Information Circular 9XXX

Coal Dust Explosibility Meter Evaluation and Recommendations for 

Application

Marcia L. Harris, Michael J. Sapko, Floyd D. Varley, and Eric S. Weiss

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Office of Mine Safety and Health Research

Pittsburgh, PA • Spokane, WA

Month 2012
This document is in the public domain and may be freely 

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DHHS (NIOSH) Publication No. 20XX–XXX

Month 2012 

SAFER • HEALTHIER • PEOPLETM
Table of Contents

Executive Summary ........................................................................................................ 1 

Introduction ..................................................................................................................... 3 

Background on Coal Dust and Explosibility Testing ........................................................ 4 

CDEM Operation............................................................................................................. 6 

Comparison of Laboratory Results and CDEM Results................................................... 7 

Joint Study between NIOSH and MSHA ......................................................................... 8 

Results and Discussion................................................................................................... 9 

GREEN Measurements............................................................................................. 12

RED/YELLOW Measurements................................................................................... 15

Conclusions from the NIOSH Study .............................................................................. 19

Commercial CDEM Development.................................................................................. 20

Calibration and Programming of the Commercial CDEM .............................................. 21

Commercial CDEM Changes based on Potential Customer Concerns........................... 21

The Commercial CDEM as a Verification and Compliance Tool .................................... 22

NIOSH Recommendations ............................................................................................ 22

Acknowledgments ......................................................................................................... 23

References.................................................................................................................... 23

APPENDIX A: CDEM Design ........................................................................................ 25

APPENDIX B: CDEM Training ...................................................................................... 29

APPENDIX C: Prototype CDEM Calibration and Testing Procedures Used in the Joint 

Study ...................................................................................................................... 31

Preparation of Calibration Sample.............................................................................. 32

Drying the sample..................................................................................................... 33

CDEM Calibration..................................................................................................... 34

In-Mine Testing ........................................................................................................ 35

APPENDIX D: Particle Size Effect ................................................................................ 39

APPENDIX E: MSHA Inspector Questions and Comments .......................................... 43
Figures

Figure 1. Effect of particle size of coal dust on the explosibility of Pittsburgh seam 

bituminous coal as tested within LLEM. The dashed curve represents the 

propagation/nonpropagation boundary. ..................................................................... 5

Figure 2. Coal Dust Explosibility Meter (CDEM).............................................................. 6

Figure 3. CDEM measurement comparison of NIOSH-received samples..................... 11

Figure 4. Comparison of CDEM results and lab TIC results for one representative

mine......................................................................................................................... 12

Figure 5. CDEM GREEN (G) measurement comparison of NIOSH-received samples. 

Agreeing GREEN samples have LTA results > 80% IC. Disagreeing GREEN 

samples have < 80% IC LTA results........................................................................ 13

Figure 6. GREEN sample of 73.9% IC. ......................................................................... 14

Figure 7. Comparison of MSHA and NIOSH incombustible measurements.................. 16

Figure 8. CDEM RED (R) measurement comparison of NIOSH-received samples. 

Agreeing RED samples have LTA results < 81.5% IC. Disagreeing RED have

 more than 81.5% IC LTA results.............................................................................. 18

Figure 9. Agreement between CDEM and laboratory analysis...................................... 19

Figure A1. Coal Dust Explosibility Meter (CDEM). ........................................................ 26

Figure A2. Extinction limit for normalized reflectance.................................................... 28

Figure C1. Sample tube containing molecular sieves. .................................................. 34

Figure C2. Collecting a band sample. ........................................................................... 35

Figure C3. Sifting a sample........................................................................................... 36

Figure C4. Collecting a sample into a sample tube. ...................................................... 37

Figure C5. Transferring dust to a sample cup. .............................................................. 37

Figure C6. Testing a dust sample with the CDEM......................................................... 38

Figure D1. A comparison of coal dust particle size distributions on φ values.

Rock dust particle size distributions are constant. ................................................... 40

Figure D2. A comparison of rock dust particle size distributions on φ values. Coal

dust particle size distributions are constant. ............................................................ 41

Figure E1. Cross section of a very thin (0.01-in-thick) explosible float coal dust layer 

deposited on top of a 3/4-in-thick (20-mm-thick) layer of rock dust.......................... 49
Tables

Table 1. Elapsed time between dates that samples were collected and the MSHA 

inspector was notified of laboratory results ................................................................ 9

Table 2. Comparison of CDEM results as determined by separate MSHA and NIOSH 

testing ...................................................................................................................... 10

Table 3. Particle size distribution of 72.7% IC dust sample and LTA results for three 

different size fractions of the sample........................................................................ 15

Table B1. Mixtures of RD and PPC for CDEM calibration............................................. 30

Table C1. Pittsburgh pulverized coal size data ............................................................. 32

Table C2. Reference mixtures, percentage by weight ......................................................... 33
Acronyms and Abbreviations

CD coal dust

CDEM Coal Dust Explosibility Meter

CFR Code of Federal Regulations

ETS emergency temporary standard

IC incombustible content

LLEM Lake Lynn Experimental Mine

LTA low temperature ashing

MSHA Mine Safety and Health Administration

NIOSH National Institute for Occupational Safety and Health

Φ normalized reflectance

Φ* normalized reflectance at the extinction limit

OMSHR Office of Mine Safety and Health Research

PPC Pittsburgh pulverized coal

K product of rock dust to coal dust particle density ratio and 

ratio of mean particle diameters of coal to rock dust

RI Report of Investigations

RD rock dust

SUT sample under test

Sc surface area of coal dust

Sr surface area of rock dust

TIC total incombustible content

USBM United States Bureau of Mines

H2O water/moisture
Unit of Measure Abbreviations

ft feet 

fr fraction rock dust

g grams

Ix light intensity of coal and rock dust mixture

Ic light intensity of coal dust

Ir light intensity of rock dust

μm micrometers or microns

mL milliliters

mm millimeters

oz ounces

% IC percentage of incombustible content

% CH4 percentage of methane

% RD percentage of rock dust

% TIC percentage of total incombustible content

wt weight

Coal Dust Explosibility Meter Evaluation and 

Recommendations for Application

Marcia L. Harris,

1

 

1 Research Chemical Engineer, Fires and Explosions Branch, Office of Mine Safety and Health Research 

(OMSHR), NIOSH, Pittsburgh, PA.

Michael J. Sapko,

2

2 Senior Research Physical Scientist (retired), OMSHR, NIOSH, Pittsburgh, PA.

Floyd D. Varley,

3

3 Chief (retired), Fires and Explosions Branch, OMSHR, NIOSH, Spokane, WA.

and Eric S. Weiss4

4 Senior Research Mining Engineer, Fires and Explosions Branch, OMSHR, NIOSH, Pittsburgh, PA.

Office of Mine Safety and Health Research

National Institute for Occupational Safety and Health

Executive Summary

This report details the results of a NIOSH investigation on the ability of the Coal Dust 

Explosibility Meter (CDEM) to accurately predict the explosibility of samples of coal and rock 

dust mixtures collected from underground coal mines in the U.S. The CDEM, which gives 

instantaneous results in real time, represents a new way for miners and operators to assess the 

relative hazard of dust accumulations in their mines and the effectiveness of their rock dusting 

practices. The CDEM was developed by the National Institute for Occupational Safety and 

Health (NIOSH) and successfully underwent national and international peer review. The 

intention of the device is to assist mine operators in complying with the Mine Safety and Health 

Administration (MSHA) final rule 30 CFR* 

*Code of Federal Regulations. See CFR in References.

75.403, requiring that the incombustible content of 

combined coal dust, rock dust, and other dust be at least 80% in underground areas of bituminous 

coal mines. 

As a final step towards commercialization of the CDEM, and to evaluate the performance of 

the device as a potential compliance tool, NIOSH undertook an extensive cooperative study with 

MSHA. This study, completed in 2009–2010, involved field use of the CDEM within MSHA’s 

10 bituminous coal districts. As part of their routine dust compliance surveys in these districts, 

MSHA inspectors collected sample coal and rock dust mixtures, field testing these samples for 

explosibility with the CDEM. Samples were then sent to the MSHA laboratory at Mt. Hope, 

WV, for parallel testing, first using a drying oven to determine the surface moisture followed by 

traditional low temperature ashing (LTA) method. The LTA method determines explosibility of 

a coal and rock dust sample in a laboratory by heating the mixture to burn off the combustible 

material. The results, when combined with the surface moisture, are reported as total 

incombustible content (TIC). If the TIC is ≥ 80%, the sample is deemed to be nonexplosible and 

compliant with 30 CFR 75.403. 

In the field component of this study, MSHA’s use of the CDEM indicated that 30% (175) of 

the 591 samples collected were explosible. NIOSH was able to obtain and re-measure 297 
samples, and 97% of those identified by the CDEM as being explosible (27% of samples) or 

nonexplosible (73% of samples) correlated with the results of the subsequent lab analysis using 

the LTA method. Of the remaining 3% where there were differences between the field and 

laboratory methods, subsequent NIOSH evaluation attributed these differences to the variability

(incomplete mixing, inadequate drying of the sample, the particle size of the rock dust and/or 

coal dust) of the samples being analyzed, the retained moisture in those samples, and the inherent 

ash in the coal.

In considering these results and comparing the CDEM field measurements to the LTA 

laboratory measurements, it is important to understand the fundamental distinctions between the 

two methods. The determination of TIC by the LTA method is not itself a direct measure of 

explosibility, but a surrogate that calculates a single parameter associated with full-scale 

experimental results. This method is not based on particle size and treats all particles equally 

regardless of the size. In contrast, the CDEM utilizes a different approach, using optical 

reflectance to determine the ratio of rock dust to coal dust in a mixture, with full-scale 

experiments on flame propagation having already demonstrated the effects of varying the coal 

dust particle sizes and incombustible concentrations on the explosible vs. nonexplosible dust 

mixtures. A final important distinction between the two methods is that the CDEM offers realtime measurements of the explosion propagation hazard within a coal mine entry, allowing for 

immediate identification and mitigation of the problem, while the results from the traditional 

LTA method are not known for days or weeks after a sample is collected, allowing for the 

deficiency in rock dust to continue. 

The conclusions of this study strongly support the field use of the CDEM to measure the 

explosibility of coal and rock dust mixtures, to more effectively improve the onsite adequacy of 

rock dusting for explosion prevention. Mine operators could use the CDEM on a regular basis to 

ensure that their rock dusting practices are achieving inertization requirements and meeting the 

intent of 30 CFR 75.403. MSHA inspectors could use the CDEM as a tool to immediately 

identify onsite explosibility hazards and initiate corrective action. A critical issue to both the 

LTA and the CDEM analysis methods is that the results are dependent on representative samples 

being collected for analysis.
3 

Introduction

Federal regulations require that rock dust (RD) be applied in all underground areas of a coal 

mine to mitigate the propagation of a coal dust (CD) explosion. Prior to September 2010, U.S. 

