Field evaluation of the coal dust


 explosibility meter (CDEM)
 


M.L. Harris, M.J. sapko, k.L. casHdoLLar 

and H.c. verakis 

M.L. Harris, M.J. Sapko, members sMe, and K.L. Cashdollar 

are research chemical engineer, principal research physicist and 

senior physical scientist, respectively, with the pittsburgh research 

Laboratory, national institute for occupational safety and Health, 

pittsburgh, pa; H.C. Verakis is general engineer with the Mine 

safety and Health administration, Triadelphia, Wv. 

Abstract 

In underground mines, coal dust explosions are prevented 

by the addition of rock dust sufficient to render the coal 

dust inert. The National Institute for Occupational Safety 

and Health (NIOSH) developed a hand-held instrument 

that uses optical reflectance to measure the explosibility of 

a rock dust and coal dust mixture.This instrument is called 

the Coal Dust Explosibility Meter (CDEM). In this study, 

NIOSH personnel accompanied Mine Safety and Health 

Administration (MSHA) inspectors on their routine band 

surveys in five underground coal mines in MSHA District 

2 (Pennsylvania) and three underground coal mines in 

MSHA District 11 (Alabama).While underground,NIOSH 

personnel and MSHA inspectors used the CDEM to assess the explosibilities of the dust samples. The values of 

percent incombustible content (percent IC) determined by 

the CDEM agreed well with those obtained later by lowtemperature ashing (LTA) in both MSHA and NIOSH 

laboratories. Further, the meter identified some samples 

as potentially explosible that LTA analysis had found to 

possess sufficient rock dust for inerting. The CDEM provides more information on the hazards in the mine. Rapid 

identification of areas with explosible dust mixtures using 

the CDEM allows for immediate intervention rather than 

a wait of several weeks for laboratory analysis. 

Introduction 

Experimental studies by the 

Pittsburgh Research Laboratory of 

the National Institute for Occupational Safety and Health 1 (PRL) and 

similar agencies in other countries 

have shown that mixing a sufficient 

quantity of inert rock dust (usually 

limestone) with coal dust will prevent coal dust explosion propagation 

(Nagy, 1981). The rock dust particles 

act as a heat sink, lowering the flame 

temperature below the value needed 

to continue explosion propagation. 

Previous full-scale explosion studies have illustrated the 

importance of particle size in relation to explosibility 

(Nagy, 1981; Weiss et al., 1989). As these studies demonstrate, the finer the coal dust particle size, the more rock 

dust required for inerting.The U.S.requirement specified 

in Title 30, Code of Federal Regulations, Part 75, Section 

403, mandates that the nation’s coal mines must maintain 

an incombustible content of at least 65 percent in the 

nonreturn (intake) airways and at least 80 percent in the 

return airways, where finer float coal dust is more likely 

to accumulate. 

The current procedures for determining compliance with rock 

dusting requirements begin with 

an MSHA inspector visiting a mine 

quarterly to perform a rock dust 

survey. The dust samples are then 

submitted to the Mine Safety and 

Health Administration (MSHA) 

laboratory at Mt. Hope,WV,for lowtemperature ashing (LTA) analysis, 

which will be described below. The 

MSHA laboratory determines the 

total incombustible content of each 

sample and sends the results to the 

inspector. This process, from obtaining the samples to 

reporting the analytical results, typically takes several 

weeks. 

The Coal Dust Explosibility Meter (CDEM) gives 

real-time results in the mine during the rock dust surveys 

instead of waiting weeks for laboratory results from the 

MSHA survey (Sapko and Verakis, 2006).With real-time 

results, the potential for a disaster can be mitigated im 

mediately. The CDEM is a hand-held instrument that 

uses optical reflectance to determine the explosibility of 

a rock dust and coal dust mixture. The CDEM displays 

the percent incombustible content as well as a color in 

dicating the relative explosibility of the coal and rock 

dust mixture. A red-color readout indicates that more 

incombustible material is required to inert the coal and 

rock dust sample, while a green readout indicates that 

the dust sample is sufficiently inert. When the mixture is 

marginally explosible, yellow is indicated. 

