Evaluating Factors Affecting the Performance of Three-Axis Ultrasonic Anemometers 

Charles Taylor, Emery Chilton, Linda McWilliams, and Mark Senk 

NIOSH, Pittsburgh Research Laboratory, Pittsburgh, PA USA 

ABSTRACT: Three-axis ultrasonic anemometers are used to study airflow in the NIOSH ventilation gallery 

where ventilation in an underground mine is simulated. In this study, performance characteristics of this anemometer are evaluated and the effects of sensor orientation and sampler location on flow readings are examined. The study results provide guidelines for more effective use of the ultrasonic anemometers in the ventilation gallery. 

1 INTRODUCTION 

Monitoring airflow in underground gassy mines 

is essential for maintaining methane levels below the 

explosive concentration limit and providing a safe 

environment for workers. This is particularly true in 

areas near working faces where methane liberation 

rates are generally highest. Normally a handheld 

vane anemometer is used to measure average air velocity at the end of ventilation tubing or brattice. 

Smoke tubes are used to observe short distance flow 

patterns in face areas. However, airflow is often turbulent near the face and it is difficult to accurately 

measure air velocity and direction. 

Three-axis ultrasonic anemometers are often used 

for collecting meteorological data. Two units, called 

"Windmasters," were purchased from Gill Instruments Ltd., Great Britain (Reference to specific 

products does not imply endorsement by NIOSH). 

Figure 1 shows one of the anemometers attached to 

a stand. 

Figure 1. Ultrasonic anemometer on stand. 

The "Windmaster" units were used to measure airflow velocity and direction in the NIOSH 

Ventilation test gallery. The conditions in the gallery 

simulate ventilation conditions underground and the 

measurements are used to evaluate the effectiveness 

of airflow for diluting and removing methane liberated at the mining face (Taylor 2003). 

In the test gallery the anemometers must frequently be moved to make measurements at multiple 

locations. Thus, proper calibration of the anemometers is important in order to compare flows at multiple locations or for different operating conditions. 

The anemometer 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. 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. During testing in the gallery it is important to check instrument performance to ensure 

that readings remain constant for equal flows and 

that instruments exposed to the same airflow give 

the same flow readings. 
 

This study describes how anemometer performance was evaluated by comparing readings from 

anemometers exposed to the same flows in the gallery. Factors affecting variations in readings obtained in the gallery are also discussed. 

2	 TEST PROCEDURE – COMPARING 

AIRFLOW IN THE VENTILATION GALLERY 

2.1 Test Gallery 

Tests were conducted in the NIOSH ventilation 

test gallery to evaluate how operating conditions affect the flow of intake air in mines. The gallery is 

designed to simulate ventilation conditions near the 

face in an underground mine. The research focuses 

on ways to improve the dilution and removal of 

methane liberated at a working mining face. 

One side of the L-shaped gallery is designed to 

simulate a mining entry with a 2.2-m (7 ft) high roof 

and ribs that are 5 m (16 1/2 ft ) apart (see Figure 2). 

Figure 2. Ventilation Test Gallery. 

Air enters through two windows and an exhaust fan 

removes air from the gallery at a rate of 5.9 m3

/s 

(12,500 cfm). A brattice curtain that reaches from 

the floor to the roof is supported by a wood frame 

that is constructed 0.6 m (2 ft) from the wall. The 

curtain directs outside air toward the face. The air 

quantity was increased by closing the regulator 

doors or decreased by opening them. 

The anemometers were tested in the 2-ft wide 

area between the curtain and wall. The flow turbulence was less in the area because the wall and curtain were relatively straight and the cross-sectional 

area -- 0.7 by 2.2 m (2 ft by 7 ft) -- relatively uniform. 

2.2 Instrument orientation 

The ultrasonic anemometer measures flow in 

three orthogonal directions, U, V and W (see Figure 

3), but airflow in the test facility moves primarily in 

the horizontal U and V directions. 

Figure 3. Airflow passing over sensor head . 

