USING ULTRASONIC ANEMOMETERS TO EVALUATE FACE VENTILATION CONDITIONS

E. Hall, National Institute for Occupational Safety and Health, Pittsburgh, PA 

C. Taylor, National Institute for Occupational Safety and Health, Pittsburgh, PA 

J. Chilton, National Institute for Occupational Safety and Health, Pittsburgh, PA 

Abstract 

The fact that methane ignitions continue to occur at the mining 

face indicates that monitoring with machine-mounted methanometers 

does not always indicate the presence of high methane concentrations. 

Methane concentrations at the face change quickly due to changes in 

airflow. By measuring these changes in airflow, it may be possible to 

predict changes in face methane levels more quickly. Currently there 

are no techniques or instruments to accurately measure airflow inby 

the mouth of the ventilation curtain or tubing. Tests conducted in the 

NIOSH ventilation test gallery compare the data provided by one-, two-

, and three-axis ultrasonic anemometers. The effects of changes in 

airflow direction and turbulence on instrument readings are discussed, 

and guidelines are given for selecting the type of instruments to be 

used for monitoring airflow near the mining face. 

Background 

Fresh ventilation air is needed at the face to dilute and remove 

methane and dust liberated during the mining operation. Effective face 

ventilation requires that methane liberated at the face be diluted and 

removed quickly. At present, methane measurements are made at 

least once every 20 minutes at the face and continuously on the mining 

machine to determine methane concentrations. Airflow reaching the 

face is the only adequate control that can reduce face methane 

concentrations. As long as the methane concentrations do not exceed 

1 percent, it is assumed that a sufficient air quantity is reaching the 

face. 

The quantity of air that must be provided to each working face 

where coal is being cut, mined, drilled for blasting, or loaded, and the 

locations where ventilation measurements must be taken are specified 

by the following Federal regulations: 

A minimum of 3000 cfm must reach each working face and 

this quantity must be measured “... at or near the end of the 

line curtain, ventilation tubing, or other ventilation control 

device.”[30 CFR §75:325 (a) (1)]. For faces with exhaust 

ventilation, the mean entry air velocity must be measured 

“...at or near the inby end of the line curtain, ventilation 

tubing, or other ventilation control device.” [30 CFR § 

75:326]. 

Airflow measurements made at the inby end of the tubing or 

curtains are usually the only measurements available for estimating 

face airflow. However, past studies have shown that the air quantities 

measured at the inby end of the line curtain are not good estimates of 

how much air actually reaches the face (Thimons, et al., 1999). Airflow 

measurements made between the curtain or tubing and the face might 

give better estimates of how much intake air actually reaches the face. 

However, with current instruments and monitoring techniques, it is 

difficult to make accurate flow measurements. 

For example, vane anemometers are normally used to measure 

airflow at the inby end of the ventilation curtain, but their ability to make 

accurate measurements is limited. The direction of the airflow behind 

the curtain is known and the vane anemometer is aligned with the flow 

direction to obtain an accurate air velocity reading. It is difficult, 

however, to align the anemometer at any location inby the curtain 

because the flow direction is constantly changing. Smoke from 

chemical tubes can be used to estimate flow direction, but only where 

flow speeds and turbulence are relatively low (Taylor, et al., 2003). 

Currently, there are no anemometers approved for underground use 

that can accurately measure airflow velocities between the mouth of 

the curtain and at the face. Moreover, hand held anemometers cannot 

be used under unsupported roof. 

This study evaluated techniques and instruments for making 

airflow readings inby the mouth of the ventilation curtain. The objective 

of the work was to test three different anemometers and identify how 

airflow properties in a simulated mine environment affect airflow 

measurements obtained with the three instruments. Based on the 

results of these tests, an anemometer design is recommended for 

measuring airflow near the mining face. 

