Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

RI 9663

REPORT OF INVESTIGATIONS/2004

Performance of a New Personal

Respirable Dust Monitor

for Mine Use
Report of Investigations 9663

Performance of a New Personal Respirable Dust

Monitor for Mine Use

By Jon C. Volkwein, Robert P. Vinson, Linda J. McWilliams, 

Donald P. Tuchman, and Steven E. Mischler

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

June 2004
This document is the public domain and may be freely copied or reprinted.

Disclaimer: Mention of any company or product does not constitute endorsement by NIOSH.

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DHHS (NIOSH) Publication No. 2004-151
CONTENTS

Page

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction ................................................................................... 2

Description of the PDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Methods ...................................................................................... 4

Laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Dust exposure chamber ...................................................................... 5

Coal types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Filters and pumps .......................................................................... 5

Impactor preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Experimental design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Size distribution measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Low mass measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

In-mine testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Mine sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Sampling mine workers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Research samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Mass results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Accuracy criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Low mass measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

All laboratory data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Distribution of precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Cyclone bias results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

In-mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Concentration comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Cyclone bias results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Timely data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Data trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ILLUSTRATIONS

1. Internal PDM components .................................................................... 3

2. Screen display of PDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Plan view of a typical test setup in the Marple chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Regression analysis of total laboratory data set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Regression analysis of all mine data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6. Example of individual PDM data results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7. Data trends from the cumulative data files of all PDMs sampling on a section . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
TABLES

Page

1. Overall laboratory data comparing laboratory reference mass measurement to PDM mass measurement . . . . . . . . 10

2. Laboratory accuracy results and confidence limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Low mass data results comparing reference mass to PDM mass measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4. RSD of reference samples and PDM samplers by mass loading ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Laboratory cyclone bias compared to impactor-defined ISO and MRE respirable mass concentrations and size

 distribution data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6. Statistical significance of cyclone bias testing against ISO and MRE definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Mine data results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8. Paired t-test for mine data (testing mean difference equal to 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9. Mine cyclone bias compared to impactor-defined ISO and MRE respirable mass concentrations and size 

 distribution data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter mg/m3 milligrams per cubic meter

hr hour mm millimeter

in inch :m micrometer

L/min liter per minute °C degrees Celsius

mg milligram % percent
PERFORMANCE OF A NEW PERSONAL RESPIRABLE DUST MONITOR

FOR MINE USE 

By Jon C. Volkwein,1 Robert P. Vinson,2 Linda J. McWilliams,3

Donald P. Tuchman,4 and Steven E. Mischler5

ABSTRACT

A personal dust monitor (PDM) was developed to measure respirable coal mine dust mass to provide

accurate exposure data at the end of a work shift. Additionally, the new monitor continuously displays nearreal-time dust exposure data during the shift. The PDM uses a tapered-element oscillating microbalance to

measure the mass of dust deposited on a filter and continually displays the cumulative exposure concentration

data. The accuracy and precision of the instrument was determined by comparison to gravimetric filter

samplers in the laboratory and in four mines. Laboratory results with different coal types and size distributions

showed that there is a 95% confidence that the individual PDM measurements were within ±25% of the

reference measurements. Mine test results indicate that data taken with adjacent PDM and reference samplers

are indistinguishable. The technology proved durable enough to successfully measure 108 shifts of data out

of 115 attempts in the mines. Under these specific test conditions, the PDM demonstrated that it was

convenient to wear, robust, provided accurate data, provided timely data that could be used to prevent

overexposure, and was easy to use.

1

Research physical scientist.

2

Research physicist.

3

Statistician.

4

Industrial Hygienist.

5

Physical scientist.

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA.
2

INTRODUCTION

Measurement of personal exposure to coal mine dust has

remained essentially unchanged for the last 35 years under the

Federal Coal Mine Health and Safety Act of 1969 (Public Law

91-173), the predecessor to the Federal Mine Safety and Health

Act of 1977 (Public Law 95-164). Following a long history of

developmental efforts associated with the fixed-site and

personal continuous dust monitors, the National Institute for

Occupational Safety and Health (NIOSH) embarked on research

to improve sampling instrumentation for use in the mining

industry at the recommendation of the Secretary of Labor and

the Federal Advisory Committee on the Elimination of

Pneumoconiosis Among Coal Mine Workers [U.S. Department

of Labor 1996]. In consultation with labor, industry, and

government, NIOSH issued a contract to Rupprecht &

Patashnick Co., Inc. (R&P), Albany, NY (contract 200-98-8004)

to develop a one-piece personal dust monitor (PDM). The

objective of this work was to miniaturize a tapered-element

oscillating microbalance (TEOM) technology into a form

suitable for a person-wearable monitor that would enable

accurate end-of-shift dust exposure information to be available

to miners. It was a further objective of this work to develop a

person-wearable dust monitor that minimizes the burden to the

wearer by incorporating the monitor into the mine worker's cap

lamp battery where exposure data are continually displayed

during the shift, which enables workers and management to

react to changes in dust exposure.

The current personal dust sampler used to measure exposure

to coal mine dust uses a person-wearable pump, a cyclone that

separates dust that can enter the inner lung, and a filter to collect

dust that is then weighed [MSHA 1989]. Knowing the volume of

air sampled and the mass of dust collected, a concentration is calculated. This procedure normally takes several days, but occasionally weeks before miners know the results of a given day's

dust exposure. In that time, the mining workplace has moved and

conditions may have substantially changed. Consequently, this

current sampling method cannot be used to intervene, in a timely

manner, to prevent overexposure to coal mine dust.

Coal worker's pneumoconiosis (CWP) results from long-term

overexposure to respirable coal mine dust. Federal law is quite

specific in stating that coal mine dust levels in the work environment must not exceed 2 mg/m3

 for any 8-hr work shift [30

CFR6

 70.100]. The Mine Safety and Health Administration

(MSHA) uses a periodic method to audit compliance with this

standard and to assess the effectiveness of a mine's dust control

plan. Under the current dust control strategy, MSHA relies

mainly on the implementation of a well-designed dust control plan

and not on sampling to prevent overexposures on individual shifts.

This periodic method of audit and plan verification works well in

other industries when dealing with fixed worksites because it

assumes that conditions from one sample to the next are

essentially unchanged. This may be a poor assumption in the

mining industry in view of the continuing occurrence of

6 Code of Federal Regulations. See CFR in references.

more than 1,000 annual deaths attributed to complications from

CWP in U.S. coal mines [NIOSH 1999].

Accurate real-time monitoring of coal mine dust has been a

longstanding goal of miners. In 1983, the U.S. Bureau of Mines

(USBM) and NIOSH funded the development of a prototype

TEOM personal dust monitor [Patashnick and Rupprecht 1983].

The prototype monitor was a system configured for end-of-shift

measurements. It was not a real-time monitor, but used oscillating microbalance technology to "weigh" the collection filter

before and after dust sampling. The USBM evaluated this prototype system in the laboratory for both end-of-shift and nearreal-time applications [Williams and Vinson 1986]. These early

attempts to construct a person-wearable form of the TEOM

required a substantial mass in the base of the element to dampen

the vibrations, thus reducing the concept's "wearability."

More recently, to address the continuing incidence of CWP,

the Secretary of Labor in 1995 commissioned an advisory

committee to study ways to prevent this illness. The committee

recommended that improved personal dust monitoring instruments for continuous monitoring of dust controls be developed

and that timely results be given directly to the miners. NIOSH

and MSHA began development of improved dust monitors in

1996 in support of the advisory committee's recommendations.

The NIOSH Pittsburgh Research Laboratory (PRL) issued a

contract to develop an accurate, end-of-shift, one-piece dust

monitor. The monitor directly measures mass of dust deposited

on a filter using a TEOM that was successfully being used in

large stationary environmental monitors commercially produced

by R&P. However, substantial redesign to miniaturize and electronically stabilize the microbalance was needed to enable the

sensor to be incorporated into a person-wearable monitor.

Another essential feature of this person-wearable dust monitor

was that it had to be acceptable to the miners. This was

accomplished by incorporating the device into the existing miner's

cap lamp and battery system, moving the dust sample inlet from

the lapel to the bill of the hard hat, and transporting the sample

through a tube to the belt-worn unit for analysis. The new dust

inlet location is closer to the worker's nose and mouth and easily

within an industrial hygiene definition of a breathing zone [Guffey

et al. 2001; Cohen et al. 1983; 30 CFR 70.207(e)].

The fraction of dust that is considered respirable is an important part of measuring a worker's risk from dust. The International Standards Organization [ISO 1995] has recommended that the definition of respirable dust follow the

convention described by Soderholm [1989]. Because no device

precisely follows this theoretical convention, specific size

classification devices that are used will have inherent bias when

attempting to duplicate the convention. In fact, the currently

used 10-mm Dorr-Oliver (DO) dust cyclone has bias relative to

the conventions of both the ISO and the U.K.'s former Mining

Research Establishment (MRE) [Bartley et al. 1994]. The cyclone chosen for use in the PDM required an inlet that could

accept the tube coming from the hard hat inlet. The cyclone

followed the Higgins-Dewell (HD) design, which had been
3

previously tested to have low bias relative to the ISO convention

[Maynard and Kenny 1995].

