Table Ill. Comparison Between Integrated Continuous Dust Monitor Mass Concentrations
and Those Obtained Using the MSA Personal Respirable Dust Sampler

Date TEOM® mass Personal sampler mass Difference
(August 1995) concentration, mg/m® concentration, mg/m? %
16 1.26 1.19 6
7 3.09 2.10 32
18 0.40 0.29 23
24 () 0.46 NA

"TEOM® data not used because of cable break during test day.
NA Not applicable.

The data developed during this phase of the project will be
used to finalize the design of the machine-mounted monitor. Fol-
lowing delivery of ten production units in Phase 3, MSHA will use
them in in-mine evaluations to develop use strategies for the
monitor and will explore its use in MSHA's respirable coal mine
dust control program.

SUMMARY

In response to MSHA’s Respirable Dust Task Group report,
the USBM initiated an aggressive research effort to develop
mineworthy devices to continuously monitor respirable coal mine
dust mass concentration. The dust sensor technology chosen for
development is the TEOM® manufactured by R&P Co., Inc. This
technology is desirable in that it measures collected aerosol mass
directly. Results from laboratory and initial in-mine evaluations of
a TEOM® sensor modified to reduce response to environmental
vibrations indicate that the sensor will operate well within the en-
vironmental and performance specifications of Table |.

‘Two pre-production prototype MMCRDM's will be fabricated in
mid 1996. After certification by MSHA, this MMCRDM system will
be evaluated in both continuous miner and longwall sections of an
underground coal mine. MSHA will be involved in these tests as
part of a joint venture with NIOSH-PRC. As a final phase in this

development, ten production prototype continuous respirable dust
monitors based on the TEOM® sensor will be fabricated for
mine evaluation. The production versions of the instruments will
be used underground to develop use strategies for the monitor.
If successful, this NIOSH-PRC research will foster develop-
ment of a family of commercial continuous respirable dust moni-
tors that can be mounted on mining machines, used as portable
dust monitors, incorporated in mine atmospheric monitoring sys-
tems, and perhaps even be used as personal exposure monitors
for mine workers. Such instruments could make accurate, con-
tinuous records of dust concentrations in the workplace, a signifi-
cant development for occupational health and the mining industry.
Using this record, researchers, regulatory personnel, or mine
personnel could evaluate specific mining practices to see which
‘ones expose mine workers to the least dust. This information
would permit mine personnel to optimize mining procedures to
reduce dust exposure. It will also provide mine personnel with an
immediate record of daily dust concentration levels, enabling dust
exposures to be maintained below federally mandated levels.

FUTURE WORK

MSHA has endorsed the development of a personal version of
the continuous dust monitor. This project will be a contract re-

———ae |
L =2_ © ly _ H

r I x

Figure 6. Design of MMCRDM for phase two in-mine evaluation.




jo-1 GY HE Yy Y
Luonit Hea eo
jata/Contret

retunn <—

TRACK HHH

Intake > OOwOE
oe

BELT:

Figure 3. Location of MMCRDM sensor (Monitor) and auxiliary electronics (DC)
during phase one evaluation.

section's power center. In this way, the intrinsically safe sensor
assembly could be operated at the face, while the nonintrinsically
safe equipment was operated in fresh or intake air.

During the field tests, measurements were also taken of ma-
chine vibration and respirable dust levels at the position of the
monitor. The accelerometers used to measure machine vibration
were also connected through a 300-m umbilical cable and electri-
cal barriers to signal amplifiers and a power supply at the data and
control center. Data logging for both dust monitor concentration
and vibration measurements was done using personal computer-
based data loggers. Gravimetric respirable dust measurements
were collected for comparison with the continuous monitor mea-
surements using Mine Safety Appliances (MSA) personal respi-
rable dust samplers operated at a flow rate of 1.7 Lpm. This
flowrate was used to better approximate the ISO respirable dust

criteria used by the continuous monitor.

Table II displays the measurements made during the field
evaluation of the continuous monitor. Measurements were taken
for seven days over a two-week period.