The instruments selected for these tests were one-, two-, and 

three-axis ultrasonic anemometers (see Figure 1). The three 

instruments were manufactured by the Gill Instruments, Ltd., Great 

Britain.1

 These instruments were chosen for use in this test program 

because they have the following features: 

• High resolution at low velocities [0.01 m/sec (2 fpm)] 

• Fast response (1 sample per second for these tests) 

• Ability to provide data to calculate flow direction with respect 

to some reference point (two- and three axis only) 

• Output functions that permit data to be transferred to a 

computer-based data acquisition system. 

Figure 1. One-, two-, and three-axis ultrasonic anemometers. 

The operation of an ultrasonic instrument is based on the principle 

that the speed of a sound pressure wave varies with the local air 

speed. The air velocity is calculated from measurements of air-pulse 

 

1 References to specific products do not imply endorsement by 

NIOSH. 
 

transit times between sound transmitter and receiver. All three 

anemometers have a linear response to airflow and an absolute 

calibration that depends only on sensor spacing and transit time 

accuracy. The one-, two-, and three-axis ultrasonic anemometers use 

the pulse transit times to calculate the orthogonal flow components, 

which are designated U, V, and W (see Figure 2). 

Figure 2. Comparison of flow components for three anemometers. 

The one-axis instrument measures flow speed in the direction in 

which the instrument is pointed. The velocity calculated with the oneaxis instrument is equal to the magnitude of the U flow component. 

With the instruments oriented vertically (see “Instrument Orientation” 

below) the U and V flow components are in a horizontal plane that is 

perpendicular to the sensor head. The W component is perpendicular 

to the horizontal plane. 

Velocity measurements calculated with the two-axis anemometer 

are equal to: 

22 +VU

Velocity measurements calculated with the three-axis 

anemometer are equal to: 

222 ++ WVU

The two- and three-axis instruments measure airflow speed in the 

direction of the flow. Directions were calculated using the average 

values for the U and V flow components. The calculations were 

performed in an EXCEL spreadsheet using a modified form of the 

function ATAN2(U,V). 

The angle of the flow directed above or below the horizontal UV 

plane was calculated using the three-axis data. The vertical angle is 

equal to: ATAN(W/UV), where UV = 22 +VU . 

Test Procedures

Test Gallery 

Testing was conducted in the NIOSH Pittsburgh Research 

Laboratory’s Ventilation Test Gallery (see Figure 3). The gallery is 

designed to simulate ventilation conditions in a working entry of an 

underground mine. One side of the empty gallery has the dimensions 

of a mining entry with a 2.2 m (7-ft) high roof and ribs 5 m (16-1⁄2 ft) 

apart. Air enters the gallery through two windows, and an exhaust fan 

removes air from the gallery at a rate of 5.9 m3/s (12,500 cfm). The 

face of the mining entry was 35 ft inby the mouth of the blowing 

ventilation curtain. The curtain reaches from the floor to the roof and is 

supported by a wood frame that is constructed 0.6 m (2 ft) from the 

wall. The area behind the curtain was 0.7 by 2.2 m (2 ft by 7 ft). The 

air quantity behind the curtain is varied by opening or closing regulator 

doors. Three different airflows were used for tests conducted behind 

the curtain: 

• Low flow [1 m/s (200 fpm)] 

• Medium flow [2 m/s (400 fpm)] 

• High flow [3 m/s (600 fpm)]. 

For all face tests the curtain flow was 2 m/s (400 fpm). 

Figure 3. Pittsburgh Research Laboratory ventilation gallery. 

Instrument Orientation 

For all tests, the anemometers were attached to stands and 

oriented vertically. A bubble level was placed on top of the instrument 

to set and check vertical orientation. Instrument height was adjusted 

so that the center of the sensor head was 1.1 m or 3.5 ft from the floor 

(i.e. mid-way between the gallery roof and floor). For all tests the 

reference direction was toward the face. The instruments were rotated 

until the arrow printed on each instrument was pointed toward the face 

(see Figure 2). 