Federal law 30 CFR 75.403 mandated that the nation’s coal mines maintain a total incombustible 

content (TIC) of at least 65 percent in nonreturn entries and at least 80 percent in the return 

airways.

5,

 

5 In September 2010, MSHA published an emergency temporary standard (ETS) increasing the total 

incombustible requirement in intake airways to 80%. The final rule 2011–15247, requiring 80% TIC in 

intakes, was effective June 21, 2011. The current study was conducted prior to the ETS and subsequent 

final rule.

6

6 Total incombustible content (TIC) includes measurements of the as-received surface moisture in the 

samples, the ash in the coal, and the rock dust. Incombustible content (IC) includes measurements of the 

ash in the coal and the rock dust and does not include the moisture content.

The 65% TIC requirement was based on an average particle size termed “mine-size 

dust,” which was based on an average of representative samples collected from mines in the 

1920s. To determine compliance with the Federal regulation, mine inspectors systematically 

collect dust samples from sections of underground coal mines and send the samples to a Mine 

Safety and Health Administration (MSHA) Mt. Hope laboratory for analysis of incombustible 

content. 

The TIC analysis is a gravimetric (mass) measurement of the incombustible content (IC) of a 

coal and rock dust mixture. Generally, the analysis is attained using a low temperature ashing 

(LTA) method [NIOSH 2010]. Due to the inherent time needed to collect the samples, ship the 

samples, and then test the samples, the Coal Dust Explosibility Meter (CDEM) was developed to 

allow for immediate determination of the explosible reactivity of a coal and rock dust mixture. 

The device was tested using experimental coal and rock dust mixtures and on band samples 

collected by MSHA inspectors from underground coal mines [Harris et al. 2008]. 

In 2009–10, the National Institute for Occupational Safety and Health (NIOSH) and MSHA 

conducted an extensive cooperative study to contrast explosibility assessment as determined by 

the CDEM with explosibility assessment results as determined by the laboratory gravimetric 

analysis of incombustible content. Further, the study was able to evaluate the feasibility for 

inspectors to use the CDEM within 10 of MSHA’s bituminous coal districts. This report will 

discuss the study results, with emphasis on comparisons of the CDEM explosibility assessment 

with the traditional method for determining the TIC. CDEM operation and the use and 

application of the commercial CDEM will also be discussed. Importantly, the study results are 

presented in the context of the current standard requiring not less than 80% TIC in all areas of an 

underground coal mine. 
4 

Background on Coal Dust and Explosibility Testing

Coal dust particle size has a significant impact on the explosion propagation potential of coal 

and rock dust mixtures. As the coal dust particle size decreases, the coal particles are more 

reactive and increased amounts of compliant rock dust are necessary to render the coal dust 

inert.

7

 

7 Compliant rock dust is defined in 30 CFR 75.2 as “Pulverized limestone, dolomite, gypsum, anhydrite, 

shale, adobe, or other inert material, preferably light colored, 100 percent of which will pass through a 

sieve having 20 meshes per linear inch and 70 percent or more of which will pass through a sieve having 

200 meshes per linear inch; the particles of which when wetted and dried will not cohere to form a cake 

which will not be dispersed into separate particles by a light blast of air; and which does not contain more 

than 5 percent combustible matter or more than a total of 4 percent free and combined silica (SiO2), or, 

where the Secretary finds that such silica concentrations are not available, which does not contain more 

than 5 percent of free and combined silica.”

Coal particle size has a high variability both within and between mines, with size being

dependent on factors such as mine type (i.e., longwall 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 

further down the entries. 

Numerous coal dust explosion tests have been conducted in the NIOSH Lake Lynn 

Experimental Mine (LLEM) to specifically quantify the concentration of rock dust required to 

prevent flame propagation [NIOSH 2010]. These tests studied flame propagation as a function of 

coal dust particle size while using a rock dust particle size of ~75% < 200 mesh (volume median 

rock dust particle diameter of ~ 25 microns, or μm) (Figure 1). Based on these results, the 

greatest impact on explosibility is evident between the particle size of the 20% < 200 mesh coal 

(mean coal particle diameter of 96 microns, μm) and 80% < 200 mesh (mean coal particle 

diameter of 33 μm). To ensure nonpropagation within the LLEM, the 20% < 200 mesh coal dust 

required a 70% TIC (~ 68% rock dust) and the 80% < 200 mesh coal required ~ 81.5% TIC 

(80% rock dust) to prevent sustained flame propagation. Once the 80% < 200 mesh benchmark 

had been reached, no additional incombustible content (IC) was required to prevent flame 

propagation with further decreases in coal dust particle size under these full-scale experimental 

conditions. 
5 

 

Figure 1. Effect of particle size of Pittsburgh seam bituminous coal dust on the 

explosion propagation for % TIC as tested within LLEM [NIOSH 2010]. The dashed 

curve represents the propagation/nonpropagation boundary. 

To determine compliance with current regulations set forth in 30 CFR 75.403, inspectors 

from MSHA periodically collect samples of deposited dust from specified areas in a mine. An 

MSHA Mt. Hope laboratory determines TIC and compares this TIC with the standard of 80% 

minimum TIC.

8

8 The 80% TIC requirement is based on explosion temperature thermodynamic limit models for coal and 

rock dust mixtures, extensive in-mine coal dust particle size surveys, and multiple explosion experiments 

at the Lake Lynn Experimental Mine [NIOSH 2010]. Presently, the size of the coal dust particles is not 

determined by the MSHA Mt. Hope laboratory as part of the explosibility assessment.

The TIC includes measurements of the surface moisture, the ash in the coal, and 

the rock dust. If 10% of the samples collected in a survey are < 80% TIC (in the absence of 

methane9

9

 Per 30 CFR 75.403, “Where methane is present in any ventilating current, the percent of incombustible 

content of such combined dust shall be increased 0.4 percent for each 0.1 percent of methane.” 

), MSHA considers the sample survey to be noncompliant. The mine operator is 

notified and given time to abate the citation. Abatement is accomplished by applying additional 

rock dust to the deficient areas from which the sample was collected. Abatement is confirmed 

through visual assessment by the MSHA inspector, but no followup dust samples of the abated 

area are collected or analyzed to ensure compliance with the respective intake or return airway 

TIC requirements.
6 

CDEM Operation

The CDEM is a handheld device developed to assess the explosibility of coal and rock dust 

mixtures in real time. The principle of operation of the device is based on the measurement of 

infrared radiation reflected from the surface of a homogeneous mixture of two dusts with 

different optical reflectance, in this case light-colored rock dust and dark coal dust. Near-infrared 

radiation is emitted by a light-emitting diode located behind the window of the CDEM probe. 

When the CDEM probe is inserted in the dust mixture, the infrared radiation reflects off the 

surface of the dust and back to a silicon photodiode sensor (Figure 2). 

Figure 2. Coal Dust Explosibility Meter (CDEM).

The normalized 

reflectance, Φ, is related to the rock dust to coal dust particle density ratio and the ratio of the 

mean particle diameters of coal to rock dust contained in the mixture. The normalized reflectance 

for the tested sample is compared to that of the calibration sample. If the test sample normalized 

reflectance is greater than that of the calibration sample, it is determined to be nonexplosible. If it 

is less than the calibration sample normalized reflectance, it is classified as explosible. For 

further detail on the CDEM design, calibration, and operation, see Appendices A, B, and C. 
7 

Operationally, the CDEM uses a digital readout that identifies a sample as being either 

“RED” or “GREEN.” The CDEM identifies a sample as RED (potentially explosible and 

requiring more rock dust) when the measured Φ is ~ 6% or more below the extinction limit Φ* 

needed to prevent flame propagation (Figure 1 and Figure A2). The extinction limit Φ* is the 

boundary between propagation and nonpropagation and is set at the 80% rock dust level during 

CDEM calibration.

10

 

10 For this study the extinction limit Φ* was set exactly at the experimental extinction boundary between 

propagation and nonpropagation, 80% rock dust. Due to the inherent accuracy of the CDEM being ±2% 

rock dust, to reduce the probability that an explosible mixture is not misidentified the extinction limit Φ* for 

field use should be set at 83% rock dust. The CDEM should be calibrated with an 83% rock dust sample 

instead of 80% rock dust, providing a greater margin of safety. 

The CDEM identifies a sample as GREEN (nonexplosible) when the 

measured Φ is equal to or greater than the extinction limit Φ*.11

11 In addition to RED and GREEN, the prototype CDEM also identified a sample as YELLOW when the 

measured Φ was within 5% of the extinction limit Φ*. A YELLOW reading indicated that the sample was 

marginally explosible. This feature was eliminated as unnecessary in the later-developed commercial 

version of the CDEM. However, in this report, the YELLOW readings were considered to be RED in the 

analysis of the data discussed in this report. Although the YELLOW measurements are not discussed 

separately, all of the RED, YELLOW, and GREEN measurements are included in this report.

12 Pittsburgh pulverized coal (PPC) has been used in large-scale explosion tests at the Lake Lynn 

Experimental Mine (LLEM) and is the standard upon which the 80% total incombustible content cited in 

30 CFR 75.403 is based [NIOSH 2010].

Comparison of Laboratory Results and CDEM Results

The traditional low temperature ashing approach to determine if a coal and rock dust mixture 

passing through a 20 mesh sieve (< 850 μm) is compliant with the inert requirement is 

significantly different from that approach used by the CDEM for assessing the potential 

explosibility of the coal and rock dust mixture. The current LTA method actually consumes the 

coal dust and considers the remaining material to be inert. Compliance with the law is then 

determined by comparing the measured percentage of inert material of the representative band 

sample with the pre-established requirement of 80%. The TIC of the sample includes the rock 

dust, the amount of surface moisture as received at the lab, and the inherent ash in the coal. The 

LTA method is not itself a direct measure of explosibility but is a surrogate that calculates a 

single parameter associated with full-scale experimental results and is also insensitive to particle 

size. In contrast, the CDEM determines the potential reactivity of the coal and rock dust mixture 

by optically comparing the ratio of the surface area of the rock dust particles to the surface area 

of the coal dust particles, and relates the measured ratio to a stored 80% rock dust/coal dust 

calibration sample at the extinction limit of Φ*. The 80% calibration sample is prepared with the 

particular rock dust used at the coal mine and mixed with the standard Pittsburgh pulverized coal

(PPC) dust [80% < 200 mesh (~ 74 μm)].12

When comparing methods that make accuracy determinations, the tendency is to compare the 

new method with the traditional method head-to-head. In this case, since the LTA method and 

the CDEM use different means to determine the explosibility of a coal and rock dust mixture, it 
8 

is difficult to directly compare one method with the other. The only effective approach should be 

outcomes-based, i.e., to compare the LTA prediction with the CDEM prediction and judge both 

against actual explosion tests conducted with samples using a laboratory test chamber

[Cashdollar 1996, Cashdollar and Chatrathi 1993, Cashdollar and Hertzberg 1989, Cashdollar et 

al. 1987, Cashdollar et al. 1992a, 1992b, and 1992c]. The following sections detail how this 

approach was used for this report. 

