Laboratory Evaluation of Pressure Differential-Based

Respirable Dust Detector Tube

Jon c. Volkwein, April L. Schoeneman, and Steven J. Page

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health,

Pittsburgh, Pennsylvania

Assessment of exposure to occupational dusts is a first step

in reducing exposures to harmful dust concentrations. A new

type of respirable dust sampler was developed and compared

side-by-side to personal gravimetric samplers in the laboratory. The new sampler correlates filter back pressure with

mass accumulation to provide mid-shift and end-of-shift determinations of cumulative exposure. The sampler uses a

small low flow rate pump to draw dust through a small detector tube that contains a porous urethane foam respirable

classification section and glass fiber filter that collects respirable dust.

Six different coal dusts were aerosolized in a laboratory

dust chamber and a total of 119 triplicate observations were

obtained. For individual coal types, the correlation coefficients were between 0.87 and 0.97. The precision of the two

methods was similar, with the percent relative standard deviation of the personal samplers of 12 percent and the new

detector method of 14 percent. For all coal types tested the

data were best described by a power function where M =

1.43 massO.S5, with a correlation coefficient of 0.73. The method becomes more accurate at higher dust loadings such that

all laboratory data with mass loadings greater than an equivalent concentration of 2 mg/m3 fall within + / -25 percent

of the power function. Assessment of the method under field

conditions is in progress.

a given quantity of air,(2) or through the use of instantaneous

electronic or direct-reading dust monitors.(3) The filter method

takes several weeks to process before results are reported to the

mine. This time delay, coupled with the ever-changing and moving workplace of the underground mine environment 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. Electronic dust measurement methods

that do provide immediate feedback include photometers, beta

gauge, and piezobalances. These devices have helped to understand dust generation patterns in mines and have been very

useful research tools. Their use for routine personal monitoring,

however, is limited due to their complexity, size, and expense.

The objective of eliminating occupational dust diseases by reducing worker dust exposures can be accomplished in a number

of ways. Obviously, the establishment of permissible dust exposure limits is a first step. Adoption of these permissible levels into

law and enforcing compliance of these levels has been a mainstay of reducing occupational exposures. Good business practices have also led progressive companies to prevent worker illnesses through worker education and adoption of best -available

engineering control technologies.(4) Effective monitoring with

immediate feedback of exposure results to workers is another

method that has shown benefits at reducing exposures in other

occupational settings.(5)

In the Report of the Secretary of Labor's Advisory Committee on the Elimination of Pneumoconiosis Among Coal Mine

Workers,(6) several recommendations deal with the development

of continuous respirable dust monitors to help protect workers'

health. In addition, the National Institute for Occupational Safety

and Health (NIOSH) Criteria DocumenP) lists improved sampling devices as a research need pertinent to coal miner respiratory health and prevention of disease. Several approaches are

being taken to address these needs. These studies include, but are

not limited to, a Machine Mounted Respirable Dust Monitor,(8)

light scattering dust monitor response,(9.10) pressure drop evaluation of filter media,( I 1.12) and other novel techniques. One of

the principal goals of each of these efforts has been to identify

or develop an instrument that will give short-term or real-time

Keywords Respirable Dust, Monitoring, Pressure, Mining

Assessment of personal respirable dust exposure is an important step in eliminating many dust-related occupational illnesses

and diseases. Sampling dust levels in mining presents unique

challenges because of the variable composition of the dusts and

the constantly moving workplace that is a result of the removal

of the ore.(l) Currently, dust levels in mining are measured either

gravimetrically, using filters and the accumulated dust mass in
(Type S, FiltercrestTM from [PCF Foam Corp., Hamilton, Ohio)

with a density of 50 pores per inch (ppi). This segment filters out

oversized non-respirable particulate and protects the main classifier from plugging with oversize material. The tube narrows

to a 4.0-mm-diameter section that contains a 25-mm-Iength of

90 ppi open-cell urethane foam that collects the non-respirable

dust and passes the respirable fraction of tbe dust.

Dust penetration characteristics of the 90 ppi foam section

were compared to the DOff Oliver 10-mm-diameter nylon cyclone. This comparison was done by first measuring the penetration characteristics of the cyclone and fitting the data into

the form of the International Standards Organization (ISO) respirable dust equation(16) with minor modifications. This was

done to facilitate the comparison of the foam penetration to

that of the cyclone. The nylon cyclone respirable mass fraction

(RMF) is defined as:

measurements of worker dust exposure. Another concern of the

committee's report to the secretary was the issue of how to reduce tampering with reported dust results.