The use of a different cyclone, however, complicates the

direct comparison between the PDM and the current personal

coal mine dust sampling unit because the difference in cyclones

may cause somewhat different results according to the size

distribution of the dust [Hearl and Hewett 1993; Bartley and

Breuer 1982]. Therefore, the PDM's ability to accurately measure a mass of respirable coal mine dust must be judged against

the identical HD sample inlet and cyclone and not the DO cyclone used in the traditional personal sampler. To assess the

comparison to the existing personal sampler, we must also

measure the size distribution of the dust. Knowing the size

distribution enables the respirable mass to be calculated

according to either the ISO or MRE definitions of respirable

dust. We can calculate from these measurements the bias introduced by the HD cyclone and the bias introduced by the 10-

mm DO cyclone when determining the respirable mass for

different coal mine aerosols.

This report evaluates the performance of the PDM compared

to gravimetric-based reference dust sampling methods. The

work was conducted in two parts. The first part compares the

new instrument to reference mass samplers and samplers

currently used by coal mines in a controlled laboratory dust

chamber. The second part examines instrument performance

when worn by a miner in underground mines.

DESCRIPTION OF THE PDM

The PDM is a respirable dust sampler and gravimetric analysis instrument that is part of a belt-worn mine cap lamp battery.

Components of the device include a sampling inlet tube, HD cyclone, air heater, pump, dust sensor, battery for the sampler, battery for the cap light, electronic control and memory boards, a

display screen, and Windows®-based computer interface software. Figure 1 illustrates some of the components.

The inlet of the sampler is located adjacent to the lens of the

miner's cap light that is worn on the front of the hard hat. The

air to be sampled is pumped through a rounded inlet and carried

through a 0.48-cm (0.19-in) internal-diameter conductive

silicone rubber tube running beside the cap light cord to the

beltworn sampler. At the sampler, dust is separated using an

HD cyclone into coarse and respirable fractions. When operated

Figure 1.–—Internal PDM components.
4

at a flow rate of 2.2 L/min, this cyclone [Bartley et al. 1994]

best approximates the classification of dust according to the ISO

definition of respirable dust [ISO 1995]. The coarse dust remains in the cyclone grit pot while the respirable fraction

continues into the analytical portion of the unit.

The sample is heated to a constant temperature, typically

45 °C, in an elliptical cross-section metal tube designed for low

particulate loss. The sampled dust is then deposited on a 14-

mm-diam Teflon-coated glass fiber filter. The filter is mounted

on an inertial mass detector (TEOM) [Patashnick and Rupprecht

1991]. The TEOM has been miniaturized and stabilized using

proprietary technology to enable its use as a person-wearable

device [Patashnick et al. 2002].

Custom software is used to program the PDM through any

personal computer. The mass on the TEOM filter is analyzed by

the internal electronics, and several concentrations based on

flow rate and times are calculated. These data are displayed on

the top of the battery, as seen in figure 2. Concentration data

and other operational parameters (flow rate, filter pressure, tilt

status, shock status, temperature, and TEOM frequency data) are

simultaneously recorded to internal memory.

Lithium-ion battery packs independently power both the

sampler and cap light. A combination charging and down loading

cradle is used to charge both cap lamp and dust monitor batteries

simultaneously. In addition, the cradle provides contacts that connect the sampler to a computer's RS-232 data port.

The instrument may be operated in shift or engineering modes.

The shift mode is programmed through the personal computer

software interface. In this mode, a technician programs the instrument to start at a specific time and run for the expected

duration of the shift. Also during programming, various sample

identification codes may be entered into the instrument in a form

typical of the currently used dust sampling data card. Once

programmed, the only way to alter the instrument is to use the

original computer interface. At the end of the programmed shift

time, the unit retains the final exposure data in the screen display

until the memory from the sampler is downloaded by a personal

computer. Depending on the number and frequency of recording

data, several shifts of data can be retained in the instrument's

internal 2-megabyte memory. A typical shift file size varies from

40 to 250 kilobytes. Shift data are retained in the instrument until

memory capacity is reached, then the oldest data are overwritten.

If a new program is not loaded into the PDM after a download,

the instrument may be operated in the engineering mode. This

mode allows manual startup and control of the instrument through

a series of button presses on the top of the battery pack without

need of a personal computer.

METHODS

Performance of the PDM was evaluated in the laboratory

and through in-mine testing. The laboratory portion of the

testing determined the PDM mass measurement accuracy and

precision compared to existing personal samplers. The bias of

the HD cyclone used in the PDM and the DO cyclone used in

the personal sampler was compared to the ISO and MRE

definitions of respirable dust. In-mine testing measured the

durability of the instrument, compared the PDM concentration

measurements to those of side-by-side reference samplers, and

determined cyclone bias.

LABORATORY

Laboratory testing was conducted in a dust chamber at PRL.

We first determined if the PDM mass measurement was accurate

when compared to the filter mass measurement method using a

defined accuracy criterion. We also compared the PDM to the

existing personal sampler method of dust measurement using a

more complex study design that accounted for the PDM's use of

a different cyclone to define the respirable dust fraction. This

bias analysis procedure [Bartley et al. 1994] was used to

Figure 2.—Screen display of PDM.
5

determine if the HD cyclone had less than or equivalent bias

compared to the DO cyclone when using either the MRE or ISO

definition of respirable dust.

Samplers

Six identical PDM dust monitors were produced by R&P.

Four units were available for laboratory evaluation, and two

units were provided for the in-mine testing. Instruments were

used as delivered to PRL from R&P. Other samplers used for

gravimetric analysis included the personal coal mine dust

sampling unit (MSA Co., Inc., Pittsburgh, PA) and the BGI-4CP

dust sampler (BGI, Inc., Waltham, MA). The personal coal

mine dust sampling unit, hereafter referred to as the personal

sampler, uses a 10-mm DO nylon cyclone to select the respirable portion of the total dust aerosol. The HD cyclone used

in the PDM unit was designed to perform identically to the

cyclone used in the BGI-4CP sampler.

Size distributions of the dust in the chamber were measured

using a Marple personal cascade impactor (Model 290, Thermo

Electron Corp., Franklin, MA) operated at a flow rate of 2

L/min. The device was operated according to the manufacturer's

instructions, including correction factors to account for wall loss

[Rubow et al. 1987].

Dust Exposure Chamber

A Marple chamber provided a uniform atmosphere for

comparing dust-measuring instruments while maintaining good

control of test variables [Marple and Rubow 1983]. The chamber was operated to produce dust concentrations nominally

ranging from 0.2 to 4 mg/m3

. While this is the concentration

range recommended in NIOSH's Guidelines for Air Sampling

and Analytical Method Development and Evaluation [Kennedy

et al. 1995], it was viewed as a guideline since it pertains to

analytes that have very good reference standards. In our case,

the reference was the personal gravimetric sampler. These personal samplers have been demonstrated [Kogut et al. 1997] to

have significantly higher relative standard deviations (RSDs) in

multiple sampler comparisons at mass concentrations of less

than 0.5 mg/m3

. To minimize error in the accuracy measurement of the PDM caused by inaccuracy of the reference

sampler, mass loadings were maintained above 0.5 mg/m3

.

A turntable in the Marple chamber that holds the instruments

was rotated at a rate of one to two revolutions per minute. This

eliminated the need for a randomized block design and ensured

that each sampling device was exposed equally to all radial portions of the chamber. Chamber environment was regulated to

between 20 and 25 °C and a relative humidity between 40% and

60%.

Chamber dust concentrations were monitored with a commercially available Model 1400 TEOM (R&P, Albany, NY).

This was used to help select the correct time intervals to achieve

desired mass loadings for the testing.

Coal Types

Three types of coal dust were used: Keystone, Illinois, and

Pittsburgh. The Keystone coal was a commercially available

ground coal manufactured by Keystone Filler and Manufacturing Co., Muncy, PA. The Pittsburgh and Illinois No. 6

coal dusts were obtained from The Pennsylvania State University's coal collection. The target median mass aerodynamic

diameters of the Keystone and Illinois coals were 3 and 8 :m,

respectively. The Pittsburgh coal was ground at Penn State into

three separate sizes to provide nominal median mass aerodynamic diameters of 4, 10, and 20 :m. Five laboratory experiments were conducted, three with Pittsburgh coal of three sizes

and two with the other coals. These coal types were chosen to

represent a range of coal types and a range of size distributions

within one coal type.

Filters and Pumps

Filters for the gravimetric samples were preweighed at PRL's

controlled atmosphere weighing facility using established procedures. The filter cassettes used in the personal sampler differ

from commercially available units in that the aluminum wheel

assembly and check valve were not used. The filters used in the

BGI-4CP sampler were 37-mm-diam, 5-:m pore size, polyvinyl

chloride (PVC) filters similar to those used in the coal mine

personal cassette filter. Flow-controlled MSA Escort ELF

pumps were calibrated on-site at the beginning of each test week

using a Gilibrator (Sensidyne, Inc., Clearwater, FL) primary

standard flow meter to 2.0 ± 0.020 L/min for personal coal mine

gravimetric pumps and impactor pumps and 2.2 ± 0.022 L/min

for the BGI-4CP sampler pumps. An equivalent pressure restriction for the respective samplers was used during pump

calibration. The PDM sampler flow rate was checked before

each coal type and mine test and recalibrated if flow variance

was greater than 5% of the set rate of 2.2 L/min.

Three filter blanks for each type of filter were also used for

each day of testing and were kept with experimental filters, but

not exposed to the dust atmosphere. Average blank filter

weights were used to correct the filter mass results for each test.