Joint Study between NIOSH and MSHA

A cooperative study between NIOSH and MSHA entailed an MSHA inspector from each of 

the 10 bituminous coal districts using the CDEM in mines to identify potentially explosible dust 

mixtures in real time during routine band sample surveys. The study was initiated with one 

training session held at the National Mine Health and Safety Academy in Beckley, WV (see 

Appendix B). The mine inspectors were to collect the band sample during their routine survey, 

measure the explosibility with the calibrated CDEM (per the methods described in Appendix C), 

and then send the mixed sample to the MSHA laboratory, Mt. Hope, WV, for the routine 

laboratory determination of the percentage of TIC (% TIC). The remainder of the samples would 

then be sent to NIOSH for parallel testing. The CDEM output obtained by the mine inspectors 

was shared with NIOSH researchers as well as the corresponding MSHA TIC for each sample. 

After the in-mine band samples were collected by the inspectors, the samples were submitted 

to the Mt. Hope laboratory for routine testing to determine % TIC per the MSHA standard 

sampling protocol and procedure. The MSHA inspectors were directed to record the CDEM 

readings in the “Location in Mine” column on their Rock Dust Sample Submission Forms. 

The inspectors began using the CDEMs in December 2009 (within one month of receiving 

the training on the proper use of the CDEM). NIOSH requested the inspectors send pure rock 

dust samples from each mine sampled directly to the NIOSH Office of Mine Safety and Health 

Research (OMSHR) Pittsburgh location. Once the % TIC was determined, MSHA was asked to 

send NIOSH the remainder of the dust samples from the surveys in which a CDEM was used by 

the inspectors, along with the TIC analyses and moisture content data. 

A brief look at the dust sample processing time involved in this study reveals a fundamental 

problem related to the traditional LTA method. Although the average dust sample processing 

time in this survey was less than 2 weeks, the total elapsed time from the sample collection date 

until the laboratory informed the MSHA inspector of the results ranged from 1.7 weeks to 5.9 

weeks, with an average of 3.6 weeks (Table 1). This period does not include the time elapsed 

between receipt of the results by the inspector and notification to the mine. During this 

processing period, the mine entry where the band sample was taken could be deficient in rock 

dust, thereby representing a potentially unrecognized and unmitigated hazardous condition.
9 

Table 1. Elapsed time between dates that samples were collected and the MSHA 

inspector was notified of laboratory results

Period Average Std. Dev.

Sample collection—results sent to inspector (days) 25.3 8.0

Sample collection—results sent to inspector (weeks) 3.6 1.1

Lab receipt of samples—results sent to inspector (days) 12.9 6.7

Lab receipt of samples—results sent to inspector (weeks) 1.8 1.0

Results and Discussion

The discussion that follows is based on the verifiable data that allowed NIOSH to identify 

potential issues with the CDEM instrument and procedures that could be improved. 

NIOSH received 591 samples from 37 separate surveys performed in 32 different mines in 5 

districts. However, NIOSH received pure rock dust samples for CDEM calibration for only 16 of 

the 32 mines. Therefore, NIOSH researchers were able to verify sample measurements on 297 

samples. The set of 297 (~ 50%) of the 591 field study samples which were verifiable are the 

basis of this study. 

The set of 297 field study samples was both random and similar in relation to accuracy and 

test conditions for the 591 total samples and represented 5 of the 10 bituminous coal districts.

Verification of data included the calibration of CDEMs with the local rock dust supply and 

testing the coal/rock dust sample provided by Mt Hope. Without a local rock dust sample for 

calibration, no conclusions could be made as to why results differed between the low 

temperature ash (LTA) analysis and CDEM.

Of the 591 samples that the MSHA inspectors tested with the CDEM, 416 were measured as 

GREEN, 57 were YELLOW, and 118 were RED (Table 2). The GREEN measurements made up 

70% of the total, with the remaining 30% containing RED and YELLOW measurements. The 

RED and YELLOW measurements indicated that the measured Φ was less than the 

propagation/extinction limit, Φ*, signaling the need for more rock dust to mitigate a potentially 

explosible concentration of that dust (as determined by the Sc/Sr ratio in Appendix A). For the 

purposes of instrument performance assessment and explosibility determinations, both RED and 

YELLOW results are considered to be RED, i.e. explosible. Of the 297 samples that NIOSH 

received for evaluation, 73% were GREEN and thus did not require more rock dust to inert 

(Figure 3).
10 

Table 2. Comparison of CDEM results as determined by separate MSHA and NIOSH 

testing13

 

13 When verifying the samples, NIOSH measured at least three different full cups of each sample to 

ensure adequate mixing of the sample.

 MSHA-Conducted—Total Survey NIOSH-Conducted—Verifiable Samples

CDEM 

Readings

TIC = IC 

+ H2O 

Moisturefree IC = 

TIC - H2O 

Assuming

8% IC in CD, 

80% RD = 

81.6% IC

Verifiable 

samples; 

assuming 8% IC 

in CD, 80% RD 

= 81.6% IC

Moisturefree IC = 

TIC - H2O 

Assuming 8% 

IC in CD, 

80% RD = 

81.6% IC; 

rounded to 

82%

NIOSH 

LTA 

verification

Total number 

of samples 591 591 591 296 297 297 297

< 80% and

GREEN 15 19 19 11 2 2 2 

# of GREEN 416 416 416 215 222 222 222

> 80% and

RED/YELLOW 58 50 37 28 17 11 7 

# of 

RED/YELLOW 175 175 175 81 75 75 75

RED

percentage of 

total # of 

samples

30% 30% 30% 27% 25% 25% 25%

GREEN

percentage of 

total # of 

samples

70% 70% 70% 73% 75% 75% 75% 

Disagreeing 

RED

percentage of 

total RED

33% 29% 21% 35% 23% 15% 9%

Disagreeing 

GREEN 

percentage of 

total GREEN

4% 5% 5% 5% 1% 1% 1%

Agreeing RED

percentage of 

total RED

67% 71% 79% 65% 77% 85% 91%

Agreeing 

GREEN 

percentage of 

total GREEN

96% 95% 95% 95% 99% 99% 99%
11 

Figure 3. CDEM measurement comparison of NIOSH-received samples.

The samples for measurement with the CDEM needed to be dried prior to determining the 

reflectance ratio of Sc/Sr. Moisture in a dust sample will darken the sample and thus give an 

incorrect measurement. MSHA includes the percentage of as-received surface moisture as part of 

the TIC. Therefore, in the NIOSH testing, to better compare the sample results from the CDEM 

to the TIC analysis, the percentage of as-received moisture was subtracted from the % TIC to 

obtain a moisture-free IC measurement. Subsequently, 19 of the 416 GREEN measurements 

were < 80% TIC on a moisture-free basis.

All of the GREEN measurements on the CDEM would be > 80% IC if the Sc/Sr were smaller 

than the Sc/Sr at the extinction limit Φ*. Twenty-eight of the 57 YELLOW measurements and 22 

of the 118 RED measurements were > 80% TIC. All YELLOW and RED measurements on the 

CDEM should be < 80% TIC if the Sc/Sr is larger than the Sc/Sr at the Φ* extinction limit.

Because the CDEM and TIC method use different techniques for determining the 

explosibility of a coal and rock dust mixture, Figure 4 is used to provide a visual comparison of 

the output results for both methods. Shown in Figure 4 is data collected from one representative 

dust survey from one mine. The extinction limit, Φ*, of 0.4 for this mine is indicated on the 

graph by a horizontal blue line. When the CDEM measured samples with Φs greater than 0.4, the 

CDEM indicated GREEN (green background). For samples with Φs less than 0.4, 

RED/YELLOW was indicated (red background). The vertical green line indicates the 80% TIC 

limit specified for compliance with 30 CFR 75.403. 
12 

Based on Figure 4, the following sections interpret the disagreements between the two 

methods. 

Figure 4. Comparison of CDEM results and lab TIC results for one representative mine. 

The terms “disagreement,” “agreement,” “disagree,” and “agree” are used throughout 

this report to discuss the differences in the CDEM explosibility readings to that of the LTA 

incombustibility determinations; however, the use of these terms should not be implied as an 

endorsement of the LTA method as that is not the intent. The focus will be on locations where 

the points fall in the upper left section of the green background (i.e., the CDEM determined 

samples to be nonexplosible but the TIC results indicate explosible) and lower right section of 

the red background (the TIC results indicate nonexplosible but the CDEM indicates explosible).

GREEN Measurements

Of the 416 GREEN measurements documented by MSHA inspections while using the 

CDEM, 19 (5%) were < 80% IC (< 20 mesh material), which may indicate a failure to detect a 

hazard (Figure 5). NIOSH was able to identify in 11 of these 19 samples the source of the 

disagreement. The other 8 samples belonged to surveys and mines for which NIOSH did not 

have rock dust to calibrate the CDEM. 
13 

Figure 5. CDEM GREEN (G) measurement comparison of NIOSH-received samples. Agreeing 

GREEN samples have LTA results > 80% IC. Disagreeing GREEN samples have < 80% IC LTA

results.

In 11 verifiable samples (Table 2, row 2, column 4), MSHA determined these to be 

nonexplosible (GREEN) when using the CDEM but explosible (< 80% IC) with the LTA 

method. NIOSH CDEM measurements indicated that only 2 (Table 2, row 2, column 5) of these 

11 samples were nonexplosible (GREEN). Although the source of the error cannot be confirmed

for the other 9 samples, NIOSH researchers believe that simplification of the calibration 

procedure and more robust training would likely eliminate the variability in results. Upon further 

examination of the 2 samples, one sample was 79.4% IC and the other was 73.9% IC. These two 

cases represent < 1% verifiable disagreeing GREEN measurements (Figure 5). Further analysis 

of these perceived failures to detect was then undertaken. 

