‘Maes Concentration (mg/m?)

FIGURE 3. Baseline distornons caused by a vertical sinusoidal vibra~
tion sweep.

this vibration, but the small component of vibration reaching
the sensor at this frequency causes significant baseline devia-
con. The mass concentration averaging interval used to collect
the data in Figure 3 was 30 seconds. The magnitude of the
baseline deviations depends on the averaging interval. Increas-
ing the averaging interval decreases baseline deviations.

The system's resonance frequencies, indicated in Figure 3,
were then examined using slow-sweep sinusoidal vibration
signals. The results of a slow sinusoidal sweep across the
resonance at 252.66 Hz (the tapered element oscillating fre-
quency) are shown in Figure 4. This figure shows results fom
2 preliminary test with horizontal vibrations. The direction of
vibration in this test was the same as the direction of the
oscillation of the tapered element. Initial indications suggest
that horizontal vibrations in the direction of tapered element
oscillation and near the tapered element oscillating frequency is
the worst-case vibration scenario for the TEOM sensor. The
mass concentration averaging interval for these data is 15
seconds,

Increasing the mass concentration averaging interval reduces
the baseline distortions. Additonal vibration tests at the ta
pered elementoscillating frequency examined the influence of
the averaging interval on the magnitude of the baseline dis-
tortion. For these tests the vibration signal was maintained at
252.66 Hz and 3.5 g, and the averaging interval was increased.
The magnitude of the mass concentration distortion decreased
by factors of 12 and 60 when the averaging interval was
increased from 15 seconds to 5 and 10 minutes, respectively.
Since the MMCRDM specifications of Table 1 require data
only every 30 minutes, it appears that baseline distortions from
sinusoidal vibrations can be minimized by selecting a suffi-
ciently long averaging interval

A preliminary coal dust concentration measurement com-
parison between this sensor and respirable dust samplers was
conducted for sample air humidities of 30 to 90 percent and
dust concentrations ranging from 0.2 to 10 mg/m?. Results
from these tests indicate 2 measurement variance less than 10
percent at the 95 percent confidence level. Tests with diesel,
diesel-and-coal dust, and diesel-coal-and-rock dust mixtures
have yet to be conducted.

Maes Concentration
(agi)

:
3. :
= T 8
? *
i> *
7 PD
7 -maneiunnensenney sss iiorereeveis |
8 a
! iy
2
i? =

Ecce arr ce Ce er)

Frequancy (Ez)

FIGURE 4. Baseline distortions trom slowly sweeping sinusoidal vi-
bration near the tapered element oscillanng frequency. The mass
concentration and collected mass averaging ames were both 15 sec-
‘onds, and the sweep rate for the test was 1 Hz every 16 minutes.

Summary and Status

In response to MSHA’s Respirable Dust Task Group, the
BOM is engaged in an aggressive research effort to develop
mine-worthy devices to monitor respirable coal mine dust
mass concentrations continuously. One of the dust sensor
technologies that the BOM has chosen for development is the
TEOM manufactured by R&P. This technology is desirable in
that it measures collected aerosol mass directly. Preliminary
results from laboratory evaluations of a TEOM sensor, modi-
fied to reduce response to environmental vibrations, indicate
that the sensor will operate well within the environmental and
performance specifications of Table 1.

A contract was awarded to deliver two prototype TEOM
sensors in mid 1995 for in-mine evaluation. To verify the
in-mine performance of the sensor without having to finalize
the design of permissible electronics, these sensors will use an
umbilical system for power, signal, and sample air flow. After
certification by MSHA, the umbilical system will allow the
prototype sensors to be evaluated underground by locating the
support circuitry in a remote, nonhazardous location. MSHA
will be involved in these tests as part of a joint venture with the
BOM. The contract also requires fabrication of ten production
prototype continuous respirable dust monitors based on the
TEOM sensor developed for in-mine evaluation. After certi-
fication by MSHA, the production versions of the instruments
will also be evaluated underground.

