TECHNICAL DEVELOPMENT OF THE COAL DUST EXPLOSIBILITY METER 

M. J. Sapko, NIOSH, Pittsburgh, PA 

H. Verakis, MSHA, Triadelphia, WV 

Abstract 

The Pittsburgh Research Laboratory (PRL)1

 

 

 

1

 Disclaimer: The Findings and conclusions in this report are those of 

the author(s) and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. 

of the National 

Institute for Occupational Safety and Health (NIOSH) in collaboration 

with the Mine Safety and Health Administration (MSHA) Technical 

Support Center has devised a prototype hand-held instrument which 

can provide a direct assessment of the potential explosibility of a coal 

and rock dust mixture. 

Current Federal regulations require that rock dust be applied 

in all underground areas of a coal mine to mitigate the propagation of a 

coal dust explosion. To determine compliance with the Federal 

regulation, mine inspectors collect dust samples from sections of 

underground coal mines for an MSHA laboratory analysis of 

incombustible content. However, the laboratory analysis may not be 

completed for more than a week or two, during which time the mine 

area from which samples are taken may be inadequately protected. 

The operation of the Coal Dust Explosibility Meter (CDEM) is 

based on the difference in optical reflectance between light rock dust 

and dark coal dust. For a given coal volatility, the optical reflectance of 

such mixtures is relatively constant at the limit of explosibility, 

independent of the coal dust particle size. The CDEM is not intended 

to replace the current MSHA laboratory analysis of coal mine dust 

samples for incombustible content, but to serve as a supplemental 

device for enhancing mine safety through improved rock dusting 

practices. 

Introduction 

Despite the worldwide research on coal mine safety, coal 

dust explosions involving fatalities and injuries still occur each year. 

Research by the U.S. Bureau of Mines (USBM) and agencies in other 

countries has shown that mixing a sufficient quantity of inert rock dust 

with coal dust will prevent explosions. U.S. Federal law 30 CFR, 

75.403 mandates that the nation's coal mines maintain a total 

incombustible content of at least 65 percent in non-return entries and 

at least 80 percent in the return airways where potential for 

accumulation of finer float coal dust is greater. The 65% total 

incombustible content is based on a fairly coarse "mine-size dust," a 

term used often in coal dust explosion research. It was adopted about 

1925 and refers to coal dust all of which passes a U.S.A. Standard 

No.20 sieve (841 micron) and contains 20 percent minus 200-mesh 

(74 micron) particles. The justification for adopting it is given in Bureau 

of Mines Technical Paper 464 (Rice et al 1929). Information on the 

size of dust in mines was obtained by collecting representative 

samples from passageways not rock-dusted and subjecting them to 

screen analysis. 

Reported in Technical Paper 464, dust collected from ribs, 

roof, and timbers were the finest, and 40% to 70% of the material 

passed a 200-mesh sieve. Floor dusts were much coarser, and 

samples contained 5% to 40% of material passing through a 200-mesh 

sieve. The sizes were found to vary considerably between points in a 

single mine, and the quantity of dust also varied. The averages were 

weighted and for 80% of the mines the final values ranged from 15% to 

25% through 200-mesh. Averages higher than 25% were found 

occasionally and those below 15%, rarely. The dust used in explosion 

tests was of the standard size nearest to or next finer than the 

weighted average for the mine furnishing the coal. Thus, dust having 

20% through 200-mesh was used most in explosion-hazard 

investigations and in determining the current 65% total incombustible 

requirement for intake roadways. 

To comply with regulations, mine personnel periodically dust 

the mine interior with an inert material, such as pulverized limestone 

(rock) dust. In determining compliance with the law, inspectors for the 

Mine Safety and Health Administration (MSHA) periodically collect 

samples of deposited dust from different areas in a mine. In making a 

determination as to whether samples are to be collected, visual 

inspection is often used. Typically, poorly rock-dusted areas along a 

mine entry appear dark, and well rock-dusted areas appear light. When 

samples are collected in any given mine, they are usually collected at 

intervals of 500 feet of entry. The preponderance of samples collected 

are relatively dry (< 1 % surface moisture). The inspector screens the 

samples through a 10-mesh sieve and sends about 200 grams of the 

sieved sample to MSHA's facility in Mount Hope, West Virginia for 

laboratory analysis. 

