aforementioned disadvantages as it provides a particulate monitor which gives a direct meter readout in mass/volume in a short period of time, for example, on the order of one minute. The monitor is less expensive than other instruments, can be completely portable (i.e., hand carried and battery operated), and has a signal output which is independent of the particulate diameter.

SUMMARY OF THE INVENTION

The advantages of the invention are attained by flowing the gaseous fluid containing the suspended particles down a bounded flow path, ionizing the fluid at an upstream portion of the flow path to charge the particles, repelling the charged particles from a charge deposition electrode and thus driving the charged particles toward a grounded electrode, and collecting (depositing) a portion of the charged particles on a dielectric surface positioned in front of the grounded electrode. The surface potential built upon the dielectric surface is then measured after a given sampling time by a static charge induction electrode which is connected to an electrometer. Since the current density is a linear function of the particulate mass concentration per unit volume of the gaseous flow, and also is independent of the particulate size, the mass/volume of the particulate matter can be read directly from the electrometer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more effectively understood by referring to the drawings, wherein:

FIG. 1 illustrates a particulate sampling apparatus utilizing the present invention;

FIG. 2 is a top-view of the sampling module of the apparatus shown in FIG. 1;

FIG. 3 is a cross-sectional view of the sampling module taken along the line and in the direction of the arrows 3--3 in FIG. 2.

FIG. 4 is a side-view in partial section and with the side wall removed of the electrometer module of the apparatus shown in FIG. 1;

FIG. 5 is a schematic view of the support module of the apparatus shown in FIG. 1;

FIG. 6 illustrates an operator using an embodiment of the invention;

FIG. 7 is a schematic view of a holding plate and lance for use with the embodiment of the invention shown in FIG. 6;

FIG. 8 is a schematic view of a continuous particulate sampling apparatus in accordance with the present invention; and

FIG. 9 is a schematic view of another continuous particulate sampling apparatus in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the preferred embodiment of a mass/volume particulate measuring apparatus 10 utilizing the present invention. The apparatus includes three sections which for illustrative purposes will be described and discussed separately. The three sections are an electrogasdynamic sampling module 11, an electrometer module 12, and a support module 13. For operation, the three sections are connected together to form one unit 10 which can be either portable or permanently installed.

FIGS. 2 and 3 illustrate the electrogasdynamic sampling module 11. The sampling module forms a flow channel 20. The particulate-laden gas which is to be sampled enters the channel 20 at the upstream end 21 and exits at the downstream end 22. In the channel 20 near its upstream end is an unipolar corona discharge electrode (ionizing element) 23. Such element 23 can be a wire, sawblade, or the like, and is charged by a constant current power supply 24 of any conventional type. For unit 10 the power supply 24 can be located in the support module 13. The electrode 23 is connected to the power supply 24 by means of a high-voltage connecting rod 25. To facilitate the separability of the three modules of the unit 10, the connecting rod 25 terminates at the outside perimeter of the sampling module 11 in an anti-corona metal sphere 26. When the sampling module 11 is placed adjacent the support module 13, the metal sphere 26 comes in contact with the constant current power supply 24.

The current supplied by the power supply 24 is preferably on the order of 100 μA. A constant current source is used because the ionization current flowing from a corona element supplied by a constant voltage decreases with increasing moisture content of the gas (relative humidity). It is well known that the electrical mobility of molecular ions is reduced by water vapor attachment. The constant current power supply thus maintains the particle charging current constant as relative humidity varies.

The top and bottom surfaces 27 of the channel 20 are constructed of any conductive metal and are maintained at ground potential. The side surfaces (walls) 29 are constructed of a non-conducting material, such as Teflon. During operation, a corona discharge is established between the corona discharge element 23 and the grounded surfaces 27. The corona element 23 should be well insulated from the channel surfaces 27 and this is accomplished in the preferred embodiment by terminating the element 23 at supports (not shown) located near the end of clearance tunnels 28 and by having non-conductive side surfaces 29.