The MSHA Laboratory where the samples were tested reports a ±0.5% error in its TIC 

results. The 79.4% IC sample is within this error margin. The 79.4% IC would be explosible only 

if the particle size of the coal dust were at the finest range (80% < 200 mesh) found in modern 

underground coal mines. The GREEN nonexplosible result on the CDEM is reasonable 

considering the likely particle size of the sample presented and the IC reported. 

Visual inspection of the 73.9% IC sample indicated larger coal particles as shown in Figure 

6. NIOSH measured the percentage of IC (% IC), which does not include surface moisture, on 

the < 20 mesh fraction of this sample to verify that the sample received was labeled correctly and 

matched the MSHA % IC data minus the surface moisture (i.e., that no clerical error occurred in 

sample transfer from MSHA to NIOSH). The sample was then subdivided into three particle size 

ranges to determine the distribution of the inert component within these size ranges. The 

NIOSH-determined IC was 72.7% IC for the < 20 mesh sample as compared to MSHA’s 73.9% 

IC, showing reasonable agreement given the coarse nature of the sample.
14 

Table 3 shows data for three separate size fractions of the sample. Larger particles in the 20–

60 mesh (250–850 μm) size fraction make up 28.5% of the sample mass and contained 52.4% 

IC. The finest particles in the < 200 mesh range make up 38.7% of the sample mass and 

contained 86.1% IC. The range from 60–200 mesh particles make up 32.8% of the sample mass 

and contained 74.9% IC. Experimental studies have shown that the 20–60 mesh (250–850 μm) 

size fraction does not contribute significantly to flame propagation or inerting. If the sample 

were normalized for the < 60 mesh portion, the sample would be 81% IC. Further, a GREEN

measurement would be expected in this case from the CDEM, because the optical approach is 

strongly favored by the finer reactive and inerting components. In contrast, the TIC analysis does 

not distinguish between the larger, nonreactive coal particles and the finer particles for 

explosibility assessment. Further discussion of particle size effects can be found in Appendix D. 

Figure 6. GREEN sample of 73.9% IC.
15 

Table 3. Particle size distribution of 72.7% IC dust sample and LTA results for three 

different size fractions of the sample

Mesh Size, μm Percentage of size 

fraction % IC of size fraction

20–60 250–850 28.5 52.4

< 60 < 250 71.5 81.0

60–200 75–250 32.8 74.9

< 200 < 75 38.7 86.1

The NIOSH sample analyses detailed above demonstrated that the measurement 

discrepancies (where the CDEM did not identify a hazard but the laboratory TIC analysis did 

identify a hazard) do not represent a true deficit in safety or an instrument inaccuracy. The 

MSHA data included 19 GREEN CDEM measurements that the TIC data indicated were < 80% 

IC (Table 2). Potential reasons for why the CDEM provided a different explosibility assessment 

may be due to the sample containing coarser coal dust, finer inert particles, incomplete mixing of 

the sample before testing on the CDEM within the mine, operator error, and/or incorrect CDEM 

calibration. If the sample was not homogeneous, the CDEM field of view could include an area 

of lighter color where more rock dust was deposited. Researchers are unable at this time to 

confirm, for these samples, what caused the difference in explosibility assessment in the field 

study. However, the variance was eliminated in the 11 verifiable samples through proper 

calibration and operating techniques. 

When correctly used, the CDEM shows agreement with TIC in 99% of cases in identifying 

fully protected areas of the mine. NIOSH also verified the ability of the CDEM to properly 

assess the explosibility of a dust sample based on particle size where the laboratory result 

indicated a deficiency based on TIC alone. 

RED/YELLOW Measurements

The CDEM RED and YELLOW measurements required further analysis, with a few of the 

measurements containing greater amounts of IC based on ashing of the < 20 mesh material.

Preliminary analysis of these samples identified several potential reasons for the differences in 

explosibility assessment using the CDEM and the ashing method. Initially, the YELLOW

measurement indicated a marginally explosive dust mixture, signaling that more rock dust should 

be added. MSHA recorded 175 RED and YELLOW CDEM measurements, and TIC analysis

indicated that 58 of these samples were > 80% TIC. Some of these discrepancies can be easily 

explained, as discussed below. 

Insufficient Mixing of the Band Sample

It is not known whether the band sample tested on the CDEM by the inspector was the same 

as that sent to the MSHA laboratory. If the band sample was not sufficiently mixed by the 

inspector, the portion tested by the CDEM could be different from the portion retained and 

analyzed by MSHA. It is not uncommon that there are differences in the % IC measured by 
16 

MSHA and those measured by NIOSH following the same laboratory LTA method. Previous 

studies by Harris et al. [2008] have shown that there can be ± 7% IC variation in the LTAs if the 

sample was not adequately mixed after being collected and before being subdivided and tested 

(Figure 7).

Figure 7. Comparison of MSHA and NIOSH incombustible measurements [Harris et al. 2008]. 

Insufficient Drying of the Band Sample

The CDEM sample is dried prior to testing. Surface moisture in the sample will cause the 

dust mixture to reflect less light and appear to contain less rock dust than is actually contained in 

the sample. Previous CDEM reflectance studies have shown that for each 1% added surface 

moisture, the reflectance equivalent produced is of 7% less rock dust. For example, when 1% 

water is added to a 65% rock dust/coal dust mixture, the reflectance is equivalent to a mixture 

containing 58% rock dust [Sapko and Verakis 2006]. Hence, a “false” YELLOW or RED 

measurement would be obtained. Based on the results of the published study, after the surface

moisture analysis was subtracted from the MSHA TIC, there were 50 RED/YELLOW samples 

(reduced from 58) > 80% IC. This sample number was then further reduced, as described in the 

next section.

On a related note, insufficient drying of the sample before CDEM measurement in the field 

would cause the sample to appear artificially dark, and the CDEM would measure the reflected 

light accordingly. By the time NIOSH received the samples from the Mt. Hope laboratory, the 

samples may have been subjected to additional drying during the shipping process. NIOSH also 

dried the samples with molecular sieves as part of the study process.
17 

Higher % IC of the Coal Mined

The CDEMs were calibrated with an 80% rock dust mixture for the GREEN/YELLOW

boundary. However, coal has a certain percentage of IC material also. If there is an assumed 8% 

IC in the coal, the 80% rock dust actually contains 81.6% IC. There were 50 MSHA CDEM 

RED/YELLOW measurements that were above 80% IC (without factoring in surface moisture). 

When comparing the dry MSHA IC analysis of 81.6% or higher (assuming 8% IC inherent in the 

coal) with the CDEM measurements, the number of RED/YELLOW measurements that are > 

81.6% IC is 37 individual band samples. By calibrating with a percentage of rock dust (% RD) 

rather than % IC, a margin of safety is included in the explosibility assessment.

NIOSH had received pure rock dust for calibration of the CDEMs from MSHA inspectors in 

various districts, but not for all of the mines where MSHA used the CDEM. This limited the 

number of surveys for which NIOSH could confirm the MSHA inspector’s measurements. Out 

of the 37 surveys, NIOSH could only calibrate the CDEM and verify the results of 16 surveys. 

Therefore, the individual band samples with RED/YELLOW measurements that were > 81.6% 

was reduced to 28 samples which NIOSH researchers could confirm (Figure 8). Of these 28 

samples, NIOSH was able to replicate RED/YELLOW measurements for 11 of them. TIC results 

conducted at NIOSH identified 4 of these samples as being < 81.6% TIC, likely indicating 

sample variance. NIOSH could not replicate RED/YELLOW results for 17 of the 28 samples, 

indicating problems with calibration or operation of the CDEM. The 11 of the 28 samples that 

were verified as RED/YELLOW but > 81.6% TIC represent cases where the CDEM errs on the 

side of safety.

Finally, if the operator is mining material that is dark (such as darker shale), the dust sample 

may appear darker, yet still contain a higher IC than what the CDEM detects and produce a 

conservative result.
18 

Figure 8. CDEM RED (R) measurement comparison of NIOSH-received samples. Agreeing RED

samples have LTA results < 81.5% IC. Disagreeing RED have more than 81.5% IC LTA results.

CDEM Calibration Error

Other sources of error related to the RED/YELLOW measurements could be due to 

calibration of the CDEM by the user who prepared the calibration samples. If the proportions of 

rock dust and PPC were not accurately measured out for the 75% and 80% rock dust calibration 

samples, the set points for the results would not be correct. If the proportions of rock dust and 

PPC were correct but the calibration samples were not sufficiently mixed, the calibration for 

color would be affected. In addition, the CDEM was to be calibrated for each mine with the rock 

dust from that mine before CDEM measurements of the band samples were taken from that 

mine. If the CDEM had been previously calibrated for use in a different mine, differences would 

occur if the rock dusts were different. 

Of the 28 RED/YELLOW samples that were > 81.6% IC, only 7 were found in the lab to be 

in disagreement with the TIC results. Of the RED/YELLOW samples measured by NIOSH, 90% 

of the measurements identified areas that required more rock dust. NIOSH was unable to find an 

explanation as to why the 7 measurements were at odds with the ashing approach. As with the 

GREEN cases, there will be some disagreement between the methods because of their 

fundamental differences. This disagreement represents 7 verifiable cases where the CDEM was 

more conservative than the laboratory technique. As seen in Figure 9, after accounting for the 

differences between the two methods, there is 97% agreement between the CDEM and the 

laboratory analysis.
19 

Figure 9. Agreement between CDEM and laboratory analysis.

Conclusions from the NIOSH Study

The CDEM was able to identify (in situ) coal and rock dust mixtures that were not capable of 

supporting flame propagation for 99% of the samples tested, subsequently confirmed by 

laboratory analysis as containing > 80% IC. The one sample that was determined to be < 80% IC 

by laboratory analysis was investigated in more detail. After further analysis of this questionable 

sample, it was found that it contained a large fraction of nonreactive coal and rock dust particles 

in the range of 20 to 60 mesh (250 to 840 μm). Rock dust particles in this range have been shown 

to be ineffectual for inerting flame propagation [USBM 1933]. The sample was passed through 

60 mesh screen and the < 60 mesh material was ashed to determine the % IC content of the 

reactive particles. The resulting 81% IC supported the CDEM GREEN reading of a 

nonexplosible mixture. Therefore, no cases of failure to detect an explosion hazard were 

identified in this study.

Assuming that the CDEM is properly calibrated and the test sample dried, a sample can be 

considered to be nonexplosible if the CDEM indicates a GREEN measurement; i.e., no 

additional rock dust is needed at that time. However, it is important to continue to sample that 

area from time to time because of changing conditions caused by the dynamic nature of the 

mining environment. 

