Information Circular 9529



### Coal Dust Explosibility Meter E valuation 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

August 2012

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. DHHS (NIOSH) Publication No. 2012–172

August 2012

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### Table of Contents

Executive Su mmary ........................................................................................................ 1
Introduc tion .................................................................................................................. ... 3 Background on Coal Dust and Explosibility Testing ........................................................ 4 CDEM Operat ion ............................................................................................................. 6 Comparison of Laboratory Results and CDEM Results ................................................... 7 Joint Study between NIOSH and MS HA ......................................................................... 8
Results and Di scussion ................................................................................................... 9

GREEN Measurements ............................................................................................. 13

RED/YELLOW Measurements ................................................................................... 16
Conclusions from t he NIOSH St udy .............................................................................. 20
Commercial CDEM De velopment ................................................................................. 21 Calibration and Programming of the Commercial CDEM .............................................. 22 Commercial CDEM Changes based on Potential Customer Concerns ........................... 22 The Commercial CDEM as a Verification and Compliance Tool .................................... 23
NIOSH Recomm endations ............................................................................................ 23 Acknowledg ments ......................................................................................................... 24
Referenc es .................................................................................................................... 25
APPENDIX A: CDE M Design ........................................................................................ 27
APPENDIX B: CDE M Training ...................................................................................... 31 APPENDIX C: Prototype CDEM Calibration and Testing Proc edures Used in the Joint Study ...................................................................................................................... 33 Preparation of Calibration Sample .............................................................................. 34 Drying the sample ..................................................................................................... 35

CDEM Calibration ..................................................................................................... 36 In-Mine Testing ........................................................................................................ 37
APPENDIX D: Particle Size E ffect................................................................................. 41 APPENDIX E: MSHA Inspector Questions and Comment s .......................................... 45

### Figures

Figure 1. Effect of particle size of Pittsburgh seam bituminous coal dust on the explosion propagation for % TIC as te sted within LLEM. The dashed curve represents the propagati on/nonpropagation boundary. ............................................. 5 Figure 2. Coal Dust Explos ibility Meter (CDEM). ............................................................. 6 Figure 3. CDEM measurement comparis on of NIOSH-receiv ed sample s. .................... 12 Figure 4. Comparison of CDEM results and lab TIC results for one representative mine. ........................................................................................................................ 13 Figure 5. CDEM GREEN (G) m easurement comparison of NIOSH-received samples. Agreeing GREEN samples have LTA re sults > 80% IC. Disagreeing GREEN samples have < 80% IC LTA resu lts. ....................................................................... 14 Figure 6. GREEN sample of 73.9% IC. ......................................................................... 15 Figure 7. Comparison of MSHA and N IOSH incombustible measurements .................. 17 Figure 8. CDEM RED (R) measurement co mparison of NIOSH-received samples. Agreeing RED samples have LTA result s < 81.5% IC. Disagreeing RED have more than 81.5% IC LTA resu lts. ............................................................................. 19 Figure 9. Agreement between CDEM and laboratory analysis. ..................................... 20 Figure A1. Coal Dust Explos ibility Meter (CDEM). ........................................................ 28 Figure A2. Extinction limit fo r normalized refl ectanc e .................................................... 30 Figure C1. Sample tube cont aining molecu lar siev es.................................................... 36 Figure C2. Collecti ng a band samp le. ........................................................................... 37 Figure C3. Sifting a sample . .......................................................................................... 38 Figure C4. Collecting a sample into a samp le tube. ...................................................... 39 Figure C5. Transferring dust to a sample cup. .............................................................. 39 Figure C6. Testing a dust sa mple with t he CDEM. ........................................................ 40 Figure D1. A comparison of coal dust particle size distributions on φ values. Rock dust particle size dist ributions are constant

. ................................................... 42 Figure D2. A comparison of rock dus t particle size distributions on φ values. Coal dust particle size distri butions are c onstant . ............................................................ 43 Figure E1. Cross section of a very thin (0.0 1-in-thick) explosible float coal dust layer deposited on top of a 3/4-in -thick (20-mm-thick) la yer of rock dust. ......................... 51

### 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 testi ng ...................................................................................................................... 11 Table 3. Particle size distribution of 72. 7% IC dust sample and LTA results for three different size fractions of the sample ........................................................................ 16 Table B1. Mixtures of RD and PPC for CDEM ca libration ............................................. 32 Table C1. Pittsburgh pulverize d coal size data ............................................................. 34 Table C2. Reference mixtures, percentage by weight

......................................................... 35

### 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
S c surface area of coal dust
S r

surface area of rock dust
TIC total incombustible content
USBM United States Bureau of Mines
H 2 O water/moisture

### Unit of Measure Abbreviations

ft feet
f r

fraction rock dust

g grams I x light intensity of coal and rock dust mixture
I c light intensity of coal dust
I r

light intensity of rock dust μm micrometers or microns
mL milliliters
mm millimeters

oz ounces % IC percentage of incombustible content
% CH 4 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 Michael J. Sapko, 2 Floyd D. Varley, 3 and Eric S. Weiss 4

Office of Mine Safety and Health Research National Institute for Occupational Safety and Health



