Bureau of Mines Report of Investigations/l987 

Method Factors for Anemometer 

Measurement at Pipe Outlets 

By F. Garcia and J. Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 9061 

Method Factors for Anemometer 

Measurement at Pipe Outlets 

By F. Garcia and J. Cervik 

UNITED STATES DEPARTMENT OF THE INTERmR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
L~brary of Congress Cataloging in Publication Data : 

, 

t 

Garcia, F. (Fred) 

Method factors for anemometer measurement at pipe outlets. 

(Report of investigations ; 9061) 

Bibliography: p. 10. 

Supt. of Docs. no.: I 28.23: 9061. 

1. Coalbed methane. 2. Mine ventilation. 3. Anemometer. 4. Coal mine waste. I. Cervik, Joseph. 11. Title. 111. Series: Report of investigations (United States. Bureau of 

Mines) ; 9061. 

TN23.U43 [TN305] 622 s [622'.81 86-600296 
CONTENTS 

Page 

Abstract ........................................................................ 1 

Introduction .................................................................... 2 

Test procedures ................................................................. 4 

Data analysis ................................................................... 7 

Summary and conclusions ......................................................... 10 

References ...................................................................... 10 

ILLUSTRATIONS 

1 . Typical vane anemometer calibration curve ................................... 2 

2 . General test setup .......................................................... 4 

3 . Anemometer measurement at pipe outlet ....................................... 5 

4 . Anemometer measurement at 4-in pipe outlet .................................. 5 

5 . Vertical upward flows.... ................................................... 6 

6 . Anemometer measurements closer than 7.5 pipe diameters from an elbow ........ 6 

7 . Inserted anemometer ......................................................... 7 

8 . Method factors for 4- to 30-in pipe ......................................... 8 

TABLES 

1 . Method factors for Biran-type anemometer at the discharge end of 12- to 30- 

in pipe .................................................................... 3 

2 . Method factors for centerline Biran-type anemometer (low-speed) measurement 

taken at horizontal pipe outlets. .......................................... 8 

3 . Method factors for 4-in pipe ................................................ 8 

4 . Effects of an elbow on downstream velocity measurements ..................... 9 

5 . Effects of a lS-mi/h wind speed on flow measurements at a pipe outlet ....... 9 

6 . Effects of a 15-mi/h wind speed on an anemometer mounted inside the pipe .... 9 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter km/h kilometer per hour 

diam diameter lb/f t3 pound per cubic foot 

ft foot m/s meter per second 

f t/min foot per minute mi/h mile per hour 

ID inside diameter mi n minute 

in inch Pet percent 

kg/m3 kilogram per cubic meter rad radian 

I 
METHOD FACTORS FOR ANEMOMETER MEASUREMENT 

AT PIPE OUTLETS 

By F. Garcia1 and J. Cervik* 

ABSTRACT 

Gob holes are routinely used by the mining industry to vent methane 

from gobs and to prevent it from entering the mine ventilation system. 

In some cases, flows from gob holes are measured by centering an anemometer on the end of a discharge pipe. These measurements are erroneous 

and tend to be high by as much as 30 pct. The Bureau of Mines determined method factors (correction factors) for anemometer measurements 

taken in this manner. 

Method factors for 4-, 6-, and 8-in (10.2-, 15.2-, and 20.3-cm) ID 

pipe are 0.68, 0.71, and 0.78, respectively. Comparisons between horizontal and vertical flows indicated no significant change in method 

factors. Consequently, the method factors for horizontal pipe flows can 

be applied to vertical upward flows. 

'Mining engineer. 

2~upervisory geophysicist. 

Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

Vertical gob holes are routinely used 

to control methane in gobs. In most 

cases, vane anemometers are used to measure the velocity of gas flowing from the 

gob holes and subsequently to calculate 

the volume flow of gas. Generally, the 

anemometer is centered on the end of the 

pipe during the measurement. If the 

cross section of the anemometer is appreciably larger than one-sixth of the pipe 

diameter, then erroneous velocity readings occur, owing to the blocking effect 

of the instrument 

The pitot tube isregarded as the fundamental standard for determining velocFigure 1 shows a typical calibration 

curve whose equation is of the form 

VT = A + BV,, (1) 

where A = intercept (35), 

and B = line slope or VT to V, ratio 

(0.95). 