The CDEM and TIC method use two different surrogates for estimating explosibility of coal 

and rock dust mixtures. Neither measures explosibility directly and therefore neither is 

indisputable. The only direct measure of explosibility is by use of a 20-L or larger explosion 

apparatus or through full-scale experiments [Cashdollar 1996, Cashdollar and Chatrathi 1993, 

Cashdollar and Hertzberg 1989, Cashdollar et al. 1987, Cashdollar et al. 1992a, 1992b, and 

1992c]. In the 1950s, the TIC measurement replaced the volumetric method as a means of 
20 

measuring explosibility. The CDEM reading is weighted by relative coal and rock dust particle 

sizes in assessing sample explosibility, whereas the LTA method gives equal weight to all dust 

particles < 20 mesh (840 μm). 

Even though the CDEM and the TIC methods for determining the explosibility of coal and 

rock dust mixtures use different techniques, the results of this field study show excellent

agreement (97%) between the two methods. It is unrealistic to expect the CDEM to replicate all 

laboratory TIC determinations. Rather, the goal is for the CDEM to accurately identify the 

potential in situ explosibility of a dust mixture at the time of sample collection. In this respect, 

the CDEM can be used to both identify potential hazards in real time and for use as a followup 

tool to ensure that adequate rock dust was added to abate the potential explosion hazard. 

In this study, the CDEM indicated that 25% of the samples collected were potentially 

explosible. Since each sample in this study represents 500 linear ft of mine development entry, 

the 175 RED samples represent about 17 miles of mine entries deficient in rock dust that would 

have been identified as a hazard at the time of the sample collection. The GREEN results, 

indicating that at the time of testing no additional rock dust is needed to make the sample 

nonexplosible, were accurate. The RED and YELLOW results, indicating that the area is 

deficient in rock dust, err on the side of safety in that these results require the addition of more 

rock dust even if subsequent detailed laboratory analysis indicates otherwise. With the 

immediate results, the mine can remedy the situation directly rather than wait days or weeks for 

laboratory results in order to effect change and protect miners. 

The CDEM offers an opportunity to immediately identify and correct explosible 

accumulation of coal and rock dust mixtures. This explosibility assessment is based on the 

analysis of a representative coal and rock dust mixture collected along the entries. In contrast to 

the current laboratory analysis method that takes days or weeks to obtain results, the CDEM can 

immediately identify deficient areas that can support flame propagation and trigger corrective 

measures to abate the hazard.

Finally, based on the field study results, a need for enhanced training in the use of the CDEM 

and a simpler method to calibrate the CDEM to minimize errors was identified. This input was 

used to develop a simpler calibration procedure and improve on-screen instructions to the user in 

executing calibration and testing procedures. 

Commercial CDEM Development

The term CDEM originated with the U.S. Bureau of Mines (USBM) and NIOSH 

development of the tool. The commercial CDEM is known as the CDEM–1000 as named by the 

manufacturer. The commercial CDEM developed in 2010–11 is similar to the CDEMs used in 

the study. However, some changes to the CDEM have been made based on analysis of the study 

results, input from the inspectors, and input by the instrument manufacturer.

In response to coal mine inspector comments (Appendix E), updates to the CDEM include 

changes in screen prompts to plain language and more explicit instructions, a simplified 

calibration process (using three instead of four samples), and consideration of atmospheric 

methane in results displayed. 
21 

The changes in the CDEM screen prompts now lead the user through the use and calibration 

of the device so that all terms are common and well understood by the average user. This 

improvement is accompanied by a user manual with thorough descriptions and explanations of 

the calibration and use process. 

Calibration and Programming of the Commercial CDEM

The commercial CDEM–1000 is calibrated using the same standards as those used in this 

study, except only three samples are required: 100% PPC, 100% RD, and 80% RD. All former 

YELLOW measurements are now considered to be RED. In the analysis of the study, all 

YELLOW measurements were considered to be RED readings and thus deemed to require more 

rock dust. The 80% rock dust sample was chosen as the RED/GREEN boundary based on fullscale experimental results conducted at the NIOSH LLEM [NIOSH 2010]. This value is also 

reflected in the recent MSHA final rule requiring 80% IC in all entries of an underground coal 

mine as recommended by NIOSH Report of Investigations (RI) 9679. The changes in the 

calibration procedure are software changes and not changes to the components or operation 

principles of the CDEM.

The CDEM programming was also changed to accommodate the presence of methane. In 

accordance with 30 CFR 75.403, where methane is present in any ventilating current, the percent 

of incombustible content of such combined dust shall be increased 0.4 percent for each 0.1 

percent of methane. To allow the CDEM to determine the amount of rock dust required in a mine 

entry, the user is prompted to enter ambient methane levels (as % CH4) measured with an 

MSHA-approved methane meter. The CDEM will adjust the RED/GREEN boundary based upon 

the % CH4 input, and the requirements of 75.403. 

Another change in the CDEM output is to indicate the level of noncompliance to aid mine 

operators in defining the level of action required to abate a dust explosion hazard. If a sample is 

indicated as RED, i.e., explosible, the display will indicate a range of % rock dust measured 

which is based on the measured value and the maximum observed variance in readings (+/- 2%). 

This range does not include the IC inherent in the coal nor the variable as-received moisture of 

the sample. 

Commercial CDEM Changes based on Potential Customer Concerns

Based on the outcomes of this study, an anticipated potential customer concern may be the 

availability of PPC for calibrating the CDEM. The manufacturer and NIOSH have a PPC supply 

available to support the initial build of 3,000 instruments. PPC will be provided with each unit 

for calibration. An evaluation of a manufactured PPC product is currently being pursued to 

ensure that additional PPC can be produced and supplied to customers to sustain CDEM support 

indefinitely.

Another potential customer question involves the frequency of calibration. The manufacturer 

is requiring that the CDEM be recalibrated after 200 measurements. However, if a mine changes 

its rock dust supplier, the CDEM would need to be recalibrated with new calibrations samples 

made from the rock dust received from the new supplier. Further, if an operator changes rock 

dust supplies frequently, the operator will need to recalibrate the CDEM more often. It should be 

further noted that a highly variable rock dust supply (in color and in particle size) would make 

visual hazard assessment without a meter extremely difficult and current visual determination of 
22 

adequate abatement of a violation equally suspect. Only if the current particle size specifications 

were tightened and if rock dust supplies were certified/controlled would any significant 

variations from batch to batch and from supplier to supplier be significantly minimized. 

The Commercial CDEM as a Verification and Compliance Tool

Discussion of the commercially developed meter would not be complete without 

identification of potential uses of the instrument in managing dust explosion hazards and 

compliance with regulations. The CDEM as a compliance assistance tool can be used by mine 

operators to evaluate accumulations of dust in areas where dust is generated such as returns from 

producing sections, coal crushing and transfer points, and conveyors or other coal transport 

systems in high-velocity airways. The collection and analysis of samples during normal 

operations will provide an indication of the level of safety provided by a mine’s rock dusting 

program. Areas deficient in rock dust can be identified and improvements to either rock dusting 

practices or dust control can be enacted. Mine safety inspectors can utilize the CDEM to screen 

samples for compliance immediately upon collection and take immediate action to have the 

hazard abated. Samples identified as deficient can then be sent to laboratories for analysis per the 

appropriate existing practice for determining the potential explosibility of the mixture. Samples 

identified as adequate may not require laboratory analysis. Finally, should an area be identified 

as deficient, the CDEM can be used to verify that the hazard was abated in place of the existing 

qualitative visual assessment.

In addition to these routine applications of the CDEM in traditional dust surveys and spot 

checks, it can also be used to evaluate compliance with mine ventilation standards. 30 CFR 

75.323.d.1.iii requires rock dust to be continuously applied to allow operations up to 1.5% CH4

in return air splits. The commercial CDEM with the methane input capability can evaluate the 

adequacy of the effort at rock dusting to ensure that an explosion hazard is not permitted to exist. 

NIOSH Recommendations 

The CDEM is recommended for use as a compliance tool to allow real-time assessment of 

coal dust explosion hazards in underground coal mines for the prompt initiation of corrective 

actions. The following considerations should be used as part of this recommendation: 

• If the criteria of 30 CFR 75.403 remains TIC, then the CDEM should be used by the 

mine operator and the MSHA inspector to determine the onsite need for immediate

corrective action. Inspectors would send samples to the laboratory for TIC 

determination.

• If the use of TIC as the surrogate for explosion hazard enforcement were changed to 

% RD and if other changes such as inclusion of dust particle size and elimination of 

the as-received moisture are considered, the CDEM could serve as the regulatory tool 

to take enforcement actions, and onsite corrective action in the deficient areas could 

be taken immediately by both the mine operator and the MSHA inspector. 

• The sample collected, dried, and tested with the CDEM can be retained for laboratory 

analysis to independently verify the % IC relevant to current regulatory standards. For 
23 

any discrepancies between the two methods, explosion chamber tests could be 

conducted to determine the inherent explosibility of that representative sample.

• Upon receipt of a RED measurement, the area where the sample was collected should 

be treated with more rock dust in order to inert the explosible conditions present. 

Rock dust should immediately be applied to the area of entry where the RED sample 

was collected until a GREEN measurement is obtained. 

Acknowledgments

The authors would like to acknowledge H. Erik Sherer, MSHA, Senior Mining Engineer, for 

coordinating the MSHA study of the CDEM and the invaluable cooperation of the many MSHA 

field inspectors who calibrated and used the CDEM on collected dust samples. The authors also 

acknowledge Terry Montgomery, Supervisor Chemist, of the MSHA analytical laboratory, Mt. 

Hope, WV, for providing the dust samples, the incombustible percentage data, and the 

incombustible analysis procedures. The authors also recognize the laboratory assistance of all

NIOSH OMSHR personnel for their efforts in the CDEM testing of the dust samples and the dust 

samples particle size testing. The authors would also like to acknowledge the work of Henry 

Perlee and Ed Hay of H&P Prototype and Harry Verakis, Senior Projects Engineer in MSHA 

Technical Support, for assisting with the development of the technology that led to the precursor 

prototype instruments. 

References

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Cashdollar KL, Chatrathi K [1993]. Minimum explosible dust concentrations measured in 20-L 

and 1-m3 chambers. Combustion Science and Technology, 87(1–6):157–171. 

Cashdollar KL, Hertzberg M [1989]. Laboratory study of rock dust inerting requirements: effects 

of coal volatility, particle size, and methane addition. In: Proceedings of the 23rd International 

Conference of Safety in Mines Research Institutes (Washington, DC, September 11–15, 1989). 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, pp. 965–977. 