### Executive Summary

This report details the results of a NIOSH i nvestigation 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. 5 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 mi nes and the effectivenes s of their rock dusting practices. The CDEM was developed by the Nati onal Institute for Occupational Safety and Health (NIOSH) and successfully underwent national and inte rnational peer review. The intention of the device is to assi st mine operators in complying w ith the Mine Safety and Health Administration (MSHA) final rule 30 CFR* 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, NIOS H undertook an extensive cooperative study with MSHA. This study, completed in 2009–2010, involve d field use of the CDEM within MSHA’s 10 bituminous coal districts. As part of their ro utine dust compliance survey s in these districts, MSHA inspectors collected sample coal and rock dust mixtures, field testing these samples for explosibility with the CDEM. Samples were th en sent to the MSHA National Air and Dust Laboratory at Mt. Hope, WV, for parallel testing, first us ing a drying oven to determine the moisture followed by the trad itional low temperature ashi ng (LTA) method. The LTA method determines explosibility of a coal and rock dust sample in a laboratory by heating the mixture to

1 Research Chemical Engineer, Fires and Explosions Branch, Office of Mine Safety and Health Research (OMSHR), NIOSH, Pittsburgh, PA.
2 Senior Research Physical Scientist (retired), OMSHR, NIOSH, Pittsburgh, PA. 3 Chief (retired), Fires and Explosions Branch, OMSHR, NIOSH, Spokane, WA.
4 Senior Research Mining Engineer, Fires and Ex plosions Branch, OMSHR, NIOSH, Pittsburgh, PA.
5 The testing was conducted with a prototype vers ion of the CDEM that was available in 2009. The commercial version, CDEM-1000, was released in 2011 and includes improvements in both software and hardware. These improvements are listed in the “C ommercial CDEM Development” section and take into consideration many of the MSHA inspec tor comments detailed in Appendix E. *Code of Federal Regulations. See CFR in References.burn off the combustible material. The results, wh en combined with the 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 remeasure 297 samples, and 97% of those identified by the CDEM as being explosible (27% of samples) or nonexplosible (73% of samples) correlated with th e results of the subsequent lab analysis using the LTA method. Of the remaining 3% where th ere were differences between the field and laboratory methods, subsequent NI OSH evaluation attributed these differences to the variability (incomplete mixing, inadequate drying of the samp le, the particle size of the rock dust and/or coal dust) of the samples being an alyzed, 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 im portant to understand the fundament al 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 de monstrated the effects of varying the coal dust particle sizes and incombustible concentrations on the explosible vs. nonexplosible dust mixtures. A final important distinction between th e 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 a nd rock dust mixtures, to more effec tively improve the onsite adequacy of rock dusting for explosion prevention. Mine opera tors could use the CDEM on a regular basis to ensure that their rock dusting practices are achieving inertization requi rements 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 critic al 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 dus t (CD) explosion. Prior to September 2010, U.S. Federal law 30 CFR 75.403 mandated th at the nation’s coal mines ma intain a total incombustible content (TIC) of at least 65% in nonreturn entries and at le ast 80% in the return airways. 6,7 The 65% TIC requirement was based on an average part icle 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 insp ectors systematically collect dust samples from sections of underground coal mines and send th e samples to the Mine Safety and Health Administration (MSHA) National Air and Dust La boratory at Mt. Hope, WV, for analysis of incombustible content. The TIC analysis is a gravimetric (mass) measur ement of the incombustible content (IC) of a coal and rock dust mixture. Generally, the analys is 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 Du st Explosibility Meter (C DEM) was developed to allow for immediate determination of the explosib le 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 undergr ound 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, th e study was able to evaluate the feasibility for inspectors to use the CDEM with in 10 of MSHA’s bituminous coal districts. This report will discuss the study results, with emphasis on compar isons 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.

6 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 conduct ed prior to the ETS and subsequent final rule.
7 Total incombustible content (TIC) includes measurem ents of the as-received moisture in the samples, the ash in the coal, and the rock dust. Incombustible content (IC) includes me asurements of the ash in the coal and the rock dust and does not include the moisture.4 

### Background on Coal Dust a nd Explosibility Testing

Coal dust particle size has a significant impact on the explosion propagation potential of coal and rock dust mixtures. As the co al dust particle size decreases, the coal particles are more reactive and increased amounts of compliant rock dust are nece ssary to render the coal dust inert. 8 Coal particle size has a high variability both within and between mines, with size being dependent on factors such as mine type (i.e., l ongwall or continuous miner), bit 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 spec ifically quantify the concentra tion of rock dust required to prevent flame propagation [NIOSH 2010]. These tests studied flam e propagation as a function of coal dust particle size wh ile 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.

8 Compliant rock dust is defined in 30 CFR 75.2 as “P ulverized 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 (SiO 2 ), or, where the Secretary finds that such silica concentrat ions are not available, which does not contain more than 5 percent of free and combined silica.”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 regula tions set forth in 30 CFR 75.403, inspectors from MSHA periodically collect samples of deposited dust from sp ecified areas in a mine. The MSHA National Air and Dust Laboratory determin es TIC and compares this TIC with the standard of 80% minimum TIC. 9 The TIC includes measurements of the moisture, the ash in the coal, and the rock dust. If 10% of the sample s collected in a survey are < 80% TIC (in the absence of methane 10 ), MSHA considers the sample surv ey to be noncompliant. The mine operator is issued a citation and a timeframe is stipulated to abate the citation. Abatement is accomplished by applying additional rock dust to th e deficient areas from which the sample was collected. Abatement is confirmed through vi sual assessment by the MSHA inspector, but no followup dust samples of the abated area are collect ed or analyzed to ensure compliance with the respective intake or return airway TIC requirements.