Thus, the equation for the air calibration curve is 

VT = 35 + 0.95 V,. (2) 

ities in ducts or piping. Generally, The calibration curve does change when 

point traverse measurements must be made the anemometer is used where there are 

to accurately determine the average ve- large density changes, such as in a pure 

locity and subsequently the gas flow methane gas stream. The curve's slope 

through a pipe. These measurements are (0.95) remains the same; however, the 

time consuming and very sensitive to gas intercept (A) changes to 

density changes, which for a gob hole can 

vary from 0.042 lb/ft3 (0.673 kg/m3) when A = (d,/dg)1/2 (3) 

flow is pure methane to 0.075 lb/ft3 

(1.202 kg/m3) when flow is all air. In where d, = air density at which the 

addition, the accuracy of the pitot tube calibration was made (0.075 

is limited at velocities below 1,000 lb/ft3 [1.202 kg/m3]), 

ft/min (5.08 m/s) for air and 2,000 

ft/min (10.16 m/s) for methane because and dg = density of methane (0.042 lb/ 

the manometer is not precise enough to ft3 [0.673 kg/m3]). 

accurately measure the small velocity 

pressures (2). - The 4-in (10.2-cm) diam vane anemometer 600 is an instrument commonly used by the 1 I I I I 

01 

mining industry. It is practically independent of diminutive air density 

changes encountered in mines and is furnished by the manufacturer with a calibration to eliminate small differences 

between individual anemometers, whose 

readings usually lie within 10 pct of the 

true velocity (3). These calibrations 

are usually in the form of tables of 

plus-or-minus corrections to be made to 

the observed velocities. The application 

of these corrections can be simplified by 

plotting the true air velocity (VT) 

against the registered air velocity (V,), 

which approximates a straight line. 

loob/ --- Calibration KEY curve for air 

Calibration curve in methane 

1 I I I I I 1 ----- - 0 100 200 300 400 500 600 

3~nderlined numbers in parentheses re- REG1 STERED VELOCITY, ft/min 

fer to items in the list of references at 

the end of this report. FIGURE 1 .-Typical vane anemometer calibration curve. 
Thus, the calibration curve for a pure 

methane gas stream becomes 

VT = 47 + 0.95 V,. (4) 

Figure 1 (dotted curve) shows the methane 

calibration curve for this particular 

anemometer. The difference between the 

two curves in this example is 12 ft/min 

(0.06 m/s), which is negligible where 

registered velocities are greater than 

1,000 ft/min (5.08 m/s). For registered 

velocities below 1,000 ft/min (5.08 m/s), 

the appropriate calibration curve should 

be used. Where the gas flow is a mixture 

of methane and air, the corrected velocity can be obtained by calculating a new 

intercept that lies somewhere between the 

two curves. The slope remains the same. 

The vane anemometer is normally used to 

make measurements in mine airways, where 

cross-sectional areas are much larger 

than the cross-section of the anemometer. 

Thus, the anemometer causes no appreciable obstruction to airflow. A simple 

and fast method used to determine the 

average velocity in a mine airway is to 

take a one-point measurement at the center of the airway. The anemometer should 

be mounted on the end of a short shaft 

with the starting and stopping done remotely to reduce measurement errors due 

to obstructions such as the operator's 

body. If the airway is straight, clear 

of obstruction, and has a uniform crosssectional area, fair results are obtained 

after the centerline measurement is multiplied by a "method factor." A method 

factor becomes necessary because the velocity profile in the airway is highest 

at the center and decreases towards the 

rib, roof, and floor. Multiplying the 

centerline stickheld measurement by 0.85 

(method factor) corrects it to the true 

average air velocity in the airway (4). 

Very of ten, mine conditions prevent 

one-point measurements from providing 

accuracy. Consequently, approximate continuous traverses, made by moving the 

anemometer slowly through a plane at 

right angles to the axis of the airway, 

are recommended. A popular method is to 

divide the airway in half and traverse 

each half separately over a given time 

interval. With a little practice, this 

method can yield results within 5 pct of 

the true average velocity (3). 

