NEW TOOLS TO MONITOR PERSONAL EXPOSURE TO RESPIRABLE 

COAL MINE DUST 

J.C. Volkwein E.D. Thimons 

NIOSH/Pittsburgh Research Laboratory 

Pittsburgh, PA 

NIOSH/Pittsburgh Research Lab 

Pittsburgh, PA 

ABSTRACT 

The compliance sampling method for coal mine dust in the United States has not changed appreciably in the last 30 

years. A call for more frequent sampling with immediately available results is leading to new instruments that may 

supplement or replace the existing sampler. This report discusses two such instruments that provide immediate 

results of dust levels. The first, called the Respirable Dust Dosimeter (RDD), is designed to supplement the existing 

U. S. cyclone compliance sampling method. It uses the pressure drop across a filter to provide an inexpensive 

screening type of measurement. The RDD has been tested in the laboratory and a limited number of coal mines. Side 

by side testing has compared the RDD performance with personal coal mine samplers in triplicate area sample 

measurements. Results show that the differential pressure of specific filtration media can be an effective surrogate 

for respirable mass. Data show that there is a dependence on coal type and an effect of relative humidity. There also 

appear to be two distinct responses related to coal type. For specific coals the coefficients of determination, R2

 , are 

better than 0.9. For general use the detector tube, using laboratory generated calibration curves, can be used to 

estimate respirable dust levels. However, a calibration to a specific coal type may substantially improve the 

accuracy. 

Another sampler in development is called the Personal Dust Monitor (PDM). Designed to give compliance quality 

accuracy, the PDM uses the frequency change of a vibrating element to measure dust and give immediate feedback of 

results. Results of laboratory comparison of the prototype PDM sampler to personal gravimetric samplers indicate 

R2

 values of better than 0.99 for three different coal types. Work is in progress to package the components into a 

person-wearable unit combined with a cap lamp system with a total projected weight of less than 1.4 kg (3 lbs). 

RESPIRABLE DUST DOSIMETER 

Introduction

A prototype of the RDD concept was previously 

published. That work described the performance of a 

detector tube made from glass and brass that had good 

correlation between differential pressure and mass 

(Volkwein et. al., 2000; Page et. al., 2000). 

Subsequently, a new tube was manufactured from 

conductive plastic using similar specifications. This 

work describes the performance of the new dust detector 

tubes in the laboratory. Another paper describes the 

results from underground testing of the same units 

(Ramani et. al. 2000). 

Description of device

The RDD is analogous to a conventional gas detector 

tube in that a small, low flowrate pump is used to pull a 

sample into a small diameter tube where measurement 

occurs. A uniform dust mass loading results in a 

proportional pressure increase across the filter. Any 

pressure transducer or one integral with the pump can 

be used to correlate with filter mass. After the detector 

tube has been used to make a measurement, the tube can 

be discarded, and a fresh tube used for the next 

measurement. 

Dust enters the inlet of the detector tube, illustrated in 

Figure 1, through a 6.3 mm diameter by 8 mm length of 

polyurethane open cell foam (Type S, FiltercrestTM from 

PCF foam, Corp., Hamilton, OH) with a density of 50 

pores per inch. This segment filters out oversized nonrespirable particulate and protects the main classifier 

from plugging with over size material. The tube narrows 

to a 4.0 mm diameter section that contains a 25 mm 

length of 90 pore per inch open cell urethane foam that 

collects the non- respirable dust and passes the 

respirable fraction of the dust. 

The flow path of the classified respirable fraction of 

the dust gradually expands in the detector tube to 6.3 

mm diameter and travels 55 mm, or about 8 tube 

diameters, to uniformly deposit onto the collection 

filter. The respirable dust deposits onto an 8 mm 
diameter Pallflex Fiberfilm1

 T60A20 fluorocarboncoated glass fiber filter supported by a porous fiber 

backup pad. The respirable classification section and the 

filter holder are made of conductive plastic that is 

ultrasonically welded together. 