9 The 80% TIC requirement is based on explosion te mperature thermodynamic limit models for coal and rock dust mixtures, extensive in-mine coal dust partic le size surveys, and multiple explosion experiments at the Lake Lynn Experimental Mine [NIOSH 2010]. Present ly, the size of the coal dust particles is not determined by the MSHA National Air and Dust La boratory as part of the explosibility assessment.
10 Per 30 CFR 75.403, “Where methane is present in an y ventilating current, the percent of incombustible content of such combined dust shall be increased 0.4 percent for each 0.1 percent of methane.”6 

### CDEM Operation

The CDEM (Figure 2) is a handheld device devel oped to assess the explosibility of coal and rock dust mixtures in real time. The principl e of operation of the device is based on the measurement of near-infrared radiation reflected from the surface of a homogeneous mixture of two dusts with different optical reflectance, in this case light-c olored rock dust and dark coal dust. Near-infrared radiation is emitted by a li ght-emitting diode located behind the window of the CDEM probe. When the CDEM probe is inserted in the dust mi xture, the near-infrare d radiation reflects off the surface of the dust and back to a silicon photodiode sensor. The normalized reflectance, Φ , is related to the rock dust to coal dust particle de nsity 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 sa mple. If the test sample normalized reflectance is greater than that of the calibrati on sample, it is determined to be nonexplosible. If it is less than the calibration sample normalized re flectance, it is classified as explosible. For further detail on the CDEM design, calibration, and operat ion, see Appendices A, B, and C.

Figure 2. Coal Dust Explosibility Meter (CDEM).7 Operationally, the CDEM uses a digital readout that identifies a sample as being either “RED” or “GREEN.” For the prot otype CDEM used in this study, the meter identifies a sample as RED (potentially explosible and requiri ng 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). 11 The extinction limit Φ * is the boundary between propa gation and nonpropagation and is set at the 80% rock dust leve l during CDEM calibration. The CD EM identifies a sample as GREEN (nonexplosible) when the measured Φ is equal to or greater than the extinction limit Φ *. 12



### Comparison of Laboratory R esults 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 co al 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 moisture as received at th e lab, and the inherent as h in the coal. The LTA method is not itself a direct measure of explosib ility 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 th e surface area of the rock dust particles to the surface area of the coal dust particles, and relates the measur ed 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 st andard Pittsburgh pulverized coal (PPC) dust [80% < 200 mesh (~ 74 μm)]. 13 When comparing methods that make accuracy dete rminations, the tendency is to compare the new method with the traditional method head-to-h ead. In this case, since the LTA method and the CDEM use different means to determine the e xplosibility of a coal and rock dust mixture, it is difficult to directly compare one method with the other. The onl y effective approach should be

11 The commercialized version of the CDEM i dentifies a sample as RED when the measured Φ is below the extinction limit Φ * needed to prevent flame propagati on—i.e. when less than 80% rock dust.
12 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 eliminat ed 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. Al though the YELLOW measurem ents are not discussed separately, all of the RED, YELLOW, and GREEN measurements are included in this report.
13 Pittsburgh pulverized coal (PPC) has been used in large-scale explosion tests at the Lake Lynn Experimental Mine (LLEM) and is the standard upon whic h the 80% total incombustible content cited in 30 CFR 75.403 is based [NIOSH 2010].8 outcomes-based, i.e., to compare the LTA predic tion with the CDEM prediction and judge both against actual explosion tests conducted with samples using a laboratory test chamber [Cashdollar 1996, Cashdollar and Ch atrathi 1993, Cashdollar and Hertzberg 1989, Cashdollar et al. 1987, Cashdollar et al. 1992a, 1992b, and 1992c, Lucci et al. 1995, Wei ss et al. 1989]. 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 samp le surveys. The study was initiated with one training session held at the Na tional Mine Health and Safety Academy in Beckley, WV (see Appendix B). The mine inspectors were to coll ect the band sample duri ng their routine survey, measure the explosibility with the calibrated CD EM (per the methods described in Appendix C), and then send the mixed sample to the MSHA Na tional Air and Dust Labor atory 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. Th e 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 MSHA National Air and Dust Laboratory fo r 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 “Lo cation 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). NI OSH 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 th e % TIC was determined, MSHA was asked to send NIOSH the remainder of the dust samples fr om the surveys in which a CDEM was used by the inspectors, along with the TIC an alyses 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 to tal 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 veri fiable 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 su rveys performed in 32 different mines in 5 districts. However, NIOSH received pure rock du st 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 fiel d study samples which were verifiable are the basis of this study. 14 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 repr esented 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.

14 In order to ensure that the sa mples were representative of the dust samples collected from all MSHA coal districts, a one-sample binomial test was conducted. This test allows for the determination of whether a proportion of “successes” on a two-level categorica l variable differs from a hypothesized value. The 591 samples and 297 verifiable samples were compared to a database of 50,914 dust samples collected from all MSHA districts in 2010. Prior to the analysis of these data, the dust variable was recoded to either “Green” or “Red” based on an 80% cut point. The null hypothesis that the proport ion of “Green” samples in the database of 591 records was equal to the proportion of “Green” samples in the 2010 MSHA dust survey in underground coal mines was not rejected (two-sided p-value = 0.9744). Additionally, the null hypothesis that the proportion of “Green” samples in the database of 297 records was equal to the proportion of “Green” samples in the MSHA dat abase was not rejected (two-sided p-value = 0.1193). Thus the 591 and 297 samples could have come from the 2010 MSHA dust survey and therefore could be considered as representative samp les from all MSHA coal districts. The one-sample binomial test was also used to determine whether the 297 su bset of CDEM samples were representative of the 591samples. The null hypot hesis that the proportion of “Green” samples in the database of 297 records was equal to the proportion of “Green” samples in the database of 591 records was not rejected (two-sided p-value = 0.1138). Based on t he results of this statistical test, the subset of 297 dust measurements could have come from the 591 dust measurements.10 Of the 591 samples that the MSHA inspectors test ed with the CDEM, 416 were measured as GREEN, 57 were YELLOW, and 118 were RED (T able 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 th at dust (as determined by the S c /S r ratio in Appendix A). For the purposes of instrument performance assessment a nd 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 requi re more rock dust to inert (Figure 3). To quantify the degree of association, or re liability, between the 296 CDEM samples tested by MSHA and verified by NIOSH (Table 2, row 1, column 4), a Kappa statistic ( K ) was calculated. The Kappa statistic is used when the da ta are categorical. The data in this analysis were dichotomized as “Green” and “Red.” The calculated value of the Kappa coefficient was 0.6868 (95% CI: 0.5923, 0.7814). This value denot es good reproducibility between MSHA and NIOSH data [Rosner 1990].11 Table 2. Comparison of CDEM results as determined by separate MSHA and NIOSH testing 15