In addition to the problem of waiting weeks for the 

results of MSHA inspections, the MSHA regulations are 

based on two sizes of coal — a coarse size in mine in 

takes and a finer size in returns. However, these MSHA 

rock-dusting regulations are based on coal particle size 

surveys conducted in the 1920s and experimental mine 

measurements of the amount of incombustible required 

to inert those sizes of coal (Rice and Greenwald, 1929; 

Nagy, 1981). A recent coal dust particle size survey in 

dicates that current mining methods generate finer par 

ticles; therefore, more inert material is required (Sapko 

et al., 2007). One advantage of the CDEM is that it ac 

counts for the coal particle size in its determination of 

explosibility.The CDEM color readout takes the various 

sizes of coal into account when determining the explo 

sibility of the dust mixture. Details regarding the design 

and operation of the CDEM can be found in Sapko and 

Verakis (2006). 

To field test the effectiveness of the CDEM, NIOSH 

researchers accompanied MSHA inspectors on their rou 

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tine rock dust surveys in eight underground coal mines. 

While underground, NIOSH and MSHA inspectors used 

the CDEM to assess the explosibility of the dust samples. 

A complete description of the procedures and results 

from these field studies is reported in the following sec

tions. 

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Field evaluation procedures 

To measure percent incombustible contnet (IC), the 

CDEM is calibrated for each mine using a dried sample 

of mine dust with a known percent incombustible, preferably between 63 percent and 75 percent, as determined 

by prior LTA analysis. This sample is considered to be 

representative of that particular mine and is used as a 

midpoint calibration sample. Prior to each mine visit, 

the meter is calibrated using a sample of the mine’s rock 

dust, coal dust and midpoint calibration sample. The explosibility function of the meter only requires a sample 

of pure rock dust and pure coal dust for calibration. The 

explosibility mode (red, yellow or green color) indicates 

the level of explosion protection based on the fineness of 

the coal dust and is preset in the CDEM. 

In this study, to determine the incombustible content 

of the dust mixtures in the entries, MSHA inspectors gathered dust samples per procedures outlined in the MSHA 

Handbook Series titled “General Coal Mine Inspection 

Procedures” (MSHA, 2006). The band sample method 

was used, meaning that dust was collected from around 

the perimeter (roof, ribs and floor) to a depth of 25 mm 

(1 in.) in a mine entry. When the roof was too high, only 

rib and floor samples were collected. The band samples 

were collected every 152 m (500 ft) along the entries. Part 

of the band sample was bagged, labeled and sent to the 

MSHA Mt. Hope laboratory for analysis, while part was 

analyzed in situ using the CDEM. The MSHA inspector 

collected the band sample into a metal pan using a brush, 

and the dust was then sifted through a 10-mesh screen to 

remove particles larger than 2 mm (0.08 in.). The minus 

10-mesh dust sample was then mixed by a combination 

of using fingers to mix it in the tray and by shaking the 

tray. Mixing continued for about five seconds until the 

dust appeared to be homogeneous. However, this method 

of mixing was not always sufficient, as will be shown and 

discussed later. 

NIOSH personnel used tubes (45 mL or 1.5 oz) to 

collect a small dust sample from the tray and to prepare 

the dust for CDEM measurement (Figs. 1 and 2).

Figure 1 

Sample tube containing molecular sieves. 

Figure 2 

Collecting a sample into a sample tube. 

 The 

tubes were about half-filled with molecular sieve material 

(Grace Davison, Columbia, MD2

), which was used to dry 

the dust. The remainder of the dust within the MSHA 

inspector’s tray was placed in a plastic bag and labeled for 

subsequent MSHA LTA analysis. The dust sample was 

added to the molecular sieves in the sample tube until 

the tube was mostly filled. The tube was shaken for at 

least one minute so that the dust sample would mix with 

the sieves and dry. With the moisture removed from the 

dust sample, a funnel with a 20-mesh screen was attached 

to the end of the tube. The CDEM sample cup was then 

fitted onto the end of the funnel and a dust sample was 

shaken from the sample tube into the sample cup (Fig. 3). 

Figure 3 

Transferring dust to a sample cup. 

The 20-mesh screen inside the funnel prevented the molecular sieves from entering the sample cup. The MSHA 

Mt. Hope laboratory similarly sieves all dust samples 

through a 20-mesh screen before determining the incom ­


bustible content. 

