RI 9643 REPORT OF INVESTIGATIONS/1998

Full-Scale Testing of the Float Dust Deposition 

Meter

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Report of Investigations 9643

Full-Scale Testing of the Float Dust Deposition

Meter

Robert A. Cortese and Henry E. Perlee

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

May 1998
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ........................................................................................... 1

Introduction ........................................................................................ 2

Background ........................................................................................ 2

Theory .......................................................................................... 2

Bench studies .................................................................................... 3

Experimental procedure .............................................................................. 3

Results and discussions ............................................................................... 7

Dust dispersion ................................................................................... 7

Mathematical simulation ........................................................................... 9

Conclusions ........................................................................................ 10

Acknowledgments ................................................................................... 10

References ......................................................................................... 10

ILLUSTRATIONS

1. Normalized particle size distribution of both coal and rock dust ........................................ 3

2. Principle of operation of float dust deposition meter .................................................. 3

3. Experimental Mine at Lake Lynn Laboratory ....................................................... 4

4. Float dust deposition meter used during full-scale studies at the Lake Lynn Experimental Mine ............... 5

5. Electronic circuitry ............................................................................ 6

6. Unagglomerated volumetric median particle diameters of coal and rock dust deposits as a function of distance

downwind of the dust source .................................................................. 7

7. Loadings of coal and rock dust deposited at various distances downwind of the dust source for three 

ventilation velocities ......................................................................... 8

8. Wt-pct of rock dust deposited as a function of distance from the dust source for three ventilation velocities ...... 8

9. Float dust deposition meter versus loading density ................................................... 8

10. 95% confidence band of total mass fraction as a function of the combined rock dust/coal dust loading density 

over a simulated 10 cycles .................................................................... 9

TABLES

1. Best-fit "'s with a 95% confidence interval ......................................................... 9
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

A ampere mg/cm milligram per square centimeter 2

cm centimeter mg/L milligram per liter

cm /mg square centimeter per milligram min minute 2

cm /L cubic centimeter per liter mm millimeter 3

ft foot, feet nm nanometer

kg kilogram s second

m meter V ac volt, alternating current

m square meter V dc volt, direct current 2

m cubic meter wt-pct weight percent 3

m/cm meter per centimeter μm micrometer

m/s meter per second % percent

mA milliampere

Mention of any company name or product does not constitute endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other occupational safety and health problems, call 1-800-35-NIOSH (1-800-

356-4674), or visit the NIOSH Home Page on the World Wide Web at http://www.cdc.gov/niosh
FULL-SCALE TESTING OF THE FLOAT DUST DEPOSITION METER

By Robert A. Cortese and Henry E. Perlee 1 2

ABSTRACT

Coal dust and float coal dust, produced during normal mining operations, in underground coal mines, are carried

from the point of origin downstream by the ventilating air, where it deposits on the surfaces of the mine entry. In

an explosion, this dust is lifted from the surfaces by the aerodynamic disturbances and, if of sufficient quantity, can

continue to propagate the explosion. To prevent the surface coal dust from contributing, it must be inerted,

typically by spreading pulverized limestone, i.e., rock dust, over the coal dust surface. To facilitate the dusting

operation, the National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory

(PRL), developed an automated system that continuously monitors the accumulation of coal dust. This system

could activate a rock-dusting machine that disperses rock dust into the ventilation air when dangerous deposits

accumulate and deactivate the machine when sufficient inert has been deposited on top of the coal dust.

The system consists of a microprocessor-controlled optical float dust deposition meter. This device measures

the light intensity reflected from a deposited layer of dust. A standard cap lamp is used as a fixed-position light

source. From the reflected light signal, the microprocessor determines the hazard level of the deposited layer and

performs the appropriate action.

Full-scale studies conducted in the Experimental Mine at PRL's Lake Lynn Laboratory, using alternating thin

layers of rock dust and coal dust, successfully demonstrated the operation of the device. Based on the statistical

data from these studies, a mathematical model was developed to predict long-term error in total rock dust content

using this inerting procedure. Downwind dust dispersion was also examined to provide optimum placement of the

dust sensors and to determine the need for better rock dust dispersion techniques.

