Reviewer: 

Affiliation: Consol Energy

Comment:

RE: Docket Number NIOSH - 255

CONSOL Energy Response

Response to the NIOSH CDEM Report – “Coal Dust Explosibility Meter Evaluation and 

Recommendations for Application”

Please find attached information to be included in the CDEM information from CONSOL 

Energy Inc.

The attached is a marked up copy of the DRAFT Report and other information we want 

included in the final.

We at CONSOL have been very active in the use of the CDEM and see issues with the report 

as written. As of the End of April 2012 we’ve taken and analyzed 2,600 Samples. Our 

Samples are first Analyzed by the CDEM then send to for Lab Analysis to compare. The 

numbers in the Draft NIOSH Report do not reflect what we are seeing. This has been 

communicated this to one of the Authors of the Report.

Our Numbers show the accuracy as –

When 686 Samples were Analyzed – (CDEM vs. LAB)

CDEM’s that PASSED – the Lab Failed 4%

CDEM’s that FAILED - the Lab Passed 11% of those. Result would be these 11% could be 

Violations Issued Wrongfully!

This information was also given to one of the Authors of the DRAFT Report. We also gave a 

report to Assistant Secretary, Joe Main in January in an open MSHA / Industry Meeting.

Other issues –

 For Mines NOT in the Pittsburgh 8 Coal Seam – The base coal dust used in the CDEM 

is not available for those mines.

 The Mixing of Rock Dust at each mine is a variable that cannot be known, some rock 

dust applied by machine / bulk and some applied by hand from bagged rock dust.

 The Mixing of Bulk Rock Dust in the Surface Tanks cannot be controlled, and a key

 The Difference in Lab Samples Analyzed and the “Reflective” Properties are two 

completely different Analysis, probably will never match-up for accuracy between them

 Lab Samples take many days to weeks to get the analysis back.

The bottom line is the CDEM is a good tool, and this tool Should NOT be used as an tool for 

MSHA to issue Violations, accuracy is not there!

Thank you for the Opportunity to Respond. If you have questions, please contact me.
Attachments: 

Marked up Copy of the NIOSH CDEM Report

CONSOL Count of Samples Taken

Response:

Thank you for the interest and feedback regarding your use of the CDEM. The authors have 

reviewed the information provided here and previously. Actually, it does appear that your 

results reflect what is shown in this report. They appear to closely correlate with the results 

MSHA had received. 

You stated that the CDEM passed 4% of the samples whereas these samples were deficient in 

incombustible content based on the laboratory low temperature ashing (LTA) results. This is 

down from the 5.6% of GREEN measurements that the LTA confirmed were RED that you had 

reported to Assistant Secretary, Joe Main in January at the open MSHA/Industry meeting. If 

you will refer to Table 2 of the paper, Column 1, Row 9 indicates that MSHA also had 4% of 

GREEN measurements that did not correlate with the LTA analyses. 

The authors are encouraged that you determined only 11% of the CDEM RED measurements 

had subsequently been shown to have sufficient incombustible based on the LTA method. This 

number is significantly lower than 79% of CDEM RED measurements that were confirmed to 

be GREEN by the LTA method as reported to Assistant Secretary, Joe Main in January at the 

open MSHA/Industry meeting. If you will refer to Column 1, Row 8 of Table 2, MSHA had 33% 

of the RED measurements not correlate with the laboratory analyses. To reduce from 79% to 

11% is a tremendous improvement which may be attributed to better understanding and usage 

of the CDEM within your mines. It is anticipated that with more usage and familiarity, the 

MSHA inspectors’ correlating measurements would also improve and decrease from 33%. 

One purpose of this paper was to contrast the differences between methods to assess the 

potential for a dust mixture to support flame propagation.

The authors would also like to thank you for the spreadsheet of data sampling. However, only 

the number of samples collected was supplied in the spreadsheet. We would like to meet with 

you to discuss in more detail the measurements and experiences. For example, some 

information missing from the spreadsheet that would be of interest to the authors is the number 

of RED and GREEN measurements, the %TIC, the %IC, % moisture, and whether the fraction 

of the sample used for the LTA analysis was the same fraction used on the CDEM. 