Blank filters were also used to calculate the limit of detection

(LOD) and limit of quantification (LOQ) of the experiments.

All filters were returned to PRL's weighing facility for posttest

mass determination using procedures identical to those of the

pretest weighing.

Impactor Preparation

Model 290 Marple impactors, connected to MSA Escort ELF

pumps operating at 2.0 ± 0.020 L/min, were used to measure the

particle size distributions of the various test dusts. The Model

290 impactor has eight collection stages with cut points from 0.7

to 21.3 :m and a final filter (PVC 34-mm-diam, 5-:m pore

size). At each collection stage, dust particles impact on the 34-

mm-diam Mylar substrates at six impaction zones. Before using
6

the substrates, the impaction zones were coated with grease to

hold the collected particles on the substrates. This was done by

covering the 34-mm-diam Mylar substrate with a metal template

having six slots that expose the impaction zones. These slots

were then sprayed with a 1- to 10-μm-thick layer of impaction

grease (Dow Corning 316 Silicone Release Spray, Dow Corning

Corp., Midland, MI). After spraying, the substrates were kept

at constant temperature and humidity for 3 days to allow the

volatile ingredients of the silicone spray to evaporate and to

allow outgassing of the Mylar. Substrates and the PVC final

filters were then preweighed and loaded into the eight-stage impactors. Each lab test run used 51 Mylar substrates and 6 final

filters. Three substrates and three filters were used as controls.

Experimental Design

For each of the five coal types or size distributions, three

replicate test runs were conducted. An individual test run used

12 personal samplers and 12 BGI-4CP samplers. To accurately

compare the mass measurement capability of the PDM to

gravimetric filter methods, the BGI-4CP samplers were modified to use identical inlet and tube configurations to eliminate

these as variables. These samplers were uniformly arrayed

around a central point in the Marple chamber. Three to six

PDM units, depending on availability, were uniformly interspersed into that array. Each gravimetric sampler type was divided into four test-time interval groups of three samplers.

Figure 3 illustrates a typical chamber test run setup.

The average mass of the three individual samplers in each

time group was used to determine the gravimetric dust mass

during a specific test-time interval. In addition, there were

three blank control filters for each test run for each type of

filter used. Control filters were handled in a manner identical

to that of the experimental filters except that the end caps were

not removed or for the PDM, the closed filter holders were not

opened.

Figure 3.–Plan view of a typical test setup in the Marple chamber.
7

RSD = RSD - RSD2

T

2

t

x x i

We selected test-time intervals to achieve filter mass target

loadings over the range of about 0.5 to 4 mg. For a typical test

run, the internal computer for each PDM was programmed to

automatically start and all gravimetric samplers were manually

started at the same time. Because of the large number of

gravimetric samplers started manually, they were started

sequentially by group and stopped in the same sequence to

minimize any time differences between samplers caused by

starting and stopping. As mass loaded onto the samplers with

time, groups of gravimetric sampling pumps were turned off at

predetermined mass loadings as determined by the Model 1400

TEOM. The mass loading then determined the test-time

interval. This procedure resulted in four test-time intervals with

averaged mass loadings from corresponding groups of personal

samplers, BGI-4CP samplers, and impactor samples. For each

test-time interval, the PDM measured mass, recorded in each

data file, was read to determine the mass measured by the

individual PDM for that test-time interval.

The three test runs were essentially replicate runs except that

the mass loadings varied as follows:

Run 1: 8-hr duration, test-time interval Nos. 1-4. The

chamber was brought to an MRE equivalent concentration of

about 2 mg/m3

. Gravimetric filters were turned off at equivalent

mass target loadings of 0.5, 0.8, 1.6, and 2 mg.

Run 2: 8-hr duration, test-time interval Nos. 5-8. The

chamber was brought to an MRE equivalent concentration of

about 4 mg/m3

. Gravimetric filters were turned off at equivalent

mass target loadings of 1, 2, 3, and 4 mg.

Run 3: 12-hr duration, test-time interval Nos. 9-12. The

chamber was brought to an MRE equivalent concentration of

about 2 mg/m3

. Triplicate sets of filters were turned off at

equivalent mass target loadings of 0.7, 1.2, 1.8, and 2.5 mg.

Size Distribution Measurements

Impactor size distribution samples were taken for a

representative portion of each test-time interval. The Marple

personal impactors used were susceptible to mass overloading

that could invalidate the sample. To prevent overloading and to

obtain a representative size distribution over the entire sampling

time, an intermittent sampling strategy was used. One impactor

was assigned to each test-time interval of a test run. All

impactors were started with the gravimetric samplers. The run

time of each impactor, TR, contained a portion of each time

interval. These portions were determined as follows:

Interval 1 (T1): TR = T1

Interval 2 (T2): TR = T1/2 + T2/2

Interval 3 (T3): TR = T1/3 + T2/3 + T3/3

Interval 4 (T4): TR = T1/4 + T2/4 + T3/4 +T4/4

The size distribution for interval 4 was determined using the

average of three impactors, identically operated to obtain the

experimental precision of the size distribution measurement. In

one case, all of the single impactors failed, but previous data

indicated that chamber size was constant, so the averaged results

from interval 4 were used as representative of time intervals 1-3.

Size distribution mass median aerodynamic diameter

(MMAD) and geometric standard deviation (GSD) were determined from a straight line regression of impactor stage data

plotted as the probit of cumulative mass percentages versus the

logarithm of stage cut point. The use of least squares regression

to find the best-fitting straight line for this type of plot is

recommended only if the regression is truly linear because it

overemphasizes the tails of the distribution. Cumulative

lognormal plots often show curvature toward the tails, resulting

in regression error of the distribution parameters. To account

for this, data were used only if the R-squared values for the

regression were greater than 0.95.

Low Mass Measurements

After the completion of the initial test series, a separate test

was conducted to determine the performance of the PDM in

measuring low mass loadings between 0.20 and 0.50 mg. This

test series was also used to confirm the performance of the PDM

units after some minor electronic modifications for intrinsic

safety approval were made to the units. These tests were conducted in a manner similar to that of the laboratory tests using

only the Pittsburgh 20-μm coal. No size distribution or DO

cyclone reference measurements were taken. To minimize error

with the reference samplers at low mass loadings, the number of

reference samplers was increased from three to six when mass

loadings were less than 0.5 mg.

Analysis

The accuracy and precision were calculated from the data

pairs of individual PDM mass measurements to the average

gravimetric reference standard. Accuracy, bias, and precision

were calculated from the method of Kennedy et al. [1995]. For

these tests, the mass ratio for each data pair was calculated by

dividing the individual PDM mass by the average value for the

triplicate gravimetric reference mass of the corresponding time

interval. The individual concentration ratios were then averaged

over all laboratory data, and by coal type or size. The RSDs

were calculated for both PDM and gravimetric reference

standards.

To reduce the impact of error in the personal sampler

measurement, the experimental pooled estimate of the RSD of

the gravimetric samplers was subtracted from the RSD of the

ratios such that the corrected RSD was:

where RSDx ' Relative standard deviation of mass ratio t
8

and the experimental pooled RSD of the gravimetric samplers

was:

 2

n

RSD

RSD =

2

gravimetric

Ti

∑

Bias was then calculated based on the mean concentration

minus one. Accuracy was calculated from the method of

Kennedy et al. [1995]. Confidence limits were calculated based

on the method used by Bartley [2001] using a noncentral

Student's t-distribution. These laboratory data were used mainly

to judge the PDM's mass measurement capability.

The precision of the PDM was analyzed by examining the

RSD of the PDM and reference samplers over different mass

loadings. These loadings were 0.2 to 0.49 mg, 0.5 to 0.99 mg,

1.0 to 1.49 mg, 1.5 to 1.99 mg, and 2.0 to 6.5 mg. The average

and confidence limits of the RSDs were reported.

Data from all laboratory testing were combined on a scatter

plot to help visualize the agreement and range of differences

between the PDM and reference samplers. This included a

linear regression equation and a computation of the R-squared

value of the entire data set.

A second analysis determined how well the PDM compared

to the currently used personal sampler. An indirect analysis was

used for the comparison. Here the bias between the HD and DO

chamber gravimetric mass determinations was calculated against

both the ISO and MRE respirable mass definitions as determined from the size distribution measurements. This fraction

varied with dust size distribution and coal types used. The size

distribution data were used to calculate the ISO respirable

fraction as defined by Soderholm [1989]. This calculation used

the mass from each impactor stage multiplied by the percentage

defined as respirable for that stage to arrive at the ISO respirable

mass for that stage. The summation of all respirable stage

masses determined the ISO defined mass. The procedure of the

American Industrial Hygiene Association [Lodge and Chan

1986] was used. A similar procedure was used for the MRE

fraction. From the calculated ISO or MRE respirable mass data,

differences from the HD and DO gravimetric reference

standards were calculated. All DO concentration data were converted to MRE equivalent concentration basis by multiplying by

a factor of 1.38. This second analysis was also done on a coal

type or size basis, and results were averaged. The mean bias

was computed for each cyclone by coal type and overall. A

95% confidence interval (CI) was then calculated for each mean.

IN-MINE TESTING

In-mine testing used pair-wise testing to partially take into

account the increased variability associated with personal

sampling in mining conditions and examined the mine worthiness issues of the instrument when worn by miners performing

their normal duties. Limited testing was conducted for five

shifts in each of four coal mines. This testing compared the

end-of-shift gravimetric concentration measured by the PDM

with the end-of-shift gravimetric concentration measured with

a reference filter sampler using an HD cyclone and an analytical

balance. The HD cyclone used an inlet and tubing configuration

identical to the PDM inlet and tube configuration to minimize

the number of variables.