Most profiles of 

flow are drawn in a UV plane that is parallel to the 

gallery roof and floor. Using anemometer readings, 

airflow profiles for the gallery are drawn to show the 

air direction and relative velocity in the horizontal 

plane that is mid-way between the roof and floor. 

To measure flow in a horizontal plane, the anemometer is attached vertically (W direction) to a 

stand. In this position, the flow velocity and direction are measured in a horizontal plane that is perpendicular to and passes through the center of the 

sensor head. Flow velocities in this horizontal plane 

are described by the resolved magnitude of the U 

and V vectors and calculated by the equation: 

Velocity = 2 2 U + V . 

2.3 Data Acquisition 

Each anemometer is powered by a "Power Communication Interface box" (PCI) which collects signals from the instruments and transfers the information to a computer. To collect data from two 

anemometers, the two instruments are networked 

through separate PCI boxes as shown in Figure 4. 

Figure 4. Networking of two anemometers. 

 A software program, ANEMVENT 2003, was 

written and developed by NIOSH personnel for acquisition of ultrasonic anemometer data. More information about the ANEMVENT 2003 program is 

available in the software instruction manual (Senk 

2003). ANEMVENT 2003 sequentially polls the 

two anemometers at intervals of 0.2 seconds. 

ANEMVENT 2003 then creates EXCEL spread-
sheets for storing and analyzing the data. EXCEL 

functions are used for calculating the air flow velocity and direction. 

During a test, data from each instrument was recorded once per second for a period of 3 minutes. 

The 3-minute sampling period (180 samples) was 

chosen to provide average flow magnitude and direction readings that would include the expected 

range of values. 

2.4 Comparing Readings from the Two Instruments 

To compare the flow readings the two instruments (designated instruments A and B) were alternately placed at locations 3 and 6.1 m (10 or 20 ft) 

from the end of the curtain. The sensor heads were 

positioned at the centerline of the area behind the 

curtain i.e. 0.3 m (1 ft) from the curtain and 1.1 m 

(3.5 ft) from the floor (see Figure 5).

Figure 5. Anemometer sampling locations behind curtain. 

 To maintain 

the same orientation for each instrument, a reference 

arrow on top of the sensor head was pointed toward 

the face, and the anemometer body was positioned 

vertically. A round bubble level was used to check 

the vertical orientation. Flows of approximately 100, 

400, and 700 fpm were directed over the two anemometers. 

A two-way analysis of variance (ANOVA) was 

conducted to compare the effect for instrument (A 

and B) and the effect of sampling at the two locations (20 ft and 10 ft). The results are presented in 

Table 6 which can be found in the Appendix. Because this was a balanced design, the analysis was 

robust to any deviations from normality and variance homogeneity (Keppel 1982). Due to the large 

sample size of 720 measurements, the level of statistical significance was set at p <.0001. 

No main effect due to anemometer was found at 

any of the three airflow velocities (100, 400, or 700 

fpm). There was a significant anemometer-location 

interaction at 100 and 400 fpm. The interaction at 

100 fpm was disordinal (i.e., at 10 ft the mean for B 

was larger while at 20 ft the mean for A was larger); 

thus the significant main effect for location could 

not be interpreted without considering the individual 

anemometer. At 400 ft, the interaction was ordinal, 

with the means at 10 ft being consistently higher 

than the means at 20 ft. At 700 fpm there was only a 

significant main effect for sampling location (10 ft 

means greater than 20 ft means). 

For concurrent measurements with instruments A 

and B, airflow velocities were 4 to 6 pct lower at the 

20-ft location most of the tests. Differences in cross 

sectional areas at the 10- and 20 ft locations account 

for most of this difference. The cross sectional area 

at the 20- ft location was 3 percent greater than at 

the 10-ft location. 

Flow measurements were also made at locations 

3.0 and 3.7 m (10 and 12 ft) from the end of the curtain. The mean airflows and standard deviations for 

anemometers A and B are given for flows of 100, 

400, and 700 fpm in Table 2. 

Table 2 Flows measured at 10 and 12 ft (fpm). 

Anemometer A Anemometer B 

Mean 

(fpm) 

Std. 

Dev. 

Mean 

(fpm) 

Std. 