Comment: For Mines NOT in the Pittsburgh 8 Coal Seam – The base coal dust used in the 

CDEM is not available for those mines.

Response: Pittsburgh pulverized coal (PPC) dust is always used to calibrate the CDEM 

regardless of the coal seam being mined at any given operation. The importance of the coal 

dust particle size distribution of PPC overrides other differences such as the volatility of 

different coal seams. See footnote 13 and the discussion in Appendix A on p. 28 regarding the 

use of PPC for calibration.
Comment: The Mixing of Rock Dust at each mine is a variable that cannot be known, some 

rock dust applied by machine / bulk and some applied by hand from bagged rock dust.

Response: That may be true. OMSHR NIOSH had been informed by the Industrial Minerals 

Association – North America (IMA-NA) that the controlling factor of the particle size distribution 

is the grinding mill process employed. As such, it was assumed that if the rock dust supplier 

did not change, the particle size distribution of the rock dust would not either. In a review of 

data upon which the NIOSH Hazard ID 16 - Non-Conforming Rock Dust was based, rock 

dusts supplied by the same manufacturer in different shipping containers (50 lb bag, 

superbag, or bulk) generally had the same median diameters.

Comment: The Mixing of Bulk Rock Dust in the Surface Tanks cannot be controlled, and a key

Response: The authors are unclear as to the intent of your comment. Does this comment imply 

that the company has no control over what material the supplier adds to the silo? 

Comment: The Difference in Lab Samples Analyzed and the “Reflective” Properties are two 

completely different Analysis, probably will never match-up for accuracy between them

Response: Indeed, these are two different approaches for determining if a dust mixture is 

capable of supporting explosion flame propagation. As such, one of the aims of this report was 

to discuss the principle operation of each method and provide a better understanding of the 

reasons the two methods can provide different outcomes.

Comment: Lab Samples take many days to weeks to get the analysis back.

Response: That is correct and is why the authors believe that the CDEM is a good tool to 

readily determine if sufficient rock dust is present in the mine.
Numbered comments from the Word document attachment.

1. How rock dust samples are analyzed at MSHA

Response: This section does describe the MSHA methods for analyzing rock dust samples.

2. These are issues that need attention so the correct analysis is done 

Response: The authors agree. These are issues that need attention so the correct 

interpretation is made.

3. CDEM if used properly can assist with compliant rock dusting.

Response: The authors agree.

4. Will MSHA issue citations on this information or suggest the Operator add more rock 

dust? Stress accurate samples taken

Response: Your question needs to be directed to MSHA. NIOSH OMSHR agrees that dust 

samples need to be collected in a proper manner and be representative of the dust that may 

be dispersed and participate in a potential dust explosion.

5. Is MSHA going to purchase and use CDEM’s as an enforcement tool

Response: Your question needs to be directed to MSHA. 

6. Particle size, different mines and sample location play a big part in the analysis of the 

rock dust sample

Response: The authors agree with that observation.

7. MSHA analysis takes all of this into account

Response: That is correct. The authors agree.

8. This becomes subjective rather than objective by the MSHA Inspector 

Response: Unless MSHA collects another sample for either CDEM or LTA analysis, the 

determination of hazard abatement would be subjective based on visual observation.

9. Again particle size plays a big part in the analysis by the CDEM based on reflectivity

Response: Yes, that is correct. The CDEM accounts for particle size in its measurement of 

explosibility which the LTA method does not. The finer the coal particle the more rock dust 

needed to prevent flame propagation. The coarser coal particle requires less rock dust.

10.Appears to be built in safety factor

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

either “RED” or “GREEN.” For the prototype CDEM used in this study, the meter identified a 

sample as RED (potentially explosible and requiring more rock dust) when the measured 

was ~ 6% or more below the extinction limit * needed to prevent flame propagation. The 
commercialized version of the CDEM identifies a sample as RED when the measured  is 

below the extinction limit * needed to prevent flame propagation; i.e. when less than 80% 

rock dust.

