TEST REPORT ON THE MACHINE-MOUNTED CONTINUOUS 

RESPIRABLE DUST MONITOR 

F.N. Kissell E.D. Thimons 

National Institute for Occupational Safety 

and Health 

Pittsburgh Research Laboratory, Pittsburgh, PA 

15217 USA 

National Institute for Occupational Safety 

and Health 

Pittsburgh Research Laboratory, Pittsburgh, PA 

15217 USA 

ABSTRACT 

The machine-mounted continuous respirable dust monitor (MMCRDM) is a fixed-location area sampling device 

developed for possible use at the working face of an underground coal mine. This device, based on proprietary 

technology known as the tapered element oscillating microbalance, has evolved over the past eight years through a 

cooperative effort of the former Bureau of Mines, MSHA, and the Rupprecht & Patashnick Company in Albany, NY. 

The capability to measure respirable coal mine dust levels on a continuous basis, rather than depending solely on 

periodic samples obtained from the traditional coal mine dust samplers, has been a goal in the mining industry for 

nearly two decades. Recently, an extensive series of laboratory and underground tests was conducted by NIOSH with 

the cooperation of MSHA and coal operators to test the performance of the MMCRDM. 

In preliminary laboratory testing, the MMCRDM seemed to work well. However, in every underground test, when 

compared to reference samplers placed close to the inlet, the MMCRDM failed to meet the 25% accuracy criterion 

specified in the contract under which it was developed. Two reasons explain this failure: First, in most tests the bias 

(the relative discrepancy between the average MMCRDM concentration and the average reference sampler 

concentration) was too great. Second, the variability of the samplers used for reference comparison was too large. 

Finally, the underground testing of the MMCRDMs showed that they are quite unreliable at this stage of 

development. In the majority of mine tests, no more than 10 shifts of data were taken before the MMCRDM failed to 

function properly. Major breakdowns, requiring the return of the MMCRDM to the factory for repairs, occurred on 

average every 28 days. To be considered mine-worthy, MMCRDM reliability must be substantially improved.

KEYWORDS 

Dust, coal dust, dust instrumentation, dust sampling, dust monitoring, black lung, pneumoconiosis 

INTRODUCTION 

The machine-mounted continuous respirable dust 

monitor (MMCRDM) measures dust with a tapered 

element oscillating microbalance (TEOM)(Cantrell et al., 

1997). TEOM ambient particulate monitors are 

proprietary instruments sold by the Rupprecht and 

Patashnick Company, Inc. (R&P), in Albany, NY. 

TEOM-based monitors are used around the world to 

measure combustion particulate and ambient air quality 

levels (Patashnick and Rupprecht, 1991). From the U.S. 

EPA, TEOM instrumentation has received regulatory 

certifications for measuring PM-10 concentration; from 

the German EPA this instrumentation has received 

regulatory certifications for monitoring TSP concentration. 

The TEOM operating principle uses a replaceable 

filter cartridge mounted on the narrow end of a hollow 

tapered tube. The wide end of the tube is fixed. Air 

passes through the filter and down through the tube to a 

pump. The tapered tube with the filter on the end is 

maintained in oscillation. The oscillation frequency is 

controlled by the characteristics of the tube and the filter 

mass at its end. As dust collects on the filter, the mass 

change is measured as a frequency change in the 

oscillation of the tube. The e xact mass of dust collecting 

on the filter is then determined directly. Since 

frequency can be measured accurately, the method can 

measure very small mass changes. 

For this study, the MMCRDM dust particle 

preselector was specifically designed to match the size 

penetration curve of the sampler currently used for 

coal mine respirable dust compliance measurements. 

This compliance sampler uses a 10-mm Dorr Oliver 

cyclone operated at a flow rate of 2 l/min. The 
MMCRDM dust particle preselector consists of three 

components. Sample air enters an omni-directional 

inlet cap, then passes through a central tube to an 

elutriator, which removes particles larger than about 

15 μm in size, and finally enters a virtual impactor, 

which passes only respirable size particles to the 

tapered element filter. 