Laboratory analysis at Mt Hope [Montgomery 2005], consists 

of passing the sample through 20 mesh sieve (841 micron) to remove 

material larger than 10 mesh and then oven drying the minus 20 mesh 

material for one hour at 105o

C and recording the weight change. The 

dried sample is then heated in an oven which is ramped up over 1.5 

hours and held at 515o

C for about 2.5 hours to burn off the combustible 

coal fraction, thereby leaving the incombustible ash. The weight of the 

remaining ash, plus the as-received-moisture, is combined and 

reported as total % incombustible content (TIC). In general, the total 

processing time from sample collection is about two weeks. During this 

time, a section or sections of a mine may or may not be adequately 

rock dusted to protect against a coal dust explosion. Also, the 

laboratory gravimetric analysis does not take into account the effect of 

particle size on the explosibility of dust samples. For example, given 

two mine samples containing 65% TIC, one of which contains 20% of 

coal mass distributed in particles sizes less than 200 mesh and the 

other contains 40% of coal mass less than 200 mesh, both are in 

compliance with the law based on gravimetric analysis. However, in 

full-scale explosion experiments conducted in the Bruceton and the 

Lake Lynn Experimental Mines with these dusts, the 40 % <200 mesh 

will propagate while the 20% <200 mesh will not. 

This paper presents recent data on the development and 

evaluation of a digital Coal Dust Explosibility Meter (CDEM). The 

CDEM is capable of identifying a dust explosion hazard independent of 

coal particle size. The meter could be used by the mine operator to 

manage daily rock dusting operations and can also serve as a device 

to alert mine personnel or mine inspectors to potentially dangerous 

accumulations of explosible coal dust. 

Coal Dust Explosibility Meter 

The CDEM consists of an optical probe connected to a small 

electronics box with a digital display. The principle of operation of the 

CDEM is based on the measurement of infrared radiation reflected 

1 
Dust explosibility is strongly dependent on the fineness of 

from the surface of a homogeneous mixture of two powders with 

different optical reflectance, such as light colored rock dust and dark 

coal dust. Near-infrared radiation is emitted by a light emitting diode 

located behind the window of the probe shown in figure 1. When the 

meter is inserted in the dust mixture, the infrared radiation reflects off 

the dust's surface and back to the silicon photodiode sensor also 

located in the optical module. The normalized reflectance, Φ, is related 

to the mass fraction of rock dust, fr

, in the sample by the following 

equation: 

Φ = (I ) x-Ic) (I r -Ic) = ) f r (fr +K (1- fr

)) (1)

where Ix is the intensity of light reflected from a homogeneous coal and 

rock dust mixture, Ic is the intensity of light reflected from a pure coal 

dust surface, Ir

 is the intensity of light reflected from a pure rock dust 

surface, and K is a constant related to the material densities and mean 

particle diameters of both the coal and rock dust. Therefore, Φ is the 

optical reflectance normalized to 100 percent for pure rock dust. 

window

Probe Lens and Sensor 

Dust Cup 

Figure 1. Prototype Coal Dust Explosibility Meter 

the coal particles in the mixture. Particle size has a high variability both 

within and between mines, since size is dependent on factors such as 

mine type (i.e., longwa1l or continuous miner), pick cutting speed, cut 

depth and coal type. Size distribution will also vary along mine entries 

as coarser dust is deposited from ventilation streams closer to the 

production area, while finer dust is carried farther down the entries. 