The dust detector tube was developed to provide an inexpensive, short-term measurement of the cumulative personal dust

exposure of a worker during a shift. The dust detector tube models itself after the concept of a radiation dosimeter or, more

precisely, after the sorbent detector tubes used to measure exposure to various gases. The disposable single-use tube contains a

respirable size classifier and the pressure drop filter media and

can be assembled for a few dollars per tube.

The correlation between filter back pressure and mass is not

new!13) Recent work by Dobrowski et al. using coal and rock

dust mixtures at various humidities demonstrated a linear pressure versus mass response for a specific filter medium.(]]) Concurrent work on the use of porous foam as a respirable dust

classification medium(14.]5) lent itself to the disposable detector tube idea. Combining these elements in an appropriately

designed tube can detect respirable mass through the pressure

increase across the filter. An inexpensive commercially available low-flow pump with integral pressure transducer pulls dust

through the device and onto the filter. These devices are economical and could be worn daily to determine dust exposure.

RMF = [SI(d)][l -0.5{1 -erf( -x)}]

[1](d/ r) where x = , and where

[1]}:;

r = 4.25 JLm, }:; = 1: ,

SI(d) = 0.5(1 + e-O.lld)

25. and the inhalable fraction

[2]

Data from the foam penetration study was found to closely approximate this equation when the flow rate through the tube was

250 ml/min.<17)

The flow path of the classified respirable fraction of the dust

now gradually expands in the detector tube to 6.3-mm diameter and travels 55 mm, or about eight tube diameters, to uniformly deposit onto the collection filter. The respirable dust

deposits onto an 8-mm-diameter Pallflex Fiberfilm T60A20

fiuorocarbon-coated glass fiber filter supported by a porous fiber

backup pad. The filter holder was constructed from a compression tube fitting that was bored to 9.53 mm, the same outside

diameter as the glass tube. Figure 1 shows the detector tube and

the glass tube to filter interface held in place with a flanged,

barbed nylon tube fitting compressed onto the backup pad.

DESCRIPTION OF DEVICE

The dust dosimeter is analogous to a conventional gas detector tube in that a small, low flow rate pump is used to pull

a sample into a small-diameter tube where the dust is sized

and deposited onto a filter. A uniform dust mass loading results in a proportional pressure increase across the filter. Any

pressure transducer or one integral with the pump can be used

to correlate with filter mass. After the detector tube has been

used to make a measurement, the tube can be discarded, and

a fresh tube used for the next measurement. Dust enters the

inlet of the detector tube, illustrated in Figure I, through a 6.3-

mm-diameter by 8-mm-Iength of polyurethane open-cell foam

FIGURE 1

Dust detector tube,
J.

A commercially available low flow rate air sampling pump

with integral pressure transducer was used to monitor the pressure increase with mass loading. The pump had flow capacity up

to 250 ml/min. The battery supplies eight hours of power when

run at 200 ml/min at 10 inches water gauge. The combination

of this pump with the dust detector tube comprises the new dust

dosimeter that was tested.

Freeport, Pocahontas, and Beckley A seam, were ground to minus 325 mesh size. One of the Beckley A seam coal samples

was doped with a 10 percent by mass Minu-Sil (U.S. Silica Corporation, Berkley Springs, West Virginia) ground silica. Dusts

were aerosolized using a TSI fluid bed generator and disbursed

in a I m3 aerosol chamber. The aerodynamic size of each coal

aerosol was measured with an Anderson 298 Personal Impactor

operated at a flow rate of 2 lpm for time periods between 0.75

and 2 hours to obtain optimal stage loadings. Impactor substrates were coated with Dow Coming 316 Silicone Release

Spray 24 hours prior to pre-weighing. Substrate weights were

measured using procedures similar to the filter weighing. Size

distributions were calculated and reported as the mass median

aerodynamic diameter (MMAD) and geometric standard deviation(GSD).

METHODS AND MATERIALS

A direct comparison between dust concentrations determined

by personal gravimetric samplers and the pressure increase of

the dust dosimeter was made in a laboratory dust chamber by

comparing the means of triplicate measurements of each type of

sampling device. The relative standard deviation of each triplicate grouping was also determined. These measurements were

then plotted and least squares regression analysis used to determine the correlation equations.