Cashdollar KL, Sapko MJ, Weiss ES, Hertzberg M [1987]. Laboratory and mine dust explosion 

research at the Bureau of Mines. In: Cashdollar KL, Hertzberg M, eds. Industrial dust 

explosions. West Conshohocken, PA: American Society for Testing and Materials (ASTM), 

Special Technical Publication (STP) 958, pp. 107–123. 

Cashdollar KL, Weiss ES, Greninger NB, Chatrathi K [1992a]. Laboratory and large-scale dust 

explosion research. Plant/Operations Progress, 11, pp. 247–255. 

Cashdollar KL, Weiss ES, Greninger NB, Chatrathi K [1992b]. Laboratory and large-scale dust 

explosion research. In: Proceedings of the 26th Annual Loss Prevention Symposium (New 

Orleans, LA). Paper No. 72C. AIChE Spring National Meeting, 22 pp. 
24 

Cashdollar KL, Weiss ES, Greninger NB, Hertzberg M, Sapko MJ [1992c]. Laboratory and 

large-scale explosion research at the U.S. Bureau of Mines. In: Proceedings of The First World 

Seminar on the Explosion Phenomenon and on the Application of Explosion Protection 

Techniques in Practice (Brussels, Belgium). Section 1, 16 pp. 

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office. Office 

of the Federal Register.

Harris ML, Sapko MJ, Cashdollar KL, Verakis HC [2008]. Field evaluation of the coal dust 

explosibility meter (CDEM). Mining Engineering. 60(10):50–54. 

Litton CD, Chaiken RF [1996]. The role of radiation absorption in defining explosibility of 

coal/rock dust mixtures. In: Proceedings of the Twenty-Sixth Symposium (International) on 

Combustion. Pittsburgh, PA, 1996. pp. 1571–1577. 

MSHA [2010]. MSHA to publish emergency temporary standard for rock dust

[http://www.msha.gov/MEDIA/PRESS/2010/NR100921.asp.] Date accessed: 12/01/2010. 

NIOSH [2006]. Technology News 515: Float coal dust explosion hazards. U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication No. 2006–125. 

NIOSH [2010]. Report of Investigations 9679: Recommendations for a new rock dusting 

standard to prevent coal dust explosions in intake airways. By Cashdollar KL, Sapko MJ, Weiss 

ES, Harris ML, Man CK, Harteis SP, Green GM: U.S. Department of Health and Human 

Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health, DHHS (NIOSH) Publication No. 2010–151. 

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25 

APPENDIX A: CDEM Design
26 

The CDEM (Figure A1) consists of an optical probe connected to a small electronics box 

with a digital display.

Figure A1. Coal Dust Explosibility Meter (CDEM).

The optical probe consists of an infrared source and a silicon photodiode 

sensor. When the CDEM probe is inserted into the dust mixture, the infrared radiation reflects 

off the surface of the dust and back to the silicon photodiode sensor. 

The normalized reflectance, Φ, is related to the mass fraction of rock dust, fr, in the sample 

by the following equation [Sapko and Verakis 2006]: 

(1)

where Ix = intensity of light reflected from a homogeneous coal and rock dust mixture; 

Ic = intensity of light reflected from a pure coal dust surface; 

Ir = intensity of light reflected from a pure rock dust surface; and

K = proportional to the product of the rock dust to coal dust particle density ratio and 

the ratio of the mean particle diameters of coal to rock dust contained in the mixture. 
27 

Φ can also be written directly as:

(2)

where Sc = coal dust surface area and

Sr = rock dust surface area. 

For a fixed ratio of Sc to Sr, the quantity Φ is independent of explicit dependence upon K or 

fr. This is because the ratio Sc/Sr can be maintained constant while fr and K can vary.

While examining the data from many coal and rock dust mixtures, the normalized 

reflectance, Φ, of the dust mixtures near the boundary which separates mixtures that propagate 

flame and those that do not propagate flame was relatively constant, Φ*, the normalized 

reflectance at the extinction boundary. To visualize this finding, mixtures were made using 

various experimental coal sizes that contained 20, 40, and 80% < 200 mesh coal particles used in 

the full-scale explosion experiments shown in Figure 1. Since the experimental measured rock 

dust inerting limit has a precision of approximately ± 3 weight (wt) percent, rock dust and coal 

dust samples were also prepared at ± 3 wt percent around the experimental boundary, and their 

normalized reflectance was also measured with the CDEM. Figure A2 shows the Φ values 

measured at the inerting limit for the Pittsburgh seam coal dust sizes shown in Figure 1. The 

error bars represent the measured Φ of the samples containing ± 3 wt percent rock dust around 

the boundary between propagation and nonpropagation. As shown, for the Pittsburgh seam coal, 

Φ at the limit of explosibility is fairly constant over a wide range of volume median coal particle 

sizes, ranging from 57 μm (80% < 200 mesh) to 270 μm (20% < 200 mesh), while the rock dust 

decreased from 80 to 68%, respectively [NIOSH 2010]. A larger value of Φ (greater than Φ*) is 

produced by a smaller ratio Sc/Sr. This would result from an increase in the rock dust surface area 

and/or a decrease in the coal dust surface area. A smaller value of Φ (less than Φ*) is produced 

by an increase in the ratio Sc/Sr, and would result in a greater increase in Sc than in Sr. That is, a 

decrease in Sc/Sr moves the mixture into the nonexplosible range, whereas an increase in Sc/Sr

moves the mixture into the explosible range. 
28 

Figure A2. Extinction limit for normalized reflectance [Sapko and Verakis 2006].

The constant, Φ, at the extinction boundary, is also supported by a theory developed by 

Litton and Chaiken [1996]. This theory observed that, at the lower explosion limit, the product of 

specific absorption (absorption = 1 - reflection) and mass concentration of coal dust is constant 

for fixed coal volatility. Several historical coal samples from various coal seams where volatility 

varied from 15% to 42% indicated that the reflectance of the pure coal was not significantly 

different from the PPC. Theory and data both support the experimental findings that the CDEM 

can be used to determine the explosion potential of a particular coal and rock dust mixture. 

When calibrating the CDEM, Φ*, the normalized reflectance at the extinction boundary is set 

by the mixture made and used as the 80% mine rock dust and 20% PPC. 
29 

APPENDIX B: CDEM Training
30 

MSHA inspectors from the 10 bituminous coal districts were trained by NIOSH 

representatives on the calibration and use of the CDEM at the National Mine Health and Safety 

Academy in Beckley, WV, on November 5, 2009. Two presentations were given detailing the 

background and development of the CDEM and steps to calibrate and use the device. The 

training presentations were subsequently sent to Erik Sherer, Senior Mining Engineer for MSHA 

Coal Mine Safety and Health, for later distribution among the inspectors. 

Calibration samples were prepared by the inspectors using PPC and the mine’s RD in the 

following percentages by weight: 0% RD (100% PPC), 75% RD, 80% RD, and 100% RD. The 

30-g or 50-g sample mixture of 75% and 80% rock dust were to be prepared as listed in Table 

B1. 

Table B1. Mixtures of RD and PPC for CDEM calibration

Based on 30-g mix

% RD Percentage 

of PPC

RD 

(g) PPC (g) total (g)

 80.0 20.0 24.00 6.00 30

 75.0 25.0 22.50 7.50 30

Based on 50-g mix

% RD Percentage 

of PPC

RD 

(g) PPC (g) total (g)

 80.0 20.0 40.00 10.00 50

 75.0 25.0 37.50 12.50 50

The 75% RD sample is used to set the boundary between RED and YELLOW. The 80% RD 

mixture is used to set the GREEN/YELLOW boundary. Calibration samples were required for 

each mine. If more than one mine used the same RD supplier, the calibration samples may be the 

same for those mines.

MSHA supplied the meters, scales, molecular sieves, and sampling tubes so that the 

inspectors could prepare the calibration samples for the mine surveyed and collect dust samples 

to test with the CDEM within the mine. The inspectors were asked to collect a pure rock dust 

sample from each mine, with some of the sample kept by the inspector in order to mix the 

calibration samples for each mine and the remainder of the sample sent to NIOSH. NIOSH 

would then use the pure rock dust samples to make its own calibration samples for subsequent 

testing and verification of inspector-collected samples after MSHA analysis.
31 

APPENDIX C: Prototype CDEM Calibration and Testing Procedures

Used in the Joint Study
32 

Prior to use, the CDEM was calibrated using samples of that mine’s pure rock dust, PPC, and 

two mixtures of known rock dust and PPC content. The following sections describe the 

instructions provided to MSHA on the proper preparation of the known calibration mixtures, 

calibration of the CDEM, collection of a representative dust sample, and the use of the CDEM to 

determine the explosibility of an unknown dust sample. 

The following paragraphs describe the calibration and testing directions used by MSHA in 

this joint study. The procedures for the use of the commercialized CDEM have since changed. 

The CDEM used for this study was a prototype meter that the manufacturer has since enhanced 

in the commercialized version. The calibration procedure, meter displays, and operation of the 

commercialized meter have been simplified for ease of use. Therefore, all of the manufacturer’s 

calibration and use instructions provided with the commercialized CDEM must be followed. 

Preparation of Calibration Sample

One sample is used to set the GREEN/YELLOW boundary and a second sample is used to 

set the YELLOW/RED boundary. The particle size of PPC is characterized in Table C1. 

Table C1. Pittsburgh pulverized coal size data

Mesh 

Size

Size Range, 

μm

Cumulative 

%

50 x 40 300–425 100

70 x 50 212–300 100

100 x 70 150–212 99

140 x 100 106–150 94

200 x 140 75–106 78

270 x 200 53–75 57

400 x 270 38–53 37

500 x 400 25–38 24

635 x 500 20–25 12

For 

these calibrations, an 80% rock dust mixture was used to set the GREEN/YELLOW boundary 

and a 75% rock dust mixture was used to set the YELLOW/RED boundary. 

The coal dust (CD) samples used to calibrate the CDEMs are PPC samples supplied by 

NIOSH. For proper calibration, the samples should be dried using molecular sieves. The rock 

dust (RD) samples as received for each mine are dried using molecular sieves.

The term “color sample” refers to a dust sample consisting of CD and RD to determine the 

boundary between RED/YELLOW and YELLOW/GREEN. Color samples are made by mixing 

RD and CD with the percentage determined by weight—e.g., an 80% RD sample is made by 

mixing 40 grams (g) of RD with 10 g of CD (Table C2). This mixture does not account for the 

incombustible content within the coal. 
33 

Table C2. Reference mixtures, percentage by weight

Based on 50-gram mix

Percentage 

of RD

Percentage 

of CD RD (g) CD (g) Total (g)

85 15 42.50 7.50 50

82 18 41.00 9.00 50

80 20 40.00 10.00 50

77 23 38.50 11.50 50

75 25 37.50 12.50 50

73 27 36.50 13.50 50

Drying the sample

Each sample must be dried prior to applying it to the CDEM. If the sample is not dried first, 

the moisture contained in the sample may make the sample appear artificially dark and the 

CDEM assessment may therefore be incorrect. The following procedure is used to ensure a dry 

sample:

• Tubes (11⁄2 ounces, oz, or 45 milliliters, mL) half-filled with predried molecular sieves 

are used to collect the samples (Figure C1).