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

CDEM Readings TIC = IC + H 2 O Moisturefree IC = TIC - H 2 O 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 - H 2 O Assuming 8% IC in CD, 80% RD = 81.6% IC; rounded to 82% NIOSH LTA verification

Total number

of samples 591 591 591 296 16 297 297 297

< 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%

15 When verifying the samples, NIOSH measured at l east three different full cups of each sample to ensure adequate mixing of the sample.
16 MSHA did not provide NIOSH with a CDEM measurement on 1 sample. Thus, 296 samples were analyzed using the CDEM by both MSHA and NIOSH.12 Figure 3. CDEM measurement comparison of NIOSH-received samples. The samples for measurement with the CDEM n eeded to be dried prior to determining the reflectance ratio of S c /S r . Moisture in a dust sample will darken the sample and thus give an incorrect measurement. MSHA includes the percenta ge of as-received 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. Subs equently, 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 S c /S r were smaller than the S c /S r at the extinction limit Φ *. Twenty-eight of the 57 YELLOW measurements and 22 of the 118 RED measurements were > 80% TI C. All YELLOW and RED measurements on the CDEM should be < 80% TIC if the S c /S r is larger than the S c /S r at the Φ * extinction limit. Because the CDEM and TIC method use diffe rent 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 mi ne is indicated on the graph by a horizontal blue line. When the CDEM measured samples with Φ s greater than 0.4, the CDEM indicated GREEN (green background). For samples with Φ s 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 75.403.13 Based on Figure 4, the following sections interpret the disagreements between the two methods. The terms “disagreement,” “agreement, ” “disagree,” and “agree” are used throughout this report to discuss the differences in the CD EM explosibility readings to that of the LTA incombustibility determinations; however, the us e 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 th e 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 result s indicate nonexplosible but the CDEM indicates explosible).

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

GREEN Measurements

Of the 416 GREEN measurements documente d by MSHA inspections while using the CDEM, 19 (5%) were < 80% IC (< 20 mesh materi al), 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.14 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 erro r cannot be confirmed for the other 9 samples, NIOSH researchers believe that simplification of the calibration procedure and more robust training would likely e liminate 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 erro r 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 nonexplosib le result on the CDEM is reasonable considering the likely particle size of th e sample presented and the IC reported. Visual inspection of the 73.9% IC sample indica ted larger coal particles as shown in Figure 6. NIOSH measured the percentage of IC (% IC ), which does not include 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 moisture (i.e., that no clerical error occurred in sample transfer from MSHA to NIOSH). The sample was th en subdivided into three particle size ranges to determine the distribution of the inert co mponent within these size ranges. The NIOSHdetermined IC was 72.7% IC for the < 20 mesh sample as compared to MSHA’s 73.9% IC, showing reasonable agreement given th e coarse nature of the sample.15 Table 3 shows data for three separate size fractio ns 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 propagati on 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 fr om 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 la rger, nonreactive coal particles and the finer particles for explosibility assessment. Further discussion of particle size eff ects can be found in Appendix D.

Figure 6. GREEN sample of 73.9% IC.16 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 a bove 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 defici t 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 CD EM provided a different explosibility assessment may be due to the sample containing coarser coal dust, finer inert particle s, incomplete mixing of the sample before testing on the CDEM within th e mine, operator error, and/or incorrect CDEM calibration. If the sample was not homogeneous, the CDEM field of view c ould include an area of lighter color where more rock dust was depos ited. 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 operati ng techniques. When correctly used, the CDEM shows agreemen t 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 wher e the laboratory result indicated a deficiency based on TIC alone.

RED/YELLOW Measurements

The CDEM RED and YELLOW meas urements 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, signali ng 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% TI C. Some of these disc repancies can be easily explained, as discussed below.

