‘Mine 1 (Pgh)
ya tsne+000
Me 4a) Reto

i yatate-015

£40 Ros7

i y= ox 005

3 30 Rat.

H

g 20

§

qe

Ba r=100

a9 10 20 20 4 80

Figure 4. Individual mine response. Regression equations are
for units 12 and 14 respectively.

9.4 Data analysis and statistics

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. Data were converted to MRE equivalent
mass using the factor of 1.38. In the case of the
PDM-2, this conversion takes place in the K factor
constant, which converted oscillation frequency into
mass. In the case of the personal gravimetric sam-
plers, this conversion was made following the mass
calculation.

The average gravimetric filter mass measure-
ment and each PDM-2 mass measurement consti-
tuted one paired measurement. The ratio of the
paired measurements was computed, and the accu-
racy criteria method of Kennedy et al. (1985) was
used with the addition of a method to account for
the inaccuracy of the reference method. For these
tests, the concentration ratio for each data pair was
calculated by dividing the individual PDM mass by
the average value for the personal sampler mass.
The individual concentration ratios were then aver-
aged over appropriate data sets (i.e., laboratory,
mine, overall, etc.) and the relative standard devia-
tion was calculated.

10 DISCUSSION

Fundamentally, the objective of this study is to de-
termine how well the PDM agrees with the personal
sampler. We use the accuracy criteria method of
Kennedy et al. to describe such comparisons, and the
accuracy criteria statistical calculations are supple-
mented by a correction for the experimentally de-
termined relative standard deviation of the personal
gravimetric samplers. This correction results in the
relative standard deviation of the ratios being re-
duced by between 0.04 and 0.09. Accuracy can then
be calculated from the corrected relative standard
deviation and bias of the ratios. This results in calcu-

lations of the upper (95%) and lower (5%) confi-
dence limits. According to Kennedy et al., an in-
strument meets the accuracy criteria if the 95% up-
per confidence limit is 0.25 or less and if the
absolute bias is no greater than 0.10. However, if the
method’s lower confidence limit exceeds 0.25, the
method does not meet the accuracy criteria. If nei-
ther of these conditions exist, the results are incon-
clusive and additional research will be required to
accept or reject the method.

One of the difficulties of filter dust sampling and
testing the equivalency of devices is the loss of
measurement precision at low filter mass loadings.
Kogut et al. (1997), for example, demonstrate that
measurement precision for 16 measurements inside
of a Lippmann-type chamber decreased significantly
at mass loadings of less than about 0.5 mg. As mass
levels decrease, the imprecision of the reference
mass measurement increases. At low mass loadings,
it is not possible to determine if the error in the pair-
wise difference is attributable to the gravimetric
sampler, the PDM-2, or a combination of the two in-
struments.

10.1 Accuracy analysis

Table 2 summarizes the accuracy criteria calcula-
tions for every pair-wise comparison measured as
well as logical subsets of the data. For all data col-
lected, the accuracy analysis of the data is inconclu-
sive at the 95% confidence interval because the up-
per confidence interval exceeds 0.25. We can neither
accept nor reject that the PDM-2 meets a 25% accu-
racy criteria. However, when the imprecision of the
reference gravimetric samplers is taken into consid-
eration, by examining only the data greater than 0.5
mg/m’, the analysis indicates that the PDM-2 meets
the 25% accuracy criteria because the upper confi-
dence interval is less than 0.25.

For the subset of mine data greater than 0.5
mg/m’, the accuracy analysis is inconclusive be-
cause the lower confidence interval is less than 0.25.
It must be noted, however, that mine data results, are
strongly influenced by the two mines in the Pitts-
burgh seam, where the bias' between the PDM and
gravimetric samplers varied from 0.15 to 0.27.
Three of the four individual instrument results from
the other two mines meet the accuracy criteria, al-
though the sample size for confidence interval test-
ing is quite small (n<8).

10.2 Temperature effects on accuracy

During the laboratory testing the data indicated a
dependency between mass and temperature. The
data showed that for every degree centigrade drop
in temperature, the PDM-2 apparent mass meas-
urement increased by about 5.5ug. The converse

"Bias equals the mean concentration ratio minus 1.


and for the overall data. This analysis used the Excel
regression format.

