Bruce K. Cantrell,' Kenneth L. Williams,' Stephen W. Stein,2 

David Ha~sel,~ and Harvey Patashnick3 

'National Institute for Occupational Safety and Health, Pisburgh Research Center, Pittsburgh, PA; 

23M Co., Inc.; 3Rupprecht and Patashnick 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 inmine tests have indicated that, with modification, this sensor can 

meet the humidity and vibration requirements for underground 

coal mine use. N~OSH is continuing that effort by developing prototypes 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 venture with MSHA, is nearing completion with every promise of success. 

The immediate benefit of this effort will be to researchers, 

regulatory personnel, and mine personnel, by permitting evaluation of specific mining practices to see which expose mine workers to excessive dust levels. Using this information, mine personrespirable dust samples taken by mine operators for complianc

assessment. These allegations and a directive from the Secr

tary prompted MSHA to appoint a special Respirable Dust Tas

Group to study options to improve monitoring and control of resp

rable coal mine dust. 

In June 1992, MSHA published the report of the dust tas

group (2). This report called for the accelerated development 

two mine dust monitors: (1) a fixed-site monitor capable of pr

viding information on dust levels to the miner, mine operator, an

to MSHA, if necessary, and (2) a personal sampling device c

pable of providing both a short-term personal exposure measur

ment as well as a full-shift measurement. 

In response to MSHA's request, the former U.S. Bureau 

Mines (USBM) investigated several sensor technologies for co

tinuously monitoring respirable coal mine dust. Most instrumen

currently on the market for measuring aerosol concentratio

sense some property of the particles other than their mass. Co

verting the sensor reading to equivalent mass requires tacit a

sumptions about the relationship between aerosol mass and th

property sensed. These assumptions can lead to significant erro

A direct aerosol mass sensing instrument eliminates the potent

for error that is associated with converting sensor measuremen

to equivalent mass concentrations. 

One such technology is Rupprecht and Patashnick Co., In

(R&P) proprietary tapered element oscillating microbalanc

(TEOMO) sensor. In USBM laboratory tests of sensor respon

to vibrations and humidity (3), the TEOM dust monitor exhibite

excellent accuracy and stability. Consequently, the TEOMB du

sensor has been chosen for use in the high humidity and vibratio

environment of underground coal mines. Prototypes of a fixe

nel can optimize mining procedures to reduce dust exposure. site machine mountable continuous respirable dust monit

MSHA and the operators will also be able to use the dust concen- (MMCRDM) based on this sensor are currently being develope

tration data to judge whether dust plan parameters are adequate and evaluated in underground coal mines as they become ava

to continuously maintaining environmental dust levels below the able. The first such evaluation was completed in 1995; the se

applicable standard. ond is scheduled for mid 1996. A commercial version of the mo

The second development recommended by the Dust Task tor is due in early 1997. An effort to use the technology deve

Group was a person-wearable version of the continuous dust oped, for this monitor in the design of a person-wearable versio

monitor. It will be a monitoring system designed to provide a is being planned. It will provide short-term personal exposu

measurement of worker exposure to respirable dust, both during measurement as well as end-of-shift dust exposure measuremen

and at the end of the shift. 

MMCRDM REQUIREMENTS 

INTRODUCTION 

This fixed-site monitor will be mountable on mobile minin

The current gravimetric approach to measurement of shift av- machines, most likely continuous miners or longwall shearer

erage respirable dust concentrations in underground coal mines and will continuously sample dust in the vicinity of the machin

(I), with its inherent delays, cannot provide dust level data quick operator. While MSHA has not finalized how data from such 

enough to allow on-site correction of inadequate dust control monitor would be used, it would provide more complete and im

practices. Furthermore, in April 1991, the Secretary of Labor al- mediately available information regarding how well dust contr

leged widespread tampering at several hundred coal mines with practices are working. 
With the concurrence of MSHA, the USBM formulated target 

specifications (Table I) for the MMCRDM (4). At the request of 8mpl. 

MSHA, the sample preclassifier was designed to pass size selected aerosol to the tapered element filter based on the interna- t I +--, wnC..u 

tional standards organization (ISO) definition of respirable dust / wm 

(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 r 

The TEOMO 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 I 

hollow tube constructed of metal or an elastic, glass-like material. 

The wide end of the tube is firmly mounted on an appropriate base EoMDdr 

plate. The narrow end supports a replaceable collection medium Prooedngm 

such as a filter and is permitted to oscillate. Particle-laden air is I I 

drawn through the collection medium, where particles are deposited. The filtered air is then drawn through the hollow tube. Air111 

To n# 

flow is controlled by an automatic mass flow controller. As the Conbdkr 

collection medium collects dust, its mass increases, causing a Figure 1. Schematic of the tapered element oscillating microbalance (TEOM) dust 

decrease in the frequency of oscillation. By measuring the sensor. 

