USING ULTRASONIC ANEMOMETERS TO EVALUATE FACTORS AFFECTING 

FACE VENTILATION EFFECTIVENESS 

Charles D. Taylor , NIOSH-PRL, Pittsburgh, PA 

Robert J. Timko , NIOSH-PRL, Pittsburgh, PA 

Edward D. Thimons , NIOSH-PRL, Pittsburgh, PA 

Tom Mal , NIOSH-PRL, Pittsburgh, PA 

Abstract 

A test system was developed for measuring airflow in 

the National Institute for Occupational Safety and Health 

(NIOSH) ventilation gallery using a three-axis ultrasonic 

anemometer. The gallery was used to simulate face airflow 

conditions in underground mines having a blowing curtain. 

Airflow data collected at multiple sampling locations 

between the face and the end of the curtain were used to 

draw airflow profiles for different curtain setback distances, 

intake flow quantities and entry widths. In addition, 

methane was released at the face, measured at the sampling 

locations, and displayed as methane distribution in the area 

between the curtain and the face. Entry geometry had a 

significant effect on airflow patterns, and the flow patterns 

affected the distribution of methane in the entry. Flow 

measured at and parallel to the face was a good predictor of 

methane dilution and removal within two feet of the face. 

Background 

 In coal mines the highest methane concentrations are 

often found near development faces where coal is being 

mined. Effective face ventilation requires that methane 

liberated at the face be diluted and removed quickly. An 

important factor affecting face ventilation effectiveness is 

the quantity of intake air delivered to the face from the end 

of the face curtain or tubing. To maintain methane 

concentration below 1 pct, as required by law (30 CFR 

75.323 (b)), an adequate quantity of intake air must be 

supplied to the face. Normally the quantity of air reaching 

the working face is determined using flow readings made at 

the inby end of the entry’s curtain or tubing (30 CFR 

75.325 (a)(2)). However, past studies have shown that 

these calculated quantities may not be good estimates of 

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

1999). Better estimates of face airflow could be made if 

airflow direction and speed were measured closer to the 

face. However, it is difficult to make more accurate 

measurements between the curtain and the face because 

airflow direction and speed constantly change in this area. 

 Instruments such as a rotating vane anemometer, or 

pitot tube and magnehelic gauge, are used to measure 

airflow behind the curtain or in the tubing. A vane 

anemometer can be used to measure flow velocity inby the 

tubing or curtain, but it must be aligned with the airflow to 

give accurate readings. Most often, smoke tubes are used to 

determine flow direction. Although only qualitative in 

nature, valuable information about face airflow patterns has 

been obtained with smoke tubes (cf. Luxner, 1969). 

 A previous paper (Taylor, 2004) describes how a test 

procedure was developed and used for measuring airflow 

direction and speed in the NIOSH ventilation test gallery 

using ultrasonic anemometers. This paper explains a 

continuation of that work. Airflow direction and speed in 

empty entries are measured and compared for selected 

curtain setback distances, entry widths, and intake flow 

quantities. The effects of flow on face methane 

distributions are discussed. 

Test Conditions 

Surface Test Facility 

 Tests were conducted in the NIOSH Pittsburgh 

Research Laboratory’s Ventilation Test Gallery (figure 1). 
Figure 1. Ventilation Test Gallery. 

One side of the gallery was designed to simulate a mining 

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

apart. For half of the tests the entry width was reduced to 4 

m (13 ft) by building a wall 1 m (3 1⁄2 ft) from the right rib. 

 The exhaust fan draws approximately 5.9 m3/s (12,500 

cfm) of air through the gallery. To simulate air flow in an 

underground mine, air was directed toward the entry face 

using a 2.2-m (7- ft) high curtain that was supported by a 

wood frame constructed 0.6 m (2 ft) from the left side of 

the entry. The curtain was positioned so that setback 

distances between the curtain and the face were 10.7 m (35 

ft), 7.6 m (25 ft), and 4.6 m (15 ft). Regulator doors were 

adjusted to provide intake flows behind the curtain of either 

2.8 or 4.7 m3/s (6,000 or 10,000 cfm). Flow velocity 

behind the curtain was measured with a vane anemometer 

held at the centerline of the curtain. Flow quantity was 

determined by multiplying the average velocity times the 

cross- sectional area behind the curtain. 