9 RESULTS

Presented here are results of the PDM to personal
sampler mass comparison from the Marple chamber
for the four coal types, longwall gallery, under-
ground mine data, and overall summary data. Also
included are the results for the temperature, tilt, zero,
and shock testing.

9.1. Marple chamber

A summary of the results for all Marple chamber
testing is in Figure 2. The figure displays the calcu-
lations for the linear regression and correlation coef-
ficient for each coal type. Also displayed on the fig-
ure is the ideal 1:1 comparison line. Table 1 contains
the mass median aerodynamic particle size and
geometric standard deviation for each of the coals
used in the Marple chamber testing.

9.2 Longwall gallery testing

Figure 3 contains the linear regression and correla-
tion coefficient for the longwall gallery tests. Note

+) tone rename Be

ee

rectors Ryatan-aoe @ yotaueceos

POM.2 MRE equivalent mass (ma)

Personal samplar MRE equivalent mass (mg)

Figure 2. PDM-2 response to individual coal types.

Table 1. Size distribution of Marple chamber aerosols.

MMAD GsD
um
Keystone 3.91 3.05
Pittsburgh 531 2.20
Illinois #6 154 3.08
Pocahontas 3.93 2.63

that both PDM-2 units gave essentially parallel lines
to the 1:1 response, with a positive offset of about
0.10 mg. Data for November 28 were not obtained
for unit 14 due to a mechanical failure of a micro-
switch that turns the tapered element off when the
PDM is opened in order to change the filter. This
broken switch prevented the tapered element from
oscillating. Subsequent tests were able to be con-
ducted by wedging the broken switch into the on po-
sition. This also required the filter to be changed
with the tapered element oscillating, but this has no
effect on test results.
4o
| unit2 ¢ — y=09996x+0.1008
al R= 0.9081 Jt

as] Unite m y=1.0252x+0.0736

R= 09017

Creer en)
Personal sample MRE equvalnt mass (rg)

Figure 3. Longwall gallery regressions.

The starting mass loadings of the PDM-2 for this
testing were occasionally negative. Reasons for this
were unknown at the time, but subsequent analysis
showed that this was related to ambient temperature.
Other parameters in the data file suggest that the os-
cillation frequency of the tapered element was valid
and therefore, the negative value was considered a
valid zero point and the mass added (rather than sub-
tracted) to the ending mass for the test. The maxi-
mum mass added in this manner was 0.0587 mg.

Use of the Lippmann chamber in this testing re-
duced spatial variability. The average relative stan-
dard deviation for the personal gravimetric sampler
in the longwall gallery data was 6.1%. This value is
similar to that obtained in the Marple chamber tests,
where the average RSD was 5.67.

9.3. Underground testing

Figure 4 shows the linear regressions and correlation
coefficients for all of the individual mine and in-
strument results. Note that the individual results
with larger deviation from the 1:1 correspondence
also have a reduced correlation coefficient. Mines 1
and 4 were in the Pittsburgh coal seam near the
Pennsylvania and West Virginia border. Mine 2 was
in the Eagle seam of central West Virginia. Mine 3
was a diesel powered face haulage equipped mine.
The use of the Lippmann chamber resulted in an av-
erage RSD for the mine testing of 4.32%.

636


Table 2. Accuracy criteria data,

Number
Data Set Unit No. a
All Data 12 97
All Data 14 96
Data >0.5 mg 12 1
Data >0.5 mg 4 1
Laboratory and Gallery 12 65
Laboratory and Gallery 14 65
Laboratory and gallery > .5 mg 12 47
Laboratory and gallery > .5 mg 4 47
All Mine Data 12 32
All Mine Data 14 31
Mine data >0.5 mg 12 2
Mine data >0.5 mg. “4 26

was also true, that temperature increase resulted in
mass decrease. This was initially attributed to
changes in specific humidity in the air. Subse-
quently, R&P identified that an electronic circuit
was temperature-sensitive and this caused the mass
measurement to change in response to temperature.
When the data from the gallery and mine testing are
examined in view of the temperature sensitivity, the
agreement between the PDM and personal samplers
improves.

