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Reviewer: 

Affiliation: 

Comment:
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Response: The authors respectfully disagree. The report demonstrates that the CDEM actually provides

a more conservative estimate of explosibility in comparison to the currently acceptable low temperature 

ashing (LTA) method. The CDEM is used to determine the combustibility of the sample presented to it, 

just as the LTA method is. The mine dust sampling practices used to collect the dust is a separate issue 

that affects the determinations of both the CDEM and the LTA methods. If a “false sense of security” is 

fostered with the CDEM due to the current rock dusting practices and mine dust sampling practices, it is 

also fostered in the use of the LTA method.

Response: The authors respectfully disagree. The results provided by the CDEM apply to all mines, not 

just for the mines in the Pittsburgh (hvb) seam. The limit of explosibility for many coals was determined 

in U.S. Bureau of Mine’s 20-L explosibility chamber using several types of coal dust with different 

volatility (Lucci et al). The percentage of rock dust required to inert the coal dust increases for finer coal 

dust size. For each mixture, the CDEM optical reflectance was measured at the limit of explosibility and 

the normalized reflectance of coal and rock dust mixtures at their limits of explosibility were reasonably 

constant for a given volatility of coal. Although the coal volatility does affect the explosibility of the coal,

it has been also demonstrated through full-scale explosion experiments that the particle size distribution 

of the coal can have as much of an effect on the rock dusting inerting requirements as the volatility of 

the coal. A lower volatile coal (e.g., Pocahontas seam) will generally require less incombustible to inert 

at a given similar particle size distributions as compared to a higher volatile coal (e.g., Pittsburgh or 

Sunnyside seams) but these differences in incombustible requirements decrease as the coal particle size 
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becomes finer (see listed references). However, historically and to this day, there is no relaxation in 30 

CFR 75.403 of 80% incombustible requirement for lower volatile coals; therefore, the CDEM provides a 

more conservative explosibility assessment for a lower volatile coal of similar dust particle size 

distribution using the 80% incombustible requirement in all mines.

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

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

Safety in Mines Research Institutes (Washington, DC, September 11–15, 1989). Pittsburgh, PA: U.S. 

Department of the Interior, Bureau of Mines, pp. 965–977.

Weiss ES, Greninger NB, Sapko MJ [1989]. Recent results of dust explosion studies at the Lake Lynn 

Experimental Mine. In: Proceedings of the 23rd International Conference of Safety in Mines Research 

Institutes (Washington, DC, September 11-15, 1989). Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, pp. 843-856. 

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

research. In: Proceedings of the 26th Annual Loss Prevention Symposium (New Orleans, LA). Paper No. 

72C. AIChE Spring National Meeting, 22 pp.

Lucci, C.E., Cashdollar, K.L., and Sapko, M.J. [1995]. Coal Dust Explosibility Meter, Proceedings of the 

26th International Conference of Safety in mines Research institutes, Katowice, Poland, Sept.4-8, 1995. 
4

Response: The authors respectfully disagree with this assessment. Please refer to the example of 

GREEN measurements less than 80% IC beginning on page 12 of the report. It is shown in this section 

that the sample in question was comprised of larger particles. In fact, given the data in Table 3, the 

combustible (coal) portion of the sample contains roughly 20% minus 200 mesh material. If you refer to 

Figure 1 in the report (also Figure 11 in RI 9679), the propagation curve indicates that a bit less than 70% 

incombustible content is needed to inert a dust mixture containing ~20% of coal dust passing through a 

200 mesh sieve. This mixture contained 72.7% incombustible content – not explosible when particle 

size is considered! Hence, this is not an error in explosibility assessment.
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In the event that finer coal dust is contained within the mine dust, again please refer to the explosion 

propagation curve in Figure 1 of the report. The propagation curve asymptotically approaches 80% 

incombustible for dusts containing more than 80% minus 200 mesh coal dust. When the CDEM would 

indicate RED for mixtures containing the finer than 80% minus 200 mesh coal dust, it would indicate 

more rock dust to be applied to the area sampled. This should be considered an added margin of safety

that would not endanger miners.