Electronics engineer. 1

Supervisory physical scientist (retired). 2

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA.
M '

I

s

& I

c

I

r

& I

c

,

M '

Vs& Vc

Vr& Vc

,

Mc' e

&("c@ Fc)

Mr' 1 & e

&("r@ Fr)

,

2

(1)

(2)

(3)

(4)

INTRODUCTION

Coal dust generated in underground coal mines at the cutting downwind by the ventilating air. The third procedure is

face and points of coal transfer is floated downwind by the referred to as "trickle dusting." Because of the uncertainty

ventilating air. The floating coal dust settles on the mine associated with visual inspection, the National Institute for

surfaces over extended distances and, if not rendered inert, Occupational Safety and Health (NIOSH), Pittsburgh Research

represents an explosion hazard. Using the minimum estimate Laboratory (PRL), designed and built a float dust deposition

for the lower explosive limit [Weiss et al. 1989], 50 mg/L, meter (FDDM). The FDDM is located several meters

surface layers as thin as 0.05 mm can form a flammable downwind of a trickle duster to automatically measure the

air/dust mixture. For an operating mine, coal dust layers of loading densities of dusts. The trickle duster is turned on when

this thickness can develop in hours. the coal dust loading density approaches dangerous proportions

Usually a visual inspection by a mine foreman determines and turned off when a sufficient amount of rock dust has been

when and where to spread a layer of rock dust. The rock dust deposited.

is usually applied in one of three ways: sprayed from a hose Due to the current limitations of the FDDM, the meter will

attached to a dust pump and hopper; sprayed from a piping be restricted to use in return airways. Federal law requires

manifold of nozzles connected to a vehicle containing the dust 80 wt-pct incombustible content in the dust deposited in return

pump and rock dust and directed at the floor, roof, and ribs; or airways in the absence of methane.

dispersed from a stationary pumping station and carried

BACKGROUND

THEORY

The operation of the FDDM is based on the difference in

optical reflectivity of coal and rock dust. Rock dust is optically

light and highly reflective; coal dust is dark with a significantly

lower optical reflectivity. The mathematical expression for the and

normalized intensity (M) from a given layer is given by Sapko

et al. [1988]:

where Ic and Ir correspond to the intensity of light reflected dust and rock dust in the given equation is expressed in

from a semi-infinite thick layer of pure coal dust and rock dust, milligrams per square centimeter and is represented by F and

respectively. I represents the intensity of light reflected from F , respectively. Also associated with each dust is a coefficient s

a thin layer of either dust deposited on a semi-infinite layer of of attenuation, " and " , respectively.

the other dust. The attenuation coefficient is a function of the optical

Because the reflected light intensity is measured with a characteristics of the individual particles and the particle size

silicon photodiode, whose photoelectric current is a linear distribution. Figure 1 shows the particle size distribution of

function of the light intensity, equation 1 can be written as: both the coal and rock dust used in these studies. For this

where V is the voltage output from a high-impedance

transconductance amplifier connected to the photodiode.

The behavior of the resultant curve traces for the two

deposition processes (rock dust on coal dust and vice versa)

follow a common exponential relationship and are given by the

following equations:

where M corresponds to the theoretical expression for coal dust c

deposited on an optically thick layer of rock dust.

Complementarily, Mr corresponds to rock dust deposited on an

optically thick layer of coal dust. The loading density of coal

c

r

c r

graph and the remainder of this report, the volumetric median

diameter is reported as representative of the dust size. Because

the function of the FDDM is to correlate reflectance with the

mass of a deposited layer, the volumetric median diameter was

chosen to best represent the mass of a dust particle.
3

 Figure 1.—Normalized particle size distribution of both coal and

rock dust.

Figure 2.—Principle of operation of float dust deposition meter.

BENCH STUDIES

A principle of operation drawing of the FDDM, used in the

bench-scale research [Cortese et al. 1992] to verify the above

theoretical expressions, is shown in figure 2. The "'s in

equations 3 and 4, obtained by regressing the two equations

using measured M values determined with the FDDM for both

Pittsburgh pulverized coal and commercial limestone dust, are

" ' 1.0±0.05 and " ' 0.3±0.02, respectively. c r

The effects of both ambient light and distance between the

dust layer surface and the source light were also examined.