Figure 1. Dust detector tube portion of dust dosimeter 

A commercially available low flowrate air sampling 

pump with integral pressure transducer is used to 

monitor the pressure increase with mass loading. Figure 

2 shows both parts of a complete RDD. The increasing 

pressure differential across the filter created by mass 

accumulation is measured and correlated with respirable 

mass. 

Figure 2. Respirable dust dosimeter showing pump with 

pressure readout and new plastic dust detector tube 

Methods 

The experimental method involved making triplicate 

measurements of respirable dust mass using the 

standard U. S. coal mine dust personal sampler unit 

(CMDPSU). This unit uses a Dorr-Oliver cyclone with 

filter mass measurement. These data were compared to 

triplicate differential pressure measurements made with 

the newly available RDD detector tube. 

 

1

References to commercial products are for informational purposes 

 and do not imply endorsement by CDC 

Lab testing was done in a Marple Chamber with very 

uniform dust flow at relative humidity levels between 

15 and 70%. A fluidized bed dust generator with charge 

neutralization was used to aerosolize 5 different types of 

coal dust. A minimum of 12 triplicate pair-wise 

comparisons was done for each coal type at low and 

high humidities. 

Flow controlled personal sampling pumps operated at 

a flow rate of 1.7 lpm were used to sample both 

laboratory and mine aerosols. Dust was classified using 

10 mm Dorr-Oliver cyclones and deposited onto 

standard coal mine sampling cassette filters. 

Flow controlled sampling pumps manufactured by 

SKC Inc. (Pocket PumpTM) were operated at a flow rate 

of 250 ml/min to draw dust into the dust detector tubes. 

The pump pressure transducer was used to measure the 

difference in pressure with clean and dust laden detector 

tubes. 

A direct comparison was made between dust mass 

collected by the CMDPSU and the differential pressure 

increase of the RDD. Tests compared the average 

response of triplicate measurements from each sampling 

device. Data were plotted and a least squares regression 

analysis was used to determine the correlation equations 

and coefficient of determination (R2

). 

Results and discussion

For individual coal type tests, very high coefficients 

of determination were found. Table 1 shows correlation 

equations for linear and power functions for each coal 

type along with their respective R2

 values. Note that the 

power function gives a slightly better R2

 than the linear 

function. This may be caused by the foam classification 

device loading of dust with time that has the effect of 

reducing the amount of dust accumulating on the 

pressure drop filter in relation to the amount of dust 

reaching the gravimetric filter. 

When the data from the table is plotted in Figure 3 

two distinct families of responses are evident. The 

reasons for these different responses are not known, but 

may be related to the coal type and are the subject of 

ongoing investigation. The correlation for each 

grouping is quite good, R2

 for sample sizes of 36 and 24 

are quite high at 0.97 and 0.96. The high R2

 for 

individual coal types and grouped data suggest that the 

dust detector tubes may provide very good estimates of 

dust mass when calibrated by coal family or by 

individual coal type. 

Very low humidities produced a significantly 

different response in the Pittsburgh and Pocohontas 

coal, and a smaller difference with the Upper Freeport 

coal. No substantial difference was seen with the Illinois 

#6 or the Keystone coal. Humidities near 15% RH were 

generated in the laboratory by compressed air dryers 

would be atypical of most underground mining 

situations. Despite this anomalous behavior, Figure 4 

shows that when both high and low humidity data are 

plotted, the family response function was evident. 
 Table 1. 

Power Linear 

Coal Type Correlation 

Equation 

Coefficient 

of determination 

Correlation 

Equation 

Coefficient 

of determination 

 y=mxb R2 y=mx+b R2

Keystone 4.49 x0.73 0.99 3.32x + 0.88 0.99 

Pittsburgh 1.60 x0.64 0.97 0.73x + 0.87 0.94 

Upper Freeport 1.77 x0.61 0.96 0.97x + .75 0.93 

Pocahontas 3.77 x0.9 0.99 3.10x + 1.05 0.99 

Illinois #6 3.61 x0.81 0.98 2.58x + 1.05 0.96 

y = 1.68x0.61

R2 = 0.96

y = 4.10x0.79

R2 = 0.97

0

2

4

6

8

10

12

14

16

012345

Respirable Mass (mg)