ACKNOWLEDGMENTS 

The authors gratefully acknowledge the important 

contribution of those who collected the data for this 

analysis: NIOSH employees Bruce Cantrell, Calvin 

Garbowsky, Gregory Green, Thomas Mal, Shawn 

Vanderslice, and Jeanne Zimmer, along with MSHA 

employees Karl Ahlen, David Atchison, Ken Fields, 

Everett Gerbec, and Brian Murphy. 

LABORATORY TESTS 

In laboratory experiments, the dust level indicated by 

the MMCRDM was compared to that measured by the 

traditional coal mine dust compliance samplers. A 

humidified dust chamber (Marple and Rubow, 1984), 

designed for instrument testing, was used. The design 

of the chamber permits the experimenter to set humidity 

and dust levels that are uniform throughout the 

chamber, so as to allow direct comparison of one 

instrument with another. The dust size distribution was 

selected to match that found in underground coal mines 

(Rubow et al., 1990). In initial laboratory 

measurements using coal dusts (8 μm mmd, 2.0 σg), the 

MMCRDM indicated a dust concentration that was 

20% less than the concentration from the dust 

compliance samplers. To ensure that the MMCRDM 

gave a similar dust reading during the field tests, the 

cut-point of the virtual impactor was adjusted by 

changing the nozzle orifice diameter until the difference 

in concentrations was less than 5%. 

After the impactor orifice size was adjusted, the 

MMCRDM was compared to three co-located dust 

compliance samplers under several different test 

conditions. The results for all of the test conditions are 

shown in Figure 1, where the accumulated mass on the 

MMCRDM filter is compared to the average mass 

collected on the three compliance sampler filters.These 

tests included two types of coal (Pittsburgh and 

Pocahontas #3), three relative humidity levels (40%, 

60%, and 80%), and four respirable dust levels that 

might be encountered underground (0.4 mg/m3

, 

0.8 mg/m3

, 1.2 mg/m3, and 1.6 mg/m3). Each test was 

conducted for a six hour period. The trend line in Figure 

1 is the best fit to the points. The calculated R2

correlation value is quite high, indicating a very good 

correlation. 

Figure 1. MMCRDM - Reference testing in lab
FIELD TESTS 

NIOSH and the Mine Safety and Health 

Administration (MSHA) planned and conducted these 

field studies according to an agreed-upon partnership. 

To determine the field accuracy of the MMCRDM, we 

used as a reference the traditional coal mine dust 

compliance samplers, the same type of sampler that had 

been used in the laboratory study. These reference 

samplers were always placed in close proximity to the 

MMCRDM inlet. Sampling was conducted on-section 

for the entire shift, with the exception of the time it took 

the MMCRDM to warm up. 

Test data were excluded from the analysis only for 

clearly defined anomalies related to data collection. 

These included instances where the reference sampler 

pump shut down during the shift, where the reference 

filter cassette was severely damaged, or where the 

MMCRDM failed to operate, mostly because the filter 

changer jammed. Operating failures of the MMCRDM 

were confirmed by examining a monitor diagnostic 

printout available at the end of each shift. In a few 

shifts, the compliance samplers were turnedon too 

early; where the operating times of the compliance 

samplers exceeded that of the MMCRDM by more than 

30 minutes, those shifts were also excluded. 

Dust concentrations measured by the MMCRDM were 

automatically calculated by the monitor. For the reference 

samples, MSHA pre- and post-weighed the filters to a 

precision of 11 μg in its automated sample weighing facility. 

One filter blank was established for each shift of samples, 

and filter post-weights were corrected for weight changes in 

the blank. The pumps used were the Mine Safety Appliances 

Co. Elf-Escort flow controlled pump. Pump calibration was 

checked before each sampling shift using a Gillibrator 

calibrator and a filter load. 