Although both factors affected the absolute light intensity at the

receiver diode, they did not affect the values of ". It was also

found that deposits of dusts on substrates with optical

reflectivities slightly different from those of pure coal and rock discovered that the "'s were dependent on the porosity, i.e.,

dust had no measurable effect on the values of ". This is an compaction, of the substrate, although no attempt was made to

important observation because for its intended use the substrate quantify this observation because it is not likely to be a factor

will rarely be pure coal or pure rock dust. It was also in the instrument's intended use.

EXPERIMENTAL PROCEDURE

Following the successful demonstration of the feasibility of positioned in the center of the entry, 2 m above the floor, and

the reflectance technique to measure the loading density of coal directed so as to dispense the dust in the same direction as the

dust deposited on rock dust and vice versa, it remained to ventilating air. The air exited the nozzle at approximately

conduct full-scale experiments at the Experimental Mine at 3 m/s. This point source arrangement produced a conical dust

PRL's Lake Lynn Laboratory near Fairchance, Fayette County, deposit on the floor for the first 10-15 m from the nozzle

PA (figure 3). The facility and supporting research have been beyond which the deposit reached the ribs. Air from the

fully documented by Triebsch and Sapko [1990]. ventilation shaft in E-drift was controlled by the speed of a

The first full-scale experiments conducted were designed to mine ventilation fan and the extent of closure of the bulkhead

measure the accuracy of the FDDM by comparing the loading door. In the initial studies, using three of the bench-model

densities measured by the FDDM with values obtained by FDDMs and a ventilation velocity of 0.9 m/s, the FDDMs were

collecting and weighing dust in petri dishes positioned down the positioned at 30, 60, and 90 m downwind of the nozzle

entry. These tests were conducted in D-drift of the mine, (figure 3). The three FDDMs were connected by electrical

approximately 6 m wide by 2 m high by 500 m long, using a cable to a microcomputer located upstream of the trickle duster.

commercially available trickle duster located 30 m outby the The microcomputer controlled the operation of the

bulkhead and equipped with a 4-cm outside-diameter flexible

hose and nozzle to disperse the dust. The nozzle was
LD '

0.25 @ ME @ V

SA @ L @ CF

,

4

Figure 3.—Experimental Mine at Lake Lynn Laboratory near Fairchance, PA.

(5)

FDDMs and acquired, digitized, and stored the FDDM analog entry (16 m for the Lake Lynn Experimental Mine entry).

sensor signals. To achieve a reliable estimate for the loading CF is the conversion factor to obtain the loading density

density, three dust-collection petri dishes (with a 6.9-cm radius (LD) in the proper units (for this case, CF is

and 1.3 cm deep) were placed 0.5 m upwind of each of the 1,000 cm /L @ 0.01 m/cm). The resulting coal dust loading

three FDDMs in the center of the entry. An electronic balance density for these studies is 0.9 mg/cm . Because the FDDM

located upwind of the trickle duster nozzle was used to weigh will initially be located only in returns, the rock dust loading

the dishes of dust to determine the mass of dust deposited. density was chosen to achieve the 80 wt-pct regulation, namely,

Preliminary dust dispersal studies were conducted in D-drift 3.8 mg/cm .

without the FDDMs to determine the deposition durations per Because the intent of this report is to simply demonstrate the

cycle required to achieve the desired loading densities. The feasibility of the FDDM, these calculations have been

desired coal dust loading density was calculated based on the simplified with a few assumptions. First, it was assumed that

following equation: the dust will deposit equally on all mine surfaces; in practice,

where (0.25 @ ME) represents restricting the maximum coal determined that 30 kg of rock dust dispersed in 24 min and

dust accumulation between rock-dusting procedures to 25% of 10 kg of coal dust deposited in 6 min will produce the desired

the minimum estimate for the lower explosive limit (mentioned loading densities. To a obtain a value for the two "'s by

earlier as 50 mg/L). V represents the volume contained in a regression for these experiments, the half-cycle deposition of

1-m-long entry (12 m for these studies). SA represents the each dust was divided into three equal-time intervals, i.e., three 3

total surface area of the ribs, floor, and roof in a 1-m-long 2-min coal dust minideposits and three 8-min rock dust

2

3

2

2

most of the float coal dust and rock dust will accumulate on the

floor. Secondly, these calculations assume that rock dust is the

only incombustible present, i.e., there is no water or ash.