The observed mass dependence on temperature
also had the effect of introducing bias to the mine
results from both Pittsburgh seam mines. The other
mine tests were conducted in warmer areas of the
country where the intake temperatures were not that
extreme. In the Pittsburgh seam mines, however,
sampling was conducted during winter months and
thus the mine temperatures, especially intake haul-
age, were quite cold. The sampling protocol required
portal-to-portal sampling and the test start was al-
ways prior to entering the elevator after emerging
from a warm indoor area. At the end of the test, the
final reading was taken while the instruments were
still cold from the mine air. The result of this proto-
col was that the final mass measurement was taken
from a cold instrument relative to the initial mass
measurement. This accounted for about a one third
of observed bias in the Pittsburgh seam mines.

11 CONCLUSIONS & RECOMENDATIONS

The analysis of all data showed that when the proto-
type PDM-2 was compared to the currently used

Confidence Intervals
Accuracy Lower Upper

bias sd 5% 95%
0.089 0.091 0.25 0.23 0.28
0.099 0,083 0.25 0.23 0.28
0.078 0.076 0.21 0.19 0.24
0.088 0.066 021 0.19 0.23
0.076 = 0.074 0.21 0.18 0.24
0.087 0.070 0.21 0.19 0.24
0.067 0.059 0.17 0.15 0.20
0.078 0.063 0.19 0.17 0.22
O17 0.118 0.33 0.28 041
0.124 0.102 031 0.27 0.38
0.100 0.094 0.27 0.23 0.34
0.108 == 0.077 0.25 021 0.30

personal samplers the results are inconclusive to
conclude that the device meets or does not meet a
25% accuracy criterion. There are two primary rea-
sons why the results are inconclusive: First is the
imprecision of the reference samplers at mass load-
ings less than 0.5 mg/m’; second is the high bias
found in the subset of mine data.

However, evaluating subsets of the data shows
that if more precise reference samplers were avail-
able, then it is likely that the analysis would con-
clude that the PDM-2 meets the accuracy criteria.
Data also show that the individual PDM-2 units did
not meet the accuracy criteria in the mine tests be-
cause of high bias. This high bias was especially as-
sociated with the Pittsburgh coal type. A portion of
this bias could be accounted for because of the tem-
perature problems seen throughout these tests. A
smaller, but similar bias was also noted for the labo-
ratory results with the Pittsburgh coal where there
was no temperature effect. Additional testing should
clarify this result.

Analysis shows that the temperature sensitivity of
the PDM-2 electronics did contribute to the observed
bias from the Pittsburgh seam mine tests. However,
we cannot be certain if the observed bias in the mine
can be entirely explained by temperature since our
laboratory data on temperature effects on mass is
limited. Presumably the instrument manufacturer
can eliminate the temperature sensitivity of the elec-
tronics, and additional testing must be conducted to
determine if other sources of bias may exist.

The high regression correlation coefficients and
low relative standard deviation of the data support
the usefulness of the instrument in predicting respir-

638


Mine Ventilation, De Souza (ed.)
© 2002 Swets & Zeitlinger, Lisse, ISBN 90 5809 387 5

Performance of a prototype personal dust monitor for coal mine use

J.C. Volkwein, D.P. Tuchman & R.P. Vinson
Centers for Disease Control and Prevention, NIOSH, Pittsburgh Research Laboratory, Pittsburgh, PA

ABSTRACT: The personal dust monitor (PDM) is a sampling device developed for measuring the personal
exposure to coal mine dust of mine workers. The device is based on proprietary technology known as the ta-
pered element oscillating microbalance (TEOM) originally developed as a fixed- site environmental particu-
late mass monitor by Rupprecht and Patashnick Co., Inc., Albany, NY. Currently, the monitoring of exposure
to coal mine dust relies on periodic samples taken by traditional coal mine dust sampling units that use a cy-
clone, filter, pump, and laboratory measurement of the filter mass. Better measurement of mine dust levels
has been the goal of industry and labor for nearly two decades.

PDM technology offers accurate, end-of-shift and near real time assessment of worker dust exposure.
Laboratory and underground tests compared measurements taken by a prototype PDM-2 (a two-piece TEOM)
to the average of multiple personal dust samplers monitoring the same space. In the range of mass loadings
between 0.5 and 4.0 mg/m” the PDM met a 25% accuracy criterion 95% of the time with 95% confidence.
However, some questions still exist about potential bias between types of coal. The prototype two-piece de-
vice is in the process of being combined into a single unit that also contains the miners’ cap lamp creating a

more ergonomic dust sampling system.