Flow and methane measurements were made for two 

entry widths of 4 and 5 m (13 and 16 1⁄2 ft), two intake 

flows of 2.8 and 4.7 m3/s (6,000 and 10,000 cfm), and three 

curtain setback distances of 4.6, 7.6, and 10.7 m (15, 25, 

and 35 ft). 

Airflow and Methane Measurements 

Methane concentrations and airflow measurements 

were made at the same sampling locations. The number of 

sampling locations between the curtain and the face varied 

from 36 (for the 35-foot setback tests) to 16 (for the 15-foot 

setback tests). The sampling locations for the 35-foot 

setback distance and two entry widths are shown in figure 

2. 

Figure 2. Sampling locations. 

Air flow measurements were made between the end of 

the curtain and the face using “Windmaster” three-axis 

ultrasonic anemometers (figure 3) manufactured by Gill 

Instruments Ltd, Great Britain (references to specific 

products does not imply endorsement by NIOSH). 

Figure 3. Three-axis ultrasonic anemometer. 

The anemometer was attached to an overhead support 

system (figure 4) which was used to move the anemometer 

toward and away from the face, and right and left across the 
entry. The anemometer was inverted to facilitate 

positioning of the sensor head 1.1-m (3.5 ft) from the floor 

and roof. All tests were conducted with the sensor head at 

this elevation. 

Figure 4. Overhead support system. 

In order to compare flow direction for data collected at 

multiple locations, the orientation of the anemometer with 

respect to a reference point or location had to be maintained 

for all measurements. The face was selected as the 

reference location. The anemometer was rotated around its 

vertical axis until an arrow, printed on top of the sensor 

head, was directed toward the face. Each time the 

anemometer was moved, the direction of the instrument 

with respect to the face was re-checked. 

The data collected by the anemometer pass through a 

Power Communication Interface (PCI) box which powers 

the anemometer and resolves each flow measurement into 

three orthogonal components, U, V, and W. For these tests, 

flow component data were simultaneously collected from 

two anemometers and stored in a personal computer using 

Anemvent 2003. Anemvent 2003 is data acquisition 

software developed by NIOSH specifically for use with the 

ultrasonic anemometers. Data from each instrument were 

collected at one-second intervals and the time interval 

between readings for the two instruments was 0.2 seconds. 

One hundred eighty sets of data points were collected 

during each three minute test. The data stored in the 

computer were analyzed using EXCEL functions and 

algorithms. The U and V components were used to 

calculate the flow angle and speed in the UV plane located 

3.5 ft from the floor and roof. EXCEL chart functions were 

used to draw, on a single sheet, flow vectors for all 

locations sampled. Each vector indicates flow direction and 

is proportional in length to the flow speed. 

Airflow and methane measurements were made in the 

test gallery for the same set of operating conditions, but the 

measurements were made at separate times. To make 

methane measurements, methane gas was released into the 

test area through four 3-m (10-ft-long) horizontal copper 

pipes that were located at the mining face. The pipes were 

located 0.1 m (4 in) away from the face, and equally spaced 

horizontally to provide a relatively uniform release of gas. 

Two-mm (1/16-in) diameter holes were drilled 6.5 cm (2.5 

in) apart on the top and bottom of each of the 10-ft-long 

pipes. For most tests, the methane flow into the gallery was 

16 lps (34 cfm). However, during some tests (see below) 

the flow rate was reduced to 3.2 lps (6.8 cfm) to prevent 

methane levels in the gallery from exceeding 2.5 pct. A 

rotameter was used to set gas flows at either 6.8 or 34 cfm. 

While methane measurements were being made, four 

air sampling tubes were suspended from the overhead 

support system. The hose inlets were positioned 1 m (3.5 

ft) from the roof and floor. A vacuum pump continuously 

pulled air samples through each tube to one of four 

Bacharacch combustible gas monitors, and concentrations 

were measured at four locations during each test. 

Methane concentrations were monitored (1 reading 

each second) for 8 minutes at each sampling location and 

the individual readings were averaged. Each test condition 

was repeated once and the results of the two tests were 

averaged. Data were stored using a commercial computerbased data acquisition system. 

Results and Discussion 

Air flow patterns and methane distribution 

Drawings showing the flow profiles and methane 

concentrations in the area between the blowing curtain and 

the face were made for each set of the test operating 

conditions. Data was collected for intake flows of 6,000 

and 10,000 cfm, but, to limit the material included in this 

paper, airflow profiles and methane concentrations are only 

shown for tests conducted with 10,000 cfm intake flow. 