Insufficient Mixing of the Band Sample
It is not known whether the band sample test ed on the CDEM by the inspector was the same as that sent to the MSHA la boratory. If the band sample was not sufficiently mixed by the inspector, the portion tested by the CDEM coul d be different from the portion retained and analyzed by MSHA. It is not uncommon that th ere are differences in the % IC measured by MSHA and those measured by NIOSH following the same laboratory LTA method. Previous studies by Harris et al. [2008] have shown that ther e can be ± 7% IC variat ion in the LTAs if the17 sample was not adequately mixed after being co llected and before being subdivided and tested (Figure 7). In comparison, the reproducibility ofthe data acquired with the LTA method on wellmixed samples resulted in a standard deviati on of 1.7% for moisture-free LTA results from a random set of 29 samples analyzed by MSHA, NIOSH, and an independent certified laboratory. 17

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

Insufficient Drying of the Band Sample
The CDEM sample is dried prior to testing. Moisture in the sample will cause the dust mixture to reflect less light and a ppear to contain less rock dust th an is actually contained in the sample. Previous CDEM reflectance studies have shown that for each 1% added 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 reflecta nce is equivalent to a mixture containing 58% rock dust [Sapko and Verakis 2006]. Hence, a “f alse” YELLOW or RED measurement would be

17 To determine the reproducibilit y of LTA measurements among thre e independent laboratories (MSHA, NIOSH, and an independent laboratory), 29 well-mixe d dust samples were analyzed and an intraclass correlation coefficient (ICC) was computed. The value of the ICC was equal to .982 (95% CI: .960, .992) with an F-test that the true value of the ICC was 0 being rejected at p = .000. The magnitude of the computed ICC shows excellent agreement among the three laboratories [Shrout and Fleiss, 1979].18 obtained. Based on the results of the published study, after the 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 reduce d, as described in the next section. On a related note, insufficient drying of the sa mple 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 MSHA National Air and Dust Laboratory, the samples may have been subjected to additional drying during the shipping process. NIOSH also dried the samples with mo lecular sieves as part of the study process.

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 certai n percentage of IC material al so. 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 f actoring in 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 in cluded in the explosibility assessment. NIOSH had received pure rock dust for calibra tion of the CDEMs from MSHA inspectors in various districts, but not for all of the mi nes 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 resear chers could confirm (Figure 8). Of these 28 samples, NIOSH was able to replicate RED/YELL OW measurements for 11 of them. TIC results conducted at NIOSH identified 4 of these samp les as being < 81.6% TIC, likely indicating sample variance. NIOSH could not replicate RE D/YELLOW results for 17 of the 28 samples, indicating problems with calibra tion 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 (suc h 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.19 Figure 8. CDEM RED (R) measurement comparis on 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 RE D/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 we re not sufficiently mixed, the calibration for color would be affected. In addition, the CDEM wa s 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 prev iously calibrated for use in a di fferent 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 th e RED/YELLOW samples measured by NIOSH, 90% of the measurements identified areas that requir ed 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 disagreem ent between the methods because of their fundamental differences. This disagreement repres ents 7 verifiable cases where the CDEM was more conservative than the labor atory 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.20 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 sa mples 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 de tail. After further analysis of this questionable sample, it was found that it contained a large fracti on of nonreactive coal a nd rock dust particles in the range of 20 to 60 mesh (250 to 840 μm). Ro ck 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 re sulting 81% IC supported the CDEM GREEN reading of a nonexplosible mixture. Therefore, in no case did th e CDEM fail to detect an explosion hazard in this study. Assuming that the CDEM is prope rly calibrated and the test samp le dried, a sample can be considered to be nonexplosible if the CD EM indicates a GREEN measurement; i.e., no additional rock dust is needed at that time. Howeve r, it is important to continue to sample that area from time to time because of changing c onditions caused by the dynamic nature of the mining environment. The CDEM and TIC method use two different surr ogates for estimating explosibility of coal and rock dust mixtures. Neither measures expl osibility directly and therefore neither is indisputable. The only direct measure of explosib ility is by use of a 20-L or larger explosion apparatus or through full-scal e 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 repla ced the volumetric method as a means of21 measuring explosibility. The CDEM reading is weighted by relative coal and rock dust particle sizes in assessing sample explos ibility, whereas the LTA method gi ves 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, th e results of this fiel d study show excellent agreement (97%) between the two methods. It is unr ealistic 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 mixt ure 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 indi cated that 25% of the sample s collected were potentially explosible. Since each sample in this study repres ents 500 linear ft of mine development entry, the 175 RED samples represent about 17 miles of mi ne 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 YELLO W results, indicating that the area is deficient in rock dust, err on the side of safety in that these re sults require the addition of more rock dust even if subsequent detailed laborat ory analysis indicates otherwise. With the immediate results, the mine can re medy the situation directly rather than wait days or weeks for laboratory results in order to eff ect change and protect miners. The CDEM offers an opportunity to immedi ately identify and correct explosible accumulation of coal and rock dust mixtures. This explosibility assessment is based on the analysis of a representati ve 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 calibra tion 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 ha ve been made based on analysis of the study results, input from the inspectors, an d input by the instrument manufacturer. In response to coal mine inspector comment s (Appendix E), updates to the CDEM include changes in screen prompts to plain language and more explicit inst ructions, a simplified calibration process (using three instead of four samples), and consideration of atmospheric methane in results displayed.22 The changes in the CDEM screen prompts now lead the user through th e use and calibration of the device so that all terms are comm on 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 calib rated 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 consid ered to be RED readings and thus deemed to require more rock dust. The 80% rock dust sample was c hosen as the RED/GREE N boundary based on fullscale experimental results conducted at the NIOSH LLEM [NIOSH 2010]. This value is also reflected in the recent MSHA final rule requirin g 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 presen ce of methane. In accordance with 30 CFR 75.403, “where methane is present in any ventilating current, the percent of incombustible content of such combin ed 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 % CH 4 ) measured with an MSHA-approved methane meter. The CDEM w ill adjust the RED/GREEN boundary based upon the % CH 4 input, and the requirements of 75.403. Another change in the CDEM output is to indi cate 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 rounded up or down to the nearest integer percentage and the rounded value minus 3%. This range does not in clude the IC inherent in the coal nor the variable as-received moisture of the sample.