To evaluate the effect of instrument rotation and tilt on velocity 

readings, data were initially taken with the instrument pointed toward 

the face. The amount of rotation, or yaw angle, is defined as the 

number of degrees in the horizontal plane that a vertically positioned 

instrument is rotated in a clockwise direction (see Figure 4). Plastic 

triangles with 30, 45, 60, and 90 degree angles were used to align the 

anemometer at the desired yaw angles. Airflow measurements were 

taken with the instrument directed toward the face (yaw angle = 0) and 

for yaw angles up to 90 degrees. To evaluate the effect of instrument 

tilt on velocity readings, an optical clinometer was used to set the 

anemometer at the desired tilt angle (accuracy +/- 5 degrees), which 

refers to the angle of instrument inclination (in degrees) from the 

vertical orientation (see Figure 5). Without rotating the sensor head, 

the instrument was tilted into the direction of the airflow. For the 

anemometer tilt, the angles used to measure velocities were 0, 30, 60 

and 90 degrees. 
 

Figure 4. Yaw angle (three-axis anemometer.

Figure 5. Tilt angle (three-axis anemometer). 

Instrument Location 

Instruments were tested individually behind the curtain at a 

location 6.1 m (20 ft) from the inby end of the curtain (see Figure 6). 

The sensor head was positioned mid-way between the curtain and the 

wall. 

Figure 6. Sampling locations behind the curtain. 

At the face, three instruments at a time were tested. Airflow 

measurements were made at each of the four locations (see Figure 7): 

• 0.6 m (2 ft) from left wall (Position 1). 

• 1.8 m (6 ft) from left wall, (Position 2). 

• 1.8 m (6 ft) from right wall and (Position 3). 

• 0.6 m (2 ft) from the right wall (Position 4). 

All sampling locations were 0.6 m (2 ft) from the face. 

Figure 7. Sampling locations at the face. 

Data Acquisition 

ANEMVENT 2003, a computer software program written by 

NIOSH (Taylor et al., 2005), was used to record three-axis instrument 

data. Windcom software (provided by the instrument manufacturer) 

was used to record two-axis instrument data and Hyperterminal 

software (Hilgraeve, Inc.) was used to record data from the one-axis 

instrument. All data were transferred to Microsoft Excel spreadsheets 

for analysis. 

The data sampling rate for all tests with each instrument was one 

sample/second, and the duration of each test was three minutes. The 

average reading for each test was calculated for the 180 data points. 

Each test behind the curtain was repeated once and the results 

averaged. Tests at the face were repeated six times and the results 

averaged. 

Results

Flows Behind the Curtain 

Air velocity readings were compared for the three instruments. 

When oriented vertically the instrument readings differed by less than 

3.5 pct (see Figure 8). 
 

Figure 8. Comparison of flow velocities behind the curtain. 

The effects of yaw and tilt angles on instrument readings are 

shown in Figures 9 and 10. Increasing the yaw angle had little effect on 

velocity readings for the two- and three-axis instruments. Furthermore, 

readings measured with the one-axis instrument decreased as the yaw 

angle increased. 

Velocity readings for all three instruments decreased as tilt angle 

increased, but each instrument responded differently at different 

angles. 

Across the Face 

Flow velocities measured at the face with the two- and three-axis 

instruments were comparable, but the differences were greater than 

those measured behind the curtain (8 to 10 pct at the face versus 3.5 

pct behind the curtain), as shown in Figure 11. At all four face 

sampling locations, the one-axis readings were much lower than 

readings obtained with the two- and three-axis instruments. At location 

4 the one-axis readings were negative, indicating the flow direction 

was away from the face. 

Figure 9. Effect of yaw angle on measured velocities. 

Figure 10. Effect of tile angle on measured velocities. 

