U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health TRAPED

MENT OF HEALTH & HUMAN SERVICES . USA

MEASURING VERY LOW AIR VELOCITIES IN UNDERGROUND

METAL/NONMETAL MINES

 Objective

To accurately measure very low air velocities (0-100 ft/min) in

U.S. large-opening underground metal/nonmetal mines.

Background

Hundreds of metal/nonmetal underground mines operate

throughout the United States. They extract commercial products

from deposits such as limestone, salt, and gypsum. Mining generates

a wide variety of gases and dusts, such as silica, diesel exhaust,

radon, blasting byproducts, and perhaps even methane. If these

substances become airborne, they can adversely affect the health and

safety of underground workers. Many metal/nonmetal mines have

large cross-sectional airways that could be 60 times larger than an

average underground coal mine entry. These large airways require

adequate volumes of fresh air to dilute, make harmless, and exhaust

the toxic and, at times, explosive byproducts. However, in some

underground mine sections, air movement is barely perceptible. The

technology of measuring very low air velocities will become

increasingly more important for operators of large-opening stone

mines in order to monitor miner exposure to diesel particulate matter.

Conventional anemometers (e.g., vane) are capable of accurately

measuring air velocities only above 100 ft/min. Thus, it was

necessary to investigate other anemometry technology to detect low

air velocities in the range of 0-100 ft/min. Two different portable

ultrasonic anemometers were tested. Actual testing was done in an

underground limestone mine with openings about 51 ft wide by 29 ft

high.

Ultrasonic Anemometers

Vane-type anemometers are commonly used by engineering

personnel throughout the underground coal mine industry. They

work well in measuring air velocities >100 ft/min. However,

a separate technology, e.g., portable ultrasonics, is needed to measure

air velocities much below 100 ft/min at various places within the

large mine opening. Laser-doppler technology was also considered

for this application, but was ruled out because of its high cost and

other technical limitations. With no moving parts, the portable

ultrasonic anemometers are claimed to be accurate down to about 1

or 2 ft/min.

The first type of ultrasonic anemometer that was examined

underground is manufactured by Gill Instruments Ltd., Hampshire,

U.K. The U.S. distributor is IN USA, Needham, MA. The Gill

Instruments Solent Volometer (figure 1) is a battery-powered, singleaxis anemometer meant for measuring low to medium air flows up to

1,999 ft/min. It uses the ultrasonic "time of flight" principle where

pulses of high-frequency sound are transmitted along the air flow

path and the time taken to reach the receiver is measured. The faster

the air speed, the quicker the pulses reach the receiver.

The second ultrasonic anemometer that was examined underground is manufactured by Airflow Developments Ltd., Buckinghamshire, U.K. The U.S. distributor is Airflow Technical Products,

Inc., Andover, NJ. The Anemosonic UA-6 (figure 2) is a batterypowered, handheld instrument with digital display. It measures air

velocity (0 to 9,999 ft/min) or volume flow, flow turbulence

intensity, and air temperature.

Neither device is considered to be intrinsically safe for use in

underground gassy coal mines.

A comparison of salient specifications of both instruments is

shown in table 1. The total suspended weight includes the instrument

(Gill) or instrument probe (Airflow) plus about 35 ft of connecting

cable attached to the pole. This combination of instrument plus cable

length allows two operators to measure air velocities up to about 30 ft

above the mine floor.

Figures 1 and 2 show how each unit would be attached to an

extensible, graduated fiberglass rod in order to position the sensor

probe at a certain spatial point (centroid of the quadrate) within the

underground large opening.

Technology News

Milestones in Mining Safety and Health Technology

SAFER HEALTHIER PEOPLE

No. 501 January 2003
 Figure 1.—Gill ultrasonic anemometer

taped to the top end of an extensible pole. Figure 2.—Airflow UA-6 sensor probe

bolted to the top end of an extensible pole.

Table 1.—Comparison of two ultrasonic anemometers

Anemometer

Specification

Total suspended

weight, lb Digital readout location Negative measurement

Gill Volometer . . . . . . . . . . . . . . . . 1.60 % 1.00 ' 2.60 On sensor unit (high above) Within unit circuit.

Airflow Anemosonic UA-6 . . . . . . . 0.15 % 0.60 ' 0.75 On handheld unit (waist-high) Not available on current unit; visual only.

Test Results

The following conclusions can be made regarding the potential

use of these two portable ultrasonic anemometers in measuring very

low air velocities in large-opening mines.

• The two ultrasonic anemometers were compared to a vane

anemometer "standard" in the Mine Safety and Health Administration's (MSHA) anemometer calibration rig, a 6-in open jet wind

tunnel, at airspeeds between 2,000 and 100 fpm. It was found that

the coefficient of correlation between the two ultrasonic

anemometers and the MSHA standard varied from 0.9620 to 1.0226.

• During in-mine evaluations, both instruments measured low air

velocities equally well.

• Because of its remote readout, the Airflow UA-6 unit is most

convenient to mount atop an extensible rod. However, for everyday

underground use, the sensing unit on this instrument needs to be

hardened.

Specific Mine Application 

It would be advisable initially to use a nine equal-quadrate fixedpoint traverse/mine-opening method to measure the average velocity

within a large mine opening. At a given mine with extended use,

fewer measurements could be made in conducting an air flow check.

The technique of mounting the lightweight Airflow UA-6 sensor

probe to the top of an extensible/graduated rod, bringing a nonstandard 35-ft-long connecting cable down the rod, and reading

the air velocity at a convenient height above the mine floor was

developed and tested by the NIOSH coauthors. Specific information

on these procedures is available from them.

Because of the direct-readout design and the weight of the Gill

Volometer, it is better suited to smaller cross-sectional entries that

can be evaluated without resorting to extensible devices. In addition,

this instrument is ideal for periodic measurement of air velocities

within miners' working or breathing zones.

For More Information

For more information on this technology, contact William D.

Mayercheck or Robert J. Timko, NIOSH Pittsburgh Research

Laboratory, P.O. Box 18070, Cochrans Mill Rd, Pittsburgh, PA

15236-0070; phone (412) 386-6461 or (412) 386-6684, respectively;

fax: (412) 386-6891; e-mail: WMayercheck@cdc.gov or

RTimko@cdc.gov, respectively.

To receive additional information about occupational safety

and health problems, call 1-800-35-NIOSH (1-800-356-4674),

or visit the NIOSH Web site at www.cdc.gov/niosh

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety

and Health.