11.Apples vs oranges 

Response: The authors agree and that is the reason for the discussion between the two 

methods.

12.Lab analysis uses everything in sample to factor incombustibility

Response: That is true. However, the as-received moisture content can vary in the mine dust 

sample depending on level of humidity in the mine and the time of year that it is collected and 

thus cannot be counted as inert to prevent flame propagation. Also, in order to derive the inert 

ash from the coal, the coal must be burned first which is counter to the desired qualities for 

explosion inerting. i.e., the leading edge of the flame front of the propagating explosion would 

be well beyond the point where any inerting qualities due to the ash content of the coal would

be of any benefit in preventing the flame propagation.

13.Lab analysis

Response: Yes, the LTA analysis is the lab analysis.

14.Reflective surfaces

Response: Yes, the CDEM is comparing the reflectivity of the coal and rock dust surfaces.

15.Apples vs oranges 

Response: The authors agree and that is the reason for the discussion between the two 

methods.

16.Long time before sample is analyzed. Moisture can be a factor here. 

Response: Due to the long elapsed time between sample collection and lab analysis, the asreceived moisture content can vary and potentially change as compared to the moisture 

content of the sample when first collected. In addition, the moisture of the dust within the mine 

can quickly change.

17.Don’t understand why they couldn’t sample all samples. Your sampling field being cut 

in half is not good. Don’t know if the others would have been different or not. Don’t 

know if accidental or on purpose.

Response: NIOSH OMSHR could only verify roughly half the samples because pure rock dust 

samples from the remaining mines were not received. If NIOSH OMSHR did not have the rock 

dust from the mine that the mine was using at the time of the mine dust sample collection to 

calibrate the CDEM, then the CDEM could not be calibrated. It would be a gross 

misrepresentation to calibrate the CDEM with another mine’s rock dust or even rock dust from 

the same mine that was collected months afterwards to evaluate the remaining samples. 

Ideally, NIOSH OMSHR would have received the pure rock dust samples from all mines in the 

survey; however, this was not the case. In considering the samples, please refer to Table 2 on 
p. 10. The data in columns 1 and 4 are approximately the same, i.e. 30% of the 591 samples 

and 27% of the 296 verifiable samples were red. There are similarities in the total green 

samples and also with the disagreeing/agreeing red and green samples. NIOSH OMSHR 

does not believe that the smaller sample size made a significant difference in outcome.

18.Coal is different in all districts

Response: That is true, but in the evaluation of the CDEM, except for anthracite coal, the 

reflectivity of the coal dust did not vary much between seams. Therefore, this eliminated the 

need for pulverizing coal from each seam and mine and producing particle size distribution 

equal to the standard PPC used to establish current inerting standards.

19.All that had to be done was to get a 50 lb bag of rock dust. How hard was that 

Response: That is true. NIOSH OMSHR requested that MSHA obtain the pure rock dusts on 

the same day that the mine dust surveys were collected. For reasons beyond the control of 

NIOSH OMSHR, this did not occur in many instances. As was previously stated, the pure rock 

dust sample needed to be collected at the same time as the mine survey collection to better 

ensure the same rock dust was contained within the mine dust sample. Within the short time 

period of receiving dust samples from Mt. Hope and determining which surveys we did not 

have pure rock dust samples with which to calibrate the CDEM, NIOSH OMSHR had 

contacted MSHA inspectors with requests for pure rock dust samples for calibration purposes. 

NIOSH OMSHR received pure rock dust samples for some surveys but not for all.

20.Did away with yellow and made red later on for simplicity 

Response: That is correct.

21.Is this an accurate enough percentage to justify CDEM

Response: The authors are unsure of what is being asked. This particular sentence is just 

stating the number of GREEN measurements that were obtained in the 296 verifiable samples

and does not reflect an accuracy percentage relevant to the CDEM. 