The vector drawn at each sampling location shows the 

direction of airflow. The length of the vector is 

proportional to the air speed (see figures 5, 6, and 7). Flow 

direction was similar for the 10,000 and 6,000 cfm intake 

flows when the entry width and setback distances were the 

same. However, vectors at the same sampling locations 

were shorter for intake flows of 6,000 cfm. 
Figure 5. Flow diagrams for 15-foot setback. 

Figure 6. Flow diagrams for 25-ft setback. 

Figure 7. Flow diagrams for 35-ft setback. 

The effect of entry width on flow direction was 

significant for 35- and 25- ft setback distances. The airflow 

pattern in the 4-m (13-ft) entry was similar to a figure “8.” 

In contrast, air in the 5-m (16 1⁄2-ft) entry moved up the 

curtain side of the entry to the face and returned on the 

opposite side of the entry. For all tests with the 15-ft 

setback, air moved up the curtain side of the entry to the 

face and returned on the opposite side. 

The methane profiles in figures 8, 9, and 10 show how 

methane concentrations varied between the end of the 

curtain and the face for tests where intake flow was 10,000 

cfm. The methane concentrations in figures 8 - 10 were 

measured for the same operating conditions used for the 

airflow tests described above (figures 5-7). 

In figures 8, 9, and 10, the methane concentration at 

each sampling location is indicated by the pattern of the 

block that surrounds the sampling location. The 

distribution of methane for the 6,000 and 10,000 intake 

tests was similar, but the concentrations measured during 

the 6,000 cfm tests were higher. The distribution of 

methane between the curtain and the face in general 

corresponds to the air flow patterns. When air moved along 

the intake side of the entry to the face, more methane 

accumulated on the return side of the entry. When air flow 

moved to the return side of the entry before reaching the 

face, the higher methane concentrations were on the intake 

side of the entry. 
Figure 8. Methane profile for 15-ft setback, 10,000 cfm 

intake. 

Figure 9. Methane distribution for 25-ft setback, 10,000 

cfm intake. 

Figure 10. Methane distribution for 35-ft setback, 10,000 

cfm intake. 

Flow at the Face 

Airflow and methane profiles shown above help to 

explain how air moves between the blowing curtain and the 

face. However, the most critical area for the dilution and 

removal of methane is within 2 ft of the face. The 

effectiveness of a face ventilation system depends on how 

quickly methane is diluted and removed from this area. 

Therefore, data collected closest to the face (for these tests, 

locations 1, 2, 3, and 4) provide important information for 

comparing ventilation systems and evaluating factors 

affecting flow to the face. 

The vectors shown in figures 5, 6, and 7 illustrate the 

direction of the air flow and the air speed in the direction 

indicated. An easier and more direct way to compare air 

movement at the face is to look at the magnitude of the flow 

as it moves parallel to the face. Specifically, we should 

consider the component of flow that is parallel to the face. 

The data collected at locations 1, 2, 3, and 4 were used for 

this analysis. 

The ultrasonic anemometer is designed to measure the 

U and V flow components in a plane that has been rotated 

30 degrees counterclockwise from the reference direction 

(figure 11). 
Figure 11. Anemometer and directional components. 

To measure the airflow speed moving parallel to the 

face, the U, V axes were mathematically rotated 30 degrees 

clockwise using rotation of the axes equations (Flanders, 

1985). The magnitude of the rotated V flow component 

was then equal to the speed of the airflow moving parallel 

to the face. 

Figures 12, 13, 14, and 15 show the speed of the 

airflow moving parallel to the face at sampling locations 1, 

2, 3, and 4. Data are given for 25- and 35- ft setbacks and 

10,000 and 6,000 cfm intake flows. Airflow speed was 

higher for the higher intake flow, regardless of direction 

across the face. The highest airflow speed occurred near 

the center of the face (locations 2 and 3). For the same 

intake flows, air speed across the face was significantly 

lower in the 13 ft wide entry. As seen earlier with the flow 

profiles, air moved left to right across the face in the 16 1⁄2 - 

ft entry and right to left across the face in the 13-ft-wide 

entry. 

Figure 12. Twenty-five foot curtain setback, 13-ft-wide 

entry. 

Figure 13. Twenty-five ft curtain setback, 16 1⁄2-ft-wide 

entry. 