In the first mine (Garmeada), the research team 

conducted a 16-shift test in a continuous miner section 

that had a mobile bridge conveyor, a situation 

representative of a small percentage of continuous 

miner sections. The MMCRDM was located on the first 

bridge conveyor section, and two reference samplers 

were placed within 15 cm of the MMCRDM inlet. 

The second mine (Baker) was a homotropal longwall 

in which the airflow across the face moved from tailgate 

to headgate. The MMCRDM and the two adjacent 

reference samplers were placed at the headgate, 

downwind of the shearer. The two reference samplers 

were about 45 cm from the MMCRDM inlet. Data were 

obtained for 10 shifts. 

In the third mine (Shoemaker), the researchers 

conducted an 8 shift test in a continuous miner section, 

with the MMCRDM placed on the mining machine in 

front of the operator's cab. Two reference samplers were 

placed within 15 cm of the inlet. 

In the fourth mine (Federal #2) the research team 

placed the MMCRDM and four reference samplers on a 

continuous mining machine. They were located on the 

conveyor housing at the left-hand side and toward the 

front of the operator's cab. The reference samplers were 

placed within 15 cm of the MMCRDM inlet. Data were 

obtained for 38 shifts. 

In the fifth mine (Marrowbone), the MMCRDM and 

four reference samplers were placed in the return. The 

reference samplers were 15 to 23 cm from the 

MMCRDM inlet. Data were obtained for 9 shifts. 

In the sixth mine (Shamrock), the MMCRDM was 

located on a longwall shearer in a spot where it was 

saturated by mist from the water sprays on the shearer. 

It failed due to collected moisture near the end of the 

first shift; therefore, none of the data could be used. 

FIELD TEST RESULTS AND DISCUSSION 

Figures 2 through 6 give the results for the five 

successful field tests. Each plotted point represents data 

for one shift, and the MMCRDM reading for that shift is 

plotted against the average value for the reference 

samplers that were operated during the same shift. The 

line in each of these figures represents the equation 

Y=X, and if the MMCRDM had no bias, the plotted 

points would fall equally on either side of this line. Bias 

is the relative discrepancy between the average 

instrument reading and the true value, the true value 

being the average of the reference samplers. For some 

of the mine tests (Baker-Figure 3, Federal #2-Figure 5, 

and Marrowbone-Figure 6), the plotted points indicate a 

considerable MMCRDM bias. Also, all of the data are 

considerably more scattered than the laboratory results. 

The contract under which the MMCRDM was 

developed had a +/-25% accuracy criterion, meaning 

within +/-25% of the true value 95% of the time. So, 

we examined the field test results using criteria 

specified by Kennedy et. al. (1995) for determining 

whether a prospective air sampling instrument is 

sufficiently accurate. This accuracy test requires that 

readings from a prospective instrument be compared to 

a standard. Multiple readings are taken and the 

precision of these readings is calculated. Also, the bias 

is calculated. Kennedy et al. give a nomogram for 

conveniently obtaining the accuracy based on measured 

input values of precision and uncorrectable bias. 

For each mine test, we calculated a concentration ratio 

for each shift. The concentration ratio was the 

MMCRDM reading for that shift divided by the average 

value for the reference samplers operating during that 

shift. These concentration ratios were then averaged 

over all shifts in the mine test, and the standard 

deviation calculated. The precision for each mine test 

was then obtained by dividing the standard deviation by 

the mean concentration ratio. Also, the concentration 

ratio was used to calculate the bias value for each mine 

test. (The bias is equal to the mean concentration ratio 

minus 1.) The input values and the accuracy results 

from the Kennedy nomogram were as follows: 
 Concentration Ratio 

 Figure # Mine Mean Precision Bias Accuracy results 

 2 Garmeada 1.052 0.130 +0.052 29% 

 3 Baker 1.93 0.641 +0.93 

 4 Shoemaker 0.951 0.226 –0.049 40% (estimate) 

 5 Federal #2 0.888 0.159 –0.112 34% 

 6 Marrowbone 1.40 0.110 +0.40 

 --- Shamrock ------------- test failed due to water spray ---------------- 

Figure 2. MMCRDM - Reference testing in Garmeada 

Figure 3. MMCRDM - Reference testing in Baker 
Figure 4. MMCRDM - Reference testing in Shoemaker 