Six PDM units were available for mine testing. Three units

were allocated for mine workers to wear, two units were worn

by NIOSH personnel, and one was designated as a spare and

worn by various people during the testing. The spare unit was

unavailable for testing at the first mine.

Mine Sites

Mine sites were chosen to represent various areas of the

country, types of mines, ventilation systems, and types of

equipment. Both union and industry participated in the selection

of the test mines. The mines were located in Pennsylvania's

Pittsburgh Seam, central Appalachia's Eagle Coal Seam, central

Utah's Hiawatha Seam, and Alabama's Blue Creek and Mary

Lee Seams. Mine sections were selected to provide different

types of equipment and mining situations, such as a longwall

mining machine, continuous mining machines, scrubberequipped mine machines, diesel-powered equipment, and allelectric-powered equipment.

Sampling Mine Workers

Mine workers wore a PDM that replaced their normal cap

lamp battery and one personal BGI-4CP sampler with a tubing

inlet. The tube was identical in length and inlet configuration to

that of the PDM, but was connected to a BGI-4CP sampler

located at the belt of the miner. The inlets of both tubes were

co-located on the cap lamp assembly. The inlet was attached to

the cap lamp at about the 7:00 o'clock position, opposite the

PDM's 5:00 o'clock inlet position when viewed from the front

of the lens. Escort ELF flow-controlled pumps, set at a flow

rate of 2.2 L/min, were used to power the BGI-4CP dust

samplers. NIOSH personnel carried two blank control filters

into the mine each test day, but did not expose them to dust.

Work occupations to be sampled were selected to be

representative of the mine section, with emphasis given to the

MSHA-assigned designated occupation.

Sampling was conducted for the entire shift length. The

PDM was operated in program mode, and the shift length, start

time, and other identification data were entered before the start

of the shift. The PDM started automatically and warmed up in

the mine office. Miners picked up the PDM as they would normally get the cap lamp at the start of a shift. As the shift started,

the reference samplers were manually turned on to correspond

with the PDM start time. At the end of the shift, the PDM automatically turned off and the reference samplers were manually

turned off and the pump times recorded. Miners then removed

the PDM and returned it to the charging cradle or table. At

times, the shift finished before the PDMs shut down. In those
9

instances, the samplers were removed from the miners, but both

reference and PDM samplers were run in the mine office until

the PDMs finished sampling.

At the end of each shift, the PDM units were downloaded in

the mine office to a laptop computer. Tubes and cyclones were

cleaned with compressed air, the used filters were removed, new

filters were installed, and the units were programmed for the

next day's test. Batteries were charged overnight in the mine

office.

Research Samples

Two NIOSH research technicians wore PDM and reference

sampling equipment identical to those of the miners. In

addition, the NIOSH personnel wore three additional samplers

that were used to measure cyclone bias. These samplers

included a personal sampler with a DO cyclone, a BGI-4CP

sampler with an HD cyclone modified with a tube inlet, and a

Marple personal impactor. These instruments were operated

identically to those used in the laboratory. The Marple

impactors, however, were run for the entire shift. The inlets for

all samplers were located in a small quart-size paint can with a

central 1-in-diam inlet. The purpose of this arrangement was to

minimize spatial variability commonly found in field sampling.

The use of an inlet into the paint can would clearly change the

size distribution of a sampler in the can relative to a sampler

outside of the can. However, this difference is not relevant in

this experiment where only samplers inside of the apparatus are

compared.

Ten size-distribution, DO-cyclone, and HD-cyclone measurements were made at each mine. The technicians generally

shadowed, for a period of 6 to 8 hr, an occupation that was

being tested at each mine site to obtain size distribution data for

the cyclone bias calculations. Because of the can inlet and the

need for NIOSH technicians to be in safe sampling locations,

the size distribution measurement may not be exactly

representative of the size of dust to which the PDM was

exposed; however, it was representative of the dust to which the

other reference cyclones in the can were exposed. Thus, the

bias calculations were consistent.

Analysis

Mine worker sampling measurements were expected to be

less precise than the laboratory measurements because of the

increased variability associated with personal sampling. Data

from the miners and NIOSH technicians that compared PDM to

reference samplers were evaluated using a paired t-test. This

test postulates that the mean difference score of the paired

samples is equal to 0. The level of statistical significance was

set at " = 0.05. For the mine worker sampler comparisons, a

minimum of 13 pair-wise data sets were available from each

mine.

To assess the degree of agreement between the PDM and the

reference sampler, an intraclass correlation coefficient (ICC)

was computed [Shrout and Fleiss 1979; McGraw and Wong

1996]. Because systematic differences between samplers were

considered relevant, an ICC for absolute agreement was used.

This type of ICC addressed the question: "Are the two samplers

(PDM and reference) interchangeable?"

A scatter plot of all mine data was constructed to help

visualize the comparability of the two instruments. This

included a linear regression equation and a computation of the

R-squared value of the data set.

RESULTS

LABORATORY

Laboratory testing was conducted during the spring of 2003.

A total of 316 laboratory comparisons of PDM to reference

samplers were conducted. In addition, 60 laboratory determinations of cyclone bias to ISO and MRE definitions were

conducted.

Mass Results

The laboratory results in table 1 show the average mass of

dust from the triplicate BGI-4CP samplers for each test-time

interval and the corresponding RSD of the reference samplers.

The overall average RSD for the gravimetric reference sampling

for this work was 0.047. Table 1 also contains the mass

measurements for individual PDM units for each test-time

interval. The RSD for the PDM units for each test-time interval

is indicated; the average RSD for these measurements was

0.060. Note that the mass measurements from PDM serial

No. 105 were consistently low and consequently increased the

RSD of the PDM measurements. Subsequent inspection of the

cyclone to heater transition in unit 105 indicated that an

obstruction in the air sample path may have been the reason for

the lower measurements from that unit.

For the laboratory experiments, the LOD, as defined by the

mean filter blank mass value plus three standard deviations, for

the HD and DO filters was 0.055 and 0.026 mg, respectively.

The LOQ, as defined by the mean filter blank mass plus

10 standard deviations, for the HD and DO filters was 0.125 and

0.056 mg, respectively. The difference in these limits is partly

a reflection of the different filter tare weights and the different

balances used for the gravimetric mass determinations.

Accuracy Criterion

Bias, precision, accuracy, and confidence limit calculation

results presented in table 2 are for individual instruments by coal

type and for the overall laboratory experiments. For the overall
10

Table 1.—Overall laboratory data comparing laboratory reference mass measurement to PDM mass measurement

Coal type

Gravimetric PDM

Test-time

interval

Average BGI-4CP HD Classified

mass,

mg

BGI-4CP

Relative

Standard

Deviation

std/mass

Serial No.

101

Serial No.

102

Serial No.