The particles 30 entrained in the gaseous flow are charged to a prespecified fractional charge as they pass through the ionized field between the corona element 23 and the surfaces 27. To insure neutrality of the particles prior to entering the ionizing field and thus to achieve a more theoretically perfect sampling, the particles 30 can be discharged upstream of the field by a discharge electrode 31. The discharge electrode 31 can be, for example, a series of grounded corona wires stretched across the channel 20 and can be maintained at a voltage by the power supply 24 or any other conventional power supply means. Such initial discharge electrode is not required, however, as the amount of particles charged and the strength of such charges are insignificant in most instances.

Charged molecular gas ions swept from the corona region by the channel gas flow are drawn by the grounded surfaces 27 along length L1 of the channel 20 and neutralized before they can be collected (deposited) downstream on a dielectric collection surface.

The charged particles 30 are carried by the gaseous flow downstream to the deposition region 35. In such region 35, a deposition field electrode 36 and dielectric collecting surface 37 are positioned. The electrode 36 is made of conductive metal and electrically charged by a constant voltage power supply 38 of any conventional type. The power supply 38 is preferably capable of supplying 600 to 5,000 volts. For unit 10 the power supply 38 can be positioned in the support module 13. The electrode 36 is connected to the power supply 38 by means of a metal tab 39 and a conductor 40. To facilitate separability of the three modules, the conductor 40 preferably terminates in metal sphere 41. The charge on the electrode 36 is of the same sign potential as the corona element and thus of the charged particles 30 so that the particles 30 will be repulsed (repelled) by the electrode 36 and driven toward the dielectric collecting surface 37.

The surface 37 can be any good dielectric having mechanical strength in a thin section, high electrical resistivity, and the ability to operate in high temperature environments, such as 450°-500°F. Such dielectrics can be, for example, glass or ceramic plates. Excellent results have been obtained using glass microscope slide covers.

The surface 37 is accessible for cleaning and discharging after sampling by means of an access door 45. The door 45 forms the upper surface of the channel 20 in the deposition region 35 and contains the electrode 36. The surface 37 can be cleaned and discharged, for example, by wiping it with an electrically grounded paper wetted with methyl alcohol or the like.

When the unit 10 is assembled and the sampling module 11 is placed in contact with the electrometer module 12, a grounded metal backing plate 50, which is shown in dashed lines in FIG. 3 and explained in more detail with reference to FIG. 4, is positioned slightly below the sampling surface 37. An electric deposition (collection) field is established between electrode 36 and the grounded plate 50 by power supply 38 and such field drives a portion of the charged particles 30 towards the sampling surface 37 where they are deposited thereon. The charge accumulated onto the surface 37 during a specified sampling time (typically one minute) is then measured by reading its voltage with an electrometer and induction probe (FIG. 4) positioned below the surface 37.

The surface 37 is embedded flush with the lower surface of the channel 20 and is insulated from ground. The sampling surface 37 is bonded by an adhesive or any other means to the insulated support 46. The support 46 is constructed of a highly resistive material to maintain a low leakage path from the deposited charged particles to ground. The support 46 is provided with surfaces 47 which are protected ("hidden") from soiling by the particles 30 by a grounded metal shield cover 48 and an air space 49. The cover 48 and air space 49 help maintain a highly resistive surface leakage path. When the particles 30 are very conductive, the charge leakage can be lessened by constructing the support 46 of a highly resistive material, for example, Teflon.

At the downstream end 22 of the channel 20, a discharge electrode 42 can be provided if it is desired to neutralize the charged, undeposited particles entrained in the exhaust gaseous flow. The electrode 42 can be, for example, a series of wires stretched across the channel 20. The corona electrode 42 can be maintained at a voltage by power supply 24 or any other conventional power supply means.

If it is desired to insure separation of non-respirable particles from the total airborne particles so that only the respirable size particles will arrive at and be deposited on the sampling surface 37, the length of the deposition electrode 36 can be extended an additional distance L2. By proper setting of the velocity of the gaseous flow and the strength of the deposition field, the phenomenon of electrostatic deflection of charged particles can be utilized to deposit the larger, more electrically mobile particles onto the lower channel wall and onto the grounded shield cover 48.

FIG. 4 illustrates the preferred form of the electrometer module 12 for the unit 10. The module 12 is constructed so that the sampling module 11 can be placed on its top surface 51 for direct measurement. This feature eliminates any measurement error which might result if the test surface 37 had to be removed from the module 11 for measurement.