Ifsuccessful, this 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,


Power
‘Amplifier

Ceerigeecorae

Fovtion)

FIGURE 2. Block diagram of the BOM vibration testing facility.

The greatest challenge in the repackaging of R&P’s mass
monitor for mine use is thought to be ensuring the survival,
proper operation, and explosion safety of the 20+-inch Hg
sample pump, mass flow controllers, computer, and support
electronics. There do not appear to be many off-the-shelf items
suitable for these subsystems, especially in reasonable sizes. In
this effort, modified or custom components will be developed
that may be useful in other types of mine-ready sensors and
instruments.

Laboratory Evaluation

The BOM is currently evaluating the laboratory prototype of
the machine-mounted monitor based on the TEOM. The
objectives of this testing are to confirm that the R&P sensor
meets the specifications of Table 1 and to develop a test facility
and protocols capable of evaluating other sensors for potential
use in an MMCRDM. The facilities being used for this pur-
pose are a vibration and shock test facility and a humidified
dust chamber.

The laboratory evaluation of the modified TEOM sensor
will be carried out in chree phases. The initial phase will be an
evaluation of the stability of the instrument's baseline under
various vibration conditions. In the second phase, the ability of
the instrament to measure aerosol mass concentrations accu-
rately will be Studied for a range of environmental vibration,
shock, and humidity conditions, Instrument durability will be
examined in a final phase

Vibration Tests

A block diagram of the system used for vibration testing of the
modified TEOM sensor is shown in Figure 2. The system
consists of a Gen Rad 2314 vibration control computer, soft-
ware, and accelerometer feedback system: a 12.000-W power
amplifier (MB Dynamics, model M12K); an electrodynamic
\ibration table (MB Dynamics. model C-30); and an oil cool-
ing system that cools the exciter. An oil-lubnicated shp table s
used for horizontal vibration. Up to four accelerometer inputs
are available for feedback control of sinusoidal signals (19 are
available for random vibration testing). The user can define the
reference spectrum in terms of acceleration, amplitude. or
velocity. This system has a usable bandwidth of 5 to 5000 Hz
with minimal total harmonic distornon (<1%) and a noise
specification of 60 dB below full-rated ourput.

Boseline Tests

Dunng initial evaluation of the sensor, instrument baseline
stability will be determuned under various vibration conditions.
The instrument will sample under normal operating condi-
tons, except that a filter will be placed over the aerosol inlet so
no mass will accumulate on the filter mounted on the tapered
element. Devianon from the instrument baseline (i¢.. any
indication of a nonzero collected mass or mass concentration)
will be recorded along with the input acceleranon signal.
Vibration frequencies for sensor resonances will be determined
by sweeping through the specified range of vibranon and
noting any frequencies at which the indicated collected mass or
mass concentration significantly deviates from zero. Subse-
quent tests will examine the sensor response, gt these frequen-
cies by exposing the sensor to slowly swéeping sinusoidal
vibration signals. The response of the instrument's baseline to
random vibrations will also be evaluated by exposing the sensor
to vibration spectra that would be expected for an
MMCRDM.

Dust Chamber Tests

A dust chamber system, manufactured by ELPRAM Inc.. has
been modified to accept dust trom three sources and combine
these in a humidified sample air stream. Aerosol sources in-
clude two TSI fluidized-bed aerosol generators and an
ONAN, model 10.OVR, 10-kW diesel electric generator set
for diesel aerosol generation. The chamber has been equipped
with sample flow controllers to accommodate several dust
monitors simultaneously. To provide humidified sample air.
the chamber is equipped with a system that can achieve stable
humidities in the 30 to 95 percent range.

The ability of the instrument to measure aerosol mass con-
centrations accurately while challenged by a range of vibrations
and humidities will be evaluated. This will be done by com-
Paring the indicated mass concentration to concentrations in-
dicated by concurrently collected stationary filter samplers
located next to the vibrating sensor. This will be done for both
vertical and horizontal vibrations. The sensor will be exposed
to the expected vibration spectra of an MMCRDM. Humidity
tests will be conducted separately using the dust chamber
facility.