104

Serial

No. 105

PDM

Relative

Standard

Deviation

std/mass mg mg mg mg

Keystone

1 0.563 0.04 0.532 0.548 0.566 0.509 0.045

2 1.151 0.03 1.113 1.089 1.115 1.039 0.032

3 1.742 0.01 1.655 1.611 1.651 1.542 0.032

4 2.351 0.03 2.188 2.137 2.178 2.046 0.030

5 1.128 0.01 1.168 1.140 1.141 0.989 0.073

6 2.188 0.09 2.256 2.179 2.218 1.939 0.067

7 3.406 0.03 3.383 3.202 3.266 2.959 0.056

8 4.517 0.04 4.477 4.246 4.321 3.948 0.052

9 0.852 0.04 0.804 0.852 0.884 0.750 0.071

10 1.724 0.03 1.618 1.637 1.731 1.453 0.072

11 2.510 0.04 2.455 2.444 2.593 2.182 0.071

12 filters unsealed 3.311 3.258 3.459 2.931 0.069

Illinois No. 6

1 0.727 0.034 0.728 0.668 0.655 0.598 0.080

2 1.293 0.017 1.442 1.321 1.309 1.182 0.081

3 2.153 0.069 2.105 2.029 1.972 1.820 0.061

4 3.065 0.024 2.708 2.663 2.621 2.396 0.053

5 1.210 0.079 1.222 1.007 1.195 (1

) 0.102

6 2.492 0.058 2.591 2.172 2.472 (1) 0.090

7 3.932 0.104 4.015 3.354 3.787 (1

) 0.090

8 6.045 0.038 5.406 4.508 5.118 (1) 0.092

9 1.060 0.059 1.080 1.038 1.072 0.838 0.113

10 2.195 0.035 2.200 2.046 2.166 1.707 0.111

11 3.354 0.063 3.283 3.057 3.252 2.564 0.109

12 4.262 0.096 4.335 4.021 4.278 3.417 0.105

Pittsburgh

20 :m

1 0.587 0.084 0.383 0.568 0.563 0.524 0.170

2 1.281 0.034 0.989 1.149 1.193 1.129 0.079

3 1.878 0.045 1.585 1.689 1.801 1.674 0.053

4 2.549 0.027 2.118 2.193 2.375 2.203 0.049

5 1.070 0.012 0.960 1.068 1.069 0.951 0.065

6 2.211 0.087 2.006 2.169 2.162 1.948 0.054

7 3.462 0.021 3.046 3.297 3.250 2.956 0.052

8 4.787 0.049 4.119 4.412 4.365 3.957 0.051

9 0.741 0.049 (2) 0.706 0.755 0.668 0.061

10 1.489 0.054 (2

) 1.467 1.455 1.315 0.060

11 2.253 0.052 (2) 2.235 2.205 1.980 0.065

12 3.135 0.029 (2) 3.011 2.920 2.661 0.063

Pittsburgh 4 :m

1 0.683 0.021 0.663 0.660 0.658 0.634 0.020

2 1.198 0.037 1.128 1.109 1.119 1.050 0.032

3 1.741 0.017 1.625 1.579 1.620 1.514 0.032

4 2.331 0.016 2.117 2.038 2.117 1.964 0.036

5 1.034 0.014 1.012 1.017 (3

) 0.997 0.010

6 2.042 0.030 1.898 1.936 (3

) 1.902 0.011

7 3.127 0.020 2.779 2.858 (3

) 2.797 0.015

8 4.325 0.008 3.666 3.800 (3

) 3.717 0.018

9 0.762 0.029 0.726 0.729 0.722 0.756 0.021

10 1.551 0.046 1.442 1.466 1.425 1.490 0.019

11 2.389 0.014 2.166 2.205 2.142 2.247 0.021

12 3.124 0.056 2.912 2.919 2.848 2.993 0.020

Pittsburgh

10 :m

1 0.570 0.037 (2

) 0.530 0.553 0.465 0.088

2 1.100 0.108 (2) 1.006 1.050 0.883 0.088

3 1.542 0.116 (2

) 1.491 1.530 1.285 0.092

4 2.164 0.157 (2) 1.971 1.965 1.694 0.084

5 0.965 0.088 (2

) 0.891 0.976 0.883 0.056

6 2.041 0.032 (2) 1.698 1.926 1.792 0.063

7 2.999 0.057 (2) 2.622 2.829 2.635 0.043

8 4.248 0.069 (2

) 3.592 3.764 3.503 0.037

9 0.715 0.066 (2

) 0.644 0.704 0.708 0.052

10 1.417 0.083 (2) 1.263 1.348 1.392 0.049

11 2.366 0.004 (2

) 1.894 2.009 2.116 0.055

12 3.212 0.025 (2

) 2.533 2.670 2.812 0.052

Average HD

Exp. RSD 0.047 Average PDM Exp. RSD 0.060 1

Incorrect program start time.

2

Bad heater.

3

No display.
11

Table 2.—Laboratory accuracy results and confidence limits

Coal type Unit serial

No. Bias RSDx/r Accuracy 

Confidence

Limits

Upper

95%

Keystone 

101 -0.03 0.04 7.80 11.8

102 -0.04 0.03 8.40 11.6

104 -0.01 0.04 6.70 10.4

105 -0.12 0.02 15.00 17.00

Illinois No. 6 

101 0.00 0.06 10.40 15.70

102 -0.10 0.08 20.80 28.4

104 -0.05 0.06 13.10 19.1

105 -0.19 0.06 25.40 31.7

Pittsburgh

20 :m 

101 -0.11 0.02 14.90 22.6

102 -0.05 0.05 11.10 15.20

104 -0.04 0.03 7.20 9.5

105 -0.13 0.02 16.10 18.4

Pittsburgh

4 :m 

101 -0.07 0.04 12.60 16.3

102 -0.07 0.04 11.80 15.00

104 -0.07 0.03 11.00 13.4

105 -0.08 0.05 15.8 21

Pittsburgh

10 :m 

101 

102 -0.12 0.06 18.4 22.90

104 -0.06 0.06 13.2 17.80

105 -0.13 0.08 21.7 27.80

Overall 

101 -0.04 0.06 12.5 15.10

102 -0.08 0.06 15.80 17.7

104 -0.05 0.05 11.30 12.9

105 -0.12 0.06 20.00 21.9

data, there is a 95% confidence that the individual PDM

measurements were within ±25% of the reference measurement

according to the method of Kennedy et al. [1995]. From the CI

data, we can predict that 95% of future random samples will be

within ±25% of a reference sampler measurement. The bias

data in table 2 are consistently negative, which indicates that the

PDM undersamples relative to the gravimetric standard. The

instruments have high precision, indicated by the low RSDx/r.

Subsets of data for PDM serial No. 105 again illustrate high

negative bias. This negative bias was traced to a poorly

constructed heater transition by the instrument manufacturer.

After repair of this defect by the manufacturer, subsequent data

indicate that the bias of unit 105 is now equivalent to that of the

other PDM units (see table 3). PDM serial No. 102 also

exceeded the upper confidence limit for the subset of Illinois

No. 6 coal.

Low Mass Measurements

Results from the additional testing to investigate the low

mass measurement capabilities of the PDM are in table 3. Data

from unit 105 were not used in one of the test runs because there

was an abnormal pressure spike that corresponded with a

decrease in the mass of unit 105. This is believed to have been

caused by a pinched or blocked inlet tube for that sampler.

Also, there was a communications port failure with unit 104 of

unknown origin that resulted in lost data for that run. An

accuracy analysis of this low mass data set had values of 15%,

10%, 14%, 10%, and 16% for PDM unit Nos. 101, 102, 104,

105, and 106, respectively.

All Laboratory Data

Both initial and low mass data are combined in the plot in

figure 4. The linear regression of individual pairs of data lends

support to the accuracy analysis in that the trend of the data shows

a largely negative bias of the PDM toward the reference samplers.

Distribution of Precision

Table 4 shows the precision of all laboratory data as determined by the RSD of both the BGI-4CP gravimetric sampler

and PDM for various concentration ranges. The RSD for the

BGI-4CP sampler increased as expected for mass loadings less

than 0.5 mg. The PDM RSD did not increase as much as the

BGI-4CP RSD for the low mass measurements.

Cyclone Bias Results

For the combined laboratory data, the bias of the HD cyclone

was less than that of the DO cyclone using either the ISO or MRE

definitions of respirable dust. Table 5 presents the impactor-defined MRE and ISO respirable concentrations compared to the

measured DO and HD cyclone sampler measurements. The DO

measurements were corrected to the MRE equivalency with a

factor of 1.38.

Table 5 also contains the MMAD and GSD for each testtime interval. Good agreement of the size data is evident within

each set of coal type data. Further evidence of the precision of

the size data is evident in the triplicate size distribution measurements of the T-4 time interval, where the calculated MMAD

had an average RSD of 0.06. To compare the bias between the
12

Table 3.—Low mass data results comparing reference mass to PDM mass measurement

 Test-time

interval

target 

Average BGI-4CP

HD Classified 

mass,

mg

BGI-4CP

Relative

Standard

Deviation

std/mass

 PDM PDM

Relative

Standard

Deviation

std/mass

Serial

No. 101

Serial

No. 102

Serial

No. 104

Serial

No. 105

Serial

No. 106

mg mg mg mg mg

T-1 . . . . . . 0.26 0.02 0.24 0.22 0.22 0.22 0.24 0.036

T-2 . . . . . . 0.71 0.08 0.68 0.63 0.63 0.64 0.62 0.035

T-3 . . . . . . 1.13 0.05 1.14 1.09 1.09 1.09 1.13 0.023

T-4 . . . . . . 1.75 0.04 1.72 1.61 1.62 1.62 1.65 0.027

T-5 . . . . . . 2.48 0.02 2.42 2.27 2.26 2.26 2.29 0.029

T-1 . . . . . . 0.29 0.04 0.24 0.28 0.25 0.27 0.25 0.067

T-2 . . . . . . 1.04 0.01 0.91 0.97 0.92 0.99 0.90 0.042

T-3 . . . . . . 2.06 0.01 1.80 1.93 1.79 1.91 1.74 0.045

T-4 . . . . . . 3.00 0.01 2.70 2.87 2.65 2.79 2.54 0.047

T-5 . . . . . . 4.09 0.01 3.67 3.88 3.57 3.77 3.44 0.047

T-1 . . . . . . 0.38 0.06 0.37 0.39 0.36 NU 0.38 0.029

T-2 . . . . . . 0.50 0.01 0.46 0.48 0.45 NU 0.46 0.021

T-3 . . . . . . 0.84 0.04 0.74 0.78 0.74 NU 0.72 0.035

T-4 . . . . . . 1.60 0.07 1.27 1.28 1.24 NU 1.21 0.027

T-5 . . . . . . 1.92 0.04 1.82 1.79 1.75 NU 1.70 0.029

T-1 . . . . . . 0.23 0.12 0.25 0.26 0.24 0.27 0.26 0.052

T-2 . . . . . . 0.29 0.07 0.29 0.30 0.32 0.31 0.30 0.041

T-3 . . . . . . 0.38 0.11 0.37 0.40 0.37 0.40 0.38 0.042

T-1 . . . . . . 0.26 0.06 0.28 0.24 DL 0.26 0.23 0.085

T-2 . . . . . . 0.28 0.09 0.33 0.28 DL 0.31 0.25 0.109

T-3 . . . . . . 0.41 0.09 0.42 0.37 DL 0.40 0.36 0.079

DL Data lost; could not download data.

NU Not used - pressure spike in file; pinched tube.