Figure 14. Thirty-five ft curtain setback, 13-ft-wide entry. 
Figure 15. Thirty-five ft curtain setback, 161⁄2-ft-wide 

entry. 

Figures 16, 17, 18, and 19 show the methane 

concentrations measured at sampling locations 1, 2, 3, and 4 

for both entry widths. Operating conditions used during 

these tests correspond to the operating conditions used 

during the airflow measurements (figures 12, 13, 14, and 

15). Methane concentrations varied depending on the speed 

and direction of the air moving across the face. The higher 

velocities resulted in lower methane concentrations. For 

both entry widths the highest methane concentration was 

measured on the “downwind” side of the face. The highest 

concentrations were on the right side of the face for the 16 

1⁄2-foot entry tests and on the left side of the face for the 13-

foot entry tests. 

Figure 16. Methane concentrations, 25-ft curtain setback, 

13-ft entry. 

Figure 17. Methane concentration, 25-ft curtain setback, 

16 1⁄2-ft entry. 

Figure 18. Methane concentration, 35-ft curtain setback, 

13-ft entry. 

Figure 19. Methane concentration, 35-ft curtain setback, 

16 1⁄2-ft entry. 
One cautionary note must be given when comparing 

data presented in figures 16 through 19. The methane 

concentrations measured during 13-ft entry tests were lower 

than during the 16 1⁄2-ft-wide entry tests, for both the 25- 

and 35-ft setback data. However, to maintain methane 

levels in the gallery below 2.5 pct, the gas flow rate through 

the pipe manifold had to be reduced from 34 to 6.8 cfm 

during tests where the entry width was 13 ft. For all 16 1⁄2-

ft tests, the flow rate was 34 cfm. Direct comparisons, 

therefore, should not be made for ventilation effectiveness 

in the 13- and 16 1⁄2-ft-wide entries using the measured 

methane concentrations. However, the fact that methane 

levels could be maintained below 2 1⁄2 pct methane in the 16 

1⁄2-ft entries at the higher gas flow rates indicates that more 

ventilation air was reaching the face. This was confirmed 

by the flow measurements discussed above. 

Conclusions 

In some cases, the results of this study confirm earlier 

observations concerning face airflow, e.g. increased intake 

flow results in more air reaching the face. However, past 

studies have depended largely on methane measurements to 

estimate ventilation effectiveness. The quantitative data 

collected with the three-axis ultrasonic anemometer 

provides a means to directly calculate ventilation 

effectiveness based on airflow readings. Past observations 

indicated that increasing the entry width should result in 

greater flow to the face, but the magnitude of the increase, 

indicated by the flow readings, was greater than expected. 

Thus, entry width is a factor that should be considered 

when evaluating the effectiveness of a face ventilation 

system. 

Future testing will examine airflow patterns in entries 

containing models of mining equipment. Using a model 

mining machine the effects of water spray and scrubber use 

on flow patterns and levels of turbulence will be examined. 

The effects of ventilation flow on the removal of methane 

liberated from holes drilled in the roof will be studied with 

a model roof bolting machine located near the face. 

Collecting ventilation data with the ultrasonic 

anemometer is relatively simple, but work in the gallery 

with the present test protocol is labor-intensive. For 

example, special care is needed to ensure proper orientation 

of the instrument at each sampling location. The present 

test matrix was based on the most efficient use of personnel 

and instrumentation. Sampling at more locations will 

improve the measurement of flow patterns. Further testing 

will look at improved ways to sample at more locations. 

The data obtained with the ultrasonic anemometers will also 

be used to help develop computer aided ventilation models 

that will supplement data obtained in the ventilation gallery. 

References 

1. Code of Federal Regulations (CFR-30) 75.323, (b). 

2. Code of Federal Regulations (CFR-30) 75.325 (a) 

 (2). 

3. Thimons E.D., Taylor C.D., and Zimmer J.A. 

 [1999]. “Evaluating the Ventilation of a 

 40- Foot Two-Pass Extended Cut,” NIOSH Report 

 of Investigations (RI 9648). 

4. Luxner J.V. [1969]. “Face Ventilation in 

 Underground Bituminous Coal Mines,” BuMines 

 RI 7223, 16 pp. 

5. Taylor C.D., Timko R.J., Senk M.J. and Lusin A. [

 2004]. “Measurement of Airflow in a Simulated 

 Underground Mine Environment Using an 

 Ultrasonic Anemometer,” SME Transactions, Vol. 316.