• The sample tube filled with molecular sieves and test material (either CD, RD, or a 

color sample) is shaken so that the sample mixes with the sieves and is allowed to dry 

for at least one minute.
34 

Figure C1. Sample tube containing molecular sieves.

CDEM Calibration

The CDEM must be calibrated for the specific mine before the CDEM is used to determine 

the IC of the dust samples from that mine. If not, the CDEM will operate based on the previous 

calibration which may or may not be set correctly for the mine’s rock dust used. Therefore, it is 

advisable to keep a calibration log for each CDEM with the date and rock dust used to calibrate 

that CDEM for a particular mine. A 50-g mixture is sufficient for calibration.

By preparing 80% and 75% RD mixtures, the chosen color boundaries are 

GREEN/YELLOW at 80% RD and YELLOW/RED at 75% RD. 

The following procedure is used to calibrate the CDEM:

• Turn the CDEM on while pressing the Enter button; the meter will then display 

“Meter Calibration.”

• Fill sample cup halfway with dried RD. At RD prompt display, press cup firmly onto 

probe, and press Enter on the CDEM. 

• Clean the probe and cup. 

• Fill sample cup halfway with dried CD. At CD prompt display, press cup firmly onto 

probe, and press Enter. 

• Clean the probe and cup. 

• When prompted for GREEN/YELLOW, fill sample cup with the 80% mix, press the 

cup firmly onto probe, and press Enter. 

• Clean the probe and cup. 

• When prompted for YELLOW/RED, fill sample cup with the 75% mix, press the cup 

firmly onto probe, and press Enter. 

• Clean the probe and cup. 
35 

• If the calibration was successful, the calibration is saved and the CDEM will display 

“Calibration complete.” At this point, the CDEM can be turned off or left on. If the 

CDEM is left on, it will proceed to the testing mode.

As indicated by the above procedure, it is important to clean the probe and cup between 

samples.

In-Mine Testing

The reliability of the CDEM reading is most dependent on the collection of a representative 

sample using the MSHA band sampling equipment and procedures, as follows: 

• Collection of sample (general outline of the dust collection procedures). 

o In the US, routine band surveys are performed quarterly at each mine by the 

MSHA.14

 

14 See pp. 60–66 of the General Coal Mine Inspection Procedures and Inspection Tracking System for 

detailed MSHA procedures. This publication is available at 

http://www.msha.gov/readroom/handbook/PH08-V-1GeneralCoalInspectionHandbook.pdf

Samples (up to 1 inch deep) are collected from the floor, ribs, and 

roof (when possible). Samples are taken every 500 ft in the sections of new 

development. 

o The MSHA inspector collects the band sample by brushing dust from the ribs, 

floor, and roof (when possible) into a metal pan (Figure C2).

Figure C2. Collecting a band sample.
36 

o The dust is then sifted through a 10 mesh (1.7 millimeter, mm) screen to 

remove particles larger than 2 mm (Figure C3).

Figure C3. Sifting a sample.

o Once sifted, if there is sufficient dust for a complete sample, the dust is mixed.

If not, additional dust should be collected until a sufficient sample is obtained. 

o To mix the sample dust, the inspector is to cut, cone, and quarter the mixture 

to obtain a homogeneous mixture. 

• Collecting a sample for the CDEM.

o Tubes (11⁄2-oz, or 45-mL) half-filled with molecular sieves are used to collect 

the sample from the tray and to dry the dust (Figure C1 and 

o Figure C4). It is advisable to prepare the tubes by filling them with molecular 

sieves before entering the mine.

o The sample tube filled with molecular sieves and dust is shaken so that the 

dust sample mixes with the sieves and is allowed to dry for at least one 

minute.
37 

Figure C4. Collecting a sample into a sample tube.

o With the moisture now removed from the dust sample, a funnel with a 20 

mesh screen (850 μm) is attached to the end of the tube. 

o The CDEM sample cup is fitted onto the end of the funnel and the dust sample 

is shaken from the sample tube into the sample cup (Figure C5).

o The 20 mesh screen inside the funnel prevents the molecular sieves from 

entering the sample cup.

Figure C5. Transferring dust to a sample cup.
38 

• Testing Mode: Testing a mine dust sample with the CDEM after calibration.

o Turn on the CDEM. 

o When the CDEM is ready, “PUT PROBE IN SUT THEN PRESS ENTER” 

will be displayed. SUT stands for “sample under test.”

o Once the dry dust is in the sample cup, insert the CDEM probe into the sample 

cup and hold firmly to the probe lens. While holding sample against the probe 

lens, push the Enter button on the CDEM (Figure C6).

Figure C6. Testing a dust sample with the CDEM.

o The CDEM returns a reading of explosibility. GREEN indicates there is 

sufficient RD in the sample presented. YELLOW indicates the sample is 

marginally explosible and more RD should be added to that area of the mine. 

RED indicates deficient RD in the sample, requiring more RD be applied to 

the area of the mine. 

• Between-sample cleaning of the CDEM.

o Gently tap the side of the CDEM against your palm to remove dust from 

around the probe lens. 

o Using the bare palm of your clean hand, wipe off any dust on the probe lens.

Do not use paper or other materials as it could scratch the lens or statically 

charge the lens.
39 

APPENDIX D: Particle Size Effect
40 

Finer coal particles are more explosible than coarse particles, and finer rock dust particles are 

better at suppressing an explosion than coarse particles. The reflectance of the coal mine dust 

responds in a similar manner. The CDEM measures this reflectance to determine explosibility of 

a dust sample. The CDEM is also sensitive to particle size. The ability of the CDEM to detect 

variations in particle size in relation to explosibility (see Equations 1 and 2 in Appendix A) is 

one reason that CDEM results may vary from laboratory IC measurements.

Figure D1 shows the change in the amount of rock dust required to inert three different 

particle sizes of coal dust.

Figure D1. A comparison of coal dust particle size distributions on φ values. Rock dust particle 

size distributions are constant.

 The horizontal line expressing the explosibility threshold was 

determined by NIOSH testing. Coal dust with a 57-μm mean diameter, the size of the PPC used 

in NIOSH testing and the size provided as a calibration standard with the CDEM, requires 80% 

rock dust. Coal dust 10% finer requires about 82% rock dust, while coal dust 10% coarser 

requires only about 78% rock dust. 

Figure D2 shows the same relationship as Figure D1 for variations in rock dust particle size. 

The nominal rock dust particle size used in full-scale explosion experiments by NIOSH [NIOSH 

2010] was 26 μm and should make up 80% of the mine dust for that dust to be considered inert. 

A 10% increase in rock dust particle size would require almost 82% rock dust to be inert, while a 

10% decrease in rock dust particle size would require 78% rock dust to be inert. 
41 

Figure D2. A comparison of rock dust particle size distributions on φ values. Coal dust particle 

size distributions are constant.

If a mine dust contains coal dust finer than the PPC and/or rock dust coarser than a median

26-μm particle size; the CDEM may indicate explosibility (RED) even though the dust is > 80% 

IC. This result is not an error but an indication that the dust is more explosible than measuring IC 

alone can determine.

If a mine dust contains coal dust coarser than the PPC and/or rock dust finer than a median 

26-μm particle size; the CDEM may indicate nonexplosibility (GREEN) even though the dust is 

< 80% incombustible content. This result is not an error but an indication that the dust is less 

explosible than measuring IC alone can determine.
42 
43 

APPENDIX E: MSHA Inspector Questions and Comments
44 

As a followup to this study and to evaluate the use of the CDEM, MSHA asked its coal mine 

inspectors who used the CDEM in the field four questions regarding the use of the meter: 

1. Did you have any problems using the meter? 

2. What would you like to change or improve on the meter? 

3. Do you think that this meter would be useful for rock dust sampling and enforcement 

(considering that we will have a regulation change to support the RED and GREEN

output)? and 

4. Any other comments or suggestions for this meter or on our rock dust program in 

general?

The boldfaced questions are listed below, with responses in plain text, followed by NIOSH 

commentary in italicized text. Note that these were the original NIOSH responses to MSHA 

from each of the questions asked by the inspectors. Based on this study, subsequent 

enhancements to the prototype meter were since incorporated into the commercialized version of 

the CDEM.

1. Did you have any problems using the meter? 

a. One day the meter would turn off as soon as it was turned on. 

Response: NIOSH is unable to determine a cause for this intermittent problem. 

b. Hard to use in low coal. 

Response: We understand everything done in low coal is more difficult.

c. The drying process requires you to prep a canister for each survey point. This 

makes larger surveys very burdensome, especially in lower coal seams and where 

rides are not available.

Response: We understand that all activities conducted in low coal are more 

difficult to perform and adding this additional assignment of conducting a 

parallel evaluation using the CDEM at the same time following the traditional 

sample handling added to the burden. However, it needed to be done as part of 

this field study in order for MSHA to make additional comparisons between the 

two methods for assessing explosibility. In the development, implementation, and 

use of the CDEM, NIOSH researchers envisioned, for purposes of compliance, 

only sending back the dust sample (small tube with molecular sieves) that were 

found to be deficient (RED) at the mine to Mt. Hope for followup on regulatory 

action, if needed. One goal of NIOSH is to quickly identify and correct potential 

deficiencies in rock dust preferably through the CDEM use by the mine operator. 

d. Difficult time reading the information. (This was the most common complaint and 

made by every inspector.) 
45 

Response: The output is either RED or GREEN on the display of the CDEM 

commercial version. The LCD readability has been improved. 

e. Carrying the tubes for drying the dust is extra weight and time consuming to use.

Response: As highlighted in the response to 1c above, in order to make 

comparisons, traditional sample collection and handling was required and added

more weight to carry in and out of the mine. NIOSH researchers envision through 

the implementation and use of the CDEM, the total weight of samples carried out 

of the mine for subsequent laboratory processing will be much less than what is 

now done as part of the current sample processing procedures. 

2. What would you like to change or improve on the meter? 

a. The meter needs a back light so it can be easier seen in the dark. 

Response: The meter could use this as a feature in a future model.

b. Would like to see a percentage read out as well.

