Effects of obstructions, sample 

size and sample rate on ultrasonic 

anemometer measurements 

underground

A.L. Martikainen, C.D. Taylor and A.L. Mazzella

Research engineer, industrial hygienist and technician, respectively, Office of Mine Safety and Health 

Research, National Institute for Occupational Safety and Health, Pittsburgh, PA

Abstract

In fluctuating airflow, continuous air velocity recording is the most reliable method of air velocity measurement. It allows for fast recognition of changes and the calculation of long-term averages. Also, it 

enables the mine operator to identify when the airflow has decreased to a point requiring action. Using 

ultrasonic anemometers provides an accurate option for continuous air velocity monitoring. This paper 

provides information about the effect of common obstructions in underground mining on air velocity 

readings. Stationary and moving obstructions are used to represent workers and equipment that would 

cause discrepancies in measured airflow. Also, it is important to know how large of a sample size is required 

to ensure reasonable accuracy of results. Statistical analysis is used to evaluate the required sample size. 

The sampling procedure is further studied by comparing two different sample rates. The results show that 

obstructions provide noticeable differences in air velocity measurements. Also, movement of obstructions 

can be recognized from changes in results. Surprisingly small sample sizes provide reliable air velocity 

information. Standard sample rates are found to be suitable for the underground environment.

Introduction

Ultrasonic velocity transducers have been used extensively 

in fluid flow applications, but are a relatively new addition to 

the underground mine environment. 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 transit 

times between sound transmitter and receiver (Hall et al., 2007). 

Ultrasonic anemometers have a linear response to airflow and 

an absolute calibration that depends only on sensor spacing 

and transit time measurement accuracy (Taylor et al., 2004). 

As opposed to conventional forms of velocity measurement, 

this technique requires no correction for air density, there are 

no moving parts to wear and there are no start-up friction or 

inertial problems when the air velocity changes rapidly (Casten 

et al., 1995). An important advantage of this method is the 

ability to provide a directional sign to the air velocity.

Ultrasonic instruments fall into two distinct categories: 

variable-distance instruments and fixed single-point instruments (Casten et al., 1995). Variable-distance instruments 

consist of two ultrasonic transceivers mounted on each side of 

an airway, pointed axially towards each other and measuring 

the difference in time of flight. Variable-distance instruments 

are limited to one-axis measurements, but are often capable 

of calculating airflow for a known area. Fixed-distance units 

operate on the same principle as variable-distance units, but 

the measurement is performed inside one unit, with a typical 

sensor array of about 0.2 m (0.7 ft). Fixed-distance ultrasonic 

anemometers are categorized as one-, two- and three-axis. 

From the fixed-distance ultrasonic anemometer options 

available, the one-axis instrument measures flow in one direction, the direction of instrument orientation. Thus, it has the 

same limitations as the current standard vane anemometer, 

which is dependent upon orientation with respect to airflow. 

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. 

Some new additions to the variable distance instrument 

category have been developed recently in Canada for fan 

airflow monitoring in underground mines (Synergy Controls 

Corporation, 2010; Accutron Instruments, 2010). Fixed, 

single-point instruments currently available are primarily for 

meteorological and research purposes (R. M. Young Company, 

2009; Gill Instruments, 2009; Vaisala, 2009).
Coal mine operators can use an ultrasonic instrument to 

help them comply with ventilation requirements, such as air 

velocity and direction in the belt entry and total air quantity 

in the belt entry and primary escapeway (Martikainen et al., 

2010). However, ultimate compliance is determined by the 

U.S. Mine Safety and Health Administration (MSHA) by 

taking traverse velocity measurements in the entries using 

a vane anemometer. Currently, some ultrasonic anemometer 

manufacturers have either applied for or are looking into applying for MSHA certification of permissibility, as defined 

under 30 CFR § 75.506, to enable the use of their instruments 

in U.S. underground coal mines. Several instruments have been 

classified as intrinsically safe, based on other certifications in 

countries including Poland, Canada and the U.K. Permissible, 

MSHA-certified instruments could be used in return airways 

of coal mines in the U.S. instead of only in fresh air. 