1 INTRODUCTION

Sampling dust levels in mining presents specialized
challenges because of the variable composition of
the dusts and because mining involves constantly
moving workplace (Hearl and Hewett, 1993). Moni-
toring of personal respirable dust exposure is an im-
portant step in eliminating many dust-related occu-
pational illness and diseases. Currently, dust levels
in mining are measured either gravimetrically, using
filters and the accumulated dust mass in a given vol-
ume of air (Raymond et al. 1987), or through the use
of instantaneous electronic dust monitors (Williams,
and Timko 1984). The filter method takes several
weeks to process before results are reported to the
mine. This time delay, coupled with the constant
change and movement inherent to the mining proc-
ess, makes the filter measurement useful only as an
historical data point. The results do not provide
timely feedback to detect or correct excessively
dusty conditions.

In a joint effort with the Mine Safety and Health
Administration (MSHA), the NIOSH Pittsburgh Re-
search Laboratory (PRL) has been working on an
approved sampler for coal mining. Through a con-
tract with Rupprecht & Patashnick Co., Inc. (R&P),
CDC contract 200-98-8004, a sampler was devel-

oped based on the highly successful tapered element
oscillating microbalance technology that is used
worldwide for a number of commercial environ-
mental sampling instruments. This study reports on
results of laboratory and mine testing of the proto-
type Phase II, PDM-2. This instrument represents
the first testing of a miniaturized person-wearable
TEOM respirable dust sampler. Subsequent im-
proved versions of this device are anticipated. This
work compared the dust measurement accuracy and
precision of the PDM-2 with that of the coal mine
personal dust sampling unit, hereafter referred to as
the personal sampler.

2 DESCRIPTION OF PDM-2 INSTRUMENT

The PDM-2 shown in Figure 1 is constructed in two
parts. The lapel unit contains the inlet to a Dorr-
Oliver 10-mm nylon cyclone, a U-shaped air heater
section; the filter connected to a momentum com-
pensated tapered element microbalance, and the
computer. The belt unit contains a flow controlled
pump, batteries, and data display. The cyclone has a
custom fabricated shield to protect the inlet from di-
rect water spray action and to reduce the sensitivity
of the inlet to wind direction (Cecala et al. 1983).


able coal mine dust levels. If particular coal-type
bias is still found with the improved electronics, this
may be corrected with a specific coal-type calibra-
tion constant incorporated into the instrument’s elec-
tronics or software.

Overall, these prototype PDM-2 test results dem-
onstrate the successful miniaturization of the com-
mercial TEOM technology into a person wearable
respirable dust monitor. The absolute accuracy de-
termination of the PDM-2 was hampered by inaccu-
racy of the reference methods and temperature de-
pendence of the electronics. The results presented
here, however, warrant further refinements of the
technology to produce a coal mine dust monitor that
provides accurate end of shift and short-term dust
measurements. Furthermore, this new technology
presents an opportunity to combine the dust monitor
into the miner’s cap lamp in a way that provides an
accurate, easy to wear dust sampler.

639

REFERENCES

Bartley, D.L. 2001. Definition and assessment of sampling and
analytical accuracy. Annals of Occupational Hygiene, 45;
357-364,

Blachman, M.W. & Lippmann M. 1974, Performance charac-
teristics of the multicyclone aerosol sampler. American In-
dustrial Hygiene Journal, June: 311-326.

Cecala, A.B., Volkwein J.C., Timko RJ. & Williams K.L.
1983. Velocity and orientation effects on the 10-mm dorr-
oliver cyclone. BuMines RI 8764,

Hearl, F.J. & Hewett P. 1993. Problems with monitoring dust
levels within mines. Occupational Medicine: State of the
Art Reviews, Vol 8, No. 1, Jan-Mar: 93-108,

Kennedy ER., Fischbach TJ., Ringuang S., Eller PM, &
Shulman, $.A. 1995. Guidelines for air sampling and ana-
lytical method development and evaluation DHHS
(NIOSH) Publication No. 95-117, May.

Kogut, J., Tomb T-F., Parobeck P.S., Gero A. J. & Suppers K.
L. 1997. Measurement precision with the coal mine dust
personal sampler. Appl. Occup Envron, Hyg, 12(12): 999-
1006.

Raymond, L., Tomb T.F. & Parobeck P. 8. 1987. Respirable
coal mine dust sample processing. MSHA IR 1156.

