Sonic anemometer airflow monitoring technique for use in 

underground mines 

A.L. Martikainen, H.N. Dougherty, C.D. Taylor & A.L. Mazzella 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, USA 

ABSTRACT: Satisfying statutory requirements of total air quantity and air velocity in an underground mine 

requires airflow measurements. In this paper instrumentation and sampling strategies for accurate monitoring of 

airflow in an entry using two-axis and three-axis ultrasonic anemometers are evaluated. Currently other types of 

anemometers, mainly vane anemometers, are used for measurements underground. The drawbacks of these 

devices include following strict operational procedures and an inability to measure low flows. Ultrasonic 

anemometers are known for their accuracy, capability to measure low flows, and continuous measurement 

characteristics that have made them popular in various fields of research. Several steps are required to enable the 

use of ultrasonic anemometers underground. Results and lessons learned from experiments at the NIOSH 

Ventilation Gallery and its Experimental Mine are presented. Calibration of instruments and data acquisition is 

explained. Three instruments were used simultaneously in the underground testing facility for anemometer 

comparison and sampling location analysis. Continuous single-point measurements with ultrasonic anemometers 

and vane anemometer traverse measurement results underground were compared. The comparison showed 

slightly lower air velocity values with the ultrasonic anemometers. Factors affecting the selection of sampling 

locations underground were investigated to determine optimal locations. The optimal measurement location in a 

straight tunnel was found approximately 1⁄4 diameter from the tunnel walls. For uneven flows, curves, and 

obstructed tunnels several point measurements across the entry cross-section or flow simulations are 

recommended. Two ultrasonic anemometers were assessed in the laboratory, for the influence of dust deposition. 

The effects of two different dusts, rock dust and coal dust were evaluated. The dust exposure testing showed that 

coal dust rarely affected the ultrasonic anemometers and was easy to clean off of the instruments. Rock dust, 

however, tended to stick and cake on the instruments. Cleaning was required to avoid loss of signal with heavy 

build-up of dust. 

1 Introduction 

The proper control and distribution of ventilation air is a 

key aspect in the productivity of an underground mining 

operation, and is crucial to the health and safety of mine 

workers. Accurate methods of determining the air volume 

flow rates are necessary to properly control and distribute 

the air (Thimons & Kohler, 1985). 

The airflow in a tunnel is calculated from measured air 

velocity and cross-sectional area values. In underground 

mines cross-sectional areas are usually measured with a 

tape measure. A laser device can be used in locations 

where intrinsically safe instruments are not required. Air 

velocity is typically measured by a rotating vane 

anemometer traverse or by using point measurements. The 

vane anemometer has been the primary instrument for air 

velocity measurement in mines since the early 1900s (De 

La Cruz, 1982). Other devices, like hot wire anemometers 

and pitot tubes can also be used for measuring air velocity. 

Lately, more sophisticated devices like vortex shedding 

anemometers and ultrasonic anemometers have appeared 

in other fields of research and they have already been used 

extensively in meteorology. 

In this paper the feasibility of using ultrasonic 

anemometers underground is evaluated. The measurement 

results of ultrasonic anemometers and rotating vane 

anemometers are compared. The influence of dust 

deposition was tested in the laboratory to evaluate 

maintenance requirements of these devices when installed 

underground. Also, choosing a measurement location with 

a representative air velocity is studied. The objective of the 

research is to give guidance for successful and accurate use 

of ultrasonic anemometers in an underground setting. 

A mine operator can use an ultrasonic instrument to 

help them comply with ventilation requirements such as air 

velocity in the belt entry, and total air quantity in the belt 

entry and primary escapeway. However, ultimate 

compliance is determined by MSHA by making traverse 

velocity measurements in the entries using a vane 

anemometer. Currently, ultrasonic anemometers do not 

have an MSHA certification for intrinsic safety. 

2 Measuring air velocity 

2.1 Rotating vane anemometer 

In operating mines the most common instrument for air 

velocity measurements is a rotating vane anemometer. 

Either point measurements or anemometer traverses are 

used to check for air velocity in an entry. The advantages 
of rotating vane anemometers include small size, 

portability, intrinsically safe for use throughout the mine, 

minimal maintenance, and a rather reasonable price of 

about $600 for a basic mechanical instrument that is 

typically used underground in the US. 