Discussion 

Tests were conducted to compare the performance of one-, two-, 

and three-axis ultrasonic anemometers at different sampling locations 

(curtain and face) and instrument orientations. When exposed to the 

same airflow behind the curtain, all the instruments gave similar 

velocity readings. To obtain the same readings the instruments were: 

• Oriented vertically 

• Exposed to the same airflow (placed at the same sampling 

location) 

• Oriented in the same direction as the airflow. 

Figure 11. Comparison of flow velocities at face sampling locations. 

The instruments were aligned visually with the airflow, and some 

of the variation between readings was probably due to improper 

alignment. Alignment of the instrument with the airflow is particularly 

important with the one-axis instrument. 

One-axis readings decreased as the yaw angle increased 

because velocities, with this instrument, are measured only in the 

direction of flow. The decrease in the measured velocity can be 

estimated by the cosine of the yaw angle (See Figure 12). 

Figure 12. Measured and calculated function of the cosine angle of 

flow due to yaw angle. 

The effects due to yaw angle on one-axis measurements are 

relatively small below angles of 20 degrees. These results are similar 

to results obtained by a rotating vane anemometer, which is also a 

one-axis instrument (Boshhov, 1955). The 2- and 3-axis instrument 

readings were unaffected by changes in yaw angle because both 

measure velocity in the direction of flow. 

All instrument velocity measurements decreased with increasing 

tilt angle, but the responses were different (Figure 10). For the twoaxis instrument, velocities decreased faster above 40 degrees of tilt 

angle due to the structure of the anemometer. When in the vertical 

position, the air flow can pass through the sensor head. When tilted, 

the air must move around the support structure of the sensor head 

before passing over the sensor head. 

As the three-axis instrument is tilted, the value of U decreases. 

Velocity in the direction of flow is calculated by: 

22 Velocity += WU

The measured velocity is relatively constant until approximately 
 

60 degrees. The change in velocity measured above 60 degrees is 

due to the structure of the anemometer. The three spars that hold the 

sensor probes in place are attached to the body of the anemometer 

located just below the sensor head and to a round connector at the top 

of the sensor head. At tilt angles greater than 60 degrees, the body of 

the anemometer and the upper spar connector interfere with flow over 

the sensor head (Taylor et al., 2004). The physical structures of the 

two- and three- axis instruments were different and had a different 

effect on tilt angle response.

Airflow readings behind the curtain differed by less than 3.5 

percent, while the differences at the face were greater. These 

differences were due to how air flowed over the sensor heads behind 

the curtain versus at the face. For the locations sampled at the face, 

the one-axis readings were much different than readings obtained with 

the two- and three-axis instruments. This was due to the orientation of 

the one-axis anemometer and the direction of the airflow. Flow 

directions, measured at the four face locations with the three-axis 

instrument, are shown in Figure 13. The lengths of the arrows are 

proportional to the air speed. 

Figure 13. Flow directions at four face sampling locations. 

Both the two- and three-axis instruments measured velocity in the 

direction of the airflow. The variations in the flow readings were small 

at locations 2, 3, and 4, but greater than what was measured behind 

the curtain (e.g. less than 3 pct behind the curtain and 8 to 10 pct at 

the face). 

Initially, it was unclear why the difference in the readings for the 

two- and three-axis instruments was greater at the face than behind 

the curtain. It was assumed that air flow patterns in the gallery were 

primarily in horizontal planes. Measurements with the three-axis 

anemometer behind the curtain showed the average vertical angle of 

flow above or below the horizontal sampling plane was no greater than 

3 degrees. However, at the face, the average angle varied from 3 to 8 

degrees for the four positions. In addition, the standard deviation for 

the individual angle measurements was less than 2 degrees behind the 

curtain but was 8 to 24 degrees at face sampling locations. The 

amount of turbulence indicated by the higher standard deviation 

(Hinze, 1975) is probably a significant factor affecting differences in 

two- and three-axis readings. 