22.This show how important it is to mix sample

Response: The authors agree and stress the importance of thorough sample mixing before 

filling the sample cup and presenting it to the CDEM.

23.Moisture makes the sample read low.

Response: That is correct. For this reason, the molecular sieve is used to dry the sample prior 

to taking the CDEM measurement.

24.On the lab analysis, removing the moisture resulted in some the samples failing

Response: That is correct. Removing the variable moisture content from the analysis provides 

more confidence that if or when the sample area changes, or dries out, that sufficient rock dust 

will remain to prevent flame propagation. Remember also, moisture in the mine is highly 

variable and subject to fluctuation.
25.Proper technique and attention to detail make a big difference in analysis

Response: The authors agree that, as with all analysis methods, it is important to follow the 

proper established protocol for performing that method. In addition, NIOSH OMSHR believes 

that the simpler calibration method of the commercialized unit (compared to the prototype unit) 

coupled with more robust training will reduce or eliminate many of these variations

26.Here we have another number for percent error for MSHA samples. 

Response: This sentence “The MSHA Laboratory where the samples were tested reports a 

±0.5% error in its TIC results.” refers to the error rate of the LTA method reported by MSHA. It 

has been published and reported that the MSHA National Air and Dust Laboratory claims their 

method is reproducible to ±0.5%. However, even with the LTA analysis in the laboratory, if the 

sample is not adequately mixed, the variation and reproducibility can be much greater. A 

standard deviation of 2.4% was obtained for LTA results from a random set of 29 samples 

analyzed by MSHA, NIOSH, and an independent certified laboratory. If 1 outlier was 

discounted, the standard deviation was 1.9%.

27.Particle makes a big difference in CDEM and doesn’t affect the lab testing. The smaller 

the particles the better

Response: Unlike the LTA method, the CDEM does accounts for particle size. Smaller coal 

dust and rock dust particles are more reactive as compared to larger particles. Smaller rock 

dust particles are better for inerting coal dust but smaller coal dust particles also contribute 

more to the explosion hazard.

28.Technique is key

Response: Yes, technique in the form of adequately mixing the sample (applies to the LTA 

method as well), adequately drying the sample, and proper calibration of the CDEM is key to 

an accurate measurement.

29.CDEM is very accurate with lab testing if proper sampling is conducted

Response: Yes, that is correct.

30.Must make sure well mixed, coned and quartered

Response: This is true for both the CDEM and the LTA analysis.

31.Proper mixing is must

Response: This is true for both the CDEM and the LTA analysis.

32.Must dry sample thoroughly before placed in CDEM

Response: That is correct.

33.Is this what is in the PPC coal for calibration?
Response: PPC contains approximately 8% IC. Unlike the LTA method, the CDEM does not 

account for this 8% since it cannot “see” the inherent moisture and the ash content of the coal. 

However, the %IC of the coal does not contribute to the suppression of the propagating 

explosion. The %IC of the coal is one factor that can contribute to the differences between the 

LTA data and the CDEM measurements. 

34.A safety factor is built in with the CDEM

Response: That is correct and is primarily related to the moisture content (the as-received 

surface moisture and the inherent moisture within the coal) assuming that the surface moisture 

would still be present during an explosion. The CDEM does not include the variable moisture 

whereas the LTA method does.

35.Again technique

Response: Yes, technique in the form of adequately mixing the sample (also applies to the 

LTA method), adequately drying the sample, and proper calibration of the CDEM is key to an 

accurate measurement.

36.Must make sure accurately calibrate CDEM which also means correct weights and from 

same rock sample 

Response: That is correct.

37.Very high percentage of same analysis

Response: That is correct.

38.Very accurate with lab testing

Response: That is correct.

39.Need to ensure sample is adequately dried.

Response: That is correct.