To facilitate placing and correctly positioning the sampling module 11 upon the module 12, a well ("seat") 52 is provided on the top surface 51. The well 52 is constructed to mate with the insulated support 46 and the cover 48, both of which extend below the lowermost surface of the channel 20 (see FIG. 3). The remainder of the lowermost surface of the module 11 rests on the surface 51.

When the modules 11 and 12 are mated, the deposition surface 37 is positioned approximately at the location shown by the dashed lines in FIG. 4. For testing, the grounded plate 50 is positioned directly below the deposition surface 37, as heretofore described.

The plate 50 is adjustable in order to facilitate alternately sampling the flow and measuring the accumulated charge on the surface 37. The grounded plate 50 is attached to a grounded sliding shield 53 by parallel linkages 54. The shield 53 is movable by any conventional means, such as the rack 55 and pinion 56 arrangement illustrated in FIG. 4. When the pinion 56 is rotated, the engagement of the pinion 56 with the rack 55 causes the shield 53 to travel horizontally. As the shield 53 travels rightwardly in FIG. 4 the plate 50 strikes a sharply inclined ramp 57 made preferably of very low friction material, such as Teflon, and causes the plate 50 to move vertically. The plate 50 is moved upwardly until it is near but not in contact with the sampling surface 37. The horizontal travel of the shield 53 and thus the vertical travel of the plate 50 is stopped by an adjustable screw 58. The screw 58 is adjusted so that an air gap is left between the plate 50 and the surface 37. The air gap, preferably on the order of one-sixteenth of an inch, is necessary to prevent conduction of the charge through the surface 37 to the plate 50.

During the sampling stage, the shield 53 is in its extreme rightward position and the plate 50 is in position for sampling of the shield 53 gaseous flow. After sampling, the movement of the shield 53 is reversed from that described above and the shield 53 and plate 50 are moved to a position not directly beneath the surface 37. The surface 37 is then exposed to measurement by an induction electrode 60 which is fixed permanently in vertical alignment with the well 52 and thus the surface 37.

The electrode 60 can be any static charge detection probe, such as the No. 2501 and 2503 model probes manufactured by Keithley Instruments, Inc.

Prior to movement of the shield 53 and plate 50 after sampling, however, the induction electrode 60 should be "zeroed" at ground potential. An electrical field-free space for zeroing is provided below the shield 53 when such shield is in position for sampling.

The induction electrode 60 is supported by a base 61 which in turn is supported by support 59. The base 61 is constructed of a highly non-conductive material which maintains the high input impedance necessary for the electrometer circuit 62. As illustrated in FIG. 4, the base 61 is designed to expose only a minimum amount of its surface for a residence of free charge.

When the electrode 60 is exposed to the surface 37, the accumulated charge on such surface induces a potential onto the electrode 60. The strength of the induced signal depends on the following factors: (1) the surface of the electrode 60; (2) the voltage on the surface 37; and (3) the input capacitance of the electrometer circuit 62. This induced voltage signal is the input to the electrometer circuit 62 which can be of any conventional type. The signal can be conducted to the circuit 62 by, for example, contact 65, rod 63 and conductor 64. The contact 65 is depicted for illustrative purposes as a spring contact to facilitate easy removal of the circuit 62 from the module 12, but can be of any conventional type. The output from the electrometer 60 in turn drives a voltmeter 70 of any conventional type whose deflection is a linear function of the potential on the sampling surface 37. The voltmeter can have many ranges of measurement and the appropriate range can be used depending on the potential of the charge on the surface 37. When the electrometer module 12 is constructed, the panel of the voltmeter 70 can be mounted on an outside surface (wall) 71 of the module 12 (as shown in FIG. 1) for easy viewing. The arc-type scale can be calibrated to read out in units of mass/volume, for example, milligrams per cubic meters, grains per cubic feet, or the like. As shown in FIG. 1, a control 72 for switching the range of the meter 70 also can be mounted on the wall 71 of the module 12.

The electrometer circuit 62 preferably is mounted on a slidable base 75 but it is understood that the circuit 62 can be mounted in a fixed position also. The base 75 can be constructed of any nonconductive material with good mechanical strength characteristics, such as fiberglass board. Groves 76 are provided on the inside of the walls 71 of the module 12 to facilitate sliding of the base 75. In this manner the circuit 62 is readily accessible for repair or replacement. The base 75 can be held in place by, for example, a cover-plate 77 secured by screws 78.