Table 4.—RSD of reference samplers and PDM samplers

by mass loading ranges

Mass

range

BGI-4CP PDM

Average 95% CI Average 95% CI

0.2 to 0.49 0.074 (0.058, 0.102) 0.060 (0.047, 0.082)

0.5 to 0.99 0.047 (0.038, 0.061) 0.058 (0.047, 0.075)

1.0 to 1.49 0.043 (0.035, 0.055) 0.061 (0.050, 0.078)

1.5 to 1.99 0.046 (0.036, 0.063) 0.042 (0.033, 0.058)

2.0 to 6.5 . 0.043 (0.038, 0.050) 0.056 (0.049, 0.065)

HD and DO cyclones, the CIs were inspected to determine if they

overlapped. Because of the small number of measurements for

each cyclone within coal type (n=12), the CIs tended to be wide,

so the chance for overlap was increased. A statistically significant

difference in bias was found when the two CIs did not contain an

overlapping value. These results varied by coal type; however, for

the overall bias data, there was a significant difference between the

cyclones, with the HD cyclone exhibiting smaller bias than the

DO. The results for the 95% CIs are presented in table 6.

Figure 4.–Regression analysis of total laboratory data set.
13

Table 5.—Laboratory cyclone bias compared to impactor-defined ISO and MRE respirable mass concentrations

and size distribution data

DO

MRE equiv.

conc.

mg/m3

HD

conc.

mg/m3

Impactor

ISO

conc

mg/m3

Impactor

MRE

conc

mg/m3

DO/ISO HD/ISO DO/MRE HD/MRE MMAD GSD

Keystone T-1 3.122 2.413 1.921 2.062 1.63 1.26 1.51 1.17 2.89 2.47

Run 1 T-2 2.924 2.315 2.216 2.403 1.32 1.04 1.22 0.96 3.29 2.30

 T-3 2.914 2.281 1.828 1.913 1.59 1.25 1.52 1.19 2.78 2.50

 T-4 2.866 2.288 1.808 1.918 1.59 1.27 1.49 1.19 3.51 2.72

Keystone T-1 5.413 4.204 3.797 4.104 1.43 1.11 1.32 1.02 (1

) (1)

Run 3 T-2 5.286 4.110 4.033 4.388 1.31 1.02 1.20 0.94 (1

) (1)

 T-3 5.344 4.253 3.723 3.989 1.44 1.14 1.34 1.07 (1

) (1)

 T-4 5.277 4.233 3.851 4.160 1.37 1.10 1.27 1.02 4.54 3.00

Keystone T-1 2.729 2.151 1.783 1.906 1.53 1.21 1.43 1.13 4.32 2.47

Run 4 T-2 2.684 2.165 1.700 1.810 1.58 1.27 1.48 1.20 5.12 3.10

 T-3 2.769 2.097 1.979 2.128 1.40 1.06 1.30 0.99 5.37 3.35

 T-4 2.784 (2

) 1.812 1.952 1.54 (2) 1.43 6.08 3.03

Ill. No. 6 T-1 3.122 2.413 2.498 2.889 1.25 0.97 1.08 0.84 4.85 2.25

Run 5 T-2 2.924 2.315 2.505 2.939 1.17 0.92 1.00 0.79 (3

) 

 T-3 2.914 2.281 2.459 2.831 1.18 0.93 1.03 0.81 5.50 2.35

 T-4 2.866 2.288 2.382 2.766 1.20 0.96 1.04 0.83 5.54 2.38

Ill. No. 6 T-1 4.808 4.702 4.237 4.851 1.13 1.11 0.99 0.97 5.82 2.21

Run 6 T-2 5.134 4.739 4.019 4.551 1.28 1.18 1.13 1.04 5.48 2.24

 T-3 5.220 4.965 4.258 5.000 1.23 1.17 1.04 0.99 5.09 2.03

 T-4 5.377 5.713 4.131 4.838 1.30 1.38 1.11 1.18 5.47 2.27

Ill. No. 6 T-1 2.626 2.647 2.202 2.581 1.19 1.20 1.02 1.03 5.89 2.23

Run 7 T-2 2.617 2.741 2.220 2.606 1.18 1.24 1.00 1.05 5.86 2.20

 T-3 2.634 2.798 1.990 2.339 1.32 1.41 1.13 1.20 6.04 2.17

 T-4 2.713 2.668 2.143 2.495 1.27 1.25 1.09 1.07 7.29 2.22

Pgh. 20 μm T-1 2.521 2.187 2.019 2.243 1.25 1.08 1.12 0.97 11.30 2.26

Run 8 T-2 2.625 2.405 2.136 2.343 1.23 1.13 1.12 1.03 10.82 2.77

 T-3 2.712 2.352 2.598 2.898 1.04 0.91 0.94 0.81 11.75 3.01

 T-4 2.755 2.394 2.311 2.585 1.19 1.04 1.07 0.93 12.49 2.86

Pgh. 20 μm T-1 4.675 3.988 4.136 4.563 1.13 0.96 1.02 0.87 11.14 3.00

Run 9 T-2 4.920 4.135 3.909 4.352 1.26 1.06 1.13 0.95 10.76 2.82

 T-3 5.912 4.335 3.741 4.122 1.58 1.16 1.43 1.05 9.89 2.87

 T-4 5.361 4.495 3.858 4.234 1.39 1.17 1.27 1.06 9.62 2.80

Pgh. 20 μm T-1 1.973 1.850 1.832 2.029 1.08 1.01 0.97 0.91 10.09 2.70

Run 10 T-2 2.093 1.860 1.708 1.880 1.23 1.09 1.11 0.99 10.62 2.66

 T-3 2.193 1.872 1.782 2.000 1.23 1.05 1.10 0.94 11.55 2.74

 T-4 2.262 1.958 1.748 1.964 1.29 1.12 1.15 1.00 12.57 2.75

Pgh. 4 μm T-1 3.137 2.545 2.095 2.291 1.50 1.21 1.37 1.11 2.31 2.14

Run 11 T-2 2.628 2.242 1.949 2.124 1.35 1.15 1.24 1.06 2.13 2.03

 T-3 2.491 2.174 1.822 1.971 1.37 1.19 1.26 1.10 2.33 2.45

 T-4 2.419 2.185 1.929 2.110 1.25 1.13 1.15 1.04 2.23 2.07

Pgh. 4 μm T-1 4.269 3.884 3.310 3.668 1.29 1.17 1.16 1.06 2.98 2.18

Run 12 T-2 4.144 3.836 3.108 3.415 1.33 1.23 1.21 1.12 3.06 2.11

 T-3 4.193 3.916 2.396 2.769 1.75 1.63 1.51 1.41 2.97 2.07

 T-4 4.228 4.071 3.050 3.420 1.39 1.33 1.24 1.19 2.79 1.99

Pgh. 4 μm T-1 2.127 1.902 1.691 1.873 1.26 1.13 1.14 1.02 2.62 2.00

Run 13 T-2 2.164 1.942 1.564 1.751 1.38 1.24 1.24 1.11 2.94 2.02

 T-3 2.200 1.992 1.618 1.784 1.36 1.23 1.23 1.12 2.41 1.99

 T-4 2.169 1.953 1.760 1.948 1.23 1.11 1.11 1.00 2.68 2.16

Pgh. 10 μm T-1 1.938 2.122 1.877 2.178 1.03 1.13 0.89 0.97 3.77 1.98

Run 14 T-2 1.816 2.058 1.717 1.957 1.06 1.20 0.93 1.05 4.38 2.14

 T-3 1.791 1.931 1.928 2.261 0.93 1.00 0.79 0.85 3.75 1.95

 T-4 1.769 2.028 1.895 2.248 0.93 1.07 0.79 0.90 5.18 2.13

Pgh. 10 μm T-1 3.190 3.625 3.095 3.683 1.03 1.17 0.87 0.98 3.82 1.88

Run 15 T-2 3.295 3.818 3.171 3.770 1.04 1.20 0.87 1.01 4.03 2.05

 T-3 3.372 3.735 2.831 3.337 1.19 1.32 1.01 1.12 4.10 1.94

 T-4 3.357 3.989 2.972 3.492 1.13 1.34 0.96 1.14 4.30 1.98

Pgh. 10 μm T-1 1.489 1.786 1.507 1.780 0.99 1.18 0.84 1.00 4.51 1.88

Run 16 T-2 1.656 1.770 1.496 1.715 1.11 1.18 0.97 1.03 3.91 2.08

 T-3 1.684 1.973 1.800 2.106 0.94 1.10 0.80 0.94 4.66 2.06

 T-4 1.681 2.006 1.846 2.176 0.91 1.09 0.77 0.92 4.87 2.16

Average 1.27 1.15 1.13 1.02

1

Impactors not run for intervals, calculations based on T-4 data. 2

Filter dropped.

3

Stage filter dropped.
14

Table 6.—Statistical significance of cyclone bias testing against ISO and MRE definitions

95% confidence intervals for mean DO and HD bias by coal type

Coal type DO/ISO HD/ISO Significant

Mean 95% CI Mean 95% CI difference?

Keystone . . . . . . . . . . 1.48 (1.41, 1.55) 1.16 (1.09, 1.22) Yes.

Illinois No. 6 . . . . . . . 1.22 (1.19, 1.26) 1.14 (1.04, 1.25) No.

Pittsburgh 20 μm . . . 1.24 (1.15, 1.33) 1.06 (1.02, 1.12) Yes.

Pittsburgh 4 μm . . . . 1.37 (1.28, 1.46) 1.23 (1.14, 1.32) No.

Pittsburgh 10 μm . . . 1.02 (0.97, 1.08) 1.16 (1.10, 1.23) Yes.