As for the reference to uncontrolled particle size of the coal dust in the calibration of the CDEM, the 

requirement for the use of Pittsburgh Pulverized Coal (PPC) is the control since PPC contains 80% minus 

200 mesh coal dust particles which is extremely significant and important as seen if Figure 1. For a mine 

to generate this coal dust for calibration purposes to the size specification and distribution that is 

required out of their own coal, it would be more difficult and uncontrolled. That is the purpose behind 

the supply of PPC by the manufacturer.

Response: The authors believe this has been accomplished within this report.

Response: This is incorrect. In that publication, the CDEMs used in District 11 were calibrated with rock 

dusts collected from each of the respective mines. 
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Response: The authors respectfully disagree. The report was to compare the results from the two 

methods of measurement and determine the causes of discrepancies. As such, the authors believe this 

has been done. 

Actually, the LTA method is subject to error based on the coarseness of rock dust since it gives largersized particles with more mass more credit for participation in inerting when the opposite is in fact true. 

For example, the LTA method gives large particles of rock dust, 250 (60 mesh) – 850 μm (20 mesh) in 

size, equal credit for inerting based on their weight whereas it is known that in fact the opposite is true; 

smaller particles contribute more to inertion. Neither the CDEM nor the LTA method can distinguish 

whether the rock dust in the dust mixture being tested meets the compliant size requirements or not. In 
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order to do so, a particle size analysis must be performed. Both analysis methods pass the dust through 

a 20 mesh sieve before analysis and assume that you are using a compliant rock dust. It is the 

responsibility of the mine operator to use compliant rock dust in their rock dusting practices.

Response: The compliance of a mine’s rock dust with 30 CFR 75.2 must be verified prior to its use in the 

mine for rock dusting! A non-compliant rock dust should NEVER be used to prevent a propagating 

explosion.

Response: The authors disagree. A non-compliant rock dust should never be used for explosion 

prevention. This report demonstrates that the CDEM measurements are as reliable as the LTA method 

and are better because it does consider particle size. The assumption with both methods is the use of 

compliant rock dust containing at least 70% less than 200 mesh to affect inerting at the 80% level. If rock 
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dusts are used that contain less than 70% <200 mesh particles, greater than 80% is needed to inert the 

mixture.

About footnote 3, it is understood that the CDEM cannot be used for enforcement without a 

corresponding change to the regulations including the elimination of moisture in the total incombustible 

content. Research has shown that the moisture content of dust mixtures can change quickly in a mine

environment. Such variations in moisture content can imply a safe condition on the day the sample is 

collected however, within a short period the area can dry out creating an unsafe condition.

Response: The authors respectfully disagree that the CDEM’s “optical” analysis is no less subject to 

error than a miner’s “ocular” analysis. Regarding the effectiveness of “ocular” analysis, please refer the 

following study by Bureau of mines researchers (IC 8111) that present results of studies conducted to 

evaluate the accuracy of visual estimation for incombustibility. Please note the following key Findings: 

An observer cannot estimate with precision the exact percentage of incombustible in a mine dust 

sample. The difference between visual estimation and chemical analysis ranged from -22.5 to +22.5 

percentage points of incombustible for floor samples (107 samples). The observer (an experienced mine 

inspector) judged correctly, by visual estimation, that mine dusts grossly deficient in incombustible 

contained less than the required incombustible. In a set of 211 samples mine dust samples, 17 

contained 15 to 50 % incombustible. All of these were judge by the inspector as being deficient in 

incombustible. In addition, visual observation is a poor method for estimating the moisture content of 

mine dust. 
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As an example, prior to the UBB disaster, WVMHST inspectors were using visual assessment of rock dust 

conditions at the UBB mine [McAteer et al, 2011]. However, after the UBB explosion, the MSHA 

accident investigators reported that 1412 out of 1803 post-explosion samples collected that were noncompliant with incombustible requirements in place at the time of the explosion [MSHA, 2012].

McAteer, Beall K, Beck JA Jr, McGinley PC, Monforton C, Roberts DC, Spence B, Weise, S, 2011. Upper 

Big Branch, The April 5, 2010, explosion: a failure of basic coal mine safety practices. 

MSHA, 2012. Report of Investigation, Fatal Underground Mine Explosion, April 5, 2010,

http://www.msha.gov/Fatals/2010/UBB/FTL10c0331noappx.pdf accessed 07/24/2012.