Pressure (mm Hg)

Pocahontas,

Illinois #6,

and Keystone 

Pittsburgh, and 

U. Freeport

Figure 3. Dust dosimeter laboratory data for 5 coal types at humidity levels of 50 and 70% showing two distinct coal 

groupings 

While the response of the RDD at low humidities may 

change the correlation of some coals, using the detector 

tube in mines with fairly constant environmental 

conditions will yield useful data. This was especially 

true in the region of the Permissible Exposure Limit and 

(PEL) of 1.4 mg (equivalent to 2 mg/m3

). Previous 

work also has shown that diesel particulate matter gives 

a much greater response than mineral particulate 

making correlation more difficult. 

PERSONAL DUST MONITOR 

Description of device

The original personal dust monitor (PDM-2) was 

produced under a NIOSH contract to Rupprecht and 

Patshnick Co. Inc. (R&P) The device is based on the 

highly successful tapered element oscillating microbalance 

(TEOM) technology that is the heart of a number of 

successful environmental dust monitors. Under this 

contract, the TEOM element was miniaturized for personal 

use and an electronic momentum compensation device was 

developed to eliminate the need for a large mass to 

stabilize the base of the oscillating element. 

The unit was designed to be similar to the existing 

personal dust monitor in that a lapel cyclone inlet was 

maintained. Thus, the unit consists of two modules 

(hence the designation PDM-2). The lapel module 

contains the inlet to a Dorr Oliver nylon cyclone, a Ushaped air heater section, the filter connected to a 

momentum compensated tapered element microbalance, 

and a computer. The belt module contains a flow 

controlled pump, batteries, and data display. The inlet of 

the cyclone has a custom fabricated shield to protect the 

inlet from direct water spray action and to reduce the 

sensitivity of the inlet to wind direction. 
y = 1.94x0.70

R2 = 0.88

y = 3.86x0.77

R2 = 0.85

0

2

4

6

8

10

12

14

16

0.0 1.0 2.0 3.0 4.0 5.0

Respirable Mass (mg)

Pressure (mm Hg)

Pochahontas, 

Illinois #6 and 

Keystone

Pittsburgh, and

U. Freeport

Figure 4. Dust dosimeter date for all laboratory results 

A prototype PDM-2 is shown in Figure 5. The weight 

of the total unit is 1.95 kg. The lapel module weighs 

about 0.45 kg, and measures 20 cm long, 6.3 cm wide 

and 4 cm thick. The belt module measures 18 cm tall, 

15 cm wide and 5 cm thick. 

Other required components are the battery charger for 

the lithium ion batteries that connect through a charging 

port on the belt unit, and a personal computer (lap top or 

desk top) that accesses the lapel unit computer through a 

port on the side of the lapel unit. Windows based 

software was provided by R&P to upload and download 

data from the lapel unit. 

Figure 5. PDM-2 showing lapel 

and belt pack modules 

Methods

Tests were conducted in both the Marple chamber 

described above and in a full scale model longwall dust 

gallery. Two types of coal aerosols were used in the 

laboratory Marple chamber. Pittsburgh Seam A coal 

was obtained from the Pennsylvania State University 

and Keystone coal was obtained from a commercial 

source. Only Keystone coal was used in the longwall 

gallery testing. 

Mass loading protocol

The PDM-2 was compared to the standard coal mine 

dust personal sampler unit (CMDPSU). The CMDPSU 

takes a gravimetrically determined filter sample. 

Filters were pre and post weighed in the PRL 

controlled atmosphere weighing facility according to 

established procedures. The filter cassettes differed 

from commercially available units in that the stainless 

steel wheel assembly and check valve were not used 

and mass determinations were made to ± 0.02 mg. 

Filter blanks were used for both Marple and longwall 

testing. The average of three filter blanks was 

subtracted from the Marple chamber data and the 

average of two filters was subtracted from the longwall 

gallery data. 