Personal Gravimetric Samplers

Flow-controlled personal sampling pumps operated at a flow

rate of 1,7 Ipm were used to sample coal dust aerosols from the

laboratory aerosol chamber. Dust was classified using lO-mm

nylon Dorr-Oliver cyclones and deposited onto standard coal

mine sampling cassette filters. Filters were pre- and post-weighed at the Pittsburgh Research Laboratory (PRL) under controlled atmosphere conditions. Filters were prepared without the

tamper-resistant backflow valve or the inner stainless steel support wheel. Pump flows were checked weekly with a Gilian

Bubble Flow Meter, a primary standard flow measurement device. A total of nine personal samplers were arrayed for each test

in groups of three so that each grouping was evenly spaced about

the central portion of the chamber at about the same elevation.

Test Procedure

All sampling inlets were arrayed in the central portion of the

test chamber facing toward a central point in the chamber. previous studies of the chamber showed little spatial variability (less

than 5% ) within the central portion of the chamber. Sampling

heads were connected to their respective pumps through short

sections of flexible plastic tubing that passed through a bulkhead

manifold.

The fluidized bed dust generator was loaded with the coal to

be tested and run for a minimum of one hour or until a lightscattering photometer inside the chamber indicated that an equilibrium concentration had been reached. All personal sampling

pumps and dosimeter pumps were then started. Initial back pressures from the dosimeter pumps were recorded. At 10-minute

intervals the dosimeter pump pressures and the photometer dust

concentration were recorded.

At one-hour intervals, groups of three personal sampling

pumps were switched off. The mass loadings for each grouping

of three personal samplers were averaged and the mean and standard deviation reported. Each test lasted for a total of three hours.

The pressure readings of the dosimeter pumps were recorded and

the initial pressure subtracted to determine the cumulative pressure increase caused by the dust loading for each time interval.

Each group of three dosimeter pumps was averaged for each

hour interval and the mean and standard deviation reported.

Each three-hour test yielded six results (two groups of three

dosimeters times three gravimetric sampling intervals). This test

sequence was repeated at least three times for each of the six

coal types tested for a total of 109 observations. An additional

10 observations were made with the Beckley A seam coal to

obtain heavier dust loadings by sampling for eight hours. During

ope test within each coal type, a personal impactor sample was

taken after the first hour of the test to determine the MMAD of

the aerosol in the chamber.

Dosimeters

Flow-controlled sampling pumps manufactured by SKC Inc.

(Pocket PumpTM) were operated at a flow rate of 0.250 Ipm to

draw coal dust aerosols into the dust detector tubes. Clean dust

detector tubes were prepared with the size-selective foam classifiers and new collection filters. A total of six dust dosimeters

were used for each test and divided into groups of three that were

arrayed in the test apparatus in an alternating pattern around the

central portion of the chamber and at a similar elevation to the

personal samplers.

The pump pressure transducer measures the pressure of the

entire detector tube, including the two porous foam sections.

The contribution to the total pressure from the foams was determined by measuring the pressure restriction of the combined

foam sections before and after testing during heavy dust loading conditions. A slant tube manometer was used to measure the

pressure at 0.250 Ipm. Pressure drop through the interconnecting

tubing at this low flow rate was negligible.

Analysis

Preliminary data analysis was made by comparing the cumulative dust concentrations as determined by the light-scattering photometer with the cumulative pressures recorded by the

Test Aerosols

Six different coal dust aerosols from various sources were

used in the study. Coal from the Pittsburgh, Illinois #6, Upper
FIGURE 2

Typical individual test showing the precision of three

individual dosimeter tubes response to cumulative respirable

dust loading.

dosimeter pumps. The photometer readings were normalized to

the gravimetric mass for each test at lO-rninute intervals by using

the average mass from the three-hour personal gravimetric samplers as the correct cumulative mass for that test. This analysis

compares the cumulative performance between individual detector tubes. Data was analyzed to calculate the average increase

in detector tube pressure of three dosimeters and compared to

the average personal gravimetric sampler mass at hourly intervals. The respective relative standard deviations (RSD) were also

calculated. Regression analysis used Excel calculation functions

to compute power and linear analysis of the dosimeter pressure

versus personal gravimetric sampler mass. Error bars were computed based on one standard deviation from the mean.