Response: We do not support this. The percentage readout would only be a 

prediction and may divert the focus of safety. We recommend the CDEM’s use as 

a compliance check.

The output reading would only be accurate if the median particle size of the rock 

dust and of the coal dust in the sample were identical to those used in the 80% 

calibration sample. Since the particle size in the actual samples can vary and the 

< 20 mesh (841μm) dust fraction may contain very large ineffective inert 

particles (> 250 μm and < 841 μm), outputting the % IC is of little value and may 

not be subsequently confirmed by conventional LTA analysis and thus generate 

more confusion.15

 

15 Due to several requests such as this, the commercial version has the option to display a range of 

predicted percentages of rock dust if the measurement is RED.

c. Develop a way to roll the meter along the rib, ground, and roof instead of 

sweeping a sample.

Response: Interesting idea and conceptually has merit. However, at this time, that 

is not possible with this technology. Such an approach would only be feasible if 

one could assure that the scanned surfaces were dry and the dispersible surface 

coal and rock dust is not stratified but well mixed. 
46 

d. Line or lip for testing cup. 

Response: Good suggestion, the manufacturer has included a “fill line” in the 

newly designed sample cup. 

e. The calibration process is time consuming and a little difficult. If this could be 

addressed in an automatic software process after all the initial data is analyzed it 

would be very helpful. 

Response: The calibration has been simplified so that only 3 samples are now 

needed: PPC, pure rock dust from the mine, and an 80% rock dust mixture (80% 

mine rock dust, 20% PPC).

f. The scales (to make the initial calibration samples) turn off before you have time 

to perform the measurements to make your calibration samples. With only one 

thimble if you lose or crack it, the system is down. 

Response: Thank you for informing us about the scales that MSHA ordered and 

the criticality of having only one cup with the meter. Weigh boats or a piece of 

paper to hold the dust before placing the components in a sample container for 

mixing may help provide time before your scale turns off. The commercial meter 

and kit provides extra cups in case one loses the sample cup.

g. Also it would be beneficial if the meter had a data storage system similar to the 

new Global Positioning Systems, (i.e. SD card) to keep the calibration for the 

specific mines once calibrated. This database could be updated as necessary and 

would save time on recalibrating every time you changed mine locations. 

Response: Very good suggestion and one that may be considered as an option in a 

future version of the meter. 

h. Not very sturdy. The switch cover developed a hole the second day of use. 

Response: Thank you, the switch and button covers have been changed to a more 

rugged type for the commercialized version.

i. Inconsistent results not in agreement with lab results. 

Response: Thank you for the observations: Since the CDEM and the TIC methods 

for determining the explosibility of coal and rock dust mixtures use different 

techniques, it is unrealistic to expect the CDEM to replicate laboratory TIC 

determinations. Rather; the goal is for the CDEM itself to accurately and quickly 

identify the potential in-situ explosibility of a dust mixture. 
47 

j. The use of the single thimble sized sample cup to scan multiple samples, rock 

dust, and known mixtures made inspectors worry about contamination of samples. 

There was always concern the sample cup would be lost. They thought multiple 

throw away insert sample cups might be better. 

Response: If the sample cup is completely emptied (by tapping on a hard surface), 

any small residual dust on the walls of the cup, based on lab tests, was not 

sufficient to indicate a source of error. The CDEM probe is immersed in the dust, 

picking up reflections from a small amount of dust remaining on the walls of the 

cup is unlikely. However, the manufacturer is providing a brush to aid in 

removing the dust from the cup and around the probe and is providing five sample 

cups in the kit. Therefore, 3 sample cups could be set aside and used with each 

individual calibration sample only if the user desired.

k. With only one thimble if you lose or crack it, the system is down. 

Response: Multiple sample cups will be supplied and more available for sale.

3. Do you think that this meter would be useful for rock dust sampling and 

enforcement (considering that we will have a regulation change to support the 

Red and Green output)?

a. As long as it replaces collecting bag samples.

Response: Since the CDEM and the TIC methods for determining the explosibility 

of coal and rock dust mixtures use different techniques, it is unrealistic to expect 

the CDEM to replicate laboratory TIC determinations. Rather; the goal is for the 

CDEM itself to accurately and quickly identify the potential in-situ explosibility of 

a dust mixture. NIOSH researchers believe that the CDEM does a better job of 

assessing the explosibility of a sample presented to it since it considers the very 

important factor of the particle size of the coal and rock dust variability whereas 

the current laboratory ashing method does not. 

b. No. Regardless of the outcome an inspector will have to return to an area of the 

mine that dust samples went out and resample. Right now we only have to 

observe the area to terminate a citation issued to the area.

Response: Just as your visual inspection of the area from which the sample was 

collected could not determine inert content without lab analysis, can one 

determine if adequate rock dust was added to mitigate the potential hazard by 

visual inspection alone? NIOSH researchers have shown that a visual inspection 

only for abatement purposes is not adequate to determine if the area has been 

adequately re-rock dusted. A thin surface coating of rock dust can visually 
48 

obscure a dangerous un-neutralized sub-layer of coal dust on rock dust or rock 

dust on coal dust and yet still be a potential dust explosion hazard. 

c. Yes, it could prevent fatalities by closing an area until it is properly rock dusted 

and only terminate when it is safe for miners to return. 

Response: Thank you, we agree with your comment. It also underscores the need 

to be vigilant and up-to-date with rock dusting practices. 

d. They would tend to avoid this on larger surveys; however, thought it would be 

useful to selectively use the meter (possibly even outside the mine) to scan 

questionable samples and determine if a survey is out. For example scanning 10 

bad samples from a 99 sample survey would give them the information they need 

to cite the survey that day. 

Response: Since each sample represents ~ 500 feet of mine entry, 10 out of 99 is 

perhaps a low hazard weighting factor relevant to the potential consequences if 

an explosion would occur. Perhaps 1 citation/deficient sample should be 

considered to encourage interest in maintaining adequate rock dust levels. 

e. If used to only analyze questionable areas, this would be a good tool. Did not 

understand the need of sampling areas that contained up to 6 inches of rock dust, 

either with the CDEM or by sending samples to Mt. Hope. 

Response: Thank you for the very astute observation. Some thought has been 

given to better focus inspectors’ energies on spot sampling those questionable 

areas as they carry out their routine mine inspection duties.

Although 6 inches deep of rock sounds like a lot of dust on the floor, 

understanding where the coal dust is distributed in that area is most important for 

dust explosion hazard assessment. For example, one can have 6 inches of rock 

dust on the floor with a thin layer (0.01 mm) of fine coal dust lying on top and still 

have conditions over which an explosion would propagate (Figure E1) [Sapko et 

al. 1987, NIOSH 2006]. Or there may be an accumulation of fine coal dust on the 

rib and roof area. If the rib and roof dust represents a nominal entry dust loading 

of 100 mg/l, there is sufficient dust based on full-scale float dust explosion studies 

to propagate a coal dust explosion even with 6 inches of rock dust on the floor. 

Therefore the distribution of the dust within the entry is most important in 

explosion prevention and hazard assessment. Especially the roof and rib dust. 
49 

Figure E1. Cross section of a very thin (0.01-in-thick) explosible float coal dust layer deposited on 

top of a 3/4-in-thick (20-mm-thick) layer of rock dust [NIOSH 2006].

f. Time consuming because of “double duty.” Completing rock dust survey and 

using CDEM. 

Response: Sorry for causing “double duty” but the efforts were most beneficial in 

answering some of the remaining questions about the CDEM. The double duty 

work was necessary in this field study for comparing CDEM hazard assessment 

determinations with results using the conventional explosible assessment 

approach of sending all samples back to Mt. Hope for analysis. 

g. The instrument would be great for companies to check for compliance. If this was 

used for inspectors to test areas for compliance of areas in question it would be 

great.

Response: Good point and something that should be considered as part of an 

effective dust hazard detection and prevention program in using the CDEM. 

4. Any other comments or suggestions for this meter or on our rock dust program 

in general?

a. Very time consuming!

Response: Completion of this field study did require added burdens and increased 

the time to complete standard dust surveys and to use the CDEM on all samples. 
50 

Without the fine efforts of the inspectors, the successful completion of this project 

would not have been possible. 

b. How is the meter affected by a mine that uses different types of rock dust brands 

at the same time (bulk and bag)? 

Response: There is a need for tighter rock dust specifications and certification to 

minimize these types of variations in product quality control issues. That issue is 

currently being investigated by NIOSH and MSHA. It has been observed that the 

CDEM adjusts the explosibility assessment to these variations in rock dust 

particle sizes and, in some cases, is the reason for variance in readings between 

laboratory LTA analyses and CDEM measurements. In any case, tighter rock dust 

specification should minimize any differences between bulk and bag product. 

Until rock dust specifications are tightened and product-certified, the meter needs 

to be calibrated with the rock dust in use and at least potential variance identified 

by making up calibrations samples between each source and assessing their 

impact on meter calibration. Some particle size variations between bulk and bag 

samples may have little impact on the CDEM operation. 

c. Sometimes the results were spot on and other times they may have been off by 

10%-15%. 

Response: This is not surprising since the two methods use different approaches 

for determining the explosibility of the mixture. Being off when comparing the 

results of the two tests is one thing. The most important question is: what is the 

actual explosibility of the sample being evaluated?16

 

16 The differences between the “off” measurements have been extensively discussed in the body of the 

paper (inadequate mixing of the samples when using the CDEM, moisture remaining in the sample, 

improper training, calibrating with % rock dust and not % incombustible, etc.). The explosibility of the 

sample depends on particle size in addition to the incombustible content.

d. Calibration takes a long time. 

Response: Once the calibration samples are created, the calibration of the meter 

only takes a few minutes. Once the meter is calibrated, the meter should not need 

to be calibrated frequently. The manufacturer is requiring recalibration of the 

CDEM after every 200 measurements. It is also necessary to recalibrate the 

CDEM if there is a change in rock dust being used or you are going to use it at a

different mine that uses a different rock dust. Once the calibration samples are 

made, the calibration of the meter does not take long. 
51 

Response: The manufacturer of the commercial meter provides a case with the 

CDEM.

f. Do we need to carry out our empty vials (the vials and contents may be 

combustible materials)? 

Response: The vial content may contain a explosible mixture of coal and rock 

dust just as some of the dust samples bags that you now carry out for subsequent 

analysis by the MSHA lab at Mt. Hope, WV. The empty unused vials can be used 

at a later time. In order for the dust in the tube to explode, it has to be dispersed 

in air and form a flammable dust cloud while in the presence of an ignition source 

sufficient to ignite the dust cloud. As long as the dust remains in the vial it is not 

an explosion hazard. 

e. Meter needs a case.