This study evaluates the feasibility of single-point twoaxis and three-axis ultrasonic anemometers for air velocity 

measurements in underground coal mines. The specific issues 

addressed by the study are the effects of common obstructions 

underground, required sample size to achieve accurate results 

and an adequate sample rate. 

Test setting

Tests were performed underground in three locations of 

the U.S. National Institute for Occupational Safety and Health 

(NIOSH) Bruceton Experimental Mine, which is driven into 

the Pittsburgh coal seam. The first location (Location 1) is in 

a long, straight section of a tunnel with a cross-sectional area 

of 5.3 m2 (57 sq ft). The second test location (Location 2) is 

in a curve of about 45°. The cross-sectional area of the tunnel 

is about 7.7 m2 (83 sq ft). Locations 1 and 2 were chosen to 

represent airflow in a straight and curved tunnel and because 

of instrument cable length restrictions. Location 3 is in an 

entry to an opening used to run cables through a bulkhead. 

The cables block part of the opening, causing a very uneven 

airflow. Also, the change in cross-sectional area was expected 

to result in high turbulence at measurement Location 3. The 

cross-sectional area of Location 3 is 3.0 m2 (32 sq ft), and the 

cross-sectional area of the opening is 0.7 m2 (7.5 sq ft). All 

instruments were located in the larger area. Air velocity was 

varied in the mine by opening and closing doors and changing 

fan settings. All testing locations are shown in Fig. 1.

Figure 1 — Underground test locations.

At all three test locations, the ultrasonic anemometers were 

set up in the tunnels with the three-axis instrument positioned 

between the two-axis instruments. The three-axis anemometer 

was kept stationary throughout the measurements in all locations, while the two-axis instruments were attached to adjustable 

poles with swinging arms. This allowed for point measurements to be taken at several spots across the entry. A total of 

six measurements were taken across the entry simultaneously, 

two at a time with two two-axis instruments at Locations 1 

and 2. Figure 2 shows the ultrasonic anemometer setup for 

testing in Locations 1 and 2. The same setup was used during 

a previous study (Martikainen et al., 2010). A similar setup was 

used at Location 3, but due to a smaller cross-sectional area, 

the swinging arms were not turned and only two heights, high 

and low, were used for the two-axis instruments. As a result, 

only five measurements were taken over the cross-section at 

Location 3 (two two-axis measurements on each side, plus a 

three-axis measurement in the middle), while in Locations 1 

and 2 the number of measurements was 13. 

A 180-s data collection time was used with the ultrasonic 

anemometers. The sample rate used was one sample per second 

(one sample/s) except for the sample rate study, during which 

a sample rate of four samples/s was used. These sample rates 

were readily available in all measurement instruments.

Davis rotating vane anemometers were used to measure air 

velocities for comparison. Three vane anemometer traverses of 

60 s were taken in all locations to compare with the averages 

of the 180-s data measured by the ultrasonic anemometers. 

The 180-s data collected by the three ultrasonic anemometers 

at all 13 points in Locations 1 and 2, as well as at five points 

in Location 3, was averaged for reliable comparison with 

the vane anemometer traverse results. The vane anemometer 

was used with an extension rod to minimize errors caused 

by measurement-taker proximity. The averages of the vane 

anemometer readings were compared with the averages of the 

results obtained by the ultrasonic anemometers.

Obstruction analysis

Obstruction test setup. A potential difficulty with continuous airflow monitoring may arise with the presence of 

obstructions (i.e., equipment and personnel) upstream of an 

anemometer station. Depending upon the distance from the 

anemometer, such obstructions can interfere with the flow 

around the sensor head and may generate vortices and eddies 

that seriously impact the accuracy and stability of the output 

of this instrument. A series of airflow evaluations with two 

different types of obstructions, a person and a personnel and 

equipment carrier cart, were conducted to examine the impacts 

of both stationary and mobile obstruction at two different 

distances from the instrument location.