The mechanical or Biram type vane anemometer is a 

small, windmill-like instrument consisting of a number of 

radial blades forming an impeller, a gearing mechanism, a 

clutch system, and a pointer. The blades rotate at a speed 

proportional to the air velocity. The gearing mechanism 

translates the rotation directly into linear distance and the 

pointer integrates the flow on a marked dial (De La Cruz, 

1982). 

Vane anemometer errors are categorized as those due 

to the design of the instrument, variations in airflow, and 

errors due to the operation. Errors caused by the 

instrument design are usually quite well managed by 

allowing sufficient time for measurement, typically 

1 minute per reading. Variations in airflow can be avoided 

to some extent by choosing the measurement locations 

carefully. Obstructions, changes in cross-sectional area and 

tunnel intersections should be avoided. At less than ideal 

locations, measurement accuracy can be improved by 

performing a traverse, rather than a single point 

measurement (Kohler & English, 1983). However, air 

density differences and non-uniform air velocity 

distributions may also result in erroneous readings. Vane 

anemometers are sensitive to different measuring 

techniques and the proximity of the observer. It has been 

demonstrated that two observers with the same instrument, 

in the same airway, and under the same conditions may 

take measurements that differ by as much as 20 % (Smith, 

1949; Boshkov & Wane, 1955). 

Vane anemometers are incapable of measuring low air 

velocity due to the physical restriction of the blade 

rotation. Extra sensitive vane anemometers have been 

developed to measure low flows, but basic instruments can 

only measure velocity down to about 0.3 m/s (60 fpm) 

accurately. Correction factors for low velocity 

measurements with vane anemometers have been defined 

by Thimons et al. (1979). 

2.2 Ultrasonic anemometer 

Ultrasonic anemometers have been designed for measuring 

air velocity in three-dimensional space. It has a linear 

response and an absolute calibration that depends only on 

sensor spacing and transit time measurement accuracy. 

Ultrasonic pulses traveling back and forth between three 

sets of sensor probes are used to measure the air velocity 

vector. 

As opposed to conventional forms of measurement 

this technique requires no correction for air density, it has 

no moving parts to wear, there is no start up friction or 

inertial problems when the air velocity changes rapidly and 

the instrument yields a linear output when calculating air 

velocity (Casten et al., 1995). Also, the direction of the 

airflow can be recorded. 

The instrument is robust to the surrounding 

environment, but physical impacts can alter the distances 

between the probes and affect instrument calibration. 

Adjustments to an instrument to correct for changes in 

sensor spacing can only be made by the manufacturer. 

2.2.1 Ultrasonic anemometer models and prices 

Ultrasonic anemometers are currently available from 

several manufacturers. At a minimum R. M. Young, Gill, 

and Vaisala have catalogs with several different ultrasonic 

anemometers. The typical options are basic 2-axis and 3­

axis instruments designed for meteorological use. Some of 

the manufacturers offer heated versions for severe winter 

conditions and marine instruments. The prices of the basic 

2-axis instruments range from below $2000 to the basic 3­

axis instruments at approximately $3000. The prices of the 

research instruments, however, may be as high as $10,000. 

Gill currently offers the most extensive range of ultrasonic 

anemometers in the market including an intrinsically safe 

2-axis instrument. More information can be found from the 

internet pages of the manufacturers (R. M. Young 

Company, 2009; Gill Instruments, 2009; and Vaisala, 

2009). 

EMAG in Poland has developed intrinsically safe, 

ultrasonic anemometers type AS-2 and AS-3 for 

underground use (Korczak, 2001). They are used in gassy 

coal mines in Poland. 

3 Testing 

3.1 Calibration of instruments and data acquisition 

Ultrasonic anemometers were checked before and after 

testing at the NIOSH ventilation gallery by comparing 

their outputs against each other. Manufacturer calibration 

was performed when purchased, and there was no set 

schedule for regular calibration. Only when an instrument 

was not be within a close range to other operating 

anemometers in the same airflow, would it be sent to the 

manufacturer for recalibration. 

The instruments used for testing were two 2-axis 

WindSonic ultrasonic anemometer and a 3-axis 

WindMaster ultrasonic anemometer manufactured by Gill. 

Both WindSonic and WindMaster instruments record flow 

direction and speed in the XY plane which is parallel to the 

mine roof and floor. Only the WindMaster gives the 

component of flow moving up or down through the XY 

plane. 

Communication with the computer is via RS 232 

through the computer Comm port. Data from WindSonic 

and WindMaster instruments is acquired through RS 

422/485. The WindMaster is used with a PCI box which 

supplies power to the instrument and converts the RS 

422/485 signal to RS 232. Telebyte converters and a 

separate 13 V power supply were used with the WindSonic 

instruments. 