The effect of particle size of coal dust on the explosibility is best 

illustrated in figure 2, by the incombustible required to arrest explosion 

propagation from large-scale experiments conducted in both the 

Bruceton and Lake Lynn Experimental Mines. The curve shows the 

amount of incombustible required to prevent propagation for coal dust 

containing 10 to 75 % of particles passing a No. 200 sieve (-74 

microns). With 10 % minus 200-mesh, 55 % incombustible is required; 

with 20% minus 200-mesh, 65% is required; and with 90 % minus 200­

mesh, 80 % incombustible is required. This is also the data used to 

support the current 65% incombustible requirement for intake airways. 

To improve rock dusting practices, the U.S. Bureau of Mines 

(USBM) developed a portable Rock Dust Meter (RDM) based on 

optical reflectance capable of measuring the concentration of rock dust 

in dry, homogeneous samples (Sapko and Watson, 1985, Perlee, et 

al., 1986, Woods, et al., 1988, Sapko et al. 1991). However, since the 

amount of rock dust required to inert a mixture varies with coal particle 

size (Cashdollar and Hertzberg, 1985, Cashdollar et al, 1987, 

Cashdollar and Hertzberg, 1989a, Cashdollar et al 1989b, Nagy 1991), 

the measurement of the rock dust concentration is not, by itself, 

sufficient to determine the possible explosion hazard. The RDM was 

capable of ±2 weight (wt) % accuracy with prepared samples using 

mixtures of coal and rock dust with known particle distributions. 

Therefore, to accurately determine the concentration of rock dust in the 

mixture the RDM had to be calibrated for the particular type and size of 

rock dust, and the type and size of coal dust. 

2 

Propagation 

BEM- Non-Propagation 

BEM- Propagation 

LLEM- Non Propagation 

LLEM- Propagation 

0 20 40 60 80 100

Minus 200 mesh content of coal dust, % 

Figure 2 Effect of particle size of coal dust on 

explosibility. 

80 

70 

60 

50 

40 

30 

20 

10 Total Incombustible Content, % 0

100 

90 

NON-Propagation 

While examining the data from many coal and rock dust 

mixtures, it was observed that the normalized reflectance, Φ, 

measured by the optical reflectance meter, appeared to be constant for 

a given coal dust at the limit of explosibility, independent of the coal 

particle size. To help explain this observation, Litton and Chaiken 

[1996] presented a theory and supporting data that coal dust 

explosibility and inerting requirements by powdered inhibitors such as 

rock dust depend upon a material property called the specific 

absorption which, in turn, relates to the ability of the coal and inert dust 

to compete for absorption of radiant energy from a source of ignition or 

propagation. Their theory explains the experimental observation that, 

at the lower explosion limit, the product of specific absorption and 

mass concentration of coal dust is constant for fixed coal volatility. 

Theory and data also support the experimental findings that the RDM 

may also serve as a CDEM to determine the explosion potential of dust 

mixture. The CDEM is a simple optical device that measures the 

relative reflectance from bulk mixtures of coal/rock dust. The ratio of 

the coal to rock dust specific absorption parameters, as determined by 

the CDEM, allows for an a priori prediction of the mass fraction of rock 

dust necessary to inert a particular coal dust. Most importantly, for 

coal of a fixed volatility, the relative reflectance Φ measured by the 

CDEM at the inert limit is a constant, independent of particle sizes of 

the coal dust and rock dust. 

To determine Φ, or extinction limit, for Pittsburgh Coal Dust 

based on experimental full-scale explosion propagation data, mixtures 

of rock dust with various fractions of minus 200 mesh coal dust were 

prepared and the test data is shown in figure 2. Air dried coal and rock 

dust mixtures were prepared at the specific rock dust percentage 

required to inert as a function of <200 mesh coal dust fraction. The 

reflectance of this sample was measured by manually inserting the 

sample in the cup and pressing the sample against the sensor window 

and recording the reflectance Ix. The reflectance of the pure coal dust, 

Ic and rock dust, Ir

 was also measured to determine the normalized 

reflectance of the mixture, Φ. Since the measured rock dust inerting 

limit has a precision of approximately ± 3 wt percent, rock dust 

samples were also prepared at ± 3 wt percent from the inerting limit, 

and their reflectances also measured. Figure 3 shows the Φ values 

measured at the inerting limit for the Pittsburgh Coal Dust sizes shown 

in figure 2. The error bars represent the measured Φ of the samples 

containing ± 3 wt percent rock dust from the percentage to inert. 