Two different stationary obstructions were tested with two 

airflows at Locations 1 and 2. The first obstruction (Stationary 

Obstruction 1) was a test subject with a height of 1.75 m (5.9 

ft) and an approximate average width of about 0.4 m (1.3 ft) 

in full mine gear. Stationary Obstruction 1 was placed between 

the instrument poles. To distribute the effect evenly between 

instruments, the obstruction was moved to four locations from 

left to right in the entry, as follows: (1) between rib and left 

pole, (2) between left pole and the three-axis instrument, (3) 

between the three-axis instrument and right pole (Fig. 3) and 

(4) between right pole and rib. Stationary Obstruction 2, an 

electrician’s personnel and equipment carrier cart, was placed 

in front of the instrument setup, 3.0 m (10 ft) upstream from 

the instruments. The height of this cart is about 1.17 m (46 in.) 

and the width is 1.14 m (45 in.). The cart is shown in Fig. 4.

The test subject was also used as a moving obstruction. 

The test subject moved with a steady pace of about 0.25 m/s 
(50 fpm) back and forth across the entry at two different distances, first at 1.2 m (4 ft) (Moving Obstruction 1) and then 

at 3.0 m (10 ft) (Moving Obstruction 2) upstream from the 

instruments. Tests were performed at Locations 1 and 2 with 

two different airflows. 

Figure 2 — Anemometer placement for Locations 1 and 2.

Figure 3 — Stationary Obstruction 1 between two-axis and 

three-axis ultrasonic anemometers in Location 2.

Figure 4 — Stationary Obstruction 2 upwind of Location 1.

Results of obstruction testing

With both airflows in Location 1, Stationary Obstruction 1 

increased the average air velocity measured across the entry 

by about 15%. An air velocity increase was also observed at 

Location 2. The change in air velocity corresponds to an area 

decrease of about 0.7 m2 (7.2 sq ft) at both locations, which 

correlated well with the size of the obstruction. 

Similar air velocity differences due to the effect of Stationary 

Obstruction 1 were observed in previous tests when comparing the results of a rotating vane anemometer to the results 

obtained by the ultrasonic anemometer setup (Martikainen et 

al., 2010). Even when an extension rod was used according to 

the suggested practice of keeping a minimum distance of 0.9 

to 1.2 m (3 to 4 ft) between operator and instrument (Boshkov 

and Wane, 1955), with care taken to keep the anemometer 

upstream of the measurement taker, all the vane anemometer 

readings were significantly higher than those recorded by 

the ultrasonic anemometers, ranging from 13 to 22%. These 

differences were recognized to correlate well with the size 

of the measurement taker and were comparable to the effect 

of Stationary Obstruction 1 on the ultrasonic anemometer 

measurement results.

Stationary Obstruction 2 affected the airflow in Location 1 

in the same way as Stationary Obstruction 1. In this case, the 

measured change in air velocity was about 22%. The expected 

air velocity increase from the free cross-sectional area decrease 

caused by the obstruction based on a calculation was 1.4 m2

(14.9 sq ft) and the measured amount was 1.3 m2 (14.4 sq ft). 

This discrepancy can be explained by the circumstances in 

Location 2 being more complicated. The average change in 

air velocity was different for every instrument. In some cases, 

a slight decrease in air velocity was observed instead of an 

increase. It was determined that the stationary obstruction 

caused an air velocity distribution change over the entry cross 

section, moving higher airflow from the left side of the entry to 

the right. Also, the three-axis instrument recorded noticeable 

W-axis values (air moving up- or downwards).

Moving Obstruction 1 decreased the average air velocity 

values in comparison to the air velocity with no obstruction. 

Comparison of the ultrasonic anemometer outputs with the 

positions of the obstruction showed a significant decrease of 

airflow immediately after Moving Obstruction 1 had passed 

the instrument. After this disturbance, the air velocity quickly 

returned to slightly above that measured with no obstruction. 

As the obstruction moved only 1.2 m (4 ft) away from the 

instruments, the drop in air velocity due to the disturbance was 

large enough to cause a decrease in the average air velocity. 