Another likely factor is the difference in the measurement area 

between the sensor transducers due to the physical structures of the 

two- and three-axis sensor head. The areas defined by the ultrasonic 

pulse paths through which the air flows are sampled are different for 

the two and three axis instruments. The directional velocities and 

average flows measured across areas of various sizes will differ. 

Since the flow was more uniform in the area behind the curtain, the 

differences across the two- and three-axis instruments in this location 

will be smaller. 

Conclusions 

This study investigated airflow measurements made in a 

simulated mine environment using three different ultrasonic 

anemometers. The difference between the three instruments was the 

number of orthogonal components of flow (U, V, and W) used to 

calculate the flow velocity. The one-axis instrument measures flow in 

one direction, the direction of instrument orientation. The two-axis 

instrument measures flow velocity in a plane defined by the U and V 

flow components in a direction relative to a reference direction. The 

three-axis instrument measures flow in a three-dimensional space 

defined by the U, V, and W components of flow. 

After orienting the three instruments vertically and in the direction 

of the airflow at the same location behind a blowing curtain, the airflow 

measurements obtained with the three instruments were comparable 

(differences were less than 3.5 pct). At the four sampling locations at 

the face: 

• The one-axis instrument gave lower airflow measurements 

• The velocity differences between two- and three-axis 

instrument readings were greater than readings behind the 

curtain. 

Differences between two- and three-axis instrument readings 

were the result of high variability in air flows at the face and the 

physical size and shape of the sensor heads. 

The test results show that when airflow direction is known and the 

anemometer is properly aligned, such as behind the curtain, accurate 

airflow readings can be taken with one-, two- or three-axis ultrasonic 

anemometers or vane anemometers. However, at locations between 

the mouth of the curtain and the face, two- or three-axis anemometers 

are required to accurately measure flow. This is primarily due to 

changing of airflow direction that is difficult to determine without a twoor three-axis instrument. 

A comparison of two- and three-axis instrument performance 

showed that differences in the readings were greater at the face than 

behind the curtain. Higher flow turbulence at the face is believed to be 

primarily responsible for the greater differences. 

The one-axis anemometer has the same limitations as the current 

standard vane anemometer, which is dependent upon orientation with 

respect to airflow. Further ventilation gallery testing will be conducted 

with the two- and three-axis anemometers to evaluate the effects of 

sampling location on measured velocities. It is unlikely that the threeaxis design could be modified for making underground airflow 

measurements because it is too easily deformed by physical stress. 

However, it may be possible to adapt the two-axis design for 

underground use because of its size and a more robust design. The 

two-axis design could be used to monitor flow continuously almost 

anywhere underground, including near a working face or at locations 

outby the mining section. 

References

1. Boshkov, S and MT Wayne (1955), “Errors in Underground Air 

Measurements”, Mining Engineering, November, pp. 1047-1053. 

2. Hinze, JO (1975), “Turbulence”, McGraw Hill, 2nd Edition. 

3. Taylor, CD, JE Chilton, LJ McWilliams, and M Senk (2004), 

“Evaluating Factors Affecting the Performance of Three Axis 

Ultrasonic Anemometers”, Proceedings of the Tenth US/North 

American Mine Ventilation Symposium, Anchorage, Alaska, May 

16-19, pp. 465-470 

4. Taylor, CD, RJ Timko, M Senk, and AD Lusin (2003), 

“Measurement of Airflow in a Simulated Underground Mine 

Environment Using an Ultrasonic Anemometer”, Pre-prints 2003 

Annual SME Meeting, Cincinnati, Ohio, February 24-26. 

5. Taylor, CD, RJ Timko, ED Thimons, and T Mal (2005), “Using 

Ultrasonic Anemometers to Evaluate Factors Affecting Face 

Ventilation Effectiveness”, Pre-prints 2005 Annual SME Meeting, 

Salt Lake City, UT, February 28-March 2. 

6. Code of Federal Regulations, Title 30, Mineral Resources, Part 

75, www.msha.gov/30CFR.htm