 Overall . . . . . . . . . 1.27 (1.22, 1.32) 1.15 (1.12, 1.18) Yes.

95% confidence intervals for mean DO and HD bias by coal type

Coal type DO/MRE HD/MRE Significant

Mean 95% CI Mean 95% CI difference?

Keystone . . . . . . . . . . 1.38 (1.30, 1.45) 1.08 (1.01, 1.15) Yes.

Illinois No. 6 . . . . . . . 1.06 (1.02, 1.09) 0.98 (0.89, 1.07) No.

Pittsburgh 20 μm . . . 1.12 (1.04, 1.20) 0.96 (0.91, 1.01) Yes.

Pittsburgh 4 μm . . . . 1.24 (1.17, 1.31) 1.11 (1.04, 1.18) No

Pittsburgh 10 μm . . . 0.87 (0.82, 0.92) 0.99 (0.94, 1.05) Yes.

 Overall . . . . . . . . . 1.13 (1.08, 1.18) 1.02 (0.99, 1.06) Yes.

IN-MINE

Mine testing was conducted during the summer of 2003 in

four coal mines in different coal-producing regions of the

United States. A total of 72 in-mine comparisons of PDM to

reference samplers and 40 companion determinations of cyclone

bias to ISO and MRE definitions were conducted. While

additional shifts of data were successfully measured with the

PDM, not all were paired with valid reference comparison

samples.

Concentration Comparison

Table 7 compares the in-mine PDM and the adjacent BGI4CP reference concentration measurements for various occupations. The ratio of PDM to reference concentrations for all mine

data was 0.98. This agrees with laboratory observations where

the PDM demonstrated a small negative bias compared to

reference samplers.

The paired t-test was used to evaluate whether the mean

difference, computed as PDM minus BGI-4CP, was equal to 0.

If this were the case, the two samplers would be considered to

have the same reading. The data from the four mines are shown

in table 7. When these data were analyzed individually by mine,

the means of the four difference values did not significantly

deviate from 0. In all cases, the calculated test statistic was less

than the critical two-tail t-value (p > 0.05), so the hypothesis of no

difference was accepted. These results are presented in table 8.

The data from the four mines were then combined (N = 72

pairs). This large sample size greatly increased the power of the

test such that if this test finds a statistically significant difference, the difference would be near the LOD of the experiment,

in other words, it could detect a small effect size [Cohen 1988].

The mean difference between the PDM and reference sampler

was -0.024 mg/m3

. It was further noted that the distribution of

the differences between the paired observations for the entire

data set demonstrated a substantial deviation from normality because of the presence of two extreme outliers, one in each tail of

the distribution. Because the normality assumption of the paired

t-test was violated, a nonparametric, or distribution-free, test

was then used. The Wilcoxon signed ranks test is analogous to

the paired t-test. It is based on the ranks of the observations

rather than their actual values. While this test showed a significant statistical difference from 0 (p = 0.028 shown in table 8),

practically speaking, this difference was at the LOD of the reference samplers.

To further statistically test for agreement between the

sampler readings, an ICC for absolute agreement was computed

for the overall mine data. The ICC between the PDM and reference sampler was found to equal 0.93 (F-value for two-way

mixed-effects model = 29.99, p < 0.0001; 95% CI (0.90, 0.96)).

An ICC of 0.80 is considered good agreement; thus, the data

demonstrate excellent absolute agreement between the PDM and

reference sampler. These results suggest that the two samplers

could be considered interchangeable.

Table 8 also shows that mines 1 and 4 had a high correlation

between data pairs. However, the correlation is less at mine 2,

a longwall mine, which had high dust gradients and airflows.

Mine 3, which had a scrubber fan-equipped mining machine,

also exhibits a lower correlation. High variability between dust

samplers is expected when comparing single-point measurements in a mine environment due to large spatial dust gradients

that may be especially prevalent in some mines.

All mine data are further compared in figure 5. Mine concentration levels were lower than the laboratory levels and did

not exceed 2 mg/m3

. The lower R-squared values from the mine

data are a reflection of the difficulties in obtaining precise sideby-side measurements in the mine rather than any imprecision

of the instruments.

Durability

Mine testing of the PDM demonstrated successful durability.

A total of 115 unit shifts of data were available for data collection, and only 7 shifts of data were lost due to the PDM's failure to record the end-of-shift mass concentration. This is an

availability rate of 93%, compared to an availability rate of 88%

for the reference samplers. Reasons for sample losses are included in table 7. Overall, the PDM was somewhat more successful than the reference samplers in measuring dust levels in

the underground sampling environment during these tests.
Table 7.—Mine data results

Occupation

Shift 1 Shift 2 Shift 3 Shift 4 

PDM

conc.

mg/m3

BGI-4CP

conc.

mg/m3

PDM

conc.

mg/m3

BGI-4CP

conc.

mg/m3

PDM

conc.

mg/m3

BGI-4CP

conc.

mg/m3

PDM

conc.

mg/m3

BGI-4CP

conc.

mg/m3

Miner operator . . . . . . . . . 0.79 flow fault 0.36 0.44 0.56 0.6 0.44 0.57 Loader operator . . . . . . . . 0.26 flow fault 0.32 0.29 0.17 0.21 0.18 battery fault Left bolter . . . . . . . . . . . . . 1.30 battery fault 0.60 0.55 0.96 0.9 0.60 battery fault Foreman . . . . . . . . . . . . . . 1.07 battery fault 0.15 0.17 0.56 0.66 0.52 0.60 Tail shearer operator . . . . 1.50 flow fault (1) flow fault 1.20 1.21 1.40 1.25 

Head shearer operator . . . 1.10 flow fault 0.90 0.92 0.80 flow fault 0.90 0.89 Jack setter . . . . . . . . . . . . 1.20 1.271 1.30 1.30 0.90 1.045 1.00 0.98 Guest . . . . . . . . . . . . . . . . 2.06 NR 0.70 NR 0.40 NR 0.30 NR NIOSH 1 . . . . . . . . . . . . . . 0.79 0.86 0.90 1.010 water in sensor 1.23 20.6 0.89 

NIOSH 2 . . . . . . . . . . . . . . 0.64 0.49 0.90 0.69 0.80 0.98 0.66 0.95 Operator/helper . . . . . . . . 0.77 0.81 0.92 1.09 lost grit pot 0.96 0.51 0.61 Helper/operator . . . . . . . . . 0.54 (3) 1.33 0.91 0.81 0.77 0.77 0.64 

Bolter . . . . . . . . . . . . . . . . lost grit pot 0.17 40.85 1.12 lost grit pot (1) 0.45 cracked cassette 

Guest . . . . . . . . . . . . . . . . 0.54 NR 2.37 NR 0.76 NR 0.26 NR NIOSH 1 . . . . . . . . . . . . . . pump stop 0.71 pump stop 0.69 0.59 0.67 0.35 0.42 NIOSH 2 . . . . . . . . . . . . . . 0.66 0.79 0.58 cracked cassette 0.47 0.52 0.37 0.40 Miner helper . . . . . . . . . . . 0.27 0.26 0.23 0.24 0.48 0.46 0.22 0.21 Bolter . . . . . . . . . . . . . . . . 0.63 0.54 0.22 0.21 0.41 0.35 0.22 0.19 Shuttle car . . . . . . . . . . . . 0.45 0.45 0.18 0.19 0.40 0.29 0.23 0.17 NIOSH 1 . . . . . . . . . . . . . . flow fault (1) 0.32 0.36 0.53 0.53 0.45 0.51 

NIOSH 2 . . . . . . . . . . . . . . 0.18 0.21 0.20 0.19 0.40 0.43 0.45 0.49 Guest . . . . . . . . . . . . . . . . 0.64 NR 0.64 NR 0.25 NR 1.73 NR NR No reference sampler used.

1Sample prematurely terminated due to loss of PDM sample.2End-of-shift flow rate = 2.69 L/min.3Tube disconnected from reference sampler.4Water in cyclone.
16

Table 8.—Paired t-test for mine data (testing mean difference equal to 0)

n

Pearson

Correlation

Coefficient

Mean difference

(PDM minus Ref.),

mg/m3

t p-value

Mine 1 . . . . . . . . 13 0.98 -0.034 -2.16 0.052

Mine 2 . . . . . . . . 19 0.78 -0.060 -1.61 0.12

Mine 3 . . . . . . . . 16 0.78 -0.017 -0.43 0.68

Mine 4 . . . . . . . . 24 0.94 0.005 0.48 0.64

 Overall . . . . . 72 0.94 -0.024 1

-2.19 0.028 1

z-statistic for Wilcoxon signed ranks test (nonparametric test for two paired samples).