Once the dry dust was in the sample cup, the CDEM 

probe was inserted into the cup. The CDEM returned a 

reading of percent IC and the corresponding red/yellow/ 

green indicator readout (Fig. 4) was recorded. The re-
 

 

 

 

 

 

 

 

 

 

 

 

 

 

maining sample within the tube was capped and retained 

for later NIOSH laboratory analysis. 

Figure 4 

Sampling the dust with the CDEM, displaying “65Red.” 

The MSHA Mt. Hope laboratory analysis consisted 

of sieving the sample through 20 mesh and then oven 

drying the minus 20-mesh material for one hour at 105°C 

(221° F)and recording the weight change (Sapko and 

Verakis, 2006).This value would be the percent moisture. 

LTA analysis was done by heating the dried sample in an 

oven, which was ramped up over 1.5 hours and held at 

515° C (959° F) for about 2.5 hours to burn off the combustible coal fraction, thereby leaving the incombustible 

ash.This temperature is chosen because it is high enough 

to burn off the coal but not high enough to decompose 

the limestone rock dust.The weight of the remaining ash 

and weight of the as-received moisture are combined and 

reported as total incombustible content (TIC). 

The NIOSH lab analysis procedure used only LTA 

analysis because the molecular sieves had already re 

moved the moisture during the collection process. The 

NIOSH LTA procedure included a similar ramp time of 

1.5 hours to 515° C (959° F), which was maintained for 20 

hours (Cashdollar et al., 2007). 

Results and discussion 

Figure 5 shows a comparison of the incombustible 

content as measured by both the MSHA and NIOSH 

LTA analyses.

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Figure 5 

Comparison of MSHA and NIOSH incombustible measurements. 

There is a linear relationship between the 

percent IC measured at the two facilities.The average dif 

ference between samples processed at the two sites was 

about 2 percent, with the largest difference being 20 per

cent. It should be noted that MSHA reported to NIO SH 

both the total IC percentage (including moisture) and the 

moisture percentage. To determine the percent IC on a 

dry basis, a correction was made for the MSHA moisture 

percentage.A few samples had large differences between 

the analyses at the two laboratories. These could be due 

to how adequately the dust was mixed before division. 

To determine if these large differences were associated 

with mixing or with differences in ashing time, the Mt. 

Hope laboratory sent the remainder of its portion of the 

original sample to NIOSH for 20-hour ashing. NIOSH 

testing confirmed that the MSHA and NIOSH methods 

actually produced almost the same percent IC.This result 

indicates that more care needs to be taken in the mine to 

more thoroughly mix the bulk sample before extracting 

a portion for CDEM analysis. 

Figures 6 and 7 show the in situ CDEM measurement 

of percent incombustible compared to the MSHA Mt. 

Hope LTA measurements for the five mines from District 

2.

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Figure 6 

Mine D measurement comparison. 

 For the MSHA data, the percent moisture values were 

subtracted from the total incombustible percent values 

because the CDEM values were on a dry basis. The data 

points are shown as red, yellow or green, correspond ­

ing to the explosibility color reading from the CDEM. 

Red denotes an explosible dust mixture, yellow denotes a 

marginally explosible mixture and green denotes an inert 

mixture. Figure 6 displays the data for Mine D. There is a 

linear relationship between the MSHA analyses and the 

CDEM incombustible readings. The CDEM color read ­

ings show red for the samples with the lowest incombustible contents, meaning they need more rock dust to be 

rendered inert.The CDEM green readings show samples 

with sufficient rock dust. Note that there were 10 intake 

samples that met the current 65 percent IC requirement 

by the MSHA laboratory analyses but were identified as 

red by the CDEM, meaning that they were potentially 

explosible.The reason for this difference is probably that 

the actual coal dust size in the mine was finer than the 

size basis for the regulation. Figure 7 shows the com ­

bined data for Mines A, B, C and E. There appears to be 

good agreement between the Mt. Hope percent IC (LTA 

analysis) and that measured by the CDEM. As indicated 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

above, the data that appear to be major outliers were 

found to be the result of inadequate in-mine sample mix 

ing before the sample was subdivided. 

Figure 7 

Measurement comparison of Mines A, B, C and E. 