Durability Tests

The ability of the sensor to withstand extended vibration will
be examined. The sensor will be exposed to extended periods
of vibration and checked for insrument failures. These vibra-
tion tests will simulate the expected vibratons of an
MMCRDM.

Preliminary Results

Examination of the sensor baseline response to vertical sinu-
soidal vibrations has been completed and the response is indi-
cated in Figure 3. Notice that ceviations from the baseline are
significant at 14, 35, and abou: 60 Hz. These deviations are
due to resonances in the spring suspension of the sensor.
Smaller resonances occur at 75 and 310 Hz. At frequencies
higher than 310 Hz the suspension essentially isolates the
sensor from vibration. Substantial baseline deviation occurs for
vibration at the tapered eiement oscillating frequency
(~252.66 Hz). The spring suspension eliminates almost all of


incorporated in mine atmospheric monitoring systems, and
used a5 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 im-
mediately which ones tend to expose mine workers to exces-
sive dust levels. This information would also permit mine
personnel to take corrective action and optimize mining pro-
cedures to reduce dust exposure. It will also provide MSHA
with a means to monitor dust concentrations in mines when
mine inspectors cannot be present.

Disclaimer
Reference to specific companies or brand names does not
imply endorsement by the BOM

References

1. U.S. Code of Federal Regulations. Tile 30, Chapter I, Subchapter
O, Part 70. U.S. Government Prinnng Office, Washington, DC
(1992).

2. Mine Safety and Health Admuristranon: Review of the Program
to Control Respirable Coal Mine Dust in the United States—
Report of the Coal Mine Respirable Dust Task Group. MSHA,
Picsburgh. PA (1992).

3. Williams, K.L.; Vinson, R.P.: Evaluanon of the TEOM® Dust

Monitor. USBM IC 9119. U.S. Bureau of Mines, Washington.

DC (1986).

USS. Code of Federal Regulanons. Tide 40, Chapter |. Subchapter

C. Par 53. U.S. Government Printing Office, Washington, DC

(193).

Trelewicz, K.: Environmental Test Criteria for the Acceptability

of Mine Instrumentation (Conrract J0100040, Dayton T. Brown,

Inc.). USBM, OFR. 1-82, NTIS PB 82-146325. US. Bureau of

Mines, Washington, DC (1982).

6. Wilbams, K.; Fairchild, C.; Jaklevic, J: Aerosol Measurement. In
Dynamic Mass Measurement Techniques, pp. 296-312. K.
‘Willeke and P.A, Baron, Eds. Van Nostrand Reinhold, New York
(1993)



Status of a Tapered Element,
Oscillating Microbalance-Based Continuous
Respirable Coal Mine Dust Monitor

Bruce K. Cantrell,® Stephen W. Stein,® Harvey Patashnick,® and David Hassel ®
AU.S. Bureau of Mines, Minneapolis, Minnesota 55417; *Rupprecht and Patashnick, Co., Inc., Albany, New York 12203

‘The Mine Safety and Health Administration, Department of Labor,
requires coal mine operators to measure compliance with federal re-
spirable dust exposure standards periodically using approved gravi-
metric samplers. However, the gravimetric approach, with its inher-
ent delays, cannot provide dust concentration data quickly enough to
allow on-site correction of inadequate dust control strategies or re-
moval of personnel from harmful environments. Furthermore, al-
leged tampering with dust samples collected to determine compliance
with standards prompted the Secretary of Labor to appoint a special
Respirable Dust Task Group to study options to improve monitoring
and control of respirable coal mine dust. The Mine Safety and Health
Administration's Task Group recommended that the U.S. Bureau of
Mines develop a fixed-site monitor that would provide continuous
information to the mine worker and mine operator regarding dust
levels and the status of compliance with the applicable respirable dust
standard. In response to the Mine Safety and Health Administration's
request, the U.S. Bureau of Mines is investigating several sensor tech-
nologies for continuously monitoring respirable coal mine dust. One
Of these technologies is the Rupprecht and Patashnick Co. tapered
element, oscillating microbalance. The bureau is in the process of
investigating the suitability of using this sensor in the high humidity
and vibration environment of underground coal mines. Laboratory
tests have indicated that, with modification, the sensor can meet the
humidity and vibration requirements for underground coal mine use.
Currently, the bureau is developing protoypes of a continuous dust
monitor based on this technology. When available, these will be
evaluated in underground coal mines. Canree,B.K.; Sran, $.W.,; PALAS
NO, H.; Hassey:D.: StaTis OF A TARRED ELEMENT, OSCUATING MRORALANACE-
Base> Conmunis Resmeaat Com Mae Dust MOnTOR. Arm. Occur. ENVROn.