The ultrasonic instruments come with a software 

package, which can be used for data collection. Other data 

acquisition options include using Hyperterminal, Gill 

collection software, and instrument collection and control 
software languages like LabVIEW or MATLAB for 

specific software collection and development. 

Hyperterminal lacks a time stamp, which is available in the 

instruments’ software package. There are no other 

differences in the outputs. Currently, up to 4 instruments 

can be used simultaneously at the NIOSH laboratory. 

LabVIEW compatible software is being developed to 

enable simultaneous data collection and real-time handling 

of data from up to 6 instruments. 

3.2 Underground tests 

Tests were performed underground in two locations of the 

NIOSH Bruceton Experimental Mine. The first location 

(Location 1) is in a long, straight section of a tunnel with a 

cross-sectional area of 5.3 m2 (57 ft2

). The airflow was 

expected to be rather stable with similar fluid dynamics 

characteristics to a steady flow in a large tube. The second 

test location (Location 2) is in a curve of about 45°, so a 

difference in velocities was expected over the cross-section 

of the tunnel. The cross-sectional area of the tunnel is 

about 7.7 m2 (83 ft2

). The testing locations are shown in 

Figure 1. Air velocity was varied in the mine by opening 

and closing doors and changing fan settings. 

Figure 1 Underground test locations 

Davis rotating vane anemometers were used to measure 

air velocity for anemometer comparison. Two vane 

anemometer traverses of 1 minute duration were taken in 

Location 1 with the stable airflow for each test setting and 

three traverses were taken in Location 2. The vane 

anemometer was used with an extension rod. The averages 

of the vane anemometer readings were compared with the 

results obtained by the ultrasonic anemometers. 

The ultrasonic anemometer measurement duration was 

three minutes. However, the sample rate was one 

measurement per second for all the ultrasonic anemometer 

tests, so a total of 180 velocity values were obtained during 

each test. 

The ultrasonic anemometers were set up in the tunnels 

so that the 3-axis instrument was in the middle and the 2­

axis instruments were located on both sides of the 3-axis 

ultrasonic anemometer. The 3-axis anemometer was kept 

stationary throughout the measurements in both locations. 

2-axis instruments were attached to adjustable poles with 

swinging arms. This enabled point measurements at 

several spots across the entry. A total of six measurements 

were taken across the entry with both 2-axis instruments at 

both locations. Figure 2 shows the ultrasonic anemometer 

set-up for testing in Locations 1 and 2. 

Figure 2 Instrument placing in the entry 

3.3 Laboratory dust tests 

A prior study examined factors affecting the performance 

of the ultrasonic anemometers in a simulated mine 

environment (Taylor et al., 2004). During their use 

underground, the anemometers will be exposed to coal and 

rock dust (crushed limestone), which is used in coal mines 

to inert the atmosphere to prevent coal dust explosions. 

Laboratory tests were conducted at the NIOSH Pittsburgh 

Research Laboratory’s Ventilation Test Gallery to 

determine what effect dust exposure would have on 

airflow measurements. 

The effect of dust on anemometer measurements was 

evaluated by comparing readings from two instruments 

exposed to the same flows in the ventilation gallery. The 

readings were made in the area behind the blowing curtain 

(Figure 3). The test area is relatively straight with a height 

and width of 2.1 m (7 ft) and 0.6 m (2 ft) respectively. A 

wooden frame supports and maintains the shape of the 

curtain. 

Figure 3 Ventilation Test Gallery 

The two 2-axis anemometers were placed either 3 or 

6.1 m (10 and 20 ft) from the end of the blowing curtain. 

The instrument sensor heads were positioned at the 
centerline of the area behind the curtain 0.3 m (12 in) from 

the curtain and 1.1 m (42 in) from the floor (Figure 4). The 

reference arrow on each instrument was pointed in the 

direction of the airflow and the instrument was positioned 

vertically using a round bubble level. With this orientation 

the velocity was measured for flow in a horizontal plane 

(Figure 5). 

Figure 4 Instrument set-up for dust testing at the Gallery 

Figure 5	 Orientation of anemometer sensor head 

Tests were conducted with flow velocities of 

approximately 0.35 m/s (70 fpm), 1.75 m/s (345 fpm), and 

3.10 m/s (610 fpm). The flows were adjusted by opening 

or closing gallery regulator doors (Figure 3). The 

approximate flow was monitored using a vane 

anemometer. Each test was repeated once and the results 

averaged. The instrument positions were reversed and two 

additional tests were conducted. The results of the four 

tests were averaged for each velocity. 