The induction electrode 60 and the circuit 62 can be grounded for zeroing by, for example, a panel push rod (not shown) which contacts and grounds a leaf spring 80 in contact with electrode 60. The electrode 60 can also be grounded by attaching a spring contact (not shown) to the bottom of the slide 53 which grounds the circuit 62 when the slide 53 is in position for sampling.

Since, as hereinafter discussed, the particulate mass/volume readout is independent of the specific size of material being measured, the particulate volume/gas volume readout, when multiplied by the true bulk density of the material being measured, gives the mass/volume. The multiplication is done electronically by the electrometer circuit 62. A control knob 85 can be provided, as shown in FIG. 1, to adjust the circuit 62 to the proper bulk density of the specific material being measured. When the knob 85 is set at unity, the meter 70 will provide the particulate volume/gas volume.

FIG. 5 illustrates the support module 13 of the unit 10. The use of the module 13 facilitates the containment of the power supplies, the fan and the like in one easily accessible unit. It is understood, however, that any other type of means can be used for these elements or they may all be attached separately to the electrogasdynamic sampling device and the electrometer device.

In module 13, the following elements can be located: the constant current power supply 24; the constant voltage deposition power supply 38; a fan or impeller 90; a battery pack 91; a battery recharger 92; and the voltage output connections (not shown) corresponding to and adapted to mate with metal spheres 26 and 41 (FIG. 2). The power supplies 24 and 38 are described above with reference to FIGS. 1-3. The fan 90 can be of any convention type and its purpose is to induce a gaseous flow of a specified velocity in channel 20, especially when stagnant atmospheres are measured. For portable operation of the invention, battery pack 91 can be provided. If a battery pack is utilized, it is preferable also to provide a battery recharger 92.

To maintain the velocity of the flow in the channel 20 at a specified velocity, a transducer 138 (shown in FIGS. 8 and 9) can be provided. The transducer 138 is in communication with the fan 90 and is adapted to measure the flow velocity in the channel and to adjust the speed of the fan 90 accordingly to maintain the channel velocity constant.

It will be beneficial at this point to examine in more detail the theory of operation of the sampling apparatus.

Consider an applied electric field E (volts/m) established between two parallel plate conducting electrodes. Let the gas filling the space between the electrodes contain a mass concentration M (kg/m3) of particulate matter per unit volume. Mathematically, it can be demonstrated that the current density J (amperes/m2) flowing between the plates is a linear function of the particulate mass/volume concentration M.

The current density J is the product of the numerical density of the particulates np (number of particles/m3), the particle charge qp (coulombs), and the particulate velocity vp (m/sec):

J = np qp vp (1)

But

vp = kp E (2)

where kp is the mobility of the charged particulate (m2 /volt-sec), and also,

kp = qp / 6 πηa (3)

where η is the gas viscosity (kg/m-sec) and a is the particulate radius (m). Thus,

J = np qp2 E/6πηa (4) And from particulate charging theory,

qp = (12πεo Eo a2) (f) (5)

where εo is the permittivity of the gas (coul2 /nt-m2), Eo is the corona charging field (volt/m) and f is the fraction of saturation charge attained on the particulate. Thus,

Since,

mp = 4/3 π a3 δ or a3 = 3mp /4πδ (7)

where mp is the mass of a single particulate (kg) and δ is the bulk density of the particulate matter (kg/m3), then,

But,

np mp = M (9)

and thus,

For a given type of particulate matter, with a given corona charging system and a given particulate deposition field, this becomes;

J = KM (11)

where K is a constant.

Thus for any particular system, the current density J is a linear function of the particulate mass concentration per unit volume of gas. Experiments have proved the correctness of this theory.

The foregoing analysis also illustrates that the current density is independent of the particulate size (or size distribution).

The determination of particulate mass/volume by measuring the current density directly has two complications, namely, the extremely small currents dealt with and the length of time required for highly resistive particulates to transfer their charges to a metal electrode. As described above, the present sampling apparatus utilizes a dielectric surface backed by a grounded metal electrode as the deposition surface and the resultant build-up of potential (volts) on the dielectric is measured quickly and easily.