Dev. 

Air Velocity 100 fpm 

Sampling 

Location 

10 ft 125.39 9.00 138.90 8.64 

20 ft 137.87 9.56 120.22 11.23 

Air Velocity 400 fpm 

Sampling 

Location 

10 ft 415.93 11.41 405.85 12.24 

20 ft 387.82 17.69 391.91 17.08 

Air Velocity 700 fpm 

Sampling 

Location 

10 ft 692.98 19.86 687.88 17.70 

20 ft 658.16 26.26 656.23 25.58 

Table 1 presents the mean airflows and standard 

deviations for anemometers A and B for each of the 

airflow velocities. 

 

Table 1. Flows measured at 10 and 20 ft (fpm). 
Anemometer A Anemometer B 

Mean 

(fpm) 

Std. 

Dev. 

Mean 

(fpm) 

Std. 

Dev. 

Air Velocity 100 fpm 

Sampling 

Location 

10 ft 96.99 9.99 82.96 7.86 

12 ft 90.75 8.45 101.96 10.75 

Air Velocity 400 fpm 

Sampling 

Location 

10 ft 454.44 10.02 451.22 10.96 

12 ft 482.13 12.14 476.20 10.47 

Air Velocity 700 fpm 

Sampling 

Location 

10 ft 640.34 13.61 650.09 12.71 

12 ft 710.56 14.70 707.81 16.01 

A two-way ANOVA to test the effect of instrument (A and B) and the effect of sampling at the two 

locations (10 ft and 12 ft) was again performed (See 

Table 7 in the Appendix). No main effect was found 

for instrument at either the 100 or 700 fpm air flows. 

At 400 fpm, there were significant main effects for 

anemometer and location. There was a disordinal 

interaction at 100 fpm (instrument A having a larger 

mean at 10 ft and instrument B having a larger mean 

at 12 ft) and an ordinal interaction at 700 fpm (the 

mean values were consistently higher at 12 ft). 

Air flow measurements at the 12-ft location were 

consistently higher except for the 100 fpm. The 

cross-sectional area measured at the 12 ft location 

was greater (6 pct) than at the 10 ft location. The 

area at the measurement location, therefore, could 

not account for the higher velocities. 

The proximity of the instruments is the most 

likely cause for the flow differences. The two instruments were placed so that one instrument was directly upstream of the other instrument. The upstream instrument (12 ft) diverted part of the flow 

around the downstream instrument. This effect was 

seen when the instruments were 2 ft apart, but when 

the distance was 10 ft the upstream instrument appeared to have no effect on downstream flow readings. 

2.5 Orientation of the anemometer 

Tests were conducted to determine how rotation 

and tilting of the instruments affected the airflow velocity readings. To evaluate the effects of rotation, 

flow velocity measurements were made with the reference arrow on top of the sensor head directed toward the face, 180 degrees away from the face, 90 

degrees to the right, and 90 degrees to the left of the 

face (see Figure 6).

Figure 6. Varying directions of the sensor head. 

 All measurements were made 

with instruments A and B located 20 and 10 ft respectively from the end of the curtain. Each instrument was oriented vertically. Air velocities were 

measured for flows of approximately 100, 400, and 

700 fpm. The one-second readings were recorded for 

3 minutes and averaged. The 3-minute average readings for the four directions are given in Table 3. 

Table 3 Three-minute readings (fpm). 

Instrument 

A 

Direction, 

deg. 100 fpm 400 fpm 700 fpm 

0 90 374 662 

90 99 380 655 

180 97 382 660 

270 103 378 665 

Instrument 

B 

Direction, 

deg. 100 fpm 400 fpm 700 fpm 

0 92 384 727 

90 100 393 707 

180 98 392 716 

270 106 396 727 

The differences between flow measurements 

taken for the four rotational directions were numerically small but, in most cases statistically significant. The greatest variation occurred with the flow 

was approximately 100 fpm. 

The results of one-way ANOVA's conducted for 

anemometer A found significant mean differences 

for rotation direction at 100 fpm [F(3,716) = 67.48, 

p<.0001] and 400 fpm [F(3,716) = 8.44, p<.0001]. 