Hye. 11(71:624~629; 1996

he current gravimetric approach to measurement of shift-

average respirable dust concentrations in underground
coal mines,‘ with its inherent delays, cannot provide dust
level data quickly enough to allow on-site correction of inad-
equate dust control strategies or removal of personnel from
harmful environmenss. Furthermore. in April 1991 the Secre-
tary of Labor alleged widespread tampering with respirable
dust samples taken by mine operators for compliance assess-
ment at several hundred coal mines. These allegations of tam-
pering and a directive from the secretary prompted the Mine
Safery and Health Administration (MSHA) to appoint a special

Respirable Dust Task Group to study options to improve the
monitoring and control of respirable coal mine dust.

Since then, the Mine Safety Appliances Company, sole
supplier of approved samplers,'"! has modified the samplers to
make them resistant to tampering or accidental dislodgement
‘of dust from the sampler's filter. Despite these improvements,
MSHA’s task group concluded that continuous monitoring of
the mine environment and parameters used to control dust
offers the best solution for improving the dust enforcement
program. The task group report recommended that the U.S.
Bureau of Mines (BOM) develop a continuous respirable dust
monitor that can be mounted on a continuous of longwall
mining machine or operated at a fixed site in a mine. The
monitor would provide continuous information to the miner
and mine operator regarding dust levels and the status of
compliance with the applicable respirable dust standard.

In response to MSHA's request, the BOM is investigating
several sensor technologies for continuously monitoring respi-
table coal dust. To be accepted by MSHA, mine workers, and
the coal companies, a machine mounted continuous respirable
dust monitor (MMCRDM) must reliably monitor respirable
dust concentrations in the mine with sufficient accuracy to be
used as a measurement of compliance with the coal mine dust
standard. Of the instruments currently on the market to mea-
sure aerosol concentration, most sense some property of the
particles other than their mass. These require tacit assumptions
about the relationship of aerosol mass to the property sensed to
convert the sensor reading to equivalent aerosol mass. This can
lead to significant error. A direct aerosol mass sensing instru-
ment eliminates these assumptions and their potential for er-
roneous readings.

‘One direct aerosol mass sensor is the Rupprecht and Patash-
nick Co. (R&P) proprietary tapered element, oscillating mi-
crobalance (TEOM®) sensor. R&P has developed several
commercial aerosol monitoring instruments and a variety of
special purpose prototypes based on this sensor, including one
intended to fly on the space shuttle. In doing this, R&P has
accumulated a wide variety of solutions to difficult require-
ments in building custom mass measurement systems for gov-
emment and industry. Mine conditions are severe, but are bY
no means the most severe conditions yet encountered. R&P
inertial balance mass transducers now in use have operated in
temperatures from cryogenic to 600°C and pressures from hard
vacuum to 1200 psi. Special eransducers have been designed to
withstand the vibration and shock of a space shure launch
simulation, the impact of 2 multikilowatt laser gun, and the
magnetic field inside a super-conducting magnet. In the


TABLE 1. Torget Performance and Environmental Specification for a Mochine-Mounted, Continuous Respirable Dust Monitor

Performance Specification

Standard

Measurement units
Measurement range
Accuracy

Overload tolerance

Measurement period

Maintenance cycle

Safety cerufication

‘Temperature range (anticipates both underground and
some surface operation)

Thermal shock range

Temperature excursion rate

Operational altitude pressure equivalent Range

Humidity

Mechanical shock (shipping)