Commercial CDEM Changes based on Po tential Customer Concerns

Based on the outcomes of this study, an antici pated potential customer concern may be the availability of PPC for calibra ting the CDEM. The manufacturer and NIOSH have a PPC supply available to support the initial build of 3,000 in struments. 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 th e frequency of calibration. The manufacturer is requiring that the CDEM be recalibrated afte r 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 s upplier. 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 su pply (in color and in particle size) would make visual hazard assessment without a meter extremely difficult and current visual determination of23 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 batc h and from supplier to supplier be significantly minimized.

The Commercial CDEM as a Verification and Compliance Tool

Discussion of the commercially develope d meter would not be complete without identification of potential uses of the instrument in mana ging 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 po ints, and conveyors or other coal transport systems in entries. 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 e ither rock dusting practi ces or dust control can be enacted. Mine safety inspectors can utili ze 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 la boratories for analysis pe r the appropriate existing practice for determining the potentia l explosibility of the mixture. Samples identified as adequate may not require laboratory analysis . Finally, should an area be iden tified as deficient, the CDEM can be used to verify that the hazard was abat ed 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 comp liance with mine ventilation standards. 30 CFR 75.323.d.1.iii requires rock dust to be continuously applied to a llow operations up to 1.5% CH 4 in return air splits. The commer cial CDEM with the methane i nput 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 complia nce 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 se nd samples to the laboratory for TIC determination.

• If the use of TIC as the surrogate for e xplosion hazard enforcement were changed to % RD and if other changes such as inclusion of dust par ticle size and elimination of the as-received moisture are considered, the CDEM could serve as the regulatory tool to take enforcement actions, and onsite corr ective action in the deficient areas could be taken immediately by both the mine operator and the MSHA inspector.24 • The sample collected, dried, and tested with the CDEM can be retained for laboratory analysis to independently verify the % IC re levant to current regulatory standards. For any discrepancies between the two met hods, explosion chamber tests could be conducted to determine the inherent explos ibility of that re presentative sample.

• Upon receipt of a RED measurement, the ar ea 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 th e invaluable cooperation of the many MSHA field inspectors who calibrated and used the CD EM on collected dust samples. The authors also acknowledge Terry Montgomery, Supervisor Chem ist, of the MSHA National Air and Dust Laboratory, Mt. Hope, WV, for provi ding the dust samples, the incombustible percentage data, and the incombustible analysis procedures. The assistance of Linda McWilliams, Team Leader of the Surveillance Team in the Health Comm unications, Surveillance and Research Support Branch of OMSHR, was invaluable for providing statistical analysis of the collected data. The authors also recognize the laborat ory assistance of all NIOSH OM SHR personnel for their efforts in the CDEM testing of the dust samples and th e dust samples particle size testing. The authors would also like to acknowledge the work of Henry Perlee and Ed Ha y 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.25 

### References

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Cashdollar KL, Chatrathi K [1993]. Minimum explos ible dust concentrations measured in 20-L and 1-m 3 chambers. Comb Sci and Tech 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 23 rd International Conference of Safety in Mines Research Institutes (Washington, DC, September 11–15, 1989). Pittsburgh, PA: U.S. Department of th e Interior, Bureau of Mines, pp. 965–977.

Cashdollar KL, Sapko MJ, Weiss ES, Hertzberg M [1987]. Laboratory and mi ne dust explosion research at the Bureau of Mines. In: Cas hdollar 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, Chatrath i K [1992a]. Laboratory and large-scale dust explosion research. Plan t/Operations Progress 11 (4): 247–255.

Cashdollar KL, Weiss ES, Greninger NB, Chatra thi K [1992b]. Laborator y and large-scale dust explosion research. In: Proceedings of the 26 th Annual Loss Prevention Symposium (New Orleans, LA). Paper No. 72C. AI ChE Spring National Meeting, 22 pp.

Cashdollar KL, Weiss ES, Greninger NB, He rtzberg 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 (Brusse ls, Belgium). Section 1, 16 pp.

CFR. Code of Federal Regulati ons. Washington, DC: U.S. Govern ment 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). Min Engr 60 (10):50–54.

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MSHA [2010]. MSHA to publish emergency temporary standard for rock dust [ http://www.msha.gov/MEDIA/PRESS/2010/NR100921.asp

. ] Date accessed: 12/01/2010.26 NIOSH [2006]. Float coal dust explosion hazards. U.S. Department of Health and Human Services, Centers for Disease Control and Preven tion, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2006–125.

NIOSH [2010]. Recomme ndations for a new rock dusting standard to prevent coal dust explosions in intake airways. By Cashdo llar KL, Sapko MJ, Weiss ES, Harris ML, Man CK, Harteis SP, Green GM: U.S. Depa rtment of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safe ty and Health, DHHS (NIOSH) Publication No. 2010–151.

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### APPENDIX A: CDEM Design28 The CDEM (Figure A1) consists of an optical probe connected to a small electronics box with a digital display. The optic al probe consists of a nea r-infrared source and a silicon photodiode sensor. When the CDEM probe is inserted into the dus t mixture, the near-infrared radiation reflects off the surface of the dust a nd back to the silicon photodiode sensor.

Figure A1. Coal Dust Explosibility Meter (CDEM). The normalized reflectance, Φ , is related to the mass fraction of rock dust, f r , in the sample by the following equation [Sapko and Verakis 2006]:

(1)

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

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

r = intensity of light reflected fr om a pure rock dust surface; and K

= proportional to the product of the rock dus t to coal dust partic le density ratio and the ratio of the mean particle diameters of coal to rock dus t contained in the mixture.29 Φ can also be written directly as:

(2)

where S c = coal dust surface area and S

r = rock dust surface area.