Appropriate restrictions regarding the sampling time must be observed, however, in order to insure that the output from the apparatus remains a linear function of the mass concentration M. The range of linearity of the present apparatus has been established theoretically; but before stating the results, it will be helpful to examine the theory.

Consider placing a dielectric material, for example, glass or Mylar, having a thickness dm (m) and permittivity εm onto the lower of two parallel plate conducting (metal) electrodes. The upper plate is charged by a high-voltage source to a voltage V and the lower plate is grounded, thus establishing an electric field Ea in the air-dielectric system between the plates. The total voltage drop across the system is,

V = Ea da + Em dm (12)

where da (m) is the distance between the upper plate and the uppermost surface of the dielectric (da thus is the air gap) and Em is the electric field (volts/m) established in the dielectric.

Further, at the air-dielectric interface, the discontinuity of the normal component of the electric displacement vector is equal to the surface charge density ρs (coul/m2) of the free charge on the interface, Thus,

εm Ema Ea = ρs (13)

where εa is permittivity of the air (coul2 /nt-m2).

Equations (12) and (13) can be used to derive an equation for the electric field Ea in the air which is driving the charged particulates toward the dielectric as,

where the first parenthetical expression is due only to the applied field V which is constant and the second parenthetical expression is due to the build-up on the dielectric surface as a function of time.

Now the current density, from Ohm's Law is,

J = σa Ea (15) where σa is the conductivity of the air (mho/m), and also,

J = dρs /dt (16)

where t is time in seconds.

Thus, from equations (14), (15) and (16), the following first order, linear, differential equation is derived;

By separation of variables and integration, the surface charge density build-up on the dielectric deposition surface can be expressed as a function of time,

ρs = (εm V/dm) (1 - e -t/ ) (18)

where the dielectric surface-charging time constant τ (sec) characterizes the time at which the surface has attained 63 percent of its final maximum value of εm V/dm and is given by the expression,

τ = εa dmm daa dm (19)

From Ohm's Law (σa = J/Ea), equation (10) can furnish ρa for substitution into equation (19), and thus,

Equation (18) for the surface charge density has an asymptotic growth. For t <<τ the growth is practically linear, that is, the slope (J = dρs /dt) is nearly constant with time. Therefore, for a collection (deposition) time T small compared with τ,

ρs = ∫ oJdt ≅ JT (21)

Combining equations (21) and (11),

ρs = (KT) M (22)

or

ρs = K1 M (23)

where K1 is a constant.

Further, the potential φs (volts) of the dielectric surface is a linear function of the surface charge density ρs, so,

φs =K2 M (24)

where K2 is a constant.

From the above analysis, the surface charge density ρs and the surface potential φs of the dielectric deposition surface are theoretically shown to be practically linear functions of the particulate mass/volume M, provided the particle collection time T is restricted to a value which is small compared with a time constant τ as given by equation (20).

To meet this requirement the present apparatus preferably should operate at collection times less than one-half a time constant as calculated by equation (20) for the maximum anticipated M value. In most instances, the time will be on the order of one to a few minutes.

Accordingly, the method and apparatus of the present invention provide an instrument response characterized by a short measurement time, an independence from particle diameter, and a direct signal readout which is a linear function of mass/volume over a wide range of particulate concentrations. Further, since current density is measured, it is not necessary to collect all of the particulate mass from the gaseous flow in order to measure M. In fact, the particles 30 themselves can be eroded from the sampling surface 37 by the gas velocity without affecting the measure of M so long as the particles leave their charge on such surface.

In order to insure uniform particle deposition along the length of the surface 37 for a specified channel gaseous flow velocity vg, the maximum deposition voltage V is determined by the following equation,

V = vg ηd a2 /2εo Eo fL a (25)

where L is the length of the dielectric sampling surface 37. The maximum V permissible is that which allows the largest anticipated particle to just traverse the entire air gap (da) as it travels the length L. This insures that all of the smaller particles are uniformly deposited along the length of the surface 37.