Comparison of continuous dust monitor and personal dust
monitor measurements of dust concentrations were made for
three of the four sample days. The data from the fourth day were
not used due to monitor data control damage during the measure-
ment period. A pre-filter (PF) was placed on the monitor on the
other three days to explore the effect machine vibration had on
the instrument's baseline. Vibration measurements were taken
on all sampling days.

RESULTS

During the Phase 1 test, the continuous monitor measured re-
spirable mass concentration in mg/m? every 1.7 sec. A 30-sec
moving average was applied to these data. As shown in figure 4,
this average was displayed graphically in real time by the com-
puter data acquisition system. The figure shows typical results
during two coal cutting cycles and one bottom cleaning pass by
the continuous miner. Mass concentrations drop to background
levels between periods of activity, suggesting that the measure-
ment process is not severely affected by machine vibration. Also,
total collected mass values and trends indicated no dependence
on water or humidity.

Table Il. Primary measurements conducted during
the field evaluation of the MMCRDM

Date

(August 1995) Continuous Measurement Personal

Monitor Vibration Sampler

15 ‘PFE Yes No
16 Yes Yes Yes
17 Yes Yes Yes
18 Yes Yes Yes
a ?No Yes Yes
23 PF Yes No
24 PF Yes No

‘Continuous monitor operated with pre-fiter.
*Continuous monitor data/control cable damaged; data not valid.

On the last two days of the study, a pre-flter was inserted into
the sample path of the continuous monitor The monitor was oth-
erwise operated normally, At the end of this period, which in-

\ tt
\ F
aN AVIAN

Figure 4. Dust concentrations measured by the MMCRDM on 21 August 1995.




CONTINUOUS RESPIRABLE MINE DUST
MONITOR DEVELOPMENT

Bruce K. Cantrell,", Kenneth L. Williams,’ Stephen W. Stein,?

David Hassel,? and Harvey Patashnick®
‘National Institute for Occupational Safety and Health, Pittsburgh Research Center, Pittsburgh, PA;

°3M Co., Inc.;

ABSTRACT

In June 1992, the Mine Safety and Health Administration
(MSHA) published the “Report of the Coal Mine Respirable Dust
Task Group; Review of the Program to Control Respirable Coal
Mine Dust in the United States.” As one of its recommendations,
the report called for the accelerated development of two mine dust
monitors: (1) a fixed-site monitor capable of providing continuous
information on dust levels to the miner, mine operator, and to
MSHA, if necessary, and (2) a personal sampling device capable
of providing both a short-term personal exposure measurement
as well as a full-shift measurement.

In response to this recommendation, the National Institute for
Occupational Safety and Health (NIOSH), Pittsburgh Research
Center, initiated the development of a fixed-site machine-mounted
continuous respirable dust monitor. The technology chosen for
monitor development is the Rupprecht and Patashnick Co., Inc.
tapered element oscillating microbalance. Laboratory and in-
mine tests have indicated that, with modification, this sensor can
meet the humidity and vibration requirements for underground
coal mine use. NIOSH is continuing that effort by developing pro-
totypes of a continuous dust monitor based on this technology.
These prototypes are being evaluated in underground coal mines
as they become available. This effort, conducted as a joint ven-
ture with MSHA, is nearing completion with every promise of suc-
cess.

The immediate benefit of this effort will be to researchers,
regulatory personnel, and mine personnel, by permitting evalua-
tion of specific mining practices to see which expose mine work-
ers to excessive dust levels. Using this information, mine person-
nel can optimize mining procedures to reduce dust exposure.
MSHA and the operators will also be able to use the dust concen-
tration data to judge whether dust plan parameters are adequate
to continuously maintaining environmental dust levels below the
applicable standard.

The second development recommended by the Dust Task
Group was a person-wearable version of the continuous dust
monitor. It will be a monitoring system designed to provide a
measurement of worker exposure to respirable dust, both during
and at the end of the shift.