From these preliminary dust dispersal studies, it was
5

 Figure 4.—Float dust deposition meter used during full-scale

studies at the Lake Lynn Experimental Mine.

minideposits. Before beginning the experiment, 35 kg of rock

dust was dispersed in the entry to provide an optically thick

substrate for the initial coal dust deposit.

To conduct an experiment, the trickle duster, loaded with

coal dust, was started and the coal dust was dispersed for

2 min. The duster was then stopped, and time was allowed for

the remaining entrained dust to settle. The petri dishes were

collected, weighed, and replaced, and the reflectivity of the

dust surface was recorded with the FDDM. The duster was

restarted and the process was repeated two more times, after

which the same sequence was followed using rock dust

deposited on the recently deposited coal dust layer. A total of

four cycles were included in each experiment. It was

discovered during these experiments that suitable data could not

be obtained from all three stations simultaneously. In order to

collect a measurable quantity of dust at the 60- and 90-m

stations to accurately analyze the data, the 30-m station

optically saturates, preventing further measurements of the

layer loading density at this location. As a result, the

experimental setup for subsequent testing was altered to locate

the three FDDMs at one location and measure the variance

between the meters.

These initial studies also demonstrated that the bench-model

FDDM was unsuitable for in-mine use primarily because the

entrained dust deposited on the internal surfaces of the

instrument caused extraneous reflections. This problem was

solved with the design shown in figure 4. The optics are

enclosed in a housing (18 cm wide by 18 cm long by 76 cm After the controller is energized, the program enters a

high), equipped with a surface (square 15 cm on side) that can continuous loop, where it waits for a sample request from the

be extended outside the housing to collect the depositing dust, researcher. After the request is received, the optic electronics

and retracted inside the housing to measure the reflectivity of are powered and the retractable tray is retracted. At this point,

the deposit. In the bench model, an oblique optical system was the software enters a 3-cycle loop where the following sequence

used to allow the dust to properly deposit on the collection of events occurs: (1) the lamp is turned on, (2) all 6 sensors

surface. With the retractable collection surface, the oblique (2 for each unit, 1 for the receiver, and 1 for the reference) are

optical system is no longer necessary, resulting in increased read 10 times over a 3-s period and averaged, (3) the averaged

intensity of the reflected light reaching the optical receiver. A receiver-to-reference ratio is calculated, (4) the receiver,

20-cm-wide flow divider was attached to the leading edge of reference, and resultant quotient averaged values are stored,

the housing to minimize the generation of turbulent eddies over and (5) the lamp is turned off. After executing the three cycles

the extended surface as the air flows around the housing. The of this loop, the collection surface is returned to the extended

lamp battery, relays, and photodiode electronics were contained position, the optic electronics are powered down, and the

in the top compartment. system waits for the next sample request. The program is

The microprocessor-based controller (figure 5), requiring terminated when power is removed from the controller.

5 V dc at 250 mA, performs the following functions when The next series of experiments was designed to measure the

requested: (1) powers the screw drive motor, drawing 30 mA reproducibility of the FDDMs. Three different air velocities

at 120 V ac (forward and reverse) for the retractable surface, were used in these studies: 0.9, 1.2, and 2.0 m/s. In these

(2) powers the lamp (4.5 V dc at 1.2 A) and electronics tests, the three FDDMs were placed across the entry 30 m

(±15 V dc at 50 mA and 5 V dc at 50 mA), (3) records the downwind of the trickle duster nozzle, with one unit placed in

photosensors' outputs, and (4) processes the sensor data. The the center of the entry and a unit 0.7 m on both sides of the

controller consists of (1) an 8086-compatible microprocessor center unit. The controller was placed along the rib also at

chip running a C language program, (2) a 128K RAM, (3) a 30 m. A sample-request switch was located 2 m upwind of the