When the three modular units are jointed together to form a complete sampling apparatus, as shown in FIG. 1, the apparatus is used primarily to sample open atmospheres. The fan 90 is utilized, if necessary, to provide the requisite gaseous flow through the channel 20. However, when the apparatus is used for monitoring the emission of industrial stacks and the like where the temperatures of the gases are high and the gases naturally have a velocity, another embodiment can be used. This embodiment is shown in FIGS. 6 and 7.

The temperature of the gases in industrial stacks is typically on the order of 400° to 500°F. The embodiment shown in FIGS. 6 and 7 alleviates the deleterious effects that such temperatures would have on the operation of the different parts of the apparatus. As shown in FIG. 6, the sampling module 11 is detached from the electrometer module 12 and the support module 13 and is adapted for insertion into a stack or duct 125. A small hole 126, slightly larger than the cross-sectional area of the module 11, can be provided in stack 125 for the insertion. A lance 127 allows the operator to remain at a safe distance from the stack 125 during this sampling. In the preferred construction of the stack sampling apparatus, the lance 127 is about six feet long and is comprised of an aluminum tube 1 inch in diameter. The handle area of the lance 127 can be made of or covered with an insulated material, for example, wood or rubber, to ensure protection and safety of the operator. The lance 127 also can be bent along its length to facilitate ease of insertion of the module 11 into the stack 125.

For the embodiment shown in FIGS. 6 and 7, the sampling module 11 must be constructed of materials capable of withstanding the high temperatures in stacks. For example, preferably the metal parts are aluminum and the non-conductive parts are Teflon, but these parts can be made of any other materials with similar properties.

Attached to the lance 127 and adapted to hold the module 11 is a holding plate 130. In FIG. 7, finger 131 of holding plate 130 is designed for insertion into the end of the lance 127 by, the plate 130 being securely attached to the lance 127 by, for example, one or more screws (not shown). The holding plate 130 has a grounded plate 132 which is positioned so that it will be directly beneath the sampling surface 37 (FIG. 3) when the module 11 is mounted on such plate. The plate 132 is similar to the grounded plate 50 discussed above in reference to FIGS. 2, 3 and 4 and performs the same function. Also provided in plate 130 is a well 133 which acts as a seat for mating with the sampling module 11 (FIG. 2). The grounded plate 132 is attached to the bottom of the well 133.

The ionizing constant current voltage supply 24 and the deposition voltage supply 28 are connected to the module 11 by spring loaded "bullet" contacts 134 and 135, respectively. Contact 134 makes contact with metal sphere 26 (FIG. 2) of module 11 and contact 135 makes contact with metal sphere 41 (FIG. 2). The contacts 134 and 135 are, in turn, connected to wire conductors 136 and 137, respectively, which are carried by lance 127 to the support module 13. If desired, the conductors 136 and 137 can be coaxial. The electrostatic operation of the module 11 when it is placed in the holding plate 130 at the end of the lance 127 is the same as that described above with reference to FIGS. 2 and 3.

The naturally occurring velocity of the gaseous flow in the stack 125 provides the necessary flow for sampling. The velocity of the gases in most industrial stacks is in the range of 10 ft/sec to 100 ft/sec and experiments have shown that the present apparatus operates excellently within this range. The module 11 samples the flow transversely to the gaseous flow in the stack 125, but this does not affect the operation of the apparatus nor the accuracy of the resulting measurement.

At the conclusion of the sampling stage, the module 11 is removed from the stack 125, detached from the holding plate 130 and placed upon the electrometer module 12. The charge on the surface 37 and thus the mass/volume of the particulate matter in the flow is then measured and read in the same manner as that described above with reference to FIG. 4.

In some instances, it is preferable to have a sampling apparatus which provides a continuous measurement of the amount of particulate matter in the particular environment. One such instance involves mining operations where for safety reasons the amount of dust in the mine shafts should be known at all times. Even the few minutes needed to secure a sampling by the apparatus illustrated in FIGS. 1-5 possibly could be undesirable and hazardous. Continuous or automatic sampling apparatus are illustrated schematically in FIGS. 8 and 9.