Response: That is correct. The CDEM reading are influenced by the presence of moisture that is 

precisely the reason for requiring sample drying with molecular sieves before making measurements. If 

the sample is not completely dried, the net effect is an indication that more rock dust should be applied 

than may be needed to inert the mixture – a more conservative measurement.

The reported accuracy of the first version was ± 3 % using dry samples over the entire range from 6 to 

100 %. The commercial version reported accuracy of ± 2% rock dust is based on observations for 

mixtures above 70% rock dust. -

Addressing footnote 2, NIOSH OMSHR personnel had developed and tested the proto-type CDEMs. The 

information collected was shared with the CDEM manufacturer for commercialization of the CDEM. 

NIOSH OMSHR requested that any data and figures used by the manufacturer be cited/referenced. 

Hence, Sensidyne noted NIOSH in their manuals. However, since NIOSH, as an organization, does not 

make endorsements of any specific company or product, the disclaimer is included in the draft 

publication although Sensidyne is the only manufacturer of a coal dust explosibility meter to date.
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Response: The authors are not sure how that conclusion can be drawn. Yes, about 75% of the samples 

were GREEN and above 80% IC but the total sample set ranged from 29.8% IC to 99.6% IC. Another 15% 

were within 70% to 80% IC. The remaining 10% of the samples were less than 70% IC. The authors are 

unsure how having the CDEM measure more samples of low incombustible content could contribute to 

the validation of the meter or the data. The majority of the samples within this survey fall around the 

targeted 80% IC and having more samples containing less than 80% IC is highly undesirable. According to 

the data in this paper, having more samples containing less than 70% IC for further CDEM validation 

would indicate RED on the CDEM, requiring more rock dust. 

A statistical assessment was conducted concerning the results and following are the conclusions:

Comparison of MSHA and NIOSH results for verifiable CDEM samples

To quantify the degree of association between the 296 CDEM samples (there was 1 missing observation 

for an MSHA record) measured by both MSHA and NIOSH, a Kappa statistic (K) was calculated. The 

Kappa statistic is used when the data are categorical. The CDEM values were originally coded as 

“Green”, “Red” and “Yellow”. The “Yellow” values were recoded to “Red”, so the data used in the 

analysis were dichotomous. The calculated value of the Kappa coefficient was 0.6868 (95% CI: 0.5923, 

0.7814). The guidelines for the evaluation of the Kappa statistic specify that a value between 0.4 and 

0.75 denotes good reproducibility. [Fundamentals of Biostatistics, Third Edition – Bernard Rosner]. The 

test of the null hypothesis that Kappa = 0 was rejected with a two-sided p-value <.0001.

Representativeness of 591 dust samples used to test the CDEM

The following statistical analysis was conducted to investigate whether the 591 coal and rock dust 

mixture samples collected by MSHA mine inspectors in five of the MSHA coal districts were 

representative of the dust samples collected from all MSHA coal districts. A database from MSHA’s 

2010 dust survey in underground coal mines was made available to NIOSH. This database contained 

50,914 records and was considered the “population” of dust measurements. Prior to using this 

database in any statistical analysis, the “DustAnalysis” variable, which contained the results for the low 

temperature ashing (LTA) method, was aggregated and an average dust value was calculated for each 

unique sampling date at a given mine. This was done so that mines where a large number of samples 

were collected on a specific date did not unduly influence the results of any statistical test. The final 

aggregated dust survey database consisted of 2,738 valid records. This average dust analysis variable 

was recoded in the following manner: a value greater than 0 and less than 80.00 was coded as “Red”; a 

value equal to or greater than 80.00 was coded as “Green”. The proportion of “Green” values in this 

aggregated dust database was equal to 0.7772.
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The database of 591 CDEM samples also contained a dust analysis variable computed from the LTA 

method. In order to keep the comparison consistent, this variable was used in the analysis. The dust 

analysis variable was recoded according to the procedure described in the previous paragraph. The 

proportion of “Green” samples was equal to 0.7766.

Even though a visual inspection showed the proportion of “Green” samples in both databases to be 

equal, a one sample binomial test, which allows the determination of whether a proportion of 

“successes” on a two-level categorical variable significantly differs from a hypothesized value, was 

conducted to determine if the sample proportion (0.7766) was statistically equivalent to the population 

proportion (0.7772). The null hypothesis that the proportion of “Green” samples in the database of 591 

records was equal to the proportion of “Green” samples in the database of 2,738 records, was not 

rejected (two-sided p-value = 0.9744). Thus, the 591 samples could have come from the 2010 dust 

survey because the proportion of “Green” values in the sample did not significantly differ from the 

population value of 0.7772.