For a fixed ratio of S c to S r , the quantity Φ is independent of explicit dependence upon K or f

r . This is because the ratio S c /S

r can be maintained constant while f r 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 mixtur es that propagate flame and those that do not propagate flame was relatively constant, Φ *, the normalized reflectance at the extinction bounda ry. To visualize this findin g, 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 Fi gure 1. Since the experimental measured rock dust inerting limit has a precision of approximately ± 3 weight (wt) percen t, rock dust and coal dust samples were also prepared at ± 3 wt percent around the experime ntal 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 volum e 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 la rger value of Φ (greater than Φ *) is produced by a smaller ratio S c /S

r . This would result from an increase in the rock dust surface area and/or a decrease in the coal dus t surface area. A smaller value of Φ (less than Φ *) is produced by an increase in the ratio S c /S

r , and would result in a greater increase in S c than in S r . That is, a decrease in S c /S

r moves the mixture into the nonexplosib le range, whereas an increase in S c /S

r moves the mixture into the explosible range.30 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 th at, at the lower explosion limit, the product of specific absorption (absorption = 1 - reflection) and mass concentra tion 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 refl ectance of the pure coal was not significantly different from the PPC. Theory and data both support th e 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.31 

### APPENDIX B: CDEM Training32 MSHA inspectors from the 10 bituminous co al districts were trained by NIOSH representatives on the calibration and use of the CDEM at the Na tional Mine Health and Safety Academy in Beckley, WV, on November 5, 2009. Tw o presentations were given detailing the background and development of the CDEM and the 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 la ter distribution am ong the inspectors. Calibration samples were prepared by the insp ectors 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. The 75% RD sample is used to set th e boundary between RED and YELLOW. The 80% RD mixture is used to set the GREEN/YELLOW bounda ry. 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.

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

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 in spectors were asked to collect a pure rock dust sample from each mine, with some of the samp le 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 ma ke its own calibration samples for subsequent testing and verification of inspector-c ollected samples after MSHA analysis.33 

### APPENDIX C: Prototype CDEM Calibration and Testing Procedures Used in the Joint Study34 Prior to use, the CDEM was calibrated using sa mples of that mine’s pure rock dust, PPC, and two mixtures of known rock dust and PPC cont ent. 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 calibra tion and testing directions used by MSHA in this joint study. The procedures for the use of the commerciali zed CDEM have since changed. The CDEM used for this study was a prototype me ter that the manufactur er has since enhanced in the commercialized version. The calibration pr ocedure, 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. For these calibrations, an 80% ro ck dust mixture was used to set the GREEN/YELLOW boundary and a 75% rock dust mixture was us ed to set the YELLOW/RED boundary. Table C1. Pittsburgh pulverized coal size data

The coal dust (CD) samples used to calibra te the CDEMs are PPC samples supplied by NIOSH. For proper calibration, both the PPC and mine rock dust (RD) should be dried prior to making the boundary calibration samples. The rock dust 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/GREE N. Color samples are made by mixing RD and CD with the percenta ge determined by weight—e.g., an 80% RD sample is made by mixing 40 grams (g) of RD with 10 g of CD (Tab le C2). This mixture does not account for the incombustible content within the coal. 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 1235 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) ha lf-filled with predried molecular sieves are used to collect the samples (Figure C1).

• The sample tube filled with molecular siev es 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.36 Figure C1. Sample tube containing molecular sieves.

CDEM Calibration

The CDEM must be calibrated for the specific mi ne 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 fo r 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 mixtur es, the chosen color boundaries are GREEN/YELLOW at 80% RD a nd 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 sa mple 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.37 • If the calibration was successful, the calib ration 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 pro ceed to the testing mode. As indicated by the above procedure, it is im portant 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 outlin e of the dust collec tion procedures).

o In the US, routine band surveys are pe rformed quarterly at each mine by MSHA. 18 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 samp le by brushing dust from the ribs, floor, and roof (when possible) into a metal pan (Figure C2).

Figure C2. Collecting a band sample.

18 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.pdf38 o The dust is then sifted through a 10 me sh (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 th e dust (Figure C1 and Figure C4). It is advisable to prepare the tubes by filli ng 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.39 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 μ m) is attached to the end of the tube.

o The CDEM sample cup is fitted onto th e 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.40 • Testing Mode: Testing a mine dust samp le 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, in sert 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 expl osibility. GREEN indicates there is sufficient RD in the sample presen ted. YELLOW indicates the sample is marginally explosible and more RD shoul d be added to that area of the mine. RED indicates deficient RD in the samp le, 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 ag ainst your palm to remove dust from around the probe lens.

o Using the bare palm of your clean ha nd, 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.41 

### APPENDIX D: Particle Size Effect42 Finer coal particles are more explosible than co arse particles, and finer rock dust particles are better at suppressing an explosion than coarse pa rticles. The reflectance of the coal mine dust responds in a similar manner. The CDEM measures this reflectance to dete rmine explosibility of a dust sample. The CDEM is also sensitive to part icle size. The ability of the CDEM to detect variations in particle size in relation to explosibility (see Equa tions 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 requir ed to inert three different particle sizes of coal dust. The horizontal line expressing th e 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 ca libration standard with the CDEM, requires 80% rock dust. Coal dust 10% finer requires abou t 82% rock dust, while coal dust 10% coarser requires only about 78% rock dust.