Table 9.—Mine cyclone bias compared to impactor-defined ISO and MRE respirable mass concentrations

and size distribution data

Mine

Reference cyclone to impactor defined

respirable concentrations Size distributions

DO/ISO HD/ISO DO/MRE HD/MRE MMAD GSD

Mine 1 . . . . . . . . . 0.94 1.04 0.83 0.91 9.62 2.94

Mine 2 . . . . . . . . . 1.01 1.13 0.89 1.00 8.08 2.75

Mine 3 . . . . . . . . . 1.25 1.09 1.14 1.00 9.90 5.15

Mine 4 . . . . . . . . . 1.08 0.96 0.96 0.85 11.27 3.96

 Average . . . . . 1.07 1.06 0.95 0.94

Cyclone Bias Results

In the mine, the average biases of the DO and HD cyclones to

the MRE and ISO standards were determined. Table 9 contains

averaged results for the 10 samples from each mine and the overall

average for all test mines. The average ratio of the sampler to the

impactor-defined respirable mass fraction was quite similar. Note,

however, that compared to the ISO standard the DO ratio ranged

Figure 5.–Regression analysis of all mine data.
17

from 0.94 to 1.25, a difference of 0.31, whereas the HD ratio had

a range of 0.17. We see similar results when the ratio is compared

to the MRE standard, with a DO range of 0.31 and an HD range of

0.15. The HD cyclone had lower variability between mines than

the DO cyclone when compared to either the ISO or MRE

standards. This trend was also observed in the laboratory data, but

given the large RSD and small sample size of the mine data set,

statistical significance of the data was not established.

DISCUSSION

Over the past 35 years, accurate measurement of the

workplace respirable dust exposures of miners has been

difficult. During that period, the mining industry has made the

best use of existing sampling technologies. The development of

the PDM to provide timely, accurate on-shift and end-of-shift

data on worker exposure to dust concentration levels enables

heretofore unavailable approaches for labor, management, and

Government to avoid overexposure to coal mine dust on any

given shift.

FUNCTIONALITY

Development of a truly functional sampler has involved

technical compromises in several areas. These include changing

the inlet location, addition of a tube to conduct the sample to the

sampler, and adoption of a different cyclone. These changes,

when taken as a whole, do not impair the measurement of

respirable dust within an accuracy criterion of ±25%.

Mine workers have complained to the authors for years that

the current personal sampler inlet hanging from their lapel

interferes with their ability to work in the tight confines of a coal

mine. Dirty mine clothing and jacket interference with the inlet

have also been unquantified potential sources of error. The tube

and pump added to the worker further interfered with his or her

job. To improve the ergonomic acceptability of the unit, the

sample inlet was relocated from the lapel to the bill of the hard

hat, and the tube and pump were made a part of the cap light

system. The inlet is still within the breathing zone, but we lose

some comparison to historical lapel sampling.

To minimize the profile and weight of an inlet on the hard

hat, conductive rubber tubing was used to move the sample to

the cyclone and sampler located on the belt. Dust loss to the

walls of the tubing was inevitable, but careful design kept the

loss of smaller respirable dust to less than 3% [Peters and

Volkwein 2003]. This change also meant that a cyclone that

could accept a tubing inlet was required.

In the final analysis, despite the compromises in design that

intentionally traded a little accuracy for functionality, the PDM

still accurately measured coal mine dust in the laboratory within

±25% of reference samplers. In mines, the PDM mean

concentrations were equivalent to those of the reference

samplers. In addition, the data show that the HD cyclone

defines the respirable coal dust fraction as well as or, in many

cases, better than the currently used DO cyclone.

Mine workers reported that the PDM was comfortable to

wear despite the extra burden of the reference sampler that most

wore. On occasion, when the reference samplers were not worn,

most workers reported no difference between their existing cap

lamp batteries and the PDM. When a dust monitor is easy to

wear, it also becomes a more functional tool to encourage mine

workers to control dust exposure levels.

TIMELY DATA

The concept of a rugged, lightweight dust monitor that

provides the cumulative dust exposure of an individual at any

time during the shift is a powerful tool that can be used to

prevent overexposures. An example of the type of data available is illustrated in figure 6. The cumulative exposure reading

is a good estimate of the average workplace dust levels. The

cumulative exposure evens periods of high and low exposures

to provide an averaged exposure number. This value can be

reduced, for example at 13:00 in this figure, by breaking for

lunch where little dust exposure occurred and caused the cumulative exposure levels to decline. The projected exposure reading, however, never declines because it is calculated based on

the mass of dust to a given point in time divided by the sample

air volume projected for the entire shift. Another way to look at

this is to say that if the worker receives no additional dust

exposure, this would be the shift exposure. Note that the projected exposure becomes the shift exposure at the end of the

sampling time period.

During mine testing, both miners and management were able

to use the real-time data to identify dust levels higher than normal and, using the PDM-provided information, locate the

problems or devise strategies to minimize the miners' exposure.

In one case, high intake dust levels on a longwall were traced to

a defective dust control on a roof bolting machine operating in

the intake. In another instance, high levels in another intake

location were traced to an improperly sealed brattice near a face

fan. Miners commented that the screen displays were difficult

to see and suggested that an illuminated display would be

preferable. The next generation of instruments is planned to

include a larger and illuminated display.

DATA TRENDS

Figure 7 illustrates how the data from PDM units may be

used to examine trends in dust exposures. This type of analysis

can be used to spot anomalous readings, keep track of typical

exposure data, and identify where in the work cycle exposures

occur. The mine engineer, wearing the guest unit in this example, was intentionally trying to increase his dust exposure,

which created large spikes in his cumulative concentration. This

is atypical when compared to the exposures of others on the

section.
18

Figure 7.–Data trends from the cumulative data files of all PDMs sampling on a section. Note that data spikes at the beginning of the

shift are not environmental concentrations, but a result of the software attempting to calculate concentrations based on very little mass

(electronic noise).

Figure 6.–Example of individual PDM data results.
19

As expected, these trends show the relative ranking of dust

exposure by occupation, where the loader operator has the

lowest exposure and the miner operator has the highest exposure

on the section. We also can see that the foreman's exposures

were not gradual like those of the other occupations, but

occurred in steps as he would enter very dusty areas to take

measurements or adjust ventilation devices.

This type of information is not available with conventional

filter sampling. As additional experience is obtained with the

PDM, other data trend analyses should help miners understand,

control, and prevent overexposure to dust.

BIAS

The negative bias of the PDM determined in the laboratory

study was an expected result from this testing. In the PDM, the

dust sample from the HD cyclone passes through a transition

and heater section before being deposited onto the filter for mass

measurement. This additional sample flow path is not present

in the reference sampler, where dust is deposited directly onto

the filter as it leaves the cyclone. The bias, however, was

minimized through empirical testing and design choices of the

internal flow path.

Bias in the cyclone tests resulted from differences in coal

size and type being sampled. To minimize bias, Bartley et al.

[1994] had recommended that cyclones be operated at flow rates

that produce the lowest bias in the region of most commonly

sampled dust sizes and types. The PDM cyclone was operated

at the flow rate recommended to produce minimum bias,

resulting in good agreement with the ISO definition of

respirable dust in this work. Attempts to correct for bias

through use of a correction factor (the current practice with the

personal coal mine sampler) will inevitably result in some coal

types being over- or undersampled. This results from the wide

standard deviation of the data set from which the average

correction factor was computed. Selecting appropriate cyclone

flow rates to minimize bias should result in more accurate dust

measurements.

CONCLUSION

Six PDM prototype units were successfully tested in the

laboratory and in four underground coal mines. Results showed

that the units provide accurate readings of a miner's dust exposure,

were rugged enough to survive the underground mine

environment, and provided data on instrument faults or potential

tampering.

The laboratory work specifically assessed the performance

of the new dust monitor by comparing the performance to that

of currently used personal samplers in a two-step manner. The

first step demonstrated that the PDM accurately measured mass

according to accepted criterion. The second step showed that

the HD cyclone was better than the DO cyclone in meeting both

the ISO and MRE definitions of respirable dust. The combination of these two results leads to the conclusion that the

PDM is equivalent or better than the currently used personal

sampler in measuring coal dust in the laboratory.

In-mine concentration data measurements taken by PDM or

reference samplers suggest that the two samplers could be used

interchangeably. Use of the HD cyclone in mines also

demonstrated good agreement to ISO and MRE definitions of

respirable dust. The durability and comfort of the PDM led to

good acceptance by mine workers.

The timely PDM dust exposure data provided information

that resulted in quicker recognition of the failure of engineering

dust controls. This type of information enables both miners and

management to prevent overexposure to coal mine dust. The

information also shows how actions and equipment affect a

miner's dust exposure. Miners can quickly learn how to better

reduce their dust exposures by minimizing certain actions and

by better positioning themselves during given activities.

As this technology is commercialized, further applications

of the PDM data can be developed to better protect mine

workers' health. Minor shortcomings of the prototype PDM

units were discovered and are being corrected by R&P. Overall

successes documented in this work have led to an early

commercial version that promises to correct many of the minor

problems identified in the prototype. Further in-mine trials will

determine the long-term durability, stability, and maintenance

requirements for this new dust monitor.

ACKNOWLEDGMENTS

NIOSH thanks and acknowledges the support and technical

advice given by the joint Health and Safety Committee of the

Bituminous Coal Operators' Association and United Mine

Workers of America co-chaired by Joe Lamonica and Joe Main,

respectively. We also thank Edward J. Miller (MSHA Technical Support, Pittsburgh, PA) and Kevin L. Hedrick (MSHA

Approval and Certification Center, Triadelphia, WV) for their

help in navigating the technical complexities of instrument

development for mine use. We thank the individual mine

management for their hospitality and gratefully acknowledge the

miners from the underground trials who worked hard to enable

valid field data to be collected. We appreciate the generous

engineering support provided by Erich Rupprecht, Director of

Business Development, R&P. Finally, we thank the PRL support staff—Calvin D. Garbowsky, Shawn E. Vanderslice, and

Jeanne A. Zimmer—for their heroic efforts in managing the

logistics of the equipment for both the laboratory and mine

measurements.
20

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