Figure 5. MMCRDM - Reference testing in Federal #2 
Figure 6. MMCRDM - Reference testing in Marrowbone 

Figure 7. Federal #2 - Reference sample 4 versus 1,2 and 3 

 
 

As an example, for Garmeada, data were taken for 16 

shifts, resulting in 16 concentration ratio values. The 

mean of these 16 values was 1.052 and the standard 

deviation was 0.137. The precision was 0.137/ 1.052 = 

0.130. The bias was 1.052 – 1.0 = 0.052. From the 

Kennedy nomogram, a precision of 0.130 and a bias of 

0.052 yielded an accuracy of 29%. 

Accuracy values are not provided for Baker or 

Marrowbone because the bias values of 0.93 and 0.40 

were so high as to not warrant inclusion on the Kennedy 

nomogram (in other words, off the chart). The 40% 

accuracy reported for Shoemaker is an estimate because 

the precision value of 0.226 is also off the chart. 

Not a single mine test met the 25% criterion because 

either the precision and/or the bias values were too high. 

A possible reason for the high precision values (in other 

words, poor precision) is that the concentration ratios 

reflect variance in the reference samplers as well as 

variance in the MMCRDM. For example, in Figure 7, 

the four reference samplers from the Federal #2 study 

are only plotted against each other, and they show 

considerable scatter as well as bias. The average 

reference sampler-to-reference sampler precision value 

for this mine test was 0.155. This value of 0.155 does 

not meet the 25% criterion. 

Because the Kennedy method provides no allowance 

for variance in the reference, it is not surprising that the 

concentration ratio precision values are high. Thus, it is 

clear that the failure to meet the 25% criterion is due in 

some part to variability in the reference samplers. 

Three of the five mines had bias values above 10%. 

Kennedy et al. state that acceptable methods must have 

an absolute bias no greater than 10%. These high bias 

values are difficult to explain, particularly since they 

vary so much from one mine to another. One possible 

explanation is that the preselector on the MMCRDM 

failed to completely mimic the Dorr-Oliver cyclone, and 

so when the dust particulate size distribution changed 

from laboratory to mine and from mine to mine, the bias 

also changed. Also, in the laboratory tests the dusty air 

surrounding the MMCRDM and the reference samplers 

was essentially quiescent, whereas in the mine tests the 

air was always moving. This change could have 

resulted in differences in flow field and particle path 

adjacent to the inlets, which in turn contributed to the 

discrepancy in results between laboratory and field. 

MMCRDM OPERATIONAL SHORTCOMINGS 

 

During the field testing, we found that the MMCRDM 

had many operational shortcomings. These were long 

warm-up times, rock dust interference, failure due to 

moisture, and particularly a lack of reliability. 

Long warm-up times 

To ensure that the MMCRDM measures only the 

mass of the coal dust and not any moisture on the filter, 

the measuring chamber that contains the filter and 

tapered element is heated to 50 degrees C. However, 

thermal gradients in the measuring chamber cause a 

lessening of accuracy. Hence, in those field tests where 

the power was removed between shifts an average of 59 

minutes was needed for thermal equilibrium to be 

reached and for the first dust value to be computed. 

This means that no recording of dust data took place in 

the first hour. 

Rock dust interference 

Rock dusting caused large spikes in the MMCRDM 

reading. This may have added to the shift average. 

Moisture 

The only information we have about the impact of 

moisture is from the Shamrock Mine test, where exposure 

to a water spray caused the MMCRDM to fail after just 

one shift. The other mine locations were not as wet. 

Reliability 

Failure of the monitors pervaded the testing. In the 

majority of mine tests, no more than 10 shifts of data 

were taken before the MMCRDM failed to function 

properly. The average interval between breakdowns 

serious enough to require a return of the unit to the 

factory was 28 days. Most breakdowns were failures of 

a filter changer mechanism that had been designed and 

built specifically for the MMCRDM. Breakdowns 

repaired in the field were not recorded, but were many 

times as frequent. 