Therefore, for Pittsburgh coal, Φ at the limit of explosibility is fairly 

constant over a wide range of particle sizes. 
 

Lucci et al 1995 conducted a similar analysis where Φ value 

was determined based on extinction limits from tests conducted in the 

Bureau of Mines 20-L laboratory explosibility chamber for range of 

coals with different volatilities. The extinction limit Φ was shown 

experimentally to increase with increasing coal volatility. Normalized Reflectance, 

Φ

1 

0.9 

0.8 

0.7 

0.6 

0.5 65% TIC 73% TIC 

0.4 

0.3 

0.2 

0.1 

Non-Explosible 

0 

80% TIC 

Explosible 

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

Minus 200 mesh content of coal dust, % 

Figure 3. Normalized optical reflectance of Pittsburgh seam coal 

as a function of particle size of coal. 

In practice, the Φ value at which the meter would alert would 

differ depending on the volatility of the coal seam being mined. The Φ

value ranges from 0.3 to 0.4 for 18% and 36 % volatility respectively. 

The data in figure 4 and supporting absorption theory suggests that the 

CDEM can be used to indicate the potential explosibility of coal and 

rock dust mixtures, even if the coal particle size is unknown. While the 

amount of rock dust needed to inert a mixture increases with 

decreasing particle size, the normalized reflectance of coal and rock 

dust mixtures at their limits of explosibility is reasonably constant for a 

given volatility of coal. 

3 

70 

80 

100 

90 

30 

40 

50 

60 

0 2 4 6 8 10 1 2

Added moisture, wt % 

C D E M , % T I C

Dried 

Before drying Wet 

After Drying 

10 

0 

Figure 4. Effect of added surface moisture on the CDEM reading 

before and after sample drying using molecular sieve.The Effects 

of Moisture on the Accuracy of the CDEM. 

20 

The Effects of Moisture on the Accuracy of the CDEM 

Although the meter is capable of ± 3 wt % accuracy using 

dry samples (< 0.5% moisture), wet or damp samples can have a 

significant impact on the meter accuracy as shown in figure 4. The 

presence of added surface moisture changes a sample’s reflectivity 

linearly and quite dramatically with increasing surface moisture. For 

example 5 % added moisture to a sample containing 65% rock dust 

and 35 % Pittsburgh Coal dust decreases the CDEM reading from 65% 

to 30 %. The reading of a damp sample would err on the side of safety 

indicating the need for more rock dust when in fact the sample may not 

be explosive or deficient in rock dust. Such results indicate that the 

meter cannot be used on samples containing over 0.5 % surface 

moisture. Therefore, to alleviate this problem for in-mine use, NIOSH 

developed a simple drying technique using a commercially available 

molecular sieve. Molecular sieve [Davison Chemical Division [2005]*2 

2

 Reference to specific products is for information only and does not 

imply endorsement by NIOSH 

is crystalline, hydrated metal aluminosilicate that is used in the 

chemical industry for separation of chemical using adsorption. The 

sieve are manufactured in the form of beads 8 to 12 mesh size with 4 

angstroms pore used for general drying applications. The 4 angstrom 

sieve used in this study was capable of adsorbing 0.2 gram of water 

per gram of sieve. The sieve behaves as a physical adsorbent. As 

molecules of water enter the sieve, they are held by physical forces of 

the Van der Waals type. The sieve can be regenerated for reuse by 

heating in an oven to 300 o

F. For this study the sieve were not 

regenerated since the cost of regeneration by heating of small samples 

outweighed the cost of the sieve itself. Drying the sample consisted of 

placing about 2 grams of damp dust sample with 15 grams of 

molecular sieve in a capped container and then vigorously shaking the 

damp dust particles with the sieve material. The sample was allowed to 

set for 1 minute. After drying the sample contained <0.2 % bound 

moisture. Then the dried sample was screened through a 20 mesh 

sieve into the sample cup of the CDEM and a reading was taken. Also 

shown in figure 4 are the CDEM TIC readings after the samples were 

dried with molecular sieve. The CDEM reading after drying were all 

within 2 % of the TIC reading before the water was added. The method 

of drying with the molecular sieve is a viable approach for handling 

damp or moist samples within the mine environment. 