Either coal dust or rock dust was applied to sensor 

heads of the test instrument. The two dusts were similar 

with respect to particle size. Approximately 75% of each 

dust passed through a 200 mesh sieve. One instrument was 

kept clean at all times for comparison purposes. 

First the dust was applied to the tops of the sensors 

using a spatula. The instrument was then held horizontal 

with sensor head tilted 90° and tapped lightly to remove 

excess dust. For one test set with both coal dust and rock 

dust the sensor heads were first sprayed with water before 

applying the dust. Again the excess dust was discarded by 

holding the instrument horizontal. The resulted dust layer 

for coal dust is shown in Figure 6. Also, testing was 

performed without dumping the excess dust off to evaluate 

the effect of dust piling on the instruments (Figure 7). 

Figure 6	 Wetted coal dust on 2-axis and 3-axis ultrasonic 

anemometer heads 

Figure 7 Dry rock dust piled on the 2-axis instrument. 
4 Results 

4.1 Underground tests 

The point measurement results taken over a cross-section 

with the ultrasonic anemometers were averaged for 

comparison with the averaged vane anemometer results. 

The results from the ultrasonic anemometer and rotating 

vane anemometer comparison show that higher air velocity 

values were measured with vane anemometer in Locations 

1 and 2 than with the ultrasonic anemometers. The 

recorded differences shown in Figure 8 ranged from about 

0.1 m/s (20 fpm) to 0.3 m/s (60 fpm) showing larger 

differences with higher air velocities. This correlates well 

with prior results from Casten et al. (1995) who stated that 

a Gill 3-axis anemometer tended to underestimate the air 

velocity when compared to a Davis rotating vane 

anemometer. 

Anemometer comparison 

0.3 

0.7 

1.1 

1.5 

1.9 

2.3 

0 1 2 3 4 5 

Test 

Air velocity (m/s) 

Vane Ultrasonic 

Figure 8 Comparison of vane and ultrasonic anemometer 

measurement results with different airflows 

Figure 9 Air velocity contours at Location 1 

y 

Figure 10 Air velocity contours at Location 2 
Both 2- and 3-axis ultrasonic anemometers performed 

well even at rather low air velocities. In Location 2, the 

average air velocity of the lowest airflow was 0.29 m/s 

measured with the ultrasonic anemometer. At this velocity 

the vane anemometer refused to work properly. 

Defining the optimal location for measuring air 

velocity in an entry is important. The instrument should be 

able to detect an air velocity as close to the average 

velocity over the cross-section of the entry as possible. For 

location analysis, the results of the three different 

velocities at the two locations were evaluated. Average 

velocities in Location 1 were 0.76, 0.99 and 1.95 m/s and 

in Location 2 they were 0.29, 0.43 and 0.61 m/s. Twelve 

2-axis ultrasonic anemometer measurements (Figure 2) 

were taken over the cross-section in both locations. 

In Location 1, the straight tunnel, the results showed 

the expected flow pattern similar to a flow in a tube with 

the highest velocity in the center. However, the 

distribution of the flow was not as stable as in a tube due to 

the rough walls and corner areas. In Location 2, the curve, 

the airflow was non-uniform with higher airflows in two 

areas. The velocity measurements were taken at Location 1 

0.4 m (14 in), 1.1 m (44 in), 1.5 m, (58 in) and 2.6 m 

(102 in) from left rib and at Location 2 approximately 

0.2 m (7 in), 0.9 m (37 in) and 2.9 m (114 in) and 3.9 m 

from left rib. Measurement heights were 0.6 m (2 ft), 1.2 m 

(4 ft) and 1.6 m (5 ft 6 in) in both locations. Approximate 

air velocity contours (not to scale) are shown in Figures 9 

& 10. Three-axis anemometer readings, taken in the 

middle were used to confirm the 2-axis anemometer results 

closest to the center of the entry. 

The average values recorded during the 3 minute 

sampling in each of the 12 locations were compared with 

the average air velocity over the cross-section. The four 

values out of twelve (33%) closest to the overall average 

were then identified. In Location 1 it was noticed that the 

location of these close-to-average values was comparable 

through all three different air velocities (Figure 11). In 

Location 2, the situation was more complicated with the 

close-to-average value locations spread over the crosssection in a different pattern for each velocity (Figure 12). 

