1

Reviewer:

Affiliation:

Comment:2

Response: The authors respect ful ly 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 samplin g 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 practi ces, 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 mixtur es at their limits of explosibility were reasonably constant for a given volatility of coal. Although t he coal volatility does affec t the explosibility of the coal , i t has been also demonstrated through full

- scale explosion experiments that the particle si ze 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 distribution s as compared to a higher volatile coal (e.g., Pittsburgh or Sunnyside seams) but these differences in incombustible requirements decrease as the coal particle size3

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 ; t herefore , the CDEM provides a more conservative explosibility assessment for a low er volatil e 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 23 rd I nternational C onference of S a fety in M ines R esearch I nstitutes (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 23 rd International Conference of Safety in Mines Research Institutes ( Washington, DC, September 11

- 15, 1989). Pittsburgh, PA: U.S. Department of the In terior, Bureau of Mines, pp. 843

- 856.
Cashdollar KL, Weiss ES, Greninger NB, Chatrathi K [1992 b ]. Laboratory and large

- scale dust explosion research. In: Proceedings of the 26 th Annual Loss Prevention Symposium (New Orleans, LA). Paper No. 72C. AI ChE Spr ing 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% incombustibl e 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

.5

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 t han 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 requi red 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 th at publication , t he CDEMs used in District 11 were calibrated with rock dusts collected from each of the respective mines.6

Response: The authors respectfully disagree. The report was to compare the results from the two methods of measurement and d etermine the causes of discrepancies

. As such, the authors believe this has been done.
Actually, t he LTA method is subject to error based on the coarseness of rock dust since it gives larger

sized particles with more mass more credit for participation i n inerting when the opposite is in fact true. For example, the LTA method gives large particles of rock dust, 250 (60 mesh)

– 850 μ m (20 m esh) in size, equal credit for inerting based on their weight whereas it is known that in fact the opposite is true;

s maller 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. In7

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 u sing a compliant rock dust. It is the responsibility of the mine operator to use compliant roc k 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 i t does consider particle size. The as sumption 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 rock8

dusts are used that contain less than 70 % <200 mesh particles , greater than 80 % is needed to inert the mixt ure.

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

. S uch 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 stud y by Bureau of mines researchers (IC 8111) tha t 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.9

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, a fter the UBB explosion, the MSHA accident investigators reported that 1412 out of 1803 post

- explosion samples collected that were non

compliant 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 o f Investigation, Fatal Underground Mine Explosion, April 5, 2010, http://www.msha.gov/Fatals/2010/UBB/FTL10c0331noappx.pdf accessed 07/24/2012.

Response: That is cor rect. 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 observation s for mixtures above 70% rock dust

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

- type CDEM s . 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 manu facturer 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.10

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 s et ran ged 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 ma jority 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 valida tion 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 ass ociation 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 calcula ted 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

– Bernar d 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 r ock 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 availa ble 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 ashi ng (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 a ny 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 g reater than 80.00 was coded as “Green”. The proportion of “Green” values in this aggregated dust database was equal to 0.7772.11

The database of 591 CDEM samples also contained a dust analysis variable computed from the LTA method. In order to keep the com parison 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 show ed 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

c onducted 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” sa mples 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 con stituted 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 valu e 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 hypothesi s 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, bot h 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 reproducibi lity 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 <.0001.12

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 reading s 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 th e 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 a n extra margin of safety

.

Response: If the commenter is referring to errors being masked in the collection pr otocol 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.1314

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 imple mentation of the CDEM is not. I t is apparent from the reviewer comments to this report that additional training and understanding of the differences in15

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 measu rement 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 mixtu res 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 us e d for compliance purposes still meet the intent of the law by ensuring that the dust mixture will not support flame propagation ?”