Field Sample Evaluation 

The meter was first checked against prepared samples 

consisting of mixtures of Pittsburgh Pulverized Coal (PPC) dust (80% 

<200 mesh) and commercially available limestone. Figure 5 shows the 

results obtained for the PPC mixture after calibrating the meter. The 

meter readings agree with the actual rock dust concentration to within 

±2% rock dust for prepared samples. The meter simultaneously 

provides an output of either red, yellow or green. Red indicates that the 

mixture is explosible or needs more rock dust; yellow or borderline 

indicates further analysis is needed to determine if more rock dust 

should be added. Green indicates the sample is non-explosive or 

contains sufficient rock dust to prevent propagation. 

In addition to the experimentally prepared sample mixtures 

using PPC, the CDEM was evaluated using hundreds of samples 

provided to NIOSH by MSHA’s Mt. Hope laboratory. MSHA Technical 

Support personal coordinated the acquisition mine dust samples 

collected by mine inspectors from MSHA’s Coal Mine Safety and 

Health Districts 2 and 11. Subsequently the Mt. Hope Laboratory 

provided to NIOSH a selection of mine dust samples collected by 

MSHA mine inspectors that were obtained from mines in all MSHA’s 

Coal Mine Safety and Health Districts, except District 1 which covers 

anthracite mines. Thus, dust samples from various underground coal 

mines throughout the USA were analyzed with the CDEM and

compared to the gravimetric analysis for TIC obtained by the Mt. Hope 

laboratory for the same samples. Samples from each mine were used 

to calibrate the meter. Figure 6 shows the CDEM determination of TIC 

of samples collected from mine A compared with Mt. Hope laboratory 
 

 

TIC minus the surface moisture. There is more scatter in the data from 

the actual mine samples than from the prepared sample mixtures. The 

scatter in the data is probably associated with variations in the coal 

dust particle diameters contained within the Mine A samples. To 

confirm this hypothesis of variations in coal particle size from sample to 

sample, the rock dust was leached from the mixture and a sieve size 

analysis conducted on the remaining coal dust. The laboratory acid 

leaching method (1:9 dilute hydrochloric acid) was used to remove the 

rock dust from the mine dust. The leaching technique was first verified 

using synthetic mixtures of PPC and limestone dust. Samples of 65% 

rock dust and 35% PPC were mixed together, and then the rock dust 

was leached out of the samples. Size analysis of the residues resulting 

from acid leaching of the synthetic mixtures gave a surface weighted 

mean diameter D s of 30 microns and a standard deviation of ±1 micron 

(Ds = 3D3

ÎD ) 3D2ÎD), yielding good agreement and therefore 

confidence in the acid leaching method. Several samples collected 

from Mine A, were leached and particle size analysis conducted on the 

residue. The average Ds coal diameter from Mine A was 50 microns ± 

6 microns. 

5. Comparison of TIC and CDEM readings for prepared 

100
 

90
 

80
 

70
 

20 

30 

40 

50 

60 

0 10 20 30 40 50 60 70 80 90 100 

CDEM, % 

TIC by Weight,%

Explosible 

Non-Explosible 

10 

0 

Figure 
 

samples of Pittsburgh Pulverized Coal dust. 


Mine A 

100 

90 

80 

70 Mt. Hope, %TIC 60 

50 

40 

30 

20 

10 

0 10 20 30 40 50 60 70 80 90 100 

Non-Explosible 

Marginal 

Explosible 

CDEM, %TIC 

Figure 6. Comparison of TIC and CDEM readings for Mine A dust 

samples. 