Flow controlled Mine Safety Appliance (MSA) Elf 

Escort pumps were calibrated at the beginning of each 

coal type test in the Marple chamber and before the 

longwall testing. A Gilibrator primary standard flow 

meter was used to establish a flow rate 2.0 ±0.2 lpm 

using an equivalent pressure restriction of the cyclone 

and filter assembly. 
The PDM-2 units were prepared for each test 

according to instructions from R&P. The units do not 

have an on/off switch and can only be started by 

programing the units ahead of time. A mandatory 30 

minute warmup is required prior to data collection. In 

general, the units were programed to initiate a test the 

evening before a test was conducted. Prior to 

programing, the flow rates were checked and 

adjustments made to bring the flow to 2.0 ±0.1 lpm. 

Calibration was done with a Gilibrator flow meter 

attached to the bottom of a sealed inlet cyclone that was 

substituted for the PDM-2 cyclone. Unit start times and 

test duration were then programed and the units left on 

the chargers overnight. 

On the morning of the test, the units automatically 

initiated warm-up. Units were then placed into the 

Marple chamber or transported by car to the longwall 

test gallery. At the conclusion of the test, final 

cumulative concentration data from the belt screen 

display was recorded and the units were returned to the 

laboratory. The PC was then used to download data 

from the PDM-2 onboard computer. This data is then 

translated into an ASCII text file that may be read with 

a spreadsheet program. Units were then cleaned, new 

filters installed, and prepared for the next test. 

The data files were coordinated with the run times of 

the gravimetric data. Because the start time of the 

gravimetric samplers did not always match the start time 

of the PDM-2 units, the mass of the PDM-2 at the 

gravimetric start time was subtracted from the PDM-2 

end time mass. Gravimetric samplers were always 

started after the 30 minute warmup cycle. Gravimetric 

start and stop times are ±1 minute and PDM-2 times are 

to the nearest previous minute (ie. 2 min. 59 seconds 

recorded as 2 min.) 

Marple Chamber

Chamber testing was conducted under very controlled 

conditions and assessed the best performance that can 

be expected from the PDM-2. Chamber temperature 

varied between 23 and 25 degrees C. Relative humidity 

varied between 42 and 61% with a target of 50%. Low 

humidity testing was not conducted. 

CMDPSU’s were arrayed in a 1m diameter circle 

around a central point in the chamber and the two PDM2 units placed 180 degrees apart within the array as seen 

in Figure 6. The table of the chamber was then slowly 

and continually rotated approximately 356 degrees and 

then reversed. A total of 19 gravimetric filters were 

used for each test. There were four sets of four filters for 

analysis and three control filters. The control filters 

were handled in an identical fashion to the experimental 

filters with the exception that the end caps were not 

removed. Calibrated MSA Elf Escort pumps were used 

to power the personal sampling cyclone filter units. The 

PDM- 2 units and CMDPSU’S were placed in the 

Marple chamber and operated under battery power as if 

being used in a mine. 

Figure 6. Marple chamber setup for PDM testing 

For Keystone coal, 3 tests were conducted to achieve 

a range of dust concentrations from about 0.2 to 4 mg of 

MRE equivalent mass. Similar tests were run for 

Pittsburgh coal, but one additional test was conducted to 

address a question of possible differences in 

performance with low mass collection over extended 

sampling times. 

 For one test, the chamber was brought to an MRE 

equivalent concentration of 2 mg/m3 for a 10 hour day. 

The PDM-2 was operated for 10 hours. All gravimetric 

filters were started at the beginning of the test but sets 

of four filters were turned off at intervals to encompass 

the desired range of mass loadings. 

 For a second test the chamber concentration and test 

time duration was controlled to achieve an MRE 

equivalent mass loading concentration of 4 mg/m3

. 

Gravimetric filters were turned off at equivalent mass 

loading concentrations of 1, 2, 3 , and 4 mg/m3

 . 

For a third test, chamber concentration and test time 

duration was controlled to achieve an MRE equivalent 

mass loading concentration of 2 mg/m3

. Gravimetric 

filters were turned off at equivalent mass loading 

concentrations of 0.2, 0.5, 1, and 2 mg/m3. 