Figure D1. A comparison of coal dus t particle size distributions on φ values. Rock dust particle size distributions are constant.43 Figure D2 shows the same relationship as Figure D1 for variations in rock dust particle size. The nominal rock dust particle size used in fu ll-scale explosion experiments by NIOSH [NIOSH 2010] was 26 μm and should make up 80% of the mi ne dust for that dust to be considered inert. A 10% increase in rock dust particle size would re quire almost 82% rock dust to be inert, while a 10% decrease in rock dust particle size w ould require 78% rock dust to be inert.

Figure D2. A comparison of rock dus t particle size distributions on φ values. Coal dust particle size distributions are constant.
If a mine dust contains coal dust finer than th e 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 explos ible 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 nonexpl osibility (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.4445 

### APPENDIX E: MSHA Inspector Questions and Comments46 As a followup to this study and to evaluate th e use of the CDEM, MSHA asked its coal mine inspectors who used the CDEM in the field f our 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 usef ul for rock dust sampling and enforcement (considering that we will have a regula tion change to support the RED and GREEN output)? and 4. Any other comments or suggestions for th is 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 orig inal 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 in corporated 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 in termittent 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 th e 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 en visioned, 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 pref erably through the CDEM us e by the mine operator.

d. Difficult time reading the information. (T his was the most common complaint and made by every inspector.)47 Response: The output is either RED or GREEN on the display of the CDEM commercial version. The LCD readab ility 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 coll ection and handling was required and added more weight to carry in and out of th e mine. NIOSH researchers envision through the implementation and use of the CDEM, th e total weight of samples carried out of the mine for subsequent laboratory pr ocessing 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 percen tage read out as well.

Response: We do not support this. The percentage readout would only be a prediction and may divert th e 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 we re 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 c ontain very large ineffective inert particles (> 250 μm and < 841 μm), outputting the % IC is of little value and may not be subsequently confirmed by con ventional LTA analysis and thus generate more confusion. 19

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 surfa ces were dry and the dispersible surface coal and rock dust is not st ratified but well mixed.

19 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.48 d. Line or lip for testing cup.

Response: Good suggestion, the manufacture r 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 simp lified 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 in itial 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. 20

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 data base could be updated as necessary and would save time on recalibrating ever y 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 devel oped a hole the second day of use.

Response: Thank you, the switch and butt on covers have been changed to a more rugged type for the commercialized version.

20 The commercial CDEM is supplied with large cont ainers for meter calibration. These 4-oz containers provide enough volume to create 30-g or 50-g samples. The small sample cups should not be used for the preparation of a calibration sample.49 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 c oal 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 itse lf to accurately and quickly identify the potential in-situ ex plosibility of a dust mixture.

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 complete ly 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 sour ce 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 th e 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 h ave 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 met hods for determining the explosibility of coal and rock dust mixtures use different techniques, it is unrealistic to expect the CDEM to replicate laboratory TIC dete rminations. Rather; the goal is for the CDEM itself to accurately and quickly ident ify the potential in-s itu 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 par ticle size of the coal and rock dust variability whereas the current laboratory ashing method does not.50 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 dete rmine inert content without lab analysis, can one determine if adequate rock dust was added to mitigate the potential hazard by visual inspection alone? NI OSH researchers have shown that a visual inspection only for abatement purposes is not adequat e to determine if the area has been adequately re-rock dusted. A thin surfa ce coating of rock dust can visually obscure a dangerous un-neutr alized 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 closi ng 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-dat e 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 out side 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 rel evant 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 ar eas, 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 astu te observation. Some thought has been given to better focus inspectors’ ener gies on spot sampling those questionable areas as they carry out their routine mine inspection duties.

Although 6 inches deep of rock soun ds 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 examp le, 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 et51 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 dus t represents a nominal entry dust loading of 100 mg/l, there is sufficient dust base d 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.

Figure E1. Cross section of a very thin (0.01-in-thi ck) 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 dut y.” 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 ques tions 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 bac k to Mt. Hope for analysis.

g. The instrument would be great for companie s to check for compliance. If this was used for inspectors to test areas for co mpliance 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 pr evention program in using the CDEM.52 4. Any other comments or suggestions for th is 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. Without the fine efforts of the inspectors, the successful completi on 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 vari ations in product quality cont rol issues. That issue is currently being investigated by NIOSH and MSHA. It has been observed that the CDEM adjusts the explosibility assessme nt to these variations in rock dust particle sizes and, in some cases, is th e reason for variance in readings between laboratory LTA analyses and CDEM measurem ents. In any case, tighter rock dust specification should minimize any diff erences between bulk and bag product. Until rock dust specifications are tight ened and product-certified, the meter needs to be calibrated with the rock dust in use and at least potentia l variance identified by making up calibrations samples betw een each source and assessing their impact on meter calibration. Some particle size variations between bulk and bag samples may have little im pact 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? 21

21 The differences between the “off” measurements ha ve 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.53 d. Calibration takes a long time.

Response: Once the calibration samples ar e 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 manuf acturer 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 th e meter does not take long.

e. Meter needs a case.

Response: The manufacturer of the commercial meter provides a case with the
CDEM.

f. Do we need to carry out our empty vi als (the vials and contents may be combustible materials)?

Response: The vial content may contai n 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 th e tube to explode, it has to be dispersed in air and form a flammable dus t cloud while in the presen ce of an ignition source sufficient to ignite the dust cloud. As long as the dust rema ins in the vial it is not an explosion hazard.