Mechanical shock (operating)

Vibration (continuous miner)

Vibration (haulage vehicle)

Power fluctuation

Mass concentration of respirable dust (mg/m?)
0.5 to 10.0 mg/m
25% of reading with 95% confidence
100 mg/m? for 120 seconds
23 mg/m? for 60 munutes
10 mg/m? for 120 minutes
30 minutes, cumulative shift average (shift length: 8, 10, or 12 hours)
30 days unattended. data storage for 2 minimum of 90 shifts
Must be certifiable by MSHA for use in permussible areas of coal mines
40° to 40°C, typically 0° to 30°C

40° 10 °C
10°C/min

sea level £10,000 f

0 to 100% (typical operation range 30 to 95%)
1 m drop equivalent

11 millisecond period sawtooth impulse shock of 20 g
Sine vibration, 5 to 2000 Hz, 1.5 g

Sine vibration, 5 to 92 Hz, 2.5 g and 92 to 500 Hz. 3.5.g
25%

BOM's experience, the TEOM dust monitor has exhibited
excellent accuracy and stability in laboratory tests. In addi-
gon, the Environmental Protection Agency has approved the
Series 1400 for PMyq (particulate mass less than 10 pm in size)
environmental monitoring?

The BOM is in the process of investigating the suitability of
using this sensor in the high humidity and vibration enviton-
ment of underground coal mines. Laboratory tests have indi-
cated that, with modification, the sensor can meet the humid-
ity and vibration requirements for underground coal mine use.
Currendy, the BOM is developing prototypes of a continuous
dust monitor based on this technology, When available, these
will be evaluated in underground coal mines.

MMCRDM Requirements

This monitor would be mounted on mobile mining machines,
probably continuous miners or longwall shearers, and would
continuously sample dust in the vicinity of the machine oper-
ator. MSHA has not yet finalized how data from such a
monutor would be used to enforce the statutory respirable dust
standard. They are currently collecting data with experimental
gravimetnc sampling instruments that sample mine ait contin-
uously. The data obtained will help MSHA formulate an
enforcement strategy utilizing the fixed-site monitor.

With the concurrence of MSHA, the BOM has drawn up
the target specifications of Table 1°) for the MMCRDM. In
addition to measuring respirable dust concentration in the
mine environment, the monitor is to be tamper resistant and
will store the dust exposure information for later retrieval. In
operation. the unit can be programmed to display for the
Operator both current and cumulative shift-average dust con-
centratigns. Comparison of this with the shift average permis-
sible exposure limit can then be used as an indicator of a dust
Beneration problem in the mining operation

The TEOM Dust Monitor

The TEOM sensor uses the inertial behavior of a vibrating
element to measure the mass of sampled dust. The acuve
element of the system, depicted in Figure 1, is a specially

Sample
Flow

|

_ Exchangeable
Filter Cartridge

QV

[

Electronic
Feedback System
Tapered
Element
TEOM Data
Processing Unit
To Flow
Controller

FIGURE 1. Schematic of the TEOM dust sensor.


tapered hollow tube constructed of an elastic, glasslike mate-
nal. 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 oscil-
late. Particle-laden gas streams are drawn through the collec
tion medium, where particles are deposited. The filtered gas is
then drawn through the hollow tube, typically controlled by
an automatic mass flow controller. As the collection medium
collects dust, the mass increases, thereby decreasing the fre-
quency of oscillation. By measuring the change in frequency,
one can determine the gain in the mass of dust on the collec
son medium.

An electronic feedback system initiates and maintains the
oscillation of the tapered element, The details of the feedback
system have evolved over the years, but typically a light-
emitting diode (LED) photo transistor pair aligned perpendic-
ular to the plane of oscillation of the tapered element detects
the frequency of oscillation. The light-blocking effect of the
oscillating element positioned between the photo transistor
and the LED modulates the ouput signal of the photo tran-
sistor, which is then amplified. Part of the amplified signal is
used by the feedback system to provide sufficient force to
‘overcome any amplitude damping of the tapered element
oscillation. The other part of the amplified signal from the
LED-photo transistor pair is sent to a counter and data pro-
cessing stage. Here the frequency of oscillation of the tapered
element is calculated and stored in memory.