RESULTS AND DISCUSSION

This testing covered a range of concentration equivalents

from about 0.1 to 2 times the MSHA permissible exposure limit

(PEL) of2 mg/m3.(18) Dust mass loadings for the testing covered

a range from 0.23 to 3.42 mg. This is equivalent to an eight-hour

concentration range from 0.28 to 4.19 mg/m3. Not all coal types

covered the entire range.

For each test sequence, the cumulative pressure from the

dosimeters and the cumulative mass, determined from the gravimetrically corrected light-scattering measurements, were plotted versus time. A typical test result is shown in Figure 2 where

the three dosimeters can be seen to follow similar trends. When

cumulative pressure is plotted as a polynomial expression, the

regression coefficients are better than 0.99. The step like function i& the pressure accumulation in the figure is an artifact of the

low-precision output from the pump pressure digital transducer.

A more precise pressure transducer should help to improve the

accuracy and precision. The drift in dust feed to the chamber

can also be seen in the slight non-linear cumulative mass data.

The comparison between the personal sampling method and the

dosimeter method was determined for each coal type. The average mass, measured by personal sampling pumps for one-, two-,

and three-hour intervals was plotted against the corresponding

average dosimeter pressure increase.

The x error bars represent one standard deviation in accuracy of the personal sampler mass measurement and the y error

bars represent the accuracy of the dust dosimeter pressure measurement. These errors demonstrate the variation in precision of

both the reference personal sampling method and the new dust

dosimeter. The average RSD of the data presented in Figure 3

is 9.71 percent for the personal samplers and 11.11 percent for

the dosimeters. The best fit of the data follows a power function

6.00

5.00

-

01

=

e

e

-

~

...

=

~

~

~

...

c.

1.00

0.00

0 1 2 3 4

mass (mg)

FIGURE 3

Pressure to mass correlation for Beckly A seam coal.

4.00

3.00

2.00
TABLE I

Summary of laboratory correlation data and aerodynamic size distributions

Size distribution

Power

function

y=

Personal

sampler

RSD

Mass median

aerodynamic

diameter (JLm)

-

5.59

5.19

4.99

3.62

5.36

4.92

Geometric

standard

deviation

Dosimeter

Coal type R2 RSD

1.50xo.82

1.74xo.69

0.87xo.87

2. 10xo.60

1.10xo.82

1.72xo.74

1.43xo.85

2. 16xo.85

0.72xo.85

1.80xo.85

1.08xo.85

0.90

0.87

0.90

0.94

0.91

0.97

0.73

9.71

10.89

16.37

7.74

13.78

12.77

11.83

2.33

2.38

2.31

2.20

2.15

2.24

11,11

10,26

17,73

9.04

24.79

10.81

13.96

Beckley A seam

Beckley A seam + silica

Pittsburgh

Illinois #6

Upper freeport

Pocohontas #4

All data

(+ 50%)

(- 50%)

(+ 25%)

( -25%)

mass fraction defined by the cyclone with the respirable mass

fraction defined by the porous foam. Thus, similar size fractions were being compared. The MMADdata of the various test

aerosols shown in Table I range from 3.6 to 5.6 micrometers.

This range of size variability did not appear to correlate with

the different functional response between pressure and mass.

From an applications viewpoint, the dust dosimeter comparison

TABLE II

Pressure increase of combined 50 and 90 ppi foam:

attributable to dust loading during heavy loading

with Beckley A coal

Pressure

Tube

no.

Pre loading

mmHg

Post loading

mmHg

Pressure difference

mmHg

0.41

0.45

0.41

0.45

0.41

0.41

I

2

3

4

5

6

Average

7

8

9

10

II

12

Average

0.60

0.60

0.56

0.60

0.48

0.56

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.41

0.41

0.37

0.37

0.41

0.45

0.63

0.56

0.56

0.56

0.60

0.60

where y, the differential pressure, = 1.50xo.82 where x is the

mass. Accuracy of the method may be interpreted through the

+ / -25% functions also plotted on the graph.

Data in Table I summarize the results for individual coal types

and for all 119 tests. In general, the RSD for personal samplers

is lower than the corresponding RSD for the dosimeter. This

difference may be reduced by using a more sensitive pressure

transducer in the dosimeter pump. Note that for individual coal

types a regression coefficient of 0.87 or greater is achieved.