128K EPROM, and (4) a 0- to 5-V dc, 10-bit, analog-to-digital trickle duster nozzle. The collection dishes were positioned

converter. adjacent to the FDDMs on a raised platform 30 cm above the

floor (the same height as the retractable dust collection
6
7

 Figure 6.—Unagglomerated volumetric median particle diameters of coal and rock dust deposits as a function of distance

downwind of the dust source.

surfaces). These dishes were collected and weighed after each only the coal dust particle size distribution can be obtained at

minideposition. Additional dishes were placed on the floor in these locations. Additionally, a 633-nm helium neon laser was

the center of the entry at 15, 30, 46, 61, 91, 122, and 152 m installed at the 30-m station and directed across the entry (4 m)

downwind of the duster. The dishes at the first four locations to a photodiode to measure the beam attenuation due to the

were collected and weighed after each minideposition; the last intervening dust. Knowing the attenuation and the particulate

three were collected and weighed after completion of the mass flux (determined from the mass deposited in the dishes),

experiment. The particle size distribution for the pure dust the mean particle size of the airborne dust can be determined.

samples collected during the first cycle were measured using a This was necessary because the Coulter Counter measures the

Coulter Counter. A similar coal size analysis was performed unagglomerated particle size distribution after collection, and

on the dust collected at the three farthest locations (91, 122, the bulk of the airborne coal and rock dust particles most likely

and 152 m) after the rock dust was removed by acid leaching. contain significant quantities of agglomerates [Fink 1975].

The acid-leaching process also determined the coal dust to rock These results, along with a detailed explanation of the theory,

dust mass ratio. Because acid leaching dissolves the rock dust, will be reported in a future publication.

RESULTS AND DISCUSSIONS

DUST DISPERSION

The volumetric median particle diameters and dust deposition rates were obtained from the petri dishes, as indicated

above. The median diameters were measured because the

coefficient of attenuation is theoretically dependent upon this

factor and therefore could influence the results of the FDDM.

The dust deposition rates were measured to assist in determining an optimum location of the FDDM in a mine relative

to the rock dust source.

Figure 6 shows the unagglomerated (Coulter Counter, with

an aperture setting of 200 Fm) volumetric mean particle

diameters [Woods et al. 1988] of the coal and rock dust

collected in the dishes as a function of the downwind distance

from the trickle duster for the three ventilation velocities. As

previously noted, the rock dust size data are not available at the

three farthest distances. As expected, the median particle size

for both dusts decreases with distance from the source because

the larger particles are the first to settle. Figure 6 also shows,

unexpectedly, that there is no statistically significant difference measurement, it is also nonflammable downwind, assuming that

in the median particle diameters at any location for the three at the measurement station the deposition processes have

ventilation velocities for either dust. It was anticipated that the stabilized. Figure 8 shows the percentage of rock dust in the

results would show larger diameter particles farther downwind total sample of coal and rock dust that was deposited in the

for the higher ventilation velocities. dishes at the conclusion of the experiment. Data are shown as

Figure 7 illustrates the mass loading density for coal and a function of location for the three ventilation velocities. It is

rock dust, determined from the petri dishes, as a function of apparent from the figure that the rock dust concentrations in the

distance from the source. The figure shows that more of the samples increase with increasing distance, asymptotically

rock dust, being of smaller median particle diameter, is carried approaching 90 wt-pct. The dashed line in figure 8 represents

downwind than coal dust. This would result in higher rock the average 80 wt-pct of rock dust dispersed in the experiment.

dust to coal dust loading density ratios, i.e., mass per unit area The figure shows that coal dust has a lower concentration

ratios, downwind of the measuring station. This would also farther downwind; this is expected because the coal dust has a

ensure that if the deposit is nonflammable at the point of larger median particle size and therefore a larger mass

sedimentation flux than the rock dust.
8

 Figure 7.—Loadings of coal and rock dust deposited at various

distances downwind of the dust source for three ventilation

velocities.