The close proximity of the obstruction to the instruments, the 

turbulence caused by its movement and the relatively small 

size of the obstruction were considered the main reasons for 

the air velocity decrease. The disturbances are easily recognizable in comparison to the undisturbed airflow and are shown 

in Fig. 5. In this figure, the impact of the obstruction on the 

airflow can be clearly seen as pronounced dips in the plot. It 

is important to realize that the readings quickly returned to 

near normal, unobstructed levels once the obstruction passed 

the anemometer.

Similar results were observed for Moving Obstruction 2. In 

this case, however, the average air velocities were approximately 

the same as with no obstruction. The disturbances caused by 

the obstruction moving further away from the instruments were 

not long enough or large enough to affect the averages. An 

example of the air velocity averages with no obstruction and 

with obstructions from all three instruments is shown in Fig. 6. 

Air velocity values recorded by the two-axis and three-axis 
instruments are very similar in all cases, except for Stationary 

Obstruction 2. This result shows that in a straight airway, even 

with an obstruction, the performance of a two-axis ultrasonic 

anemometer is comparable to the performance of a three-axis 

instrument. The large difference with Stationary Obstruction 2 

can be explained by instrument positioning. Directly behind the 

obstruction, air velocity was decreased, but on both sides, air 

velocity increased. This was detected by the instruments when 

the three-axis instrument was placed behind the obstruction 

with the two-axis instruments located on both sides. 

Figure 5 — Results showing airflow with no obstruction 

compared to airflow disturbed by Moving Obstruction 1 

at Location 1.

Figure 6 — An example of air velocity averages with obstructions and with no obstructions in Location 1.

Sample size and sample rate

Testing. Measurements to evaluate the required sample size 

to obtain a reasonable accuracy of results with the ultrasonic 

anemometers were taken at all three locations. For a rotating 

vane anemometer, a recommended minimum traverse time is 

60 s (McPherson, 2009). A minimum number of three measurements with results within 5% of each other are suggested. To 

ensure easy comparison of results, a 180-s data collection time 

was used with the ultrasonic anemometers. The collected data 

was then analyzed statistically to determine adequate sample 

sizes for different measurement conditions. 

 The rotating vane anemometer commonly used to measure 

air velocity in underground coal mines is known to suffer from 

erroneous readings in turbulent airflow. Also, vortex-shedding 

anemometers often used for fixed point measurements perform 

well in laminar flow only (Thimons and Kohler, 1985). For 

these reasons, ultrasonic anemometer performance in a highly 

turbulent airflow was tested. The required sample size was 

expected to be very different in such a case. Location 3, which 

was close to an opening used to run cables through a bulkhead, 

was chosen for these tests, because it was expected to have an 

extremely turbulent airflow. No Reynolds number was defined 

for this location, but the two changes in cross-sectional area 

and the presence of cables in the openings resulted in severe 

disturbances of the airflow, altered airflow patterns and turbulence, as mentioned as typical for such locations by Kohler 

and English (1983). All three ultrasonic anemometers were 

used for testing in Location 3 (Fig. 7).

Only two sample rates, one sample/s and four samples/s, 

were readily available for both two-axis and three-axis ultrasonic 
anemometers. Twelve tests were performed in Location 1 with 

these two sample rates. Location 1 was chosen for these tests 

due to its similarity to typically accepted underground velocity 

measurement locations. The obtained results were compared to 

define an appropriate sample rate for underground use. 

Figure 7 — Test setup at Location 3 with the two-axis instruments at low position.

Results

Statistical calculation was used to evaluate the required 

sample size. Adequate sample size was calculated by using 

Eq. (1):

 (1)

in which n is the desired sample size, z0 is the z-value corresponding to the desired confidence, σ2 is the population 

variance and e is the maximum allowable error of estimate 

(Hoel, 1960). Confidence intervals of 95% and 98% with an 

error value of 0.01 were used to determine the sample size. An 

example data set from Location 2 is shown in Fig. 8.