For the fourth test, with Pittsburgh coal, the chamber 

concentration was brought to a dust concentration of 

about 0.75 mg/m3

 and the test run for about 7 hours. 

Gravimetric filters were turned off at equivalent mass 

loadings of 0.1, 0.2, 0.3 and 0.5 mg/m3

. 

Gallery

The full scale longwall gallery at the Pittsburgh 

Research Laboratory (PRL) enabled the assessment of 

the PDM-2 under less controlled conditions and in the 

presence of water spray mist. Performance of the device 

in motion was also evaluated. 

To reduce spatial variability, a special portable 

“Lippmann” chamber (Figure 7) was constructed for 

this testing. This is similar in design to the multiport 

chamber that MSHA used to determine intersampler 
 

variability in the paper by Kogut et. al. (1997). The 

chamber contained four personal gravimetric 

samplers, and two PDM sampling heads. While the 

inlet of the Lippmann chamber did reduce the mass 

median aerodynamic diameter of the aerosol by about 

2 micrometers, the increased precision of the four 

gravimetric sampling units and assurance that the 

PDM and gravimetric samplers were sampling a 

similar aerosol was considered an acceptable 

tradeoff. 

Figure 7. Cutaway view of portable lippmann type 

chamber used to minimize aerosol spatial variability 

from instrument to instrument 

The gallery was operated using Keystone coal dust 

with constant water and ventilation flow rates. The 

drum water sprays on the model shearer used Spraying 

Systems Fulljet QPH 3.5 and 6.5 sprays. A total water 

flow rate of 247 lpm @ 55.1 kpa was used for each test. 

Ventilation air velocities in the gallery were between 

1.5 m/sec. Keystone coal dust was commercially 

available in large quantities required for gallery testing. 

A powered continuous loop of chain conveyor, 

described below, was used in place of a person to 

simulate a person’s movement within the tailgate area of 

the gallery. This caused the inlet of the Lippmann 

chamber to be exposed to a variety of dust and water 

concentration levels. 

Because the Lippmann chamber can only hold a total 

of 6 sampling units at one time, a series of shorter tests 

were conducted. A total of 14 tests were conducted. 

Two control filters were used for each test. As with the 

laboratory testing, the target range of masses for 

equivalent concentrations ranged from 0.2 to 4 mg. One 

of these tests was of 10 hour duration. During three test 

days, two tests per day were conducted. The same setup 

and download protocol was followed with the exception 

of battery charging. 

Reduced data compared the difference between the 

average of the gravimetric filter mass loadings to the 

individual PDM-2 mass loadings in a pair wise 

manner. All data were converted to MRE equivalent 

concentrations using the factor of 1.38. In the PDM2, this conversion takes place electronically in the 

instrument constant that was used to convert 

oscillation frequency into mass. In the case of the 

personal gravimetric samplers, this conversion was 

made following the mass calculation. It makes no 

difference if this constant is applied to mass or 

concentration since it is simply multiplication by a 

constant. 

Results and Discussion

Results from the Marple chamber for the two coal 

types and longwall gallery are presented. Marple 

chamber results in Figure 8 for the Keystone coal show 

a very high correlation coefficient R2

 of > 0.99 for both 

instruments and the linear correlation equations have a 

slope of nearly 1 with a y intercept within 0.03 mg. 

Data from the Pittsburgh coal test in Figure 9 are not 

as good. While the scatter is low, unit #14 showed a 

bias of 12%. 

Results from the full scale longwall gallery in figure 

10 also have correlation coefficients > 0.99. 

Note that the linear equations of the figure are parallel 

to the theoretical one to one correlation but off set by 

about 0.1 mg. This may be a result of the PDM-2 

actually measuring the water content of the coal that is 

not present in the desiccated gravimetric mass data. 

Use of the Lippmann chamber in the longwall testing 

reduced spatial variability compared to non- enclosed 

inlets. The average relative standard deviation (RSD) 

for the personal gravimetric sampler in the Lippmann 

chamber used in the longwall gallery was 6.1%. This 

value is similar to the two Marple chamber tests where 

the RSD was 3.0% and 6.8% for Keystone and 

Pittsburgh coal respectively. 

