Technology News

No. 463 July 1997

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Machine-Mounted Continuous Respirable Dust Monitor

Objective

To continuously monitor, display, and record concentration levels

of respirable coal mine dust in mines to an accuracy of ±25% with a

95% confidence level for at least 30 days without servicing.

Approach

The National Institute for Occupational Safety and Health

(NIOSH) has developed a machine-mounted continuous respirable

dust monitor (MMCRDM) (figure 1) based on the tapered-element

oscillating microbalance (TEOM®) sensing technology. The sensor

can be mounted on mobile mining equipment and can continuously

and accurately measure respirable coal mine dust mass concentrations

despite the rigors of the underground mine environment. Readings

of dust levels are stored in computer memory and displayed to the

machine operator. The display shows dust levels averaged over

various intervals and a graph of the shift average as a function of

time. The monitor also incorporates several automatic diagnostic

functions to detect system failure or tampering.

Such a monitor will enable mine operators and regulatory personnel to identify specific mining practices that expose mine workers

to excessive dust levels. Using information provided by the monitor,

mine personnel can optimize mining procedures to reduce dust

exposure to miners.

How It Works

The TEOM® sensor in the MMCRDM uses a tapered vibrating

tube to measure the mass of dust sampled from mine air. This tube

or element (figure 2) is hollow and made of metal or an elastic,

glasslike material. The wide end of the tube is firmly anchored; the

narrow end supports a replaceable filter and is permitted to oscillate.

By drawing air through the hollow tube, the monitor collects dust

from the mine air on the filter. As the filter collects dust, its increase

in mass causes the tapered element to vibrate

slower. By measuring the change in frequency, an onboard computer

calculates how much dust was collected on the filter.

An electronic feedback system keeps the element vibrating. The

details of the feedback system vary by model; typically, however, the

tapered element moves across the light path between a light-emitting

diode and a phototransistor. Phototransistors generate electrical

signals according to how much light they receive. The light-blocking

effect of the vibrating element regulates the output signal of the

phototransistor, which is then amplified. Part of the amplified signal

is used to provide just enough energy to keep the element vibrating.

The other part of the signal from the phototransistor goes to a counter

and data-processing stage. Here, the frequency of oscillation of the

tapered element is calculated and stored in memory.

Unlike many aerosol measurement technologies (e.g., light

scattering) that measure some aerosol parameter correlated with

mass, the TEOM® technique measures mass directly. Used with a

cyclone or other appropriate dust size preclassifier, the instrument

collects and measures respirable dust mass. However, because the

TEOM® instrument indiscriminately measures mass collected on the

filter, it would falsely assess any collected water droplets as aerosol

mass. Changes in mine air humidity and temperature could also

affect the response of the instrument. To eliminate or reduce the

effects of humidity and temperature variations on the sensor, the

monitor controls the inlet temperature to 50 C. Under these conditions, water aerosols evaporate, leaving only solid particles on the

filter. Also, because TEOM® instruments operate by measuring the

change in frequency of a vibrating element, vibrations from external

sources can interfere with the measurement. A special suspension

system was designed for the MMCRDM to prevent vibrations from

the mining machine from disrupting the operation of the sensor.

Results

In initial tests, only the TEOM® transducer/sensor with the sample inlet and sample preclassifier were mounted on a continuous

miner. Measurements were taken of mining machine vibration and

respirable dust levels at the position of the sensor. Except for a
Figure 1.—Prototype preproduction monitor.

 Figure 3.—Mass comparison between referee samplers and the

continuous respirable dust monitor.

Figure 2.—Schematic of the TEOM® dust sensor.

consistent bias introduced by the preclassifier used in that version of

the MMCRDM, the respirable dust readings compared well with

those measured with conventional samplers. Baseline drift for the

unit was less than 1 g, suggesting that the sensor is adequately

protected from environmental factors such as vibration and humidity.

In subsequent tests, preproduction versions of the MMCRDM

were tested on both continuous and longwall mining sections. Figure

3 shows a comparison of total mass measured between the monitor

and referee samplers using inlets and preclassifiers identical to those

of the monitor. The comparison measurements are highly correlated

and are well with in the accuracy specifications of the monitor, ±25%

with a confidence of 95%.

The Mine Safety and Health Administration plans to test

10 production versions of the MMCRDM in mines throughout the

United States to help determine how best to use them to protect the

health of miners.

Patent Status

NIOSH is not applying for a patent on this development; however,

Rupprecht and Patashnick Co., Inc., Albany, NY, has patented the

TEOM® aerosol mass-sensing technology. Com- mercial units are

expected to be available in late 1997.

For More Information

For more information about the MMCRDM and status of the

development effort, contact Kenneth L. Williams or Bruce K.

Cantrell, Ph.D., NIOSH Pittsburgh Research Center, Cochrans Mill

Rd., P.O. Box 18070, Pittsburgh, PA 15236-0070, phone: (412)

892-6646 or (412) 892-4019, fax: (412) 892-6764, e-mail:

kfw5@cdc.gov or bec4@cdc.gov

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other

occupational safety and health problems, call 1-800-35-

NIOSH (1-800-356-4674), or visit the NIOSH Home

Page on the World Wide Web at http://www.cdc.

gov/niosh/homepage.html 

As of October 1996 the safety and health research functions of

the former U.S. Bureau of Mines are now located in the

National Institute for Occupational Safety and Health

(NIOSH).