Figure 8 — An sample data set from Location 2 showing a rather stable airflow and a trendline.

The results of the statistical calculations showed that the 

smallest sample size was required for Location 1 with a stable 

airflow. In that case, only one sample, taken in the middle of 

the airway, was sufficient to satisfy both confidence intervals, 

while two to six samples were required to reach 95% and 98% 

confidence intervals in the middle of the entry at Location 2, 

respectively. However, when samples were taken close to the 

rib, the more turbulent airflow resulted in larger sample sizes: 

two to nine samples for Location 1 and up to 16 samples for 

Location 2 were required. 

On the other hand, calculations for Location 3 with a turbulent airflow and a high air velocity showed extremely large 

sample sizes ranging from 9 to 1,180, with most sizes falling 

between 200 and 550 samples. For the three-axis instrument, 

fluctuating values for the vertical velocity component (Wvalues) were observed. 

The averages from the sample rate tests were calculated and 

then compared for both sample rates of four samples/s and one 

sample/s. The comparison showed that identical values were 

obtained with the three-axis instrument. Also, the results were 

very close to each other when using the two-axis instruments. 

The largest air velocity average difference over the cross-section 

was less than 3%. For turbulent airflow conditions, however, 

the faster sample rate of four samples/s is beneficial, because 

it results in a shorter required measurement time, in that the 

same number of samples is gathered in one-fourth of the time. 

Conclusions

Ultrasonic anemometers show tremendous potential to 

become an accurate air velocity measurement instrument for 

use in underground mining environments. There are several 

possible applications that can be considered, such as integrating 

three-axis instruments on mining machines, using handheld 

instruments with displays for ventilation surveys, using intrinsically safe instruments in return airways and using two-axis 

instruments for continuous flow and direction monitoring in 

straight entries. This series of tests evaluated the impacts of 

obstructions, sample size and sample collection rate on the 

ability of ultrasonic anemometers to accurately measure ventilation air velocities.
This study shows that the effects of moving obstructions are 

easily recognizable in continuous measurement data. If these 

types of changes are seen in data, the measurement location can 

be checked for unexpected activity. Also, the effects of stationary obstructions in a straight entry can be seen in the collected 

data set and found to correlate well with the cross-sectional 

area of the obstruction. The effects of stationary obstructions 

can be seen as similar to air velocity values recorded by vane 

anemometer traverses. 

To achieve 95% and 98% confidence in the results, the 

required sample sizes when using two- or three-axis anemometers in a relatively straight airway (Location 1) with a nearly 

stable airflow are surprisingly low. Only a few measurements 

are needed for an accurate air velocity value, assuming that the 

measurements are not taken too close to the rib. In the case of 

a less-than-optimal measurement location, 15 data points are 

recommended. With a typical sample rate of one sample/s, this 

standard results in a 15-s measurement time. In comparison to 

the recommended practice of a minimum of three 60-s traverses 

with rotating vane anemometer surveys, using an ultrasonic 

anemometer significantly shortens the required measurement 

times, resulting in increased surveying efficiency.

In very turbulent airflow, accurate results require extremely 

large sample sizes, which are not necessarily feasible in an 

underground environment. If measurements have to be taken 

in such circumstances, long measurement times or continuous 

measurement stations are recommended for higher reliability of 

results. Location 3 with its turbulent airflow conditions required 

nearly 1,180 samples which, at a rate of one sample/s, would 

result in a sample time of almost 20 minutes.

The typical sample rate of one sample/s is adequate and 

easily achievable for underground air velocity measurements. 

In comparison to the sample rate of four samples/s, the greatest 

difference in air velocity values was less than 3%. This is well 

below the 5% error value viewed as acceptable air velocity 

measurement error in an underground mine.

Acknowledgments

The authors would like to recognize and thank Paul Stefko, 

Joseph Sabo, Jack Teatino and Heather Dougherty (all from 

NIOSH) for their valuable assistance in setting up and conducting the tests at the NIOSH Experimental Mine. 

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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