Because of the potential use of this device for 

compliance determination, a more detailed statistical 

analysis of the data is warranted and will the subject of 

another article. 

Some additional observations of this testing showed 

that readings were influenced by temperature changes 

and water sprays. Subsequent investigation by the 

manufacturer showed that the temperature effect was 

electronic in origin and may be easily corrected. The 

influence of water is predictable and may be addressed 

by increasing the temperature of the TEOM, or the 

addition of a dryer to the air stream if additional 

accuracy is required. The small differences in coal type 

response was noted and is subject to further 

investigation. The PDM-2 prototypes were not sensitive 

to shock or to the tilt of the unit as might be experienced 

when worn by a person. The zero stability of the PDM 

prototype is similar to the stability of gravimetric mass 

blank filters. 
y = 1.00x + 0.02

R2

 = 0.996

y = 0.98x + 0.03

R2

 = 0.999

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

Gravimetric mass (mg)

PDM mass (mg)

1:1 Unit #12

Unit # 14

Figure 8. Keystone coal regression of Marple chamber data 

y = 1.06x + 0.04

R2 = 0.997

y = 1.12x + 0.02

R2 = 0.999

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.0 1.0 2.0 3.0 4.0 5.0

Gravimetric Mass (mg)

PDM mass (mg)

Unit 14

Unit # 12

1:1

Figure 9. Pittsburgh coal regression of marple chamber data 

y = 1.00x + 0.11

R2 = 0.995

y = 1.03x + 0.07

R 2 = 0.992

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 .0 0 .5 1.0 1.5 2 .0 2 .5 3 .0 3 .5 4 .0

Gravimetric mass (mg)

PDM mass (mg)

1:1

Unit # 14

Unit # 12

Figure 10. Longwall gallery regression using keystone coal 
 

SUMMARY 

Protection of workers’ respiratory health depends on 

many factors. The detector tube approach to personal 

dust monitoring offers many advantages. Cumulative 

shift dust exposure estimates can be easily available at 

low cost in an inexpensive, small, and light-weight 

package. Knowledge of routine dust exposure levels can 

help workers’ and companies focus on protection of 

workers respiratory health. 

The PDM offers an accurate near real time assessment 

of worker exposure. In the present two piece 

configuration, preliminary findings of the laboratory 

evaluation of the PDM have been quite positive. The 

units exhibit reasonable accuracy over the mass ranges 

tested. There were numerous startup and debugging 

difficulties, however the units held up well during the 

entire laboratory test process. Further testing is being 

conducted by NIOSH in four underground coal mines to 

confirm the laboratory results. NIOSH hopes to build on 

this technology to develop a one piece dust monitor 

combined with a cap lamp that is even more convenient 

for miners to wear and will allow them to monitor their 

environments while working. 

REFERENCES 

Volkwein JC, Schoeneman AL, and Page SJ [2000]. 

Laboratory Evaluation of Pressure Differential Based 

Respirable Dust Detector Tube. Appl. Occ. & Enviro. 

Hyg. 15(1), pp 158-164 

Page, S J , Volkwein JC, Baronn PA, Deye GJ [2000]. 

Particulate Penetration of Porous Foam as a Low 

Flow Rate Respirable Dust Size Classifier. Appl. Occ. 

& Enviro. Hyg 15(7), pp 561-568 

Ramani R V, Mutmansky JM, He H, Volkwein JC, 

Marple VA, Olson B, Luna P [2001]. A Comparative 

Evaluation of the Differential Pressure-based Dust 

Detector with the Personal Gravimetric Respirable 

Dust Sampler in Underground Coal Mines. 7th 

International Ventilation Symposium, Kracow, 

Poland, June 

Kogut JTF, Tomb, Parobeck A, Gero J, and Suppers KL 

[1997]. Measurement Precision with the Coal Mine 

Dust Personal Sampler. Appl. Occup Envron. Hyg, 

12(12), 1997 pp.999-1006