No significant differences among the means were 

detected at 700 fpm. For anemometer B significant 

mean differences across the four degrees of rotation 

were shown at 100 fpm [F(3,716) = 105.19, p< 

.0001]; 400 fpm [F(3,716) = 36.36, p< .0001]; and 

700 fpm, [F(3, 716) = 28.63, p< .0001]. 

The differences in flow measurements could be 

due to the effects of sensor head orientation, or to 

variation in flow behind the curtain between tests. 
The differences in readings for the four head orientations increase as flow variation between the tests increases. 

Measurements comparing readings 10 and 20 ft 

and 10 and 12 feet were taken 0.2 seconds apart. 

Readings taken to compare effects of head rotation 

were obtained during different 3-minute time intervals. 

To estimate how much flow varied between tests, 

5 consecutive 3-minute airflow readings were taken 

behind the curtain with instrument A. The arrow on 

the sensor head was directed toward the face for all 

readings, Measurements were made for flows of 

100, 400, and 700 fpm (see Table 4). 

Table 4. Airflows for consecutive 3-minute 

readings (fpm). 

Instrument A 

3-minute 

reading 100 fpm 400 fpm 700 fpm 

1 90 374 662 

2 97 378 659 

3 95 377 663 

4 77 377 661 

5 92 378 662 

Std. 

Dev. 7.9 1.6 1.51 

The standard deviations for the consecutive readings shown in Table 4 varied from 1.5 to 7.9. The 

variation was 5 times greater for the 100 fpm flow 

than for the higher flows. Differences in readings 

when the anemometer was rotated can be partly attributed to variations in flow, especially at the low 

flow of 100 fpm. 

To evaluate the effects of vertical orientation, 

measurements were made with the anemometer labeled as instrument B. It was clamped to a stand and 

placed in inlet air 20 ft from the end of the curtain. 

The reference arrow was pointed toward the face 

while a flow velocity of approximately 400 fpm was 

directed over the anemometer. 

The flow velocity was first measured with the 

anemometer positioned vertically and then tilted, 

into the airflow, at angles of 5, 10, 20, 30, 45, 60, 

70, 80, and 90 degrees (see Figure 7).

Figure 7. Anemometer tilt orientation. 

 Tilt angles 

were measured with an inclinometer placed on the 

side of the anemometer. Measurements of flow velocity were made for 3 minutes at each of the tilt angles. 

Table 5 gives the average horizontal flow velocity (fpm) measured for each tilt angle. 

Table 5. Tilt angles versus velocity readings. 

Tilt Angle Velocity 

0 437 

5 431 

10 425 

20 405 

30 374 

45 300 

60 216 

70 127 

80 30 

90 6

 As shown, 

the measured velocities decreased as the tilt angle 

increased. The error in measurement of flow caused 

by tilting is approximately 1 pct at 5 degrees and 3 

pct at 10 degrees. The tilt angle must be kept less 

than 10 degrees to maintain flow accuracy within 3 

pct. 

Airflow velocity in the horizontal plane is calculated from the U and V flow components. U0 is the 

magnitude of the U component when the instrument 

is vertical (0 angle of tilt). Increasing the angle of 

tilt decreases the magnitude of U component. For a 

tilt angle of 2 degrees, the measured magnitude of 

the U component is U2. The ratio U2 /U0 is equal to 

cosine 2 (see Figure 8). 
Figure 8. Effect of tilt angle (θ) on U magnitude. 

The graph in Figure 9 shows how the measured U 

vector decreased with increasing tilt angle. 

Figure 9. Change in U magnitude with tilt angle. 

The magnitude of the U vector was corrected for angle of tile 

by dividing U0 by the cosine of the angle of tile (2). 

The values for the U0/cos2 remained relatively constant until the tilt angle exceeded 60 degrees, and after which the calculated values decreased rapidly. 

This sudden change 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, 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. At a tilt 

angle of 90 degrees, flow in the U direction was essentially 0. 