INTRODUCTION

The current gravimetric approach to measurement of shift av-
erage respirable dust concentrations in underground coal mines
(1), with its inherent delays, cannot provide dust level data quick
enough to allow on-site correction of inadequate dust control
practices. Furthermore, in April 1991, the Secretary of Labor al-
leged widespread tampering at several hundred coal mines with

*Rupprecht and Patashnick Co., Inc.

respirable dust samples taken by mine operators for compliance
assessment. These allegations and a directive from the Secre-
tary prompted MSHA to appoint a special Respirable Dust Task
Group to study options to improve monitoring and control of respi-
rable coal mine dust.

In June 1992, MSHA published the report of the dust task
group (2). This report called for the accelerated development of
two mine dust monitors: (1) a fixed-site monitor capable of pro-
viding information on dust levels to the miner, mine operator, and
to MSHA, if necessary, and (2) a personal sampling device ca-
pable of providing both a short-term personal exposure measure-
ment as well as a full-shift measurement.

In response to MSHA’s request, the former U.S. Bureau of
Mines (USBM) investigated several sensor technologies for con-
tinuously monitoring respirable coal mine dust. Most instruments:
currently on the market for measuring aerosol concentration
sense some property of the particles other than their mass. Con-
verting the sensor reading to equivalent mass requires tacit as-
sumptions about the relationship between aerosol mass and the
property sensed. These assumptions can lead to significant error.
A direct aerosol mass sensing instrument eliminates the potential
for error that is associated with converting sensor measurements
to equivalent mass concentrations.

One such technology is Rupprecht and Patashnick Co., Inc.
(R&P) proprietary tapered element oscillating microbalance
{TEOM®) sensor. In USBM laboratory tests of sensor response
to vibrations and humidity (3), the TEOM® dust monitor exhibited
excellent accuracy and stability. Consequently, the TEOM® dust
sensor has been chosen for use in the high humidity and vibration
environment of underground coal mines. Prototypes of a fixed-
site machine mountable continuous respirable dust monitor
(MMCRDM) based on this sensor are currently being developed,
and evaluated in underground coal mines as they become avail-
able. The first such evaluation was completed in 1995; the sec-
ond is scheduled for mid 1996. A commercial version of the moni-
tor is due in early 1997. An effort to use the technology devel-
oped, for this monitor in the design of a person-wearable version
is being planned. It will provide short-term personal exposure
measurement as well as end-of-shift dust exposure measurement.

MMCRDM REQUIREMENTS

This fixed-site monitor will be mountable on mobile mining
machines, most likely continuous miners or longwall shearers,
and will continuously sample dust in the vicinity of the machine
operator. While MSHA has not finalized how data from such a
monitor would be used, it would provide more complete and im-
mediately available information regarding how well dust control
practices are working.


14.04

1.05
(5 min)

0.87
(30 min)

0.62 mon?
(shift)

4100.0

“4400.0 4600.0 4900,0 6000.0 5200.0 6400.0 5800.0 5800.0 6000.0

Figure 7. Real-time MMCRDM data display.

search effort with in-house evaluation of the research product by
NIOSH-PRC. The personal monitoring system would be de-
signed to operate in two modes. The first would be in the form of
a personal dust dosimeter that can be interrogated underground
or at the end of a shift using a portable reading system. The sec-
ond configuration will provide continuous monitoring of respirable
dust concentrations and would be used as a portable monitor.
The system will provide immediate dust exposure information for
the user and permit the mine operator to evaluate his dust control
system and take effective measures to correct problems as they
arise. These efforts would be part of the expanded FY96 MSHA/
NIOSH joint effort and would continue through FY97 into FY98.

REFERENCES.

U.S. Code of Federal Regulations, 1995, Title 30- eral Re-
sources; Chapter |-Mine Safety and Health Administration,
Department of Labor; Subchapter O-Coal Mine Safety and
Health; Part 70—Mandatory Health Standards—Underground
Coal Mines; 1995, pp. 412-418.

Mine Safety and Health Administration (U.S. Department of La-
bor), 1992, “Review of the Program to Control Respirable Coal
Mine Dust in the United States—Report of the Coal Mine Respi-
rable Dust Task Group,” June 1992, 60 pp.

Williams, K.L., and Vinson, R.P., 1986, “Evaluation of the TEOM
Dust Monitor’. USBM IC 9119, 1986, 11 pp.