The equation that describes the behavior of the TEOM
system derives from the equation of motion for a simple
harmonic oscillator:

Am = K,(1/6,? — 1/6?)

where:
Am = mass of the collected sample
f, = frequency of the oscillating element after
sample collection
f, = frequency before sample collection
Ko = a constant (spring constant] unique to each
tapered element

Note that although the relationship is not linear, the increase in
mass can be determined by measuring only the change in
frequency. Although external physical influences can affect the
accuracy of this mass determination, no corrections are made
to the result. Instead, the tapered element is mounted in such
a way a5 to minimize or eliminate the effects of these influ-
ences.

Benefts/Challenges

Unlike many other zerosol measurement technologies that
measure an aerosol parameter correlated with mass, the TEOM
technique directly measures mass inertially. With the appro-
priate system preclassifier, che instrument collects and measures
respirable mass. The measurement range of the instrument
depends strongly on the design of the tapered element. For
typical commercial instruments, the range also depends on
sample averaging time. Sampling at 3 L/min for 2 minutes, the
2a mass concentration resolution is typically +15 jig/m?.
TEOM-based instruments can measure up to several grams per
cubic meter. The measurement remains accurate as long as the
mass on the filter remains below 10 to 15 mg. The pressure

drop across the filter determines when the filter should be
changed.

The TEOM aerosol monitor will initially measure any water
droplets reaching the collection filter as aerosol mass. Changes
in mine air humidity and temperature could also conceivably
affect the response of the instrument. The commercial envi-
ronmental dust monitor uses a 50°C. temperature-controlled
inlet conditioning system to eliminate or reduce the effect of
humidity and temperature variations at the sensor. Under these
sampling conditions, collected water aerosols evaporate, leav-
ing 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 alo interfere with the measurement. If mounted
on a mining machine, vibrations from the machine could
conceivably cause it to operate improperly. It is this possibility
that is the greatest challenge in using the TEOM as a dust
sensor.

Stotus

Under 2 BOM contract, R&P has completed a study to
determine the feasibility of using 2 TEOM sensor in the
underground environment as a machine-mounted dust mon-
itor. They successfully redesigned the TEOM 2erosol mass
sensor to meet the most critical environmental specification
listed in Table 1, specifically, machine vibration. Under the
contract, R&P delivered to the BOM 2 prototype laboratory
version of this sensor. The BOM is evaluating the device in the
laboratory with other candidate technologies to confirm man-
ufacturer claims, especially with respect to vibration isolation.

TEOM Dust Monitor Modifications

Starting with an R&P model 1400 commercial dust monitor,
each subsystem will be subject to review and repackaging in
order to meet the more stringent requirements of underground
use. The major new requirements for mine use are to maintain
high resolution in the face of heavy shock and vibration and a
30-day maintenance cycle.

Other mine use requirements, such 2s intrinsic safety in an
environment that is potentially explosive due to the presence
of methane, ability to withstand rock falls, ability to work in
high transients of temperature and dust loadings, ability to
work in condensing humidity or water sprays, and the require-
ment to do all this in a reasonably small package are a chal-
lenge, but straightforward approaches have been identified for
each requirement. Inherent in the basic TEOM technology are
ancillary requirements such as true mass readings and real-time
data reporting. Because of the real-time reporting, the monitor
can potentially be made tamper resistant, since anomalies in the
instrument's operation can be recorded.

The repackaging effort will include work to identify and
‘quantify all the above-mentioned environmental and operating
conditions insofar as they afect the instrument's ability (0
perform accurately and reliably. As monitor development pro-
ceeds, these will be combined to provide an error budget for
the mass concentration measurement. Included in this as 2
‘major item will be the bias introduced by the instrument's dust
sample inlet system. The budget will be used to assess monitor
design tradeof, and will ultimately assure that the final design
meets the specifications of Table 1.