-ch~eirrfr~eV, efte md*rfftiRe* gaiR awmf -Me ;t~tp!ifiedused bythgfsedhacksystem bprmedust on the collection medium. sufficient force to overcome any amplitude damping of the taAn electronic feedback system initiates and maintains the 0s- pered element oscillation. The other part of the amplified signal cillation of the tapered element. The details of the feedback SyS- from the LED/photo-transistor pair is sent to a counter and data 

tern have evolved over the Years, but typically, a light emitting di- processing stage. Here, the frequency of oscillation of the taode (LED)/photo-transistor pair aligned perpendicular to the plane pered element is calculated and stored in memory. of oscillation of the tapered element, detects the frequency of oscillation. The light-blocking effect of the oscillating element posi- ~~~~fit~/~h~ll~~~~~ 

tioned between the photo/transistor and the LED modulates the 

output signal of the photo-transistor, which is then amplified. Part 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, mglmS 

Measurement range 0.5 to 2.0 mglmS * 25% 

Accuracy * 25% of reading with 95% confidence 

Overload tolerance 100 m rnSfor 10 sec 

25 rngs for 30 sec 

10 mg/ms for 120 sec 

Measurement period 30 min cumulative shii average (shi length: 8, 10, or 12 hrs) 

Maintenance cyde 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 

Temperature range (anticipates -40 to 40 "C, typically 0 to 30 "C 

both underground and some surface operation) 

Thermal shock range 40 to 0 "C 

Temperature excursion rate 10 "Clmin 

Operational altitudelpressure Sea level * 10,000 R 

~qwlent range --- 

Humidity 0 to 100% (tj@caKpeK%@onrang~~~ - 

-- 

Mechanical shock (shipping) 1 rn drop equivalent, 

Mechanical shock (operating) 1 I 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, 3.5 g 

Power fluctuation * 25% 
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 Umin for 30 min, the typical measurement 

accuracy is + 15 pg/m3. 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 droplets 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 temperature-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 

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

Phase 2 

Complete the design and construction of the two pre-production 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 

sources can interfere with the measurement. In the machine- Delivery of ten production model MMCRDM9s built to commer- mounted unit, however, vibrations from the machine are damped cial production specification, using vibration isolation mounting of the monitor sensor. 

CONTINUOUS RESPIRABLE DUST PHASE 1 FIELD EVALUATION 

MONITOR DEVELOPMENT Phase 1 laboratory and in-mine evaluation of the prototype of 

Under a USBM contract, R&P Co., Inc. embarked on a program to design, build and test ten production MMCRDM's based 

on the TEOMO 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 certifiable by MSHA for an Experimental Permit for use in permissible 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 evaluation was conducted in a continuous miner section of an operating coal mine. 

the machine-mounted monitor was conducted as a joint venture 

with MSHA. Laboratory tests of the monitor sensor were conducted 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 evaluation is illustrated in figure 2. As designed, this configuration measured 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 location. 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 assembly within 1 m inby of the machine operator while leaving his 

view of the face unobstructed. As indicated in figure 3, a 215-1-17 

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 
criteria used by the continuous monitor. 

Table I1 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 measurement 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 respirable mass concentration in mg/m3 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 computer 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 measurement process is not severely affected by machine vibration. Also, 

total collected mass values and trends indicated no dependence 

on water or humidity. 

Table 11. Primary measurements conducted during 

the field evaluation of the MMCRDM 

Date 

Figure 3. Location of MMCRDM sensor (Monitor) and auxiliary electronics (DC) (August 1995) CO~~~~UOUS Measurement Personal 

during phase one evaluation. Monitor Vibration Sampler 

section's power center. In this way, the intrinsically safe sensor 

assembly could be operated at the face, while the nonintrinsically 15 'PF Yes No 

safe equipment was operated in fresh or intake air. 16 Yes Yes Yes 

During the field tests, measurements were also taken of ma17 Yes Yes Yes 

chine vibration and respirable dust levels at the position of the 18 Yes Yes Yes 

monitor. The accelerometers used to measure machine vibration 21 2No Yes Yes 

were also connected through a 300-m umbilical cable and electri- 23 PF Yes No 

cal barriers to signal amplifiers and a power supply at the data and 24 PF Yes No 

control center. Data logging for both dust monitor concentration ,Continuous with pre-filter. and vibration measlJrements was done using personal computer- 'Continuous data/control cable data not valid, 

based data loaaers. - - Gravimetric res~irable dust measurements 

were collected for comparison with the continuous monitor measurements using Mine Safety Appliances (MSA) personal respi- On the last two days of the study, a pre-filter was inserted into 

the sample path of the continuous monitor The monitor was oth- rable dust samplers operated at a flow rate of 1.7 Lpm. This 

flowrate was used to better approximate the IS0 respirable dust erwise operated normally. At the end of this period, which inFigure 4. Dust concentrations measured by the MMCRDM on 21 August 1995. 
RMS Acceleration on Continuous Miner (8121195) 