Trelewicz, K., 1982, “Environmental Test Criteria for the Accept-
ability of Mine Instrumentation,” (Contract JO100040, Dayton
T. Brown, Inc.), BuMines OFR 1-82, NTIS PB 82-146325;
1982.

Soderholm, S.C., 1993, “Proposed International Conventions for
Particle Size-Selective Sampling,” Ann. Occup. Hyg., Vol. 33,
1989, pp. 301-320.

Williams, K.L., Fairchild, C., and Jaklevic, J., 1993, “Dynamic
Mass Measurement Techniques,” K. Willeke and P. A. Baron,
Eds. Van Nostrand Reinhold, New York; 1993, pp. 296-312.

Cantrell, B.K., et al., 1996, “Status of a Tapered Element Oscillat-
ing Microbalance-Based Continuous Respirable Coal Mine
Dust Monitor,” Appi. Occup. Environ. Hyg. 11(7), July 1996,
awaiting publication.





RMS Acceleration on Continuous Miner (8/21/95)

300801

Acceleration (Grms)

0008-08

Yea6eo AEN NMED TeseID STOO T4STI0 148400 44820 r48PD0 H4SEDD ~WoRDD WOOD. HERI ~1HOTOD 160200

Figure 5. Vibration measured at the MMCRDM on 21 August 1995.

cluded one overnight shutdown of the monitor and a cold start
half-way through the period, bas: drift for the unit was less
than 1 jg of collected mass. This indicates that the sensor is not
sensitive to environmental factors such as vibration and humidity.

Typical results for vibration measurements, in units of the
gravitational constant (G, rms), made during the last part of the
period illustrated by figure 4, is shown in figure 5. These data,
collected during the three sump cuts, show in detail the machine
operations during the cuts. In general, accelerations were an or-
der of magnitude less than those cited in the target specifications
of Table |.

Table III includes collateral respirable dust measurements
made during the sensor evaluation. The comparison points to a
positive bias in the TEOM® results, an average of 20 + 13%. This
bias can be partially explained by the difference in the
preclassifiers used in the TEOM® sensor and the personal respi-
rable dust sampler. Using the measured penetration efficiency of
the monitor’s preclassifier and the published penetration effi-
ciency of the Dorr-Oliver cyclone used with the personal sampler
in combination with an assumed particle size distribution for the
collected mine aerosol, a net bias of 12% is predicted. Although
not determined, the remaining bias has been attributed to the
Position of the inlet on the side of the sensor box. With this inlet
position, the monitor samples from a volume where air may recir-
culate, concentrating aerosol mass in the TEOM® sample and
producing the noted bias.

PROJECT STATUS

Phase 2 of the project to produce and test a redesigned, self-
contained monitor is now underway. Using the results of the
Phase 1 evaluation, the MMCRDM has been redesigned. The
insensitivity of the unit to vibration and the lower than expected
vibration levels permitted a modification in the vibration isolation

Time

member that has reduced the size of the sensor assembly. Be-
cause of the success using the umbilical configuration, the
MMCRDM will be installed in two parts: (1) an intrinsically safe
unit containing the sensor and a filter changer, and (2) an explo-
sion-proof enclosure (XPE) containing the computer, pump, power
‘supplies, and electrical barrier circuits. This arrangement will permit
‘a smaller profile sampling unit in front of the machine operator.

Also, the use of an XPE preserves the ability to install the
monitor components in the existing miner XPE.

The inlet bias problem has been solved by moving the inlet
from the side to the top of the sensor assembly, thus putting it in
the unrestricted sample airstream. Referee sampling will be done
using a newly designed personal respirable dust sampler that in-
corporates the same inlet configuration and preclassifier as that
used on the MMCRDM. This will eliminate the need to correct one
of the measurements to obtain a valid comparison of respirable
dust mass concentration.

