Reviewer: 

Affiliation: Sensidyne

Comment:

Dear Colleague: 

Sensidyne, LP is the manufacturer of the CDEM-1000, the commercial version of the CDEM 

prototype that was tested in the NIOSH Draft Information Circular, NIOSH Docket 255, Coal 

Dust Explosibility Meter Evaluation and Recommendations for Application. We have reviewed 

the draft document and have found the document to be complete, accurate and very well 

presented. We do have some comments that are collectively assembled by our engineering group 

that developed the commercial version. The main focus of the comments is to recognize the 

improvements that were made in the instrument between the prototype and commercial versions. 

A list of the major improvements is given below.

o The software was updated to improve the calibration procedure and to incorporate 

various safety features and checks in the software logic and user interface.

o The red/yellow/green format was simplified to a red/green format.

o A methane enhancement mode was added to allow the user to input the ambient 

methane level as measured from another instrument, which shifts the required RD level 

accordingly.

o An extended red mode allows the user to see the RD level after a red indication. This is 

presented as a range (e.g., 75% to 78% RD), and provides the user with a reference to 

indicate the remaining level of RD required to reach an 80% RD level.

While this information is presented in the course of the document, it should be conveyed earlier 

that the testing was conducted with prototypes that were available in 2009, and the commercial 

version released in 2011 has improvements that address perceived shortcomings of the prototype 

model. Indeed, the comments from the MSHA inspectors in this study were considered in the 

commercial unit development. Some specific comments are listed below.

1. We encourage the addition of a footnote or text to the Executive Summary section with 

the suggested verbiage as follows. The testing was conducted with the prototype version 

of the CDEM that was available in 2009, and the commercial version CDEM-1000 

released in 2011 has improvements in both software and hardware over the prototype 

model. These improvements are listed under the “Commercial CDEM Development” 

section starting on page 20. Many of the comments from the MSHA inspectors listed in 

Appendix E were considered in the development of the commercial unit.

Response: Thank you for this comment. Footnote 5 has been added and states “The testing 

was conducted with a prototype version 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 “Commercial CDEM 

Development” section and consider many of the MSHA comments listed in Appendix E.”

2. Page 7 footnote 10: Recommendation for calibrating with 83% RD and telling the 

instrument it is 80% RD. In the commercial version CDEM-1000 this would distort the % 

Methane correction as well as the Extended Red reporting, essentially rendering the 

unit good for only red/green reporting. The magnitude of the distortions would need to 

be established to evaluate this recommendation. Previous verbal discussions with 

NIOSH on this matter had determined that the 80% value was conservative since it does 

not account for inherent coal IC as well as moisture content. If there is a concern about 

the instrument accuracy, it should be addressed by the instrument red/green 

acceptance criteria instead of intentional mis-calibration. Accordingly, it should be noted 

that Sensidyne does not endorse the calibration of the CDEM-1000 with 83% RD and 

telling the unit it is 80% due to the errors such a process can introduce to the methane 

compensation built into the commercial version of the software logic.

Response: Thank you for the correction. The footnote has been removed.

3. Page 21 “Calibration and Programming of the Commercial CDEM”, paragraph 3: This 

paragraph distorts the Extended Red Reporting. We actually report the measured value, 

rounded up or down to the nearest integer % minus 3% RD, as a safety factor. Their 

paragraph implies we give a +/- reading based on the 2% error alone. The paragraph 

should state what we actually report.

Response: Thank you for the correction. The document now states “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%.”

4. Page 22 “NIOSH Recommendations” bullet # 2: Using the CDEM as an enforcement tool 

if reporting and acceptance criteria were changed – would the CDEM need to be revalidated as an analytical instrument and method? If so, who’s validation protocols 

would apply, NIOSH, MSHA, other? The commercial version CDEM-1000 as currently 

released is not designed to be and should not be used as an analytical tool.

Response: The authors' original intent was for the mine operators and the regulatory 

inspectors to use the CDEM as a screening tool to provide an immediate assessment of the 

explosibility of the mine dust sample. The use of the CDEM as an enforcement tool would 

require a rule change by MSHA from requiring % total incombustible to % rock dust, along 

with the development of a validation protocol for the use of colorimetric methods as 

acceptable enforcement. Then Sensidyne would re-validate the CDEM as an analytical 

colorimetric method for assessing explosibility of coal and rock dust mixtures. However, 

until this rule and validation protocol are established by MSHA, the CDEM could still be used 

to screen samples to determine which samples re sent to their laboratory for additional 

testing. As a screening tool, the CDEM would allow the mine operators and inspectors to 

provide immediate remediation to rock dust deficient areas.
5. Creation of calibration standards: NIOSH does not mention that the RD and CD need to 

be dried before they are weighed out for a calibration standard, this is a requirement 

that needs to be mentioned. In addition, in the MSHA response to questions section, 

response 2.f. mentions weighing out calibration standards using the sample cup as a 

materials container. Using the sample cup (not the sample vials) would mean 

measuring out a 1 to 2 gram sample; the chances for error on this are substantial. There 

needs to be a minimum standard measurement weight that would reduce the chances 

of weighing error (e.g. a 0.1g error on weighing a 1 gram sample is 10%, yet the same 

0.1 gram error is only 0.5% of a 20g sample.

Response: The authors have made the drying of samples more explicit by stating “For 

proper calibration, both the PPC and mine rock dust (RD) should be dried prior to 

making the boundary calibration samples” in the Preparation of Calibration Sample 

section of Appendix C. The following statement has also been added as a footnote (18) 

to the response to section 2 f in Appendix E to address the error in weight issue: “Also, 

the commercial CDEM is supplied with large containers 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.”

6. The light source in the commercial unit is a red LED, which may be described as a 

red/near infrared source, but probably should not be described as infrared as it is under 

“CDEM Operation”.

Response: Thank you for the correction. Those references to infrared radiation in the paper 

have been changed to near-infrared.

7. Regarding footnote 16 on page 50, there are also no established criteria for sample 

averaging. Repeated measurements of the same sample on the same unit or different 

units can readily produce different readings, as the sample mix and orientation changes.

Response: We understand that there are variations in multiple readings of the sample on 

the same unit or different units. The pressing and releasing action may cause particle reorientation against the lens as the larger coal and rock dust particles tend to rise to the top 

of the sample and favor a larger part of the field of view. In the study, we had actually taken 

three separate measurements of the collected dust sample (three separate full cups). If the 

output was two GREEN and one RED, we called it GREEN. This way we could better define 

the measurements if the sample happened to be on the boundary of the GREEN/RED.