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, baseline drift for the unit was less 

than 1 vg 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 order of magnitude less than those cited in the target specifications 

of Table I. 

Table Ill includes collateral respirable dust measurements 

made during the sensor evaluation. The comparison points to a 

positive bias in the TEOMC3 results, an average of 20 + 13%. This 

bias can be partially explained by the difference in the 

preclassifiers used in the TEOMC3 sensor and the personal respirable dust sampler. Using the measured penetration efficiency of 

the monitor's preclassifier and the published penetration efficiency 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 recirculate, concentrating aerosol mass in the TEOMB sample and 

producing the noted bias. 

PROJECT STATUS 

Phase 2 of the project to produce and test a redesigned, selfcontained monitor is now underway. Using the results of the 

Phase 1 evaluation, the MMCRDM has been redesigned. The 

insensitivitv of the unit to vibration and the lower than expected 

member that has reduced the size of the sensor assembly. Because 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 explosion-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 incorporates 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 monitor will follow the protocol followed in Phase 1 of the project. During the in-mine evaluation, the monitor will be mounted on the 

mining machine or, as an option in the case of the longwall tests, 

on a 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 personal samplers, face ventilation, water usage, and for a portion of 

the personal samples, determination of silica fraction of the colvibration l&els permitted a modification in the vibration isolation lected rmss. 
Date 

(August 1995) 

Table Ill. Comparison Between Integrated Continuous Dust Monitor Mass Concentrations 

and Those Obtained Using the MSA Personal Respirable Dust Sampler 

TEOMQ mass Personal sampler mass Difference 

concentration, mg/rn3 concentration, mg/m3 YO 

'TEOMQBdata 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. Following 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 TEOMQ 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 TEOW sensor modified to reduce response to environmental 

vibrations indicate that the sensor will operate well within the environmental and performance specifications of Table I. 

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 

pan of a joint venture with NIOSH-PRC. As a final phase in this 

development, ten production prototype continuous respirable dust 

monitors based on the TEOMQ sensor will be fabricated for inmine 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 development of a family of commercial continuous respirable dust monitors that can be mounted on mining machines, used as portable 

dust monitors, incorporated in mine atmospheric monitoring systems, and perhaps even be used as personal exposure monitors 

for mine workers. Such instruments could make accurate, continuous records of dust concentrations in the workplace, a significant 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 reFigure 6. Design of MMCRDM for phase two in-mine evaluation. 
1.05 0.87 0.62 ms/d :::{ (ti mi") 1 (30 mid (shift) 10.0 

Figure 7. Real-time MMCRDM data display. 

sbarch effort with in-house evaluation of the research product by 

NIOSH-PRC. The personal monitoring system would be designed 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 second 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 MSHN 

NlOSH joint effort and would continue through FY97 into FY98. 

REFERENCES 

U.S. Code of Federal Regulations, 1995, Title 30-Mineral Resources; Chapter I-Mine Safety and Health Administration, 

Department of Labor; Subchapter 0-Coal Mine Safety and 

Health; Part 70-Mandatory Health Standards-Underground 

Coal Mines; 1995, pp. 412-41 8. 

Mine Safety and Health Administration (US. Department of Labor), 1992, "Review of the Program to Control Respirable Coal 

Mine Dust in the United States-Report of the Coal Mine Respirable Dust Task Group," June 1992, 60 pp. 

Williams, K.L., and Vinson, R.P., 1986, "Evaluation of the TEOM 

Dust Monitor". USBM IC 91 19, 1986, 11 pp. 

Trelewicz, K., 1982, "Environmental Test Criteria for the Acceptability of Mine Instrumentation," (Contract J0100040, Dayton 

T. Brown, Inc.). BuMines OFR 1-82, NTlS 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 Oscillating Microbalance-Based Continuous Respirable Coal Mine 

Dust Monitor," Appl. Occup. Environ. Hyg. 11 (7), July 1996, 

awaiting publication.