The completed monitor, as shown in figure 6, is scheduled for
laboratory and in-mine evaluation on both a continuous miner and
a longwall section in late 1996. Laboratory evaluation of the moni-
tor will follow the protocol followed in Phase 1 of the project. Dur-
ing the in-mine evaluation, the monitor will be mounted on the
mining machine or, as an option in the case of the longwall tests,
ona shield. The monitor will derive electrical power from the
miner. It will provide the machine operator with graphical and
numeric information on dust concentrations as illustrated in figure
7. A series of collateral measurements, like those performed in
the Phase 1 evaluation, will be made during the evaluation.
These will include measurements of machine vibration, referee
shift average respirable dust concentration collected with per-
sonal samplers, face ventilation, water usage, and for a portion of
the personal samples, determination of silica fraction of the col-
lected mass.


————
; =|
Inlet = te in
== Scour}
t 2 Sten |
T fone) Maal
i ry O
inlet Elutriator

Figure 2. Cross-section view of machine mounted, continuous respirable dust monitor.

measure an aerosol parameter correlated with mass, the TEOM®
technique measures mass directly. With the appropriate
preclassifier, the instrument collects and measures respirable
mass. Sampling at 2 Limin for 30 min, the typical measurement
accuracy is + 15 1g/m®. The measurement remains accurate as
long as the mass on the filter remains below about 5 to 10 mg.

The TEOM® aerosol monitors would measure any water drop-
lets reaching the collection filter as aerosol mass. Changes in
mine air humidity and temperature could also affect the response
of the instrument. However, the monitor uses a 50°C tempera-
ture-controlled inlet conditioning system to eliminate or reduce
humidity and temperature variations of the sensor. Under these
sampling conditions, collected water aerosols evaporate, leaving
only solid particulate on the filter.

‘Since TEOM@ instruments operate by measuring the change
in frequency of a vibrating element, vibrations from external
sources can interfere with the measurement. In the machine-
mounted unit, however, vibrations from the machine are damped
using vibration isolation mounting of the monitor sensor.

CONTINUOUS RESPIRABLE DUST
MONITOR DEVELOPMENT

Under a USBM contract, R&P Co., Inc. embarked on a pro-
gram to design, build and test ten production MMCRDM's based
on the TEOM® aerosol mass sensor.

Three phases are required to complete this work:

Phase 1
Phase 1 has two parts:

(a)Design a research proof-of-concept prototype MMCRDM to be
tested using tethered electronics. The prototype must be cer-
tifiable by MSHA for an Experimental Permit for use in permis-
sible areas of coal mines. Two of these prototype MMCRDM's
were evaluated by PRC and MSHA in laboratory and field
tests, primarily for response to machine vibration. Field evalu-
ation was conducted in a continuous miner section of an oper-
ating coal mine.

(b)Begin design of two pre-production prototypes. The pre- pro-
duction prototypes will include prototypes of all the compo-
nents that will be included in the production models.

Phase 2

Complete the design and construction of the two pre-produc-
tion prototype MMCRDM's. The design was to incorporate
tamper-resistance. The prototypes are machine mountable and
certified for experimental permits by MSHA. These prototypes will
be evaluated during laboratory and field tests. During the field
test, one unit will be mounted on a continuous miner and the other
unit will be used in a longwall mining section.

Phase 3

Delivery of ten production model MMCRDM's built to commer-
cial production specification.

PHASE 1 FIELD EVALUATION

Phase 1 laboratory and in-mine evaluation of the prototype of
the machine-mounted monitor was conducted as a joint venture
with MSHA. Laboratory tests of the monitor sensor were con-
ducted by the USBM in mid 1995. The protocol followed has been
detailed elsewhere (7). Field evaluations were conducted in a
continuous miner section from August 10 to 24, 1995.