3 DISCUSSION AND CONCLUSIONS 

Three-axis ultrasonic anemometers are used for 

measuring airflow and making flow profiles in the 

NIOSH ventilation gallery. This study compared the 

performance of two anemometers in the gallery and 

evaluated the effect of instrument positioning and 

orientation on the readings. 

When exposed to the same airflow and placed 10 

ft from one another, the instrument readings on the 

anemometers were very similar. Irregularities in the 

cross-sectional areas of the locations where the 

measurements were made, and the proximity of the 

instruments, affected how much the readings differed. 

To correct for area differences in readings it is 

necessary to alternate sampling locations and average the results. When making flow measurements 

any objects which could obstruct flow should be 

kept as far away as practical. For flow conditions in 

the gallery, the results indicate that objects 10 ft 

away will not interfere with flow measurements. 

The rotation of the sensor head has little effect on 

flow velocity readings. However, any flow tilt angle 

does affect the flow velocity readings. As reflected 

in these study results, it is recommended that the tilt 

angle not exceed 10 degrees. 

To compare airflows in the test gallery requires 

that the flow behind the curtain be controlled and 

that the flow remains relatively constant. Flow did 

vary behind the curtain and the variation was greatest at the lowest flow. To reduce the effects of flow 

variation behind the curtain flow readings for multiple instruments should be compared at higher velocities (e.g. 400 to 700 fpm). 

Tests of the anemometer tilted into the airflow 

showed that although the U and V axis measurements were accurate, flow in the W, or vertical axis, 

was accurate only for airflows less than 60 degrees 

from the horizontal axis. The unit structure holding 

the six ultrasonic sensors in fixed position interferes 

with airflows at tilt angles greater than 60 degrees. 

To make accurate flow measurements when there is 

significant vertical flow, the anemometer should be 

positioned at a 90 degree tilt (horizontal). 

REFERENCES 

Taylor, C.D. Timko, R.J. Senk, M. Lusin, A. Measurement of 

Airflow in a Simulated Underground Mine Environment 

using an Ultrasonic Anemometer, Annual Meeting, Preprint 03-065, 6 pp. 

Senk, M, Taylor, C.D. Anemomvent 2003 User's Guide, 

NIOSH Internal Document. 

Keppel, G. 1982. Design and Analysis A Researcher’s Handbook 2nd edition . Prentice-Hall, Inc., Englewood Cliffs, NJ 
Appendix 

Table 6. ANOVA for 10 and 20 ft Readings.
 

Source 

Sum of 

Squares df 

Mean 

Square F 

Air Velocity 100 fpm 

Anemometer 773.07 1 773.07 8.29

Location 1730.23 1 1730.23 18.55*

Anem X Loc 43684.08 1 43684.08 468.23* 

Error 66800.67 716 93.30 

Air Velocity 400 fpm 

Anemometer 1616.22 1 1616.22 7.31

Location 79608.03 1 79608.03 360.11* 

Anem X Loc 9039.58 1 9039.58 40.89* 

Error 158284.18 716 221.07 

Air Velocity 700 fpm 

Anemometer 2218.24 1 2218.24 4.32 

Location 198819.85 1 198819.9 387.64* 

Anem X Loc 452.62 1 452.62 0.88 

Error 367235.69 716 512.90 

* p < .0001 

Table 7. ANOVA for 10 and 12 ft Readings 

Source 

Sum of 

Squares df 

Mean 

Square F 

Air Velocity 100 fpm 

Anemometer 355.47 1 355.47 4.08

Location 7329.60 1 7329.60 84.11*

Anem X Loc 28668.83 1 28668.83 328.98* 

Error 62395.03 716 87.14 

Air Velocity 400 fpm 

Anemometer 3765.91 1 3765.91 31.55*

Location 124853.72 1 124853.7 1045.93* 

Anem X Loc 332.35 1 332.35 2.78 

Error 85469.43 716 119.37 

Air Velocity 700 fpm 

Anemometer 2197.56 1 2197.56 10.73

Location 736606.97 1 736607.9 3595.88* 

Anem X Loc 7031.22 1 7031.22 34.32* 

Error 146670.93 716 204.85 

* p < .0001