 Figure 9.—Float dust deposition meter (M) versus loading

density, as measured from the petri dishes, with fitted theory for ( A)

0.9 m/s ("c = 1.38 and "r= 0.15 cm/mg), (B) 1.2 m/s ("c = 1.27 and "r

2

= 0.13 cm/mg), and (C), 2.0 m/s (" = 1.40 and " = 0.13 cm/mg). 2 2

c r

 Figure 8.—Wt-pct of rock dust deposited as a function of

distance from the dust source for three ventilation velocities.

Figures 9A through 9C show data from the FDDM. Each

is a plot of M, calculated using equation 2, as a function of the

dust loading density, as measured from the petri dish samples

over the four cycles. As expected, because the same mass of This is similar to the means in which the meter will be used

each dust was used in each cycle, the value of M oscillates under actual mining conditions, i.e., rock dust is dispersed until

between an upper and lower bound, with M ' 1 and M ' 0 M ' 0.5, then coal accumulates during mining operations until

corresponding to an infinite layer of rock dust and coal dust, alarming at M ' 0.15, etc. To compare the values of "

respectively. This is intended to simulate the starting and

stopping of the trickle duster using upper and lower M limits.
9

 Figure 10.—95% confidence band of total mass fraction as a

functionof the combined rock dust/coal dust loading density over

a simulated 10 cycles.

Table 1.—Best-fit "'s with a 95% confidence interval

Station 0.9 m/s 1.2 m/s 2.0 m/s

Rock dust Coal dust Rock dust Coal dust Rock dust Coal dust

Station 1..... 0.14±0.04 1.35±0.31 0.12±0.01 1.31±0.40 0.13±0.04 1.29±0.26

Station 2..... 0.16±0.04 1.35±0.26 0.15±0.02 1.42±0.40 0.19±0.03 2.08±0.58

Station 3..... 0.14±0.02 1.45±0.63 0.12±0.01 1.07±0.32 0.07±0.02 0.82±0.14

Composite... 0.15±0.01 1.38±0.13 0.13±0.01 1.27±0.15 0.13±0.03 1.40±0.44

determined in the bench studies with those of the full-scale dust due to a nonuniform distribution of turbulence.

studies, the results shown in figures 9A through 9C were leastsquares fit to equations 3 and 4. Equation 2 was substituted for

M, and a regression using four fit parameters in the equations,

namely Vr, Vc, "r, and "c, was performed. Ideally, all four Using the mean values of the "'s, respective tolerances, and

parameters should be statistically determined using the fit; equations 1 through 4, a Monte Carlo simulation of the FDDM

however, this technique results in a set of eight highly was undertaken. The simulation estimated the error in the total

nonlinear equations. The process was simplified by forcing the mass fraction of rock dust in the stratified deposits for a cyclic

fitted expressions through the first point at the start of each deposition of coal dust and rock dust. The simulation assumed

cycle, which in effect eliminated Vr and Vc. The remaining that the "'s are normally distributed with means and confidence

data, in each half-cycle, were then fit to equation 3 or 4 to intervals equal to the measured values, i.e., rock dust

obtain an estimate and variance for "r and "c. The "'s for the 0.14±0.02 and coal dust 1.35±0.24. For the simulation, the

four half-cycle estimates were averaged and the variances value of M required to turn on the trickle duster was set at 0.05

pooled to obtain the best estimate for the "'s and their and the value to turn off the duster was 0.55, which should

respective tolerance limits, as shown in table 1. The results result in an 80 wt-pct rock dust mixture. Figure 10 shows the

show that, at a 95% confidence interval, there is no statistical results of the simulation. The shaded area brackets the solution

difference in the "'s as measured by the three FDDMs at shown as the solid line and indicates the total mass fraction of

the same air velocity. We conclude from these results that the rock dust in the stratified mixture. This is plotted as a function

theory, as expressed in equations 1 through 4, is sufficiently of the combined loading density of coal dust and rock dust as

accurate for our purposes and the three FDDMs provide the the cyclic deposition progressed. The simulation was run 20

same results for a given layer. However, it is to be noted that times for 10 cycles; the error in total mass fraction of rock dust

the " values, as measured in the bench experiments (" ' 0.3 after the 10th cycle was found to be ±3%. The plot shows r

and " ' 1.0), are statistically different from those obtained in that, even taking into consideration the variability of the "'s, c

the full-scale experiments. Although it remains to be the meter performs within acceptable limits.