The monitor configuration tested in the Phase 1 field evalua-
tion is illustrated in figure 2. As designed, this configuration mea-
sured 0.3 x 0.55 x 0.2 meters. During the tests, however, the
computer control, data logging, sample flow control, and sample
pump were removed from the case and placed at a remote loca-
tion. Only the sample inlet, sample preclassifier, and TEOM®
aerosol mass transducer/sensor were attached to the canopy of
the continuous miner. This arrangement placed the sensor as-
sembly within 1 m inby of the machine operator while leaving his
view of the face unobstructed. As indicated in figure 3, a 215-m
umbilical cable containing instrument power, signal, and vacuum
lines, connected the sensor through a group of electrical barriers
to the electronics located at a data and control center near the




With the concurrence of MSHA, the USBM formulated target
specifications (Table |) for the MMCRDM (4). At the request of
MSHA, the sample preclassifier was designed to pass size se-
lected aerosol to the tapered element filter based on the interna-
tional standards organization (ISO) definition of respirable dust
(5). In addition to measuring respirable dust concentration in the
mine environment, the monitor is to be tamper-resistant and store
dust exposure information for 30 days. In operation, the unit will
be programmed to display dust concentrations in real time and
indicate whether the shift average permissible exposure limit will
be exceeded during the work shift.

TAPERED ELEMENT OSCILLATING MICROBALANCE

The TEOM® sensor uses the inertial behavior of a vibrating
element to measure the mass of sampled dust (6). The active
element of the system, depicted in figure 1, is a specially tapered
hollow tube constructed of metal or an elastic, glass-like material.
The wide end of the tube is firmly mounted on an appropriate base
plate. The narrow end supports a replaceable collection medium
such as a filter and is permitted to oscillate. Particle-laden air is
drawn through the collection medium, where particles are depos-
ited. The filtered air is then drawn through the hollow tube. Air-
flow is controlled by an automatic mass flow controller. As the
collection medium collects dust, its mass increases, causing a
decrease in the frequency of oscillation. By measuring the
change in frequency, one can determine the gain in the mass of
dust on the collection medium.

An electronic feedback system initiates and maintains the os-
cillation of the tapered element. The details of the feedback sys-
tem have evolved over the years, but typically, a light emitting di-
ode (LED)/photo-transistor pair aligned perpendicular to the plane
of oscillation of the tapered element, detects the frequency of os-
cillation. The light-blocking effect of the oscillating element posi-
tioned between the photo/transistor and the LED modulates the
output signal of the photo-transistor, which is then amplified. Part

tT
----~ Exchangeable
UD Fiter Cartridge
Electronic
Feedback System)
Element l
‘TEOM Data
Processing Unit
'
To Flow
Controller

Figure 1. Schematic of the tapered element oscillating microbalance (TEOM) dust
sensor.

of the amplified signal is used by the feedback system to provide
sufficient force to overcome any amplitude damping of the ta-
pered element oscillation. The other part of the amplified signal
from the LED/photo-transistor pair is sent to a counter and data
processing stage. Here, the frequency of oscillation of the ta-
pered element is calculated and stored in memory.

Benefits/Challenges

Unlike many other aerosol measurement technologies that

Table 1. Target Performance and Environmental Specifications for a Fixed-site MMCRDM

Performance Specification ‘Standard
Measurement units Mass concentration of respirable dust, mg/m?
Measurement range 0.5 to 2.0 mg/m? + 25%
Accuracy + 25% of reading with 95% confidence
Overload tolerance 100 my for 10 sec
2 ‘for 30 sec
40 mg/m’ for 120 sec
Measurement period 30 min cumulative shift average (shift length: 8, 10, or 12 hrs)
Maintenance cycle 30 days unattended, data storage for a minimum of 90 shifts
Safety certification Must be certifiable by MSHA for use in permissible areas of coal mines
ici -40 to 40 °C, typically 0 to 30 °C
Teh Underground and some suface operation) sweaty
Thermal shock range 40 to 0 °C
Temperature excursion rate 10 °Cimin
Operational altitude/pressure ‘Sea level + 10,000 ft
equivalent range
Humidity 0 to 100% (typical operation range 30 to 95%)
Mechanical shock (shipping) 1m drop equivalent,
Mechanical shock (operating) 14 ms period sawtooth impulse shock of 20 g
Vibration (continuous miner) Sine vibration, 5 to 2,000 Hz, 1.5 g
Vibration (haulage vehicle) Sine vibration, 5 to 92 Hz, 2.5 g and 92 to 500 Hz, 3.5g

Power fluctuation + 25%