Representativeness of 296 dust samples used to test the CDEM

To determine whether the 296 dust samples were representative of the 591 samples, a one sample 

binomial test was again conducted. An assumption was made that the 591 samples constituted the 

“population” of dust readings that were collected by MSHA mine inspectors for this study. 

The dust analysis variable in the database of 296 records, calculated using the LTA method, was recoded 

into a dichotomous variable. If the dust value was greater than 0 and less than 80.00, it was coded as 

“Red”; if the value was equal to or greater than 80.00 it was coded as “Green”. The proportion of 

“Green” values in this sample database of 296 observations was equal to 0.8148. The null hypothesis 

that the proportion of “Green” samples was equal to 0.7766 (the proportion of “Green” samples in the 

“population” database of 591 records) was not rejected (two-sided p-value = 0.1138). Thus, the sample 

of 296 dust measurements could have come from the “population” of 591 dust measurements.

Comparison of MSHA CDEM readings and low temperature ashing results for the 591 samples

To assess the degree of association between the CDEM and low temperature ashing method values in 

the database of 591 samples, both measured by MSHA, a Kappa statistic (K) was calculated. As 

discussed previously, the CDEM and LTA values were coded as either “Green” or “Red”. The Kappa 

coefficient was found to be equal to 0.6810 (95% CI: 0.6143, 0.7477) which showed good reproducibility 

between the CDEM and LTA measurements. The test of the null hypothesis that Kappa = 0 (no 

association) was rejected with a two-sided p-value <.0001.
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Response: Actually, 20% of the samples contained 75% - 85% IC which is the range that you are 

expressing interest but which is contradictory to your previous comment about needing more samples 

containing higher combustible contents.

Yes, 17 of the 75 verifiable samples read RED on the CDEM and contained > 80% IC according to LTA 

analysis. However, if you follow the data to the last column, NIOSH OMSHR could only reproduce the 

readings for 7 samples with the LTA analyses on the portions of samples retained and sent to Mt. Hope 

for analysis. Further, the authors do not view this as disturbing since the action to be recommended by 

the CDEM is to add more rock dust and the addition of more rock dust would not place the miner in 

danger but add an extra margin of safety.

Response: If the commenter is referring to errors being masked in the collection protocol as the 

sampling protocol, the authors only addressed that each sample collected should be well mixed and 

homogenous. Further discussion and issues pertaining to the collection of the samples is a separate 

issue and is not addressed in this paper.
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Response: Thank you for your thoughtful comments. The authors are confident in the CDEM and its 

measurements. The CDEM has been commercialized and is being manufactured and as such, the 

development phase is complete. However, the implementation of the CDEM is not. It is apparent from 

the reviewer comments to this report that additional training and understanding of the differences in 
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explosion hazard detection between the LTA method and the CDEM need to be conveyed to the end 

users and to those who need to interpret the data. 

Several times it has been pointed out that the sampling procedures themselves are in question, not 

necessarily the results obtained from the CDEM. The report was to compare the results from the two 

methods of measurement and determine the causes of discrepancies. The authors believe this has been 

accomplished.

The purpose of this field study was to conduct a one- on- one comparison of two independent methods 

identifying the potential explosibility of mine dust mixtures and attempt to explain the potential reasons 

for differences between LTA and CDEM assessment methods for the marginal samples. 

One must also consider the timeliness of explosion hazard assessment. The CDEM has the advantage of 

providing a near real time in situ assessment of the level of explosion protection compared to the LTA 

method where samples are sent to a remote laboratory for LTA analysis turnaround times ranging from 

several days to several weeks.

The LTA method for dust explosion hazard assessment (based on an unchanging combustible and inert 

dust particles size) and for determining compliance with the 30 CFR 75.403 has been used without 

question for over 40 years and thereby has gained acceptance by the mining industry and MSHA as the 

only acceptable method for determining regulatory compliance with the law. One needs to ask the 

relevant question “does the LTA method used for compliance purposes still meet the intent of the law 

by ensuring that the dust mixture will not support flame propagation?”