SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 21

Performance of a light-scattering dust 

monitor in underground mines

G.J. Chekan, J.F. Colinet, F.N. Kissell, J.P. Rider, R.P. Vinson and J.C. Volkwein

NIOSH, Pittsburgh, Pennsylvania

Abstract

The Thermo-MIE personal DataRAM dust monitor (pDR) is a light-scattering instrument approved 

for use in gassy underground mines by the U.S. Mine Safety and Health Administration (MSHA). 

However, the use of light-scattering monitors has been inhibited by concerns about inaccurate readings 

resulting from changes in dust size distribution and composition, as well as concerns about the impact 

of moisture in the mine air. Because of this concern, the National Institute for Occupational Safety and 

Health (NIOSH) evaluated the pDR using available data from laboratory tests and mine surveys. The 

data show that accuracy can vary widely from test to test and mine to mine. In most cases, the results can 

be improved by using a companion gravimetric measurement to adjust the pDR data. 

Introduction

For the measurement of respirable dust, instantaneous dust 

monitors based on light-scattering principles have an advantage 

when compared to gravimetric filter samples — they offer immediate feedback on dust levels. The Thermo-MIE personal 

DataRAM dust monitor (pDR) can be used in underground 

gassy mines because it has been tested and approved by the 

Mine Safety and Health Administration (MSHA) for intrinsic 

safety in methane-air mixtures (Approval No. 2G-4126-0). 

It uses light-scattering principles to detect dust concentrations. Light-scattering instruments offer a relative measure of 

concentrations, but provide a continuous record of particulate 

levels so that concentrations can be evaluated over any time 

interval during the sampling period. 

In some mines, the pDR is regarded as a useful tool for 

making rough relative measurements, such as those made to 

identify dust sources and select locations for subsequent gravimetric sampling. However, any expanded use of light-scattering monitors has been inhibited by concerns about inaccurate 

readings resulting from changes in dust size distribution and 

composition, as well as concerns about the impact of water 

sprays (Williams and Timko, 1984; Smith et al., 1987; Tsai 

et al., 1996). This concern has been reinforced by coal mine 

tests (Page and Jankowski, 1984) showing that light-scattering 

measurements could vary from gravimetric measurements by 

a factor of two.

Despite these findings, there is a continuing interest in using 

light-scattering instruments for broader applications in mining, 

possibly by identifying specific circumstances under which the 

pDR can be expected to be more accurate.

The work reported here details some side-by-side comparisons between the pDR and conventional gravimetric samplers 

first conducted in a laboratory test chamber, then in three different types of underground mines. The main purpose of the mine 

studies was to assess control technology; however these dust 

studies also offered the opportunity to make pDR-gravimetric 

comparisons under a variety of realistic mining conditions. 

Two specific issues were addressed. First, what was the 

instruments precision? Precision is defined as the instrument’s 

degree of agreement in a series of tests or, in other words, its 

repeatability. Information was collected to determine how much 

the precision varied from laboratory to mine and from mine 

to mine. Second, what was the instruments bias? Instrument 

bias is defined as a systematic distortion in a measurement. 

This would be the accuracy of the instrument when compared 

to the true value — in this case the Dorr-Oliver gravimetric 

sampler. Again, information was collected to determine how 

much the precision varied from laboratory to mine and from 

mine to mine.

Methods used

In this study, all of the tests were conducted with the pDR in 

the “active” sampling mode, as shown in Fig. 1. In this sampling mode, particulate was classified through a Dorr-Oliver 

cyclone before entering the pDR light-scattering chamber. 

In addition to dust measured by the pDR, reference samples 

were collected by conventional gravimetric samplers located 

within a few inches of the pDR. Each of these conventional 

gravimetric samplers consisted of a Dorr-Oliver cyclone, a 

37-mm filter and a pump. The pDR pump and the gravimetric 

Preprint number 05-069, presented at the SME Annual Meeting, Feb. 28-March 2, 2005, Salt Lake City, Utah. Revised 

manuscript accepted for publication February 2006. Discussion of this peer-reviewed and approved paper is invited and 

must be submitted to SME Publications Dept. prior to Sept. 30, 2007. Copyright 2006, Society for Mining, Metallurgy, and 

Exploration, Inc.
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 22

pumps were operated at 2 L/min in coal mines and 1.7 L/min 

in metal/nonmetal mines.

To calculate precision and bias for a typical mine test, a 

pDR and two gravimetric samplers were placed in a designated 

location and operated continuously for approximately six 

hours. When the concentration results were obtained, a ratio for 

that location was calculated. This ratio was the pDR average 

concentration divided by the average of the two gravimetric 

concentrations. All of the ratio values obtained at that mine 

were averaged, and the standard deviation calculated. The 

precision was obtained by dividing the standard deviation 

by the mean concentration ratio. A value of less than 0.25 is 

considered acceptable.