The National Coal Board of the United 

Kingdom developed method factors for correcting centerline 4-in (10.2-cm) vane 

anemometer measurements taken at the discharge end of pipes ranging in diameter 

from 12 to 30 in (30.5 to 76.2 cm) (5). 

The instrument was operated at the endof 

a rod that was held against the duct for 

support. It was started and stopped remotely to reduce errors caused by the operator's hand. Centerline measurements 

were employed because of the difficulties 

encountered in maintaining the alignment, 

position, or course of the anemometer 

when traversing methods were attempted. 

The anemometer measurements were compared 

simultaneously to upstream pitot-tube 

pipe traverses. A ratio of the true 

average velocity (determined from the 

pitot traverse) to the anemometer measurement (once corrected to its calibration) was determined for each duct size. 

Table 1 summarizes these method factors 

for each duct size. 

Unfortunately, most cased gob holes in 

the United States are generally smaller 

in diameter than 12 in (30.5 cm), and the 

use of an anemometer to measure the velocity of flow from the end of the pipe 

yields erroneous volume flows. The objectives of this report are to determine 

the method factors for pipe sizes ranging 

from 4 to 8 in (10.2 to 20.3 cm) and to 

investigate the effect of wind speed on 

pipe velocity measurements by using an 

anemometer. 

TABLE 1. - Method factors for Biran-type anemometer at the discharge 

end of 12- to 30-in pipe 

Pipe diam, 

in 

12......-. 

18.---...- 

Velocity 

range, 

ft/min 

500-2,500 

500-4,000 

Mean 

method 

factor 

0.84 

-85 

9 5-pc t 

confidence 

limit 

20.05 

+ -04 

Pipe diam, 

in 

24-....... 

3Om....... 

Velocity 

range, 

ft/min 

800-4,600 

500-2,700 

Mean 

method 

factor 

0.85 

-86 

95-pct 

confidence 

limit 

5-05 

5-04 
using the instrument calibration curve. 

The effects of air density were disgarded 

because density variations caused less 

than 1 pct error. 

The anemometers were operated for a 1- 

min period at the end of a 2-ft (61-cm) 

long by 518-in (1.6-cm) diam rod and were 

started and stopped by a simple remote 

control device (fig. 3). The rod was 

held against the pipe for support except 

on the 4-in (10.2-cm) pipe, where the 

Air velocity measurements should be 

taken at least 7.5 pipe diameters (7.5 D) 

downstream from an elbow or branch entry 

that causes a nonuniform velocity profile 

in the pipe (2). Measurements taken 

closer than 7.5 D may be erroneous; however, the 7.5 D distance may not always 

be possible so measurements taken closer 

may be necessary. Consequently, tests 

were conducted to determine the effects 

on method factors when measurements at 

Thermocouple 

KEY 

D Pipe diameter 

SIDE VIEW END VIEW 

FIGURE 2.-General test setup. 
FIGURE 4.-Anemometer measurement at 4-in pipe outlet. 
FIGURE 6.-Anemometer measurements closer than 7.5 plpe diameters from an elbow. 
FIGURE 7.-Inserted anemometer. 

the end of a pipe are taken closer than 

7.5 D from an elbow (fig 6.) or any other 

factor that causes an abnormal velocity 

profile in the pipe. 

All laboratory tests were conducted in 

a quiescent envir3nment. In practice, 

field measurements of gas flow from gob 

holes are subject to wind velocities 

that rarigt? from a quiescent stage to 15 

mi/h (24 km/h) or more. Wind, which 

blows acr~jss the end of the pipe, can 

C~ILS(:' errorleous anemometer readings. 

Consequently, tests were conducted to 

determine the effect of wind speed on 

method factors. A fan was used to generate winds ranging from 5 to 15 mi/h (8 

to 24 km/h); pipe flows ranged from 175 

to 2,900 ft/min (0.9 to 14.7 m/s). The 

method factors determined under those 

conditions were then compared with method 

factors determined in a quiescent environment. Tests were also conducted with 

the anemometer mounted about 6 in (15 cm) 

inside the pipe (fig. 7). 