MMCRDM - AREA SAMPLING INACCURACY 

It should be noted that this report only compares the 

MMCRDM with reference samplers located adjacent to 

the MMCRDM inlet. The question as to whether such a 

device, even if accurate, correctly measures what nearby 

workers are breathing is a completely different issue. 

The practice of general air monitoring for measuring 

employee exposure, as the MMCRDM does, is called 

area sampling. Modern industrial hygiene practice has 

been to avoid area sampling and to sample airborne 

contaminants using "personal sampling" equipment worn 

by workers. It is well-known (Leidel et. al., 1977) that if 

the contaminant source is nearby, personal samples are 

more accurate than area samples, simply because the 

sampler is in the workers breathing zone rather than 

several feet away. We will address this area sampling 

inaccuracy issue in a separate report, to be published. 

SUMMARY 

In preliminary laboratory testing, the MMCRDM 

seemed to work well. However, in every underground 

test, when compared to reference samplers, the 
 

MMCRDM failed to meet the 25% accuracy criterion 

specified in the contract under which it was developed. 

Two reasons explain this failure: First, the variability 

(the precision value) of the compliance samplers used 

for reference comparison was too large. Second, in 

three of the five successful mine tests the bias values 

were too great. Had the reference sampler variability 

been less, the accuracy criterion would still not have 

been met due to the large bias values obtained at the 

Baker, Federal #2, and Marrowbone mines. 

Finally, the underground testing of the MMCRDMs 

showed that they are quite unreliable at this stage of 

development. In the majority of mine tests, no more 

than 10 shifts of data were taken before the MMCRDM 

failed. Major breakdowns, requiring return of the unit to 

the factory for repairs, occurred on average every 28 

days. Electronic devices often suffer reliability 

problems when put into underground coal mines. Given 

the complexity of the MMCRDM, an extended 

shakedown period should have been expected. 

The results lead to two conclusions. First, the 

MMCRDM cannot be used to represent worker 

exposure levels to respirable dust. Second, the reliability 

of the MMCRDM must be vastly improved for the 

monitors to be considered mine worthy. 

REFERENCES 

Cantrell, B.K., Williams, K.L., Stein, S.W., Hassel, D., 

and Patashnick, H.,1997, “Continuous Respirable 

Dust Monitor Development”, Proceedings of the 6th 

International Mine Ventilation Congress, Society of 

Mining Engineers, Littleton, CO 

Kennedy, E.R., Fishbach, T.J., Song, R., Eller, P.M., and 

Shulman, S.A., 1995, “Guidelines for Air Sampling 

and Analytical Method Development and Evaluation”, 

DHHS (NIOSH) Publication No. 95-117 

Leidel, N.A., Busch, K.A., and Lynch, J.R., 1977, “The 

Inadequacy of General Air (Area) Monitoring for 

Measuring Employee Exposures,” Technical 

Appendix C in Occupational Exposure Sampling 

Strategy Manual, NIOSH Publication No. 77-173, pp. 

75-77 

Marple, V.A., and Rubow, K.L., 1984, “Upgrading and 

Evaluation of Test Chamber”, Respirable Dust 

Measurement, BuMines OFR No. 98-95, NTIS No. 

PB85-245843, March 1984, pp 15-25 

Patashnick, H., and Rupprecht, E.G.,1991, “Continuous 

PM-10 Measurements Using the Tapered Element 

Oscillating Microbalance”, Journal of the Air and 

Waste Management Association, Vol. 41, No. 8 

Rubow, K.L., Cantrell, B.K., and Marple, V.A., 1990, 

“Measurement of Coal Dust and Diesel Exhaust 

Aerosols”, Proceedings VIIth International 

Pneumoconiosis Conference, Pittsburgh, PA, pp. 645-

650