Williams, K. & Timko R.J. 1984. Performance evaluation of a
real-time aerosol monitor. BuMines IC 8968.



Figure 1

PDM-2 belt and lapel unit.

The weight of the total unit is 1.9 kg (4.3 Ib). The
lapel unit weighs about 0.45 kg (1 Ib). The lapel unit
measures 20 cm (8 in) long, 6.3 cm (2.5 in) wide,
and 3,8 cm (1.5 in) thick. The belt unit measures
17.8 cm (7 in) tall, 15.2 (6 in) wide, and 5 cm (2 in)
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 per-
sonal 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 pro-
vided by R&P to upload and download data from the
lapel unit. The PDM-2 memory has the capacity to
store 128 parameters at one-minute intervals, for an
entire 12-hour shift.

3 TEST AEROSOLS

Four types of coal aerosols were used in the labora-
tory Marple chamber. These were Pittsburgh Seam
A, Pocahontas, Illinois #6 obtained from the Penn
State Coal Collection, and a commercially available
ground coal called Keystone. Only Keystone was
used in the longwall gallery testing. Underground
tests were conducted in the Pittsburgh (PA), Eagle
(WV), and Blue Creek (AL) coal seams. Characteri-
zations of the airborne size distributions were meas-
ured with Marple personal cascade impactors for all
coal types tested in the Marple chamber.

4 MASS LOADING PROTOCOL,

Two identical PDM-2 prototypes, designated units
12 and 14, were available for this study and were
always run simultaneously. The mass determined by
each PDM-2 dust monitor was compared with the
average mass of four standard personal samplers.
The personal sampler filters were preweighed at the
PRL controlled atmosphere-weighing facility ac-

634

cording to established procedures. Filter blanks used
the average of three filter blanks for the Marple
chamber and underground data and the average of
two filter blanks for the longwall gallery data. All
filters were returned to the PRL weighing facility for
post-mass determination using identical procedures
to the preweighing.

Flow-controlled MSA Elf Escort pumps were
calibrated at the beginning of a each coal type test in
the Marple chamber, before the longwall gallery
testing, and before each underground mine test. A
Gilibrator, primary standard, flow meter was used to
establish a flow rate of 2.0 £1% Ipm using an
equivalent pressure restriction of the cyclone and fil-
ter assembly. Flow rates of the PDM-2 were checked
and recalibrated (if required) each day using the
Gilibrator attached to the bottom of a sealed inlet
cyclone that was substituted for the PDM-2 cyclone.

During the programmed warm-up, an airflow
check was usually conducted. Units were then placed
into the Marple chamber, or placed into a Lippmann
chamber (Blachman and Lippmann, 1974) that was
used to reduce spatial variability, and then trans-
ported by car to the longwall test gallery; alterna-
tively, for underground sampling, the instruments
were placed into the Lippmann chamber at the mine
prior to being carried underground. At the conclu-
sion of each test, final cumulative concentration data
from the belt screen display were recorded, airflow
rates with loaded filters were then checked, and the
units’ memories were downloaded to a PC. These
data were then translated into an ASCII text file that
was read with a spreadsheet program. Units were
then cleaned, new filters installed, and prepared for
the next test.

The data files were then coordinated with the run
times of the gravimetric data. Because the start time
of the gravimetric samplers did not always match the
automatic start times of the PDM-2 units, the mass
of the PDM-2 at the gravimetric start time was sub-
tracted from the PDM-2 end time Gravimetric
samplers were always started after the 30-minute
warm-up cycle. Gravimetric start and stop times
were +] minute and PDM-2 times were to the near-
est previous minute (i.e., 2 min. 59 sec. was re-
corded as 2 min.)

5 MARPLE CHAMBER

Chamber tests were conducted under well-controlled
conditions and assessed the best performance that
can be expected from the PDM-2. Chamber tem-
perature varied between 23° and 25°C. Relative hu-
midity varied between 42 and 61%.

Personal sampling cyclone and filter holders were
arrayed in a | m (39-in.) diameter circle around a
central point in the chamber about 0.5 m (19 in)
above the table, and the two PDM-2 units were


placed 180 degrees apart within the array. The array
slowly rotated 356 degrees and then reversed on a
continuous basis so that each sampler inlet was ex-
posed to an identical location in the chamber. A total
of 19 gravimetric filters were used for each test.
There were four sets of four filters for testing and
three control filters. The control filters were handled
in a fashion identical to the experimental filters with
the exception that the end caps were not be removed.
Calibrated Elf Escort pumps (Mine Safety Appliance
Co.) were used to power the personal sampling cy-
clone filter units. The PDM-2 and Elf units were
placed in the Marple chamber and operated under
battery power as if being used in a mine.