DATA ANALYSLS 

'rab11.l 2 shows that the method factors 

for iiorizoiital pipe flows from 4-, 6-, 

and i3-in (1-, 15.2-, and 20.3-cm) 

schedule 40 pipe are 0.68, 0.71, and 

0.78, respectively, when the low-speed 

anemoneter is ~rsed. Each method Factor 

is subject to an analysis of variance and 

is shown in ternis of a 95-pct couFiderlce 

limit. This me;llis, For example, that 

there is a95-pct certainty that the 

true method factor lies in the interval 

0.6820.07. 

Figure 8 shows that the method factor 

for 4- to 18-in (10.2- to 45.7-cm) pipe 

increases with pipe diameter and then 

remains constant (0.85) for larger pipe 

sizes. Even for an opening as large as a 

mine entry, the method factor is 0.85. 
0.90 1 1 I 1 1 I than the high-speed anemometer; there1 I fore, the low-speed anemometer registered readings are much higher than the highspeed anemometer readings. Consequent ly, 

the registered readings for a low-speed 

anemometer must be multiplied by a 

smaller method factor to obtain the true 

average velocity. In experiments involving inlet flows into 12- to 24-in (30.5- 

to 61.0-cm) tubing, no substantial differences in method factors were noted 

between high- and low-speed anemometers 

I I I I I I I (1). Apparently, the difference in vane 

.65! 8 12 16 20 24 28 32 configuration has no effect on the method 

PIPE DIAM, in factor for the larger diameter ducts. 

FIGURE 8.-Method factors for 4- to 30-in pipe. On most occasions, anemometers are calibrated and used with the plane of vane 

Apparently, the blocking effect of the rotation in the vertical. For flows from 

anemometer is more pronounced on 4- to vertical stacks, though, the plane of 

18-in (10.2- to 45.7-cm) pipe. For lar- vane rotation is horizontal when velocity 

ger openings, the relative size of the measurements are made. Table 3 shows a 

anemometer is small in comparison, and difference of 0.02 between method factor 

the anemometer measures the centerline for horizontal and vertical flow from 4- 

velocity, which is generally 15 pct in (10.2-cm) pipe. However, a statistigreater than the average velocity. cal analysis comparing the two method 

A high-speed anemometer (200 to 10,000 factors showed that this difference of 

ft/min [l to 50.8 m/s]) generally has 0.02 is not significant at the 95-pct 

four vanes, whereas a low-speed anemom- probability limit. 

eter (30 to 5,000 ft/min [0.15 to 25.4 

m/s]) generally has eight vanes. A TABLE 2. - Method factors for cenblocking effect is created when using an terline Riran-type anemometer 

anemometer to measure flows in small di- (low-speed) measurement taken at 

ameter pipes, which causes the registered horizontal pipe outlets 

velocities to be erroneously high. Table 

3 shows that the method factor for a 4-in 

(1.6-cm) pipe is 0.78 when a high-speed 

anemometer is used and 0.68 when a lowspeed anemometer is used. Apparent ly, 

the larger number of vanes causes the 

low-speed anemometer to present a greater 

blocking effect or restriction to flow 

Pipe diam, 

in 

TABLE 3. - Method factors for 4-in pipe 

Velocity 

range, 

Mean 

met hod confidence 

-- 

limit 

50.07 

t.06 

k.05 

High-speed ............. Low-speed: 

Horizontal flow....... 

Vertical flow......... 

-- 

Velocity 

range, 

f t/min 

4,000-10,000 

100- 5,000 

100- 5,000 

Mean 

method 

factor 

0.78 

.68 

.70 

--- 

95-pct 

confidence 

limit ---- +0.03 

+.07 

k.08 
Table 4 shows that anemometer measurements can be taken closer than 7.5 pipe 

diameters (7.5 D) from an elbow or any 

other factor that causes a nonuniform 

velocity profile in a pipe. For 4-in 

pipe, velocity measurements as close as 1 

D from the disturbance differ from those 

made at 7.5 D by only about 2 pct. 

For 6- and 8-in (15.2- and 20.3-cm) pipe, 

measurements should not be made closer 

than 3 and 5 D, respectively. 