investigated, it is believed that this difference is due to the

difference in the particle size distributions of both the coal and

rock dusts between the two experiments. Although the same

starting coal and rock dust were used for the experiments, the

median particle diameters of the collected large-scale samples

were smaller due to the distance from source size classification

effects. The median particle diameters for both dusts collected

in the bench studies were identical to those of the starting

materials, because for those tests the dust was confined to a

stove pipe and settled horizontally. The median diameters for

coal and rock dust in the bench studies were 49 Fm and 36 Fm,

respectively, whereas the corresponding median diameters at

30 m from the source were 42 Fm and 24 Fm (average of all

three air velocities), respectively. A significant difference with

various air velocities is not evident. This is not surprising

because the mean diameters of both rock and coal dust do not

differ statistically with air velocity at the 30-m location, as

shown in figure 6. Some inconsistencies were evident in the

2.0 m/s test; this was caused by low batteries (this was the last

of the three tests conducted).

The small variations in the "'s are most likely attributed to

the measurement errors in calculating the loading density.

Significant variations in the loading densities were observed

between the three trays placed 20 cm apart used to determine

the minideposit loading density. These variations are most

likely caused by a nonuniform cross-sectional deposition of the

MATHEMATICAL SIMULATION
10

CONCLUSIONS

An FDDM designed to monitor the mass of float coal dust the constants requires making parallel measurements of loading

depositing in underground coal mine passageways and to density and surface reflectance under actual operating

automatically control a rock-dusting machine has been conditions, i.e., rock dust dispersal techniques and float coal

successfully tested under full-scale conditions. The test dust generation during typical mining operation, at the desired

demonstrated that the unit is reproducible and capable of meter location. Further studies are being discussed that will

maintaining the total rock dust concentration in the deposited focus on determination of "'s relative to given dust properties,

dust downwind of the source well within acceptable limits. such as median diameter, density, and reflectance of pure

Still to be resolved is the technique to determine the correct material.

"'s for installing the meter. Currently, the determination of

ACKNOWLEDGMENTS

The authors acknowledge C. D. Litton, physicist, for technicians, for suggesting and developing methods to better

suggesting a means to measure the airborne particle size; conduct the research; and Gregory M. Green, physical science

Eugene M. Bazala and William Slivensky, physical science technician, for conducting particle size analyses.

REFERENCES

Cortese RM, Sapko MJ, Perlee H [1992]. Automated rock dusting Weiss ES, Greninger NB, Sapko MJ [1989]. Recent results of dust

controllers. In: Proceedings of the IEEE-Industrial Applications Society explosion studies at the Lake Lynn experimental mine. In: Proceedings of the

Annual Meeting, pp. 2034-2040. 23rd International Conference of Safety in Mines Research Institutes.

Fink ZJ [1975]. Agglomeration of particles flowing down a vertical duct. Washington, DC: pp. 843-854.

Powder Technology 12:287-289. Woods T, Sapko MJ, Perlee H [1988]. Analog optical rock dust meters.

Sapko MJ, Pinkerton JE, Bubash JR [1988]. Optical dust deposition In: Proceedings of the IEEE-Industrial Applications Society Annual Meeting,

meter. IEEE Transactions on Industry Applications 24(3):508-513. pp. 1272-1276.

Triebsch GF, Sapko MJ [1990]. Lake Lynn Laboratory: a state-of-the-art

mining research laboratory. In: Proceedings of the International Symposium

on Unique Underground Structures. Chapter 75, pp. 75-1 to 75-21.
DHHS (NIOSH) Publication No. 98-139

1998

To receive other information about occupational safety and health problems, call 

1-800-35-NIOSH (1-800-356-4674), or

visit the NIOSH Home Page on the World Wide Web at

http://www.cdc.gov/niosh