Figure 11 Air velocity values closest to the average over the cross-section in Location 1 (Straight airway) 

Figure 12 Air velocity values closest to the average over the cross-section in Location 2 (Curved airway) 

It can be seen from the results that taking a centerline 

reading will result in a higher than average value in a 

straight tunnel. The air velocity contours are comparable to 

airflow in a duct with zero velocity against the walls of the 

duct and the highest velocity in the middle (Cheremisinoff 

& Gupta, 1983). Average values are to be expected at 

about 1⁄4D from the duct walls in case of laminar flow. In a 

mine the openings may be more rectangular. In this 

situation the hydraulic diameter, DH, can be used to locate 

the average velocity. The average velocity occurs 

approximately at 1⁄4DH from the ribs, floor, and ceiling of a 

rectangular tunnel with a laminar flow (Ebadian & Dong, 

1998). 

4.2 Laboratory dust tests 

The dust exposure testing showed a small decrease in the 

measured velocity after application of dust with or without 
water in almost all cases. Wet rock dust caked on the 

instrument was found to result in instrument signal 

interruptions. Consequently, the wetted rock dust results 

were left out from the velocity difference analysis. The 

results with wet and dry coal dust and dry rock dust are 

shown in Figure 13. Air velocity decrease, m/s 0.12 

0.1 

0.08 

0.06 

0.04 

0.02 

0 

Dry coal Dry rock Wet coal 

3.10 m/s 1.75 m/s 0.35 m/s 

Figure 13 Velocity decrease with wet and dry dusts 

To test the effect of piling of dust, coal dust or rock 

dust was added to the tops of the sensors until there was 

loss of signal and no more velocity data was generated. At 

this point the instrument generated an error code. The 

manufacturer said that when the dust deposit reaches a 

critical thickness where it is attenuating the ultrasonic 

signals too much for the anemometer to compensate for, an 

error code is shown. The measurement results should be 

accurate as long as no error codes are being reported. In 

case of an error code a simple cleaning, such as wiping the 

sensor down with a wet cloth or even blowing some air to 

remove dust from the sensor should be sufficient to return 

the sensor to good working condition (Doyle, 2009). In 

these tests, brushing the sensor head lightly was found to 

work best in removing the dust. Data generation resumed 

after cleaning and velocity readings were approximately 

the same as before application of dust. In practice, the 

recorded air velocity decreases are so small, that before 

loss of signal, the results can be considered accurate 

enough for underground air velocity screening. 

5 Conclusions 

Ultrasonic anemometers can be used underground for air 

velocity screening purposes. It is recommended that for a 

permanent underground installation a location analysis be 

made to enable collection of representative velocity values. 

Point measurements should be taken at several locations 

over the cross-section and their average calculated. The 

location giving closest values to the average should be 

selected for ultrasonic anemometer measurements. In case 

of a straight tunnel with a similar width and height a good 

approximation is flow in a circular duct. Average values 

are acquired at 1⁄4D of the duct walls. In case of a 

rectangular opening, hydraulic diameter DH can be used for 

the approximation. For more complicated tunnel shapes, 

tunnels with obstructions, and curved locations flow 

simulation is recommended to find the optimal location. 

Comparison of vane anemometer and ultrasonic 

anemometer values showed that vane anemometer 

readings were higher than the results obtained with 

ultrasonic instruments. This correlates well with a previous 

study. Even so, the reason for the difference is questioned 

as ultrasonic anemometers are considered very accurate 

and this will be studied in the future. 

It was observed that coal dust was easy to clean off of 

the instrument even when wet and it did not pile up to a 

critical thickness easily. Rock dust, on the other hand, 

tended to cake on the instrument due to its hygroscopic 

behaviour. Even when added on the instrument dry, it 

resulted in the largest reduction in air velocity. Regular 

cleaning of the instrument is expected to be needed in a 

humid, dusty environment to keep it in working order. 

Acknowledgements 

The authors thank for assistance received in test planning 

and underground work from Roy Grau, Paul Stefko, 

Joseph Sabo and Jack Teatino. Also, comments from 

Gerrit Goodman are appreciated. 

Disclaimer 

The findings and conclusions in this manuscript are those 

of the authors and do not necessarily represent the views of 

the National Institute for Occupational Safety and Health 

(NIOSH). Mention of any company or product does not 

constitute endorsement by NIOSH. 

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