0 

Table 1 shows the linear regression statistics from the 

comparisons of Mt. Hope gravimetric analysis method with CDEM 

readings for prepared samples, the best mine fit and best combined fit 

to data from 26 mines. The prepared samples, with the uniform particle 

size distribution, produced the best fit, as expected, with an R2

 of 0.99 

and standard deviation (σ) of 0.8 wt %. While the best agreement with 

actual mine samples occurred with mine M, R2 of 0.97 and σ of 2.7 %. 

Comparison of Mt. Hope %TIC with CDEM %TIC readings for all mines 

combined is shown in figure 7. The combined comparison using all 

mine samples evaluated in this study is quite good with an overall R2

 of 

0.92 and σ of 4 % indicating that the meter performed reasonably well 

for its intended use. To yield the best TIC results, the meter should be 

calibrated with dry samples of pure rock dust, pure coal dust and an 

actual dry mine sample containing $ 63 and #75 TIC as determined by 

prior laboratory analysis. The explosibility function of the meter only 

requires a sample of pure rock dust and pure coal dust for operation. 

Table 1. Linear regression data from the comparison of Mt. Hope 

analysis with CDEM readings 

Linear Prepared Best All mines 

Regression 

R2

samples 

0.99 

Mine M 

0.97 

( 26) 

0.92 

Standard 0.8 2.7 4.0 

Deviation 

N 33 58 555 

Slope 1.02 ± 0.01 0.97 ± 0.02 0.94 ± 0.01 

Intercept -2.8 ± 0.8 2.6 ± 2.0 5.6 ± 1.0 

All Mines 

100 

0 

20 

40 

60 

80 

0 20 40 60 80 100

CDEM, % TIC 

Mt. Hope, % TIC

Ideal Response 

Figure 7. Comparison of % TIC and CDEM readings for all mine 

dust samples. 

Additional work is planned to assess the capability of using 

the CDEM in some actual operating coal mines by mine inspectors, 

NIOSH personnel and others. The results of the in-mine studies will be 

evaluated to determine issues that may hamper the use of the CDEM 

as another tool for assessing the explosibility hazard of mine dust in a 

particular area. Work is also in the final stages with respect to 

obtaining an experimental permit to use the CDEM in underground 

coal mine areas where methane may be present. The CDEM may also 

serve as a useful tool to reduce the number of dust samples sent by 

mine inspectors to the Mt. Hope laboratory for TIC, especially where it 

can be shown by the CDEM that the TIC is 85 % or higher. 

Conclusions 

The CDEM has been checked against prepared dust 

mixtures and found to be accurate to within ± 1 wt percent between 0 

to 100 % rock dust. The average measurement error for 26 mines 

tested was 4.0wt % total incombustible content. 

Currently, this hand held, intrinsically safe digital CDEM 

coupled with a simple drying technique using molecular sieve, could 

provide an efficient method to determine both the explosibility of the 

dust and approximate TIC content of dried samples within the mine. 

The CDEM’s in situ explosibility measurement can help mine operators 

4 
 

 

reduce the danger of operating under hazardous conditions of 

explosible coal dusts and help provide a better balance between the 

applied rock dust and generated coal dust. Relevant to compliance, an 

inspector could focus specifically on deficient or borderline samples for 

subsequent laboratory analysis. Most importantly, the CDEM shows 

promise as a useful tool for mine operators and safety inspectors for 

the in situ determination of the explosible nature of coal and rock dust 

deposits and thereby would enable immediate corrective action. 

Acknowledgements 

The authors acknowledge the Geneva College Technology 

Development Center of Beaver Falls, PA for contributions to the new 

design, testing and fabrication of the prototype digital CDEM. Special 

thanks to Ken Cashdollar, NIOSH for providing information regarding 

the dust size distributions and guidance in the project. The authors 

also acknowledge Cindy Hollerich, NIOSH for the many hours she 

spent conducting hundreds of measurements and to Greg Green, 

NIOSH for conducting particle size analysis on many of the mine 

samples. 

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5