For each coal type three tests were conducted to
achieve a range of dust concentrations from about
0.2 to 4 mg of MRE equivalent mass. All gravimet-
ric filters were started at the beginning of the test,
but sets of four filters were tured off at intervals to
encompass the desired range of mass loadings. The
PDM electronic file was then used to determine the
mass at the corresponding gravimetric sampling in-
tervals. In this way, each test resulted in four PDM
to gravimetric data pairs. Mass loadings ranged
from 0.2 mg/m’ to 4 mg/m’,

6 GALLERY

The full-scale longwall gallery model at PRL én-
abled the assessment of the PDM-2 under less con-
trolled conditions and in the presence of water spray
mist. Performance of the device in motion was also
evaluated. The full-scale model testing also enabled
the performance and logistics of the Lippmann
chamber to be assessed prior to underground testing.

The gallery was operated using Keystone coal
dust, commercially available in large quantities re-
quired for gallery testing, with constant water and
ventilation flow rates. Gallery water flow rate was
65 gpm @ 80 psi with a ventilation rate between 280
and 300 fpm. A powered continuous loop of chain
conveyor was used to simulate a person’s movement
within the tailgate area of the gallery.

A total of 14 tests were conducted in the gallery.
The Lippmann chamber contained four personal dust
samplers and two PDM-2 units. Two control filters
were used for each test. As with the laboratory test-
ing, the target range of masses for equivalent con-
centrations ranged from 0.2 to 4 mg. The same setup
and download protocol were followed for these tests
with the exception of battery charging.

7 UNDERGROUND TESTING
Four evaluations were conducted in underground

US. coal mines. Two coal mines were in the Pitts-
burgh seam, (which included coal similar to that

635

used in the laboratory test), one in a longwall sec-
tion, and one in a continuous mining section. One
coal mine was in the Eagle seam in central WV and
the fourth used diesel-powered face haulage and was
in the Blue Creek seam in AL. Full-shift under-
ground testing was conducted. Two PDM units, four
personal gravimetric sampling filters, and two con-
trol filters were placed inside the Lippmann cham-
ber. All units were tumed on as they entered the por-
tal, then were carried to the face for an 8-hour shift,
and then were returned to the portal where the units
were tumed off. One test at each mine was con-
ducted for 10 hours.

For all sampling, a NIOSH team was responsible
for transporting the instruments at all times and
shadowing a designated high-risk occupation for the
entire shift. The readings from the PDM were used
to estimate mass loadings such that a range of con-
centrations from 0.2 mg/m’ to 4 mg/m’ was achie-
ved. To achieve higher mass loadings, it was occa-
sionally necessary to move to a higher dust area.

8 DATA ANALYSIS AND STATISTICS

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. Data were converted to MRE equivalent
mass using the factor of 1.38. In the case of the
PDM-2, this conversion takes place in the K factor
constant, which converted oscillation frequency into
mass. In the case of the personal gravimetric sam-
plers, this conversion was made following the mass
calculation.

The average gravimetric filter mass measurement
and each PDM-2 mass measurement constituted one
paired measurement. The ratio of the paired meas-
urements was computed, and the accuracy criteria
method of Kennedy et al. (1995) was used with the
addition of a method to account for the inaccuracy of
the reference method. For these tests, the concentra-
tion ratio for each data pair was calculated by divid-
ing the individual PDM mass by the average value
for the personal sampler mass. The individual con-
centration ratios were then averaged over appropri-
ate data sets (i.e., laboratory, mine, overall, etc.) and
the relative standard deviation was calculated.

To reduce the impact of error in the personal gra-
vimetric measurement, the experimental pooled es-
timate of the relative standard deviation of the gra-
vimetric samplers was subtracted from the relative
standard deviation of the ratios. Bias was then calcu-
lated based on the mean concentration minus one.
Accuracy was then estimated from the chart pro-
vided by Kennedy et al. Confidence limits were then
calculated based on the method used by Bartley
(2001) using a non-central Student-t test. A linear
regression analysis was also done for each data set