However, as can be seen from the power function equations,

there remains a range of correlations between pressure and mass

from pressure = 0.87 x massO.87 to pressure = 2.10 x massO.60.

This range in response to individual coal types results in a lower

regression coefficient for all data.

The effect of the porous foam precollector in series with the

pressure measuring filter is minimal. Table II shows the clean

and loaded pressures of the combined 50 and 90 ppi foam for

the heavily loaded, eight-hour, Beckley A coal tests. Increases

in pressure attributable to the foams of 0.14 and 0.18 mm Hg

compare to total pressure increases of 4.14 and 4.30 mm Hg increases in the total detector tube. This is less than 4 percent of the

total pressure increase for dust loadings, nearly twice the PEL.

The functional relationship between pressure and mass may

be dependent on the size of dust being sampled. Dobrowski et al.

reported that the different pressure response of various coals was

attributable to different sizes of the collected dust, reasoning that

smaller-sized dust mechanically restricts air flow at a greater rate

than an equivalent mass of larger-sized dust.

Their technique used variable flow rates through a cyclone

to alter the size distribution of the collected dust. Thus the dust

deposited on the filter was not equivalent to a defined respirable

fraction. This study, however, directly compared the respirable

19

15

15

15

07

15

14

22

15

19

19

19

15

18
+ 5<1'/0

y = 1.44xo.85

~=0.73

~

~.

-5(J'/o

';r, 7.00

=

e 6.00

-! 500

...

...

~ 4.00- "'

..

a. 3.00

...

~ 200

~ 1.00 t

0.00 , I I I I I I I

0 0.5 1 1.5 2 25 3 3.5

sampling showed good correlation for individual coal types and

good correlation at higher mass loadings for all coal types. The

advantages of this new approach to dust sampling include the

immediate availability of the cumulative shift dust exposure, a

significant reduction in size of the instrumentation that a person must carry to evaluate their respirable dust exposure, and

lower cost per sample. The dust detector tube requires field verification to assure that issues of variable particle size, shape,

concentration, humidity, or other environmental factors do not

substantially alter the pressure versus mass response.

Protection of workers' respiratory health depends on many

factors. Dust assessment ools for .engineering control development and compliance determination are available. Another potentially powerful tool to help improve workers ,

health may be the empowerment of the worker and management

with the timely knowledge of what current dust exposures are

routinely occurring. The inexpensive dust detector tube may provide the knowledge that helps workers protect their respiratory

health.

~ saDJie: ~ (q)

FIGURE 4

Correlation of all laboratory data of pressure with mass

DISCLAIMER

References to commercial products are for informational purposes and do not imply endorsement by the Centers for Disease

Control and Prevention.

to personal samplers must work underground where MMADs

are expected to be on the order of 7 to 20 micrometers in size,

depending on the mining method.(19) Additional field comparisons between personal samplers and dosimeters is underway

to answer the question of comparison with typical underground

size distributions. For this laboratory work there was no apparent dependence on the functional relationship between pressure

and mass attributable to different size distributions.

Despite the variability of individual coal type response,

Figure 4 shows that the laboratory response of the dust dosimeter approach provides a qualitative tool to evaluate exposure to a

number of respirable coal dusts. As dust concentrations increase,

the accuracy of the method improves such that measurements

above 1.7 mg (2 mg/m3) fall within +1- 25% of the laboratory

functional curve. Additional research on the variability of coal

type of pressure response may help to improve the accuracy of

the technique.

The question of accuracy versus cost is pertinent to an overall evaluation of any new respirable dust assessment echnique.

The low-cost approach of the dosimeter lends itself to an increased number and frequency of samples that can be taken.

Furthermore, the cumulative shift personal dust exposure will

be immediately available to workers. This can enable quick corrections to procedures or dust controls to immediately reduce

dust exposure. Direct availability of the data to the workers may

also help to reduce tempering with exposure data. The reduced

size, weight, and noise level of the new pumps may also encourage better worker acceptance of the new technique. Even

though more accurate methods may be possible, and indeed

beneficial for certain applications, that level of accuracy may

not be required for routine monitoring of many workplace environments. Improved accuracy may be of less importance when

all other benefits are considered.

CONCLUSION

A new respirable dust sampling device has been developed

based on the principle of the correlation of pressure restriction

of a filter with increasing mass loading. The laboratory comparison of this technique with conventional personal gravimetric

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