In FIG. 8, the particles 30 entrained in the gaseous flow in the channel 20 are charged by an ionizing element 23 in the same manner as that described above with reference to FIGS. 2 and 3. Also similarly, the charged particles are repelled in a deposition region 35 and are driven toward and deposited upon a sampling surface 137. The sampling element in this embodiment, however, comprises a rotatable, grounded metal cylinder or drum 150 covered on its outside surface with a dielectric surface 137. The surface 137 can be any dielectric, such as a 3.5 mil Kapton tape which is self-adhesive and can be bonded to the drum 150.

The drum 150 is rotated in the direction of the arrow in FIG. 8 and continuously presents a portion of the surface 137 to flow for sampling. The tangential velocity of the drum 150 preferably is adjusted to be the same as the velocity of the gaseous flow in the channel 20. To measure the accumulated charge on the surface, a static charge induction electrode 60 connected to an electrometer (not shown) is provided. The electrode 60 is positioned adjacent the drum 150, preferably at the position illustrated in FIG. 8. The electrode 60 and the manner of achieving the resultant mass/volume measurement are the same as that described above with reference to FIG. 4.

After the surface 137 is "read" by the electrometer, it reconditioned (cleaned and neutralized) for further sampling. The reconditioning phase can comprise of, for example, a counter-rotating brush 151 which physically contacts the surface 137 and cleans the particles therefrom, and an A.C. corona 152 which discharges the surface 137 to zero volts (or to a base reference level). The surface 137 can also be neutralized by wiping it with a methanol wetted grounded roller (not shown). With this latter method of discharging the drum surface 137, however, it is preferable to remove the drum from the electrometer module 12. Mechanical means (not shown) for automatically sliding the drum 150 out of the module 12 can be provided.

To operate the drum 150 and cleaning brush 151, a small drive motor 153 of any conventional type is provided. The A.C. corona discharge 152 can be operated by the deposition power supply 38 (FIGS. 3 and 5) which can be contained in a supply module 13 similar to that described above.

Inserted into the channel 20 is an air velocity transducer 138 which is described above with reference to FIG. 5. In addition to being in communication with the fan and thus regulating the velocity of the flow in the channel, the velocity transducer 138 can also or alternatively be in communication with the drive motor 153 and utilized to adjust the rotation of the drum 150. In the latter instance, where the flow in the channel is caused by the gas itself, the transducer 138 can maintain the speed of rotation of the dielectric surface in a prespecified relationship to the velocity of the flow.

A second embodiment of a continuous sampling apparatus is illustrated in FIG. 9. In this design an endless dielectric belt 160 is rotated by two rollers 161 and 162. The belt 160 can be of any dielectric material, such as Mylar. One or both of the rollers 161 and 162 can be driven by a drive motor 153 of any conventional type. Positioned underneath the upper portion 237 of the belt 160 is a grounded metal plate 250. The operation of apparatus of FIG. 9 is the same as that with respect to the earlier described embodiments. The charged entrained particles 30 are driven by a deposition electrode 36 toward the grounded plate 250 and are deposited upon the portion of the belt designated by numeral 237. Also in a similar manner to that heretofore described with reference to FIG. 8, the accumulated charge on the surface 237 is measured and the belt 160 is cleaned and discharged for resampling.

For continuous sampling, the three phases of the sampling operation (the collection, measurement and discharge phases) are preferably carried out simultaneously but they do not have to be. For example, first the apparatus could be run for a period of time with only the collection phase and its corresponding power supplies operating. Then, the collection power supply could be turned off and the electrometer section put into operation. Finally, the electrometer power could be turned off and the cleaning and discharging phase put into effect, either by operating the A.C. corona 152 or by sliding out the drum 150 or belt 160 for wiping.

If any of the above embodiments (FIGS. 1-9) are used for sampling respirable dust, it is sometimes preferable to attach a particle size separation unit (not shown) at the inlet 21 of the channel 20. Such separation unit could be, for example, a cyclone unit, and would exclude from sampling all particles over a certain diameter. Alternately, an electrostatic deflection technique could be used to deposite out the more mobile, larger charged dust particles prior to entrance of the flow in the channel 20.

Although the invention has been described with reference to specific embodiments thereof, it is understood that substitutions, modifications and variations, both in form and detail, will occur to those skilled in the art. All such substitutions, modifications and variations, therefor, are intended to be included within the scope of the invention as defined in the appended claims.