Bias is the relative discrepancy between the average instrument reading and the true value, with the true value defined as 

the average of the reference gravimetric samplers. So, the bias 

was calculated by subtracting 1.0 from the mean concentration 

ratio. A value of ±0.30 is considered acceptable.

Laboratory tests

Prior to the mine tests, a brief series of laboratory tests was 

conducted to assess the pDR under more ideal conditions. The 

tests were designed to compare the responses of two pDR lightscattering samplers to the average of three separate gravimetric 

samplers at four different dust concentrations at four different 

exposure times. These tests used Arizona road dust, which is 

used as a standard for calibrating dust-monitoring instruments 

due to its consistency in mineral content and particle size. The 

aerosol test chamber used to test the instruments is known as 

a Marple Chamber and is 1.2 m (4 ft) in diameter and 2.4 m 

(8 ft) high (Marple and Rubow, 1983). Dust and dilution air 

are introduced at the top of the chamber. The air then flows 

downward through a honeycomb structure to provide a low, 

uniform velocity flow through the test section of the chamber. 

A tapered element oscillating microbalance (Patashnick et al., 

2002) was used to continuously monitor dust concentrations so 

that dust in the chamber could be adjusted and kept constant 

during each test-sampling period. 

The three gravimetric samplers in the Marple Chamber 

tracked each other reasonably well. The average gravimetric 

sampler-to-sampler precision over all of the tests was 6%. The 

two pDRs also gave like readings in all of the tests, with the 

average difference between the two pDRs being only 1.9%. A 

concentration ratio was then calculated for each measurement. 

This ratio was the pDR average reading divided by the average 

of the three gravimetric results. All of the measurements at a 

given concentration level were averaged, and the precision 

and bias were calculated as previously described. The results 

are shown in Table 1. 

As an example, for Arizona road dust at the 3-mg/m3 level, 

four measurements were conducted at 53, 113, 173 and 233 

minutes. With pDR #1, the mean concentration ratio for the 

four measurements was 0.77 and the standard deviation was 

0.09. This yielded a precision of 0.12 and a bias of -0.23. Table 

1 suggests that the major source of pDR error is the bias. 

Mine tests – metal/nonmetal

Dust data from pDRs used in several underground limestone 

mines and one gold mine were analyzed in the same manner 

as the laboratory data. Table 2 gives the type of mine, the dust 

sources, the number of locations where instruments were placed 

and the number of shifts during which testing was conducted at 

each location. These mines displayed a variety of particulates, 

including diesel smoke, water mist, and mineral dust from 

blasting, roadways and crushers. In the mines, instrument 

packages, consisting of one pDR and two gravimetric samplers, were placed at key locations depending on the sampling 

protocol for the study. Because the testing was performed in 

metal/nonmetal mines, the pumps were operated at 1.7 L/min 

in accordance with MSHA regulations. 

All sampling protocols consisted of area sampling and the 

placement of sampling packages in intake and return airways, 

on the outside and inside the operator’s cab of haulage trucks 

and at locations around dump/crusher facilities. Prior to testing, 

a zero and internal span check, as described in the instrument 

manual (Thermo Andersen, 2001) was performed on each 

pDR to ensure that the instrument’s calibration agreed with 

the original factory setting. 

For each location on each shift, the pDR/gravimetric ratio 

was calculated using the pDR reading and the average dust 

concentration from the two gravimetric filters. The average 

difference between these two gravimetric samplers in the 

metal/nonmetal tests was 7%. Also, shift average pDR readings below 0.1 mg/m3 were considered unreliable due to the 

limitations of the unit, so those tests were excluded. Overall, 

4% of tests were excluded for this reason.

Precision and bias results for the ratio values were then 

calculated in two different ways. First, the pDR/gravimetric 

ratios for all locations on a given shift were averaged, and the 

precision and bias were calculated for that shift. Because these 

measurements extended over several shifts, the precision and 

bias values from each of the shifts were averaged to obtain a 

mean precision and mean bias value. This was called the “shift 

value.” Secondly, the pDR/gravimetric ratios for all shifts at a 

given location were averaged and the precision and bias calculated for that location. Because the measurements extended 

over several locations, the precision and bias values from each 

of the locations were averaged to obtain a mean precision and 

Figure 1 — Active-mode pDR modified for a Dorr-Oliver 

cyclone.

0.6 0.053 -0.18 0.05 -0.21

1.5 0.19 -0.49 0.19 -0.5

3 0.12 -0.23 0.098 -0.22

5 0.4 -0.66 0.39 -0.66

Concentration, 

mg/m3 Precision Bias Precision Bias

pDR #1 pDR #2

Table 1 — Laboratory test results.
SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 320 • 2006 TRANSACTIONS 23

mean bias value. This was called the “location value.”