Among District 2 intake entry samples, 92 out of 104 

had ≥65 percent IC, as required by current regulations. 

However, the CDEM indicated that 27 of those samples 

were within the red or yellow bands, indicating that about 

a quarter of the samples may be deficient in incombus 

tible content. Mine areas represented by these samples 

would most likely not receive additional rock dust be 

cause they are compliant with current regulations. Yet, 

according to CDEM analysis these samples represent 

areas of the mine where a risk of explosion propagation 

is present and more rock dust is required.The reason that 

the CDEM shows these samples as potentially explosible 

is probably because the coal dust in the samples is finer in 

size than the basis for the regulation (Sapko and Verakis, 

2006; Sapko et al., 2007). 

Figure 8 displays the CDEM percent IC measure 

ments and the MSHA results for three mines within Dis 

trict 11.

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Figure 8 

Measurement comparison of Mines F, G and H. The CDEM IC percentages agree reasonably well 

with the MSHA results for Mines F and G but not as well 

for Mine H. However, Mine H values are all very high 

in incombustible content. The rock dust used in District 

2 mines is composed of limestone that is light gray in 

color, while the rock dust used at the District 11 mines 

is a marble dust that is bright white. Figure 9 is a visual 

comparison of the marble dust and limestone dust.

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Figure 9 

Different rock dusts used. 

The 

normalized reflectance takes these reflectance differ 

ences into account for the CDEM percent IC readings. 

However, the red/yellow/green explosibility boundaries 

that had been preset in the CDEM for District 2 using 

limestone dust did not apply to the District 11 marble 

dust. Based on the explosibility color settings for District 

2 using the limestone, most of the District 11 sample read 

ings were red, which was obviously in error. The color 

settings of the CDEM for the marble dust were further 

studied to address the issue. 

Figure 10 shows the normalized reflectance of the 

limestone and marble dust mixed with Pittsburgh coal 

dust of different sizes. The coal size is identified by the 

percent by weight of minus 200 mesh. The data points 

show the mixtures at the explosibility limits, which cor 

respond to the yellow-green boundaries for the CDEM 

(Sapko and Verakis, 2006). As can be seen, the normal 

ized reflectance remains relatively constant for both sizes 

of coal dust, but the normalized reflectance at the ex 

plosibility limit is higher for the limestone than for the 

marble. Therefore, mines that inert with marble dust will 

require a different normalized reflectance value at the 

explosibility limit to be programmed into the CDEM for 

the correct color output. The difference in the normal 

ized reflectances for the two rock dusts is most likely 

due to a difference in the particle sizes of the limestone 

and marble. More study on the effects of rock dusts from 

different districts will be undertaken to determine the 

appropriate color output settings for the CDEM. 

Conclusions 

After calibration with a mine sample of known incom ­

bustible content, the CDEM can give real time measurements of the percent IC in a mine during a rock dust band 

survey. Rapid determination of percent IC allows for 

immediate remedy of inadequate rock dusting. Previous 

large-scale explosion studies illustrated the importance of 

particle size in explosion hazard. The color indicators of 

red, yellow and green output of the CDEM take this coal 

dust size variation into account when determining the 

level of protection present. During the current survey, the 

CDEM color indications determined that about a quarter 

of the intake samples meeting the MSHA requirements 

(incombustible content) fell into the possibly explosible 

range requiring more rock dust. 

Acknowledgments 

The authors acknowledge Terry Montgomery with 

the MSHA laboratory for supplying LTA and moisture 

analyses for the District 2 and 11 mine samples. The au­
 

 

 

 

 

 

 

 

 

 

 

 

 

thors also acknowledge the MSHA Ruff Creek, PA, field 

office and the MSHA Bessemer, AL, field office for al

lowing NIOSH investigators to accompany its inspectors 

on their routine rock dust surveys. The authors wish to 

thank Greg Green of NIOSH PRL for his assistance with 

the LTA data. 

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Figure 10 

Comparison of normalized reflectance using different rock dust mixtures. 

Footnotes: 1 The Pittsburgh Research Laboratory was part of the 

U.S. Bureau of Mines until 1996, when it was transferred 

to the National Institute for Occupational Safety and 

Health (NIOSH). 2 Mention of any company or product does not constitute endorsement by NIOSH or MSHA. 

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