40.Down fall of CDEM is particle size

Response: Unlike the LTA method, the CDEM provides a true measurement of the explosibility 

of the mine dust sample by taking into account the reactive dust particle sizes. The presence 

of finer coal dust in the mine dust sample collected inherently makes that mine dust sample 

more explosible and the CDEM would indicate that. Based on the coal and rock dust particle 

size for a given sample, the laboratory LTA method could result in an incombustible content 

>80%TIC but the sample could still be explosible; i.e., large rock dust particles mixed with fine 

coal dust particle sizes..

41.Still have apples and oranges comparison

Response: That is true. However, the LTA method is another surrogate for mine dust 

explosibility. This method was adopted in the 1950s to measure one parameter of explosibility. 

It was the best method of the era, but given improvements in science and technology over the 
years, the mining industry now has a new and more accurate approach for determining the 

explosibility in-situ and in real time.

42.Analysis is present time, lab several days away

Response: That is correct. There could be a potentially dangerous environment that is 

undetected for several days or weeks.

43.So what, this is spread out over several mines, not all at one

Response: Although many mines are very diligent in their efforts to maintain appropriate levels 

of rock dust in their entries, some mines are not quite so vigilant. Upper Big Branch is a case 

in point where it was determined that insufficient rock dust played a role in the loss of 29 

miners. Since mine sampling is conducted on 500-ft intervals, the authors included this 

statement as a means to emphasis the length of entries represented by the deficient samples 

and the importance of maintaining proper incombustible levels in the mine.

44.CDEM is real time; no delay for analysis

Response: That is correct.

45.Training is a must

Response: That is correct. The CDEM manufacturer will provide training when units are 

purchased. NIOSH OMSHR will also have training videos on their website.

46.Improved analysis

Response: These changes are not necessarily to improve the analysis. The changes were 

aimed at simplifying the calibration by only requiring an 80% dust mixture rather than 80% and 

75% dust mixtures. Also, the software changes included different and more consistent screen 

prompts.

47.Methane can come and go, need real life analysis

Response: The authors are unsure of this comment. Yes, the methane can vary and that is 

why the user is to measure and enter the ambient methane into the CDEM. Currently, the 

inspector takes methane measurements, writes the methane measurement on a sheet of 

paper, and later enters it into the system. The results sent by the MSHA National Air and Dust 

Laboratory adjust for the methane measurement. The presence of methane in the ventilating 

airways requires additional incombustible (above the 80%IC level) in the mine dust to prevent 

explosion propagation.

48.Accuracy is fair

Response: We have since corrected the manuscript as follows: “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%.” For instance, if the CDEM estimates 76.3% rock dust, it will indicate 73% -

76% rock dust.
49.Is the PPC an ash free product or is there 8% added in?

Response: PPC is not an ash free product as it is actually Pittsburgh seam coal. According to 

proximate analysis, it contains about 8% incombustible. The %IC of the PPC is not the 

governing factor of the CDEM measurements, it is the particle size of the PPC. 

50.How can you force dust manufactures to have better quality control

Response: A mine operator can require rock dust manufacturers to demonstrate better quality 

control by requiring certifications of their product. At this time, the requirements and 

specifications for rock dust as per 30 CFR 75.2 are very general. If the definition of rock dust 

were changed to tighten these specifications, then the quality control would also improve.

51.Does this mean they will issue citations or just make recommendations to add more 

dust

Response: NIOSH OMSHR cannot predict how MSHA will use the CDEM. The authors believe 

the CDEM could be used by the MSHA inspector to immediately identify areas deficient in rock 

dust so additional rock dust can be immediately added. The issuance of any citations may

require follow-up confirmation in the MSHA National Air and Dust Laboratory but this would be 

a decision made by MSHA. 

52.Better than visual which is subjective

Response: The authors agree.

53.Does this mean spot checking, action taken, sample results to still warrant citation 

issuance 

Response: Your question needs to be directed to MSHA. 

54.Does this mean citations will be issued on CDEM analysis and not what 30 CFR states 

as lab for TIC

Response: Your question needs to be directed to MSHA. 

55.Who and where will this testing be done

Response: Your question needs to be directed to MSHA. NIOSH OMSHR is providing potential 

solutions to addressing any discrepancies between the CDEM measurements and the LTA 

method. 