Shift and location values were calculated separately because 

there was a possibility that an instrument at a given location 

might show different variability in the pDR/gravimetric ratio 

over several shifts in comparison to instruments on a given 

shift over several locations. 

Table 2 gives the mean precision and the mean bias values 

of the pDR/gravimetric ratio for each mine. The bias is actually a “mine bias,” because the change in bias from one mine 

to another is likely a result of changes in external conditions 

rather than in the instrument itself. Both a shift and a location 

value of the mean precision and mean bias are presented in 

Table 2; note that the results vary widely. Like the laboratory 

results, the hard rock results suggest that a substantial source 

of error is the bias rather than the precision. 

Mine tests — coal

Following the hard rock testing, the pDR was tested at longwall 

faces in five coal mines using the same procedure that was 

followed in the hard rock mines. Table 3 gives the location 

where the instrument package was placed and the number of 

shifts of testing conducted at each location. The arrangement 

of instruments was similar to that used in the hard rock mines, 

and two gravimetric samples were taken. The average difference between these two gravimetric samplers in the coal mine 

tests was 10%.

Table 3 gives the mean precision and mean bias of the pDR/

gravimetric ratio for tests in each of the coal mines visited. 

At the coal mines, data were available from only one or two 

sampling locations. Consequently, calculation of shift values 

was not completed. Only location values were determined. As 

with the hard rock data, the results vary widely. The bias is 

again a major source of error. 

Analysis

In the laboratory testing, the pDR values were less on average 

than the gravimetric values, leading to a consistently negative 

bias. In the mine testing, seven of the 11 bias calculations 

were also found to be negative. However, it was not clear 

what circumstances caused the bias to be positive at certain 

locations. In general, if the absolute value of the bias were 

0.3 or less, the precision value was under 0.25, as shown in 

Fig. 2. However, high bias values did not automatically lead 

to poor precision. Moisture in the air is one likely cause of the 

spread in bias values. Quintana et al. (2000) called attention 

to a strong humidity effect on pDR readings, especially above 

relative humidity values of 85%. Although relative humidity 

values were not measured in this study, it is well known that 

Number of locations 5 5 4 4 6

Number of shifts 3 3 3 3 1

Average gravimetric 0.88 0.48 0.58 0.41 0.69

concentration, mg/m3

Shift value – 0.20 0.18 0.14 0.14 0.19

mean precision

Shift value – 1.30 -0.35 -0.035 -0.18 0.20

mean bias

Location value – 0.16 0.087 -0.11 0.11 –

mean precision

Location value – 1.30 -0.36 -0.035 -0.12 –

mean bias

 Load, haul, 

 Crusher dust dump cycle dust Blast dust Blast dust Blast dust

 and and and and and 

Particulate sources diesel diesel diesel diesel diesel

Mine type: Limestone Gold Limestone Limestone Limestone

Table 2 — Test results for underground metal/nonmetal mines.

Number of shifts 3 3 3 3 3 3 3

Average gravimetric 0.26 3.20 1.00 4.50 1.10 0.79 0.80

concentration, mg/m3

Location value – mean 0.67 0.07 0.14 0.07 0.30 0.19 0.09

precision

Location value – mean 0.73 -0.17 -0.059 -0.28 0.38 0.27 -0.26

bias

Mine Mine A Mine A Mine B Mine B Mine C Mine D Mine E

Location Shield 10 Shield 160 Shield10 Shield 153 Shield 10 Shield 10 Shield 10

Table 3 — Test results from longwall mines.
2006 TRANSACTIONS • Vol. 320 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 24

air in mines can have high relative humidity (Pappas et al., 

2002) and even visible fog. Cecala et al. (1985) described a 

mist eliminator for use with light-scattering dust monitors. 

However, up to now, it has been used only when the mine air 

contained visible mist from water sprays.

Although the pDR continues to be a useful tool for making 

rough relative measurements in mines, users cannot depend 

on the absolute values of the numbers that are obtained. However, accuracy of pDR concentrations can be improved when 

the bias is corrected with a “field gravimetric calibration,” as 

described below.

Guidelines for using the pDR in the active mode 

in underground mines.

It is strongly recommended that one take an accompanying 

gravimetric sample when using the pDR. Test the pDR to 

determine if the seals are tight and the case of the pDR is not 

drawing air through the unit under the negative pressure of 

the pump. When this occurs, particulate may be drawn into 

the unit, bypassing the cyclone. As a result particulate is being 

deposited on the in-line filter but not being recorded by the 

unit. If leakage is extreme, the concentration from the in-line 

filter and the average concentration from the pDR can vary 

greatly.

Leakage in the unit is easily determined by placing the pDR 

in-line when setting the pump flow rate during pump calibration 

with a primary standard instrument. If leakage is occurring, the 

required flow rate will be difficult to achieve. In this situation, 

the pDR may need to be resealed. If no leaks are detected, the 

concentrations calculated from the in-line filter on the pDR can 

also be used in precision and bias calculations if needed.

The accompanying gravimetric concentrations are important, because they can be used to correct the individual pDR 

concentrations. This “field gravimetric calibration” (Thermo 

Andersen, 2001) is accomplished by multiplying individual 

pDR data points by the ratio of gravimetric concentration to 

the average pDR concentration. For example, if a five-hour 

gravimetric sample gives a concentration of 2.0 mg/m3 and 

the accompanying five-hour pDR concentration average is 1.0 

mg/m3, all of the individual pDR readings can then be multiplied 

by 2. If dust and environmental conditions are not expected 

to vary during multiple-shift sampling, there is a menu option 

in the pDR software so this correction factor can be directly 

programmed into the unit to improve sample efficiency.

In addition, the accuracy of pDR measurements is questionable when the average pDR concentration for the shift is 

below 0.1 mg/m3 or when the bias is greater than 0.3 (or less 

than -0.3). In these instances, additional sampling should be 

considered. 

There are many other uses for light-scattering dust monitors in mines where accuracy is not of paramount importance. 

Some of these include:

• calculate dust concentrations for different operations 

(such as characterizing dust levels during truck haulage 

cycles or headgate to tailgate cuts in longwall mining),

• identify and isolate high or problematic dust sources,

• use for short-term sampling to improve statistical validity,

• monitor dust movement through the mine as an estimator of air velocity for large opening mines that have 

extremely low air flow (Chekan et al., 2004),

• identify ambient dust concentration buildup in mines 

and mills and

• activate alarm feature to detect high dust concentrations 

so personnel can implement remedial action to lower 

dust levels.

References

Cecala, A.B., Volkwein, J.C., and Thimons, E.D., 1985, “Adding Steam to Control 

Dust in Minerals Processing,” U.S. Department of the Interior, Bureau of 

Mines, Pittsburgh, Pennsylvania, RI 8935, NTIS No. PB 85-198885.

Chekan, G.J., Colinet, J.F., and Grau, R., 2004, Evaluating ventilating air movement 

in underground limestone mines by monitoring respirable dust generated 

from production shots,” Proceedings of the Tenth US/North American Mine 

Ventilation Symposium, May 16-19, 2004, Anchorage Alaska, pp. 221-232.

Marple, V.A., and Rubow, K.L., 1983, “An aerosol chamber for instrument 

evaluation and calibration,” American Industrial Hygiene Assoc. J., Vol. 44, 

No. 5, pp. 361-367.

Page, S.J., and Jankowski, R.A., 1984, “Correlations between measurements 

with RAM-1 and gravi­metric samplers on longwall shearer faces,” AIHA J.

Vol. 45, No. 9, pp. 610-616.

Pappas, D.M., Barton, T.M., and Weiss, E.S., 2002, “Developments in sealant 

support systems for ground control,” Proc. 21st International Conference 

on Ground Control in Mining, Morgantown West Virginia, August 6-8, 2002, 

pp. 344-353.

Patashnick, H., Meyer, M., and Rogers, B., 2002, “Tapered element oscillating 

microbalance technology,” in Mine Ventilation: Proceedings of the North 

American/Ninth U.S. Mine Ventilation Symposium, Kingston, Ontario, 

Canada, June 8-12, 2002, E. DeSouza, ed., Balkema, Lisse, Netherlands, 

pp 625-631.

Quintana, P.J.E., Samimi, B.S., Kleinman, M.T., Liu, L.J., Soto, K., Warner, G.Y., 

Bufalino, C., Valencia, J., Francis, D., Hovell, M.H., and Delfino, R.J., 2000, 

“Evaluation of a real-time passive personal particle monitor in fixed site 

residential indoor and ambient measurements,” Journal of Exposure Analysis 

and Environmental Epidemiology, 2000, Vol. 10, pp. 437-445.

Smith, J.P., Baron, P.A., and Murdock, D.J., 1987, “Response characteristics of 

scattered light aerosol sensors used for control monitoring,” AIHA J., Vol. 

48, No. 3, pp. 219-229.

Thermo Andersen, 2001, “Operator Manual for Personal DataRAM Model 

pDR-1000AN and pDR-1200,” Thermo Andersen, 500 Technology Court, 

Smyrna, GA, 30082

Tsai, C.J., Shih, T.S., and Lin, J.D., 1996, “Laboratory testing of three directreading dust monitors,” AIHA J., Vol. 57, pp. 557-563.

Williams, K.L., and Timko, R.J., 1984, “Performance Evaluation of a Real-Time 

Aerosol Monitor,” U.S. Department of the Interior, Bureau of Mines, Pittsburgh, Pennsylvania, IC 8968, NTIS PB 84-199710.

Figure 2 — Precision vs. bias for all of the measurements.