56.Does this mean that MSHA will try to enforce re-dusting immediately and immediately 

checked again or what action will be taken?

Response: Your question needs to be directed to MSHA. However, this is something that a 

mine operator can do to help ensure/manage their own dust explosion prevention practices. 

57.Is this why Pittsburgh seam is used because of its consistency 
Response: The Pittsburgh seam coal is used because it is the standard coal dust used in the 

full-scale testing that has been performed in the Bruceton and Lake Lynn experimental mines. 

The Pittsburgh seam was originally used because it was available from our Safety Research 

Coal Mine in Bruceton, PA and had consistent properties as it was mined.

58.Rock dust needs to be checked on a more regular basis because it can change from 

delivery to delivery

Response: That may be true. OMSHR NIOSH had been informed by the IMA-NA that the 

controlling factor of the particle size distribution is the grinding mill process employed. As such, 

it was assumed that if the rock dust supplier did not change, the particle size distribution of the 

rock dust would not either. However, it is a good practice to periodically make new calibration 

samples for the CDEM calibration using the rock dust used in that area of the mine.

59.Pittsburgh seam appears to be more consistent

Response: The authors have found that the reflectance of Pittsburgh seam coal is similar to 

other coal seams.

60.Better unit now. Is it more accurate now than when testing with the prototype ? Are the 

sampling procedures more refined and improvements in the software package equal 

more accurate comparison between lab and CDEM testing?

Response: To the NIOSH OMSHR knowledge, the software changes made and employed with 

the commercialized CDEM did not affect the accuracy of the instrument. The sampling 

procedures and software package improvements would not affect the CDEM measurements. 

What would affect the measurements are the user and their level of training and the manner in 

which the mine dust sample was collected and mixed. The CDEM upgrades were related to 

more descriptive user screens and calibration processes.

61.Drying the sample thoroughly is very important. Improper drying is cause a sample to 

fail.

Response: That is correct. Moisture will cause the dust sample to appear artificially darker 

than what it is.

62.I personally have found that 3 minutes is better

Response: Three minutes would ensure complete drying. Drying times depends on the degree 

of activation of the molecular sieves and on the wetness of the sample. A good rule of thumb is 

to observe the dust sieve mixture after mixing for about 1 minute. If balls or clumps of 

agglomerated dust particles still exist, continue shaking the vial until the dust mixture is 

uniform, free flowing, and free of agglomerations.

63.Should be 20 mesh

Response: Actually, if you examine the mesh in the MSHA inspectors dust collection kit, it 

uses a 10 mesh screen to sieve out chunks and pieces of rock. It is further screened through 

a 20 mesh sieve when it is received and processed by the MSHA National Air and Dust 

Laboratory. The CDEM sample transfer funnel is currently 20 mesh.
64.I have never ever seen an inspector cut two samples for one. They tend to dig in ribs 

and bulldozer the floor exceeding 1 inch depth

Response: Thank you for the observation. However, these are the procedures to be used by

the MSHA inspectors as per the guidelines in the General Coal Mine Inspection Procedures 

and Inspection Tracking System of the MSHA Handbook Series (p. 60, 

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

65.Need to pay close attention to the size of coal particles in samples. Large particles will 

cause the sample to fail. Also, need to ensure the rock dust we are getting from our 

supplies meets the 70% passing a 200 mesh screen. 

Response: It is true that a mine operator needs to ensure that the rock dust supplier is meeting 

the 30 CFR 75.2 requirements. However, if you have larger coal particles, they will not cause 

the sample to fail as per the CDEM (see p. 14-15 for the discussion of “GREEN 

Measurements.”

66.I think the Pokie 3 seam at Buchanan may cause some problems with this because the 

coal has a very high grind.

Response: If the coal dust is finer than the PPC that is used to calibrate the CDEM, then more 

RED measurements may occur that may not correlate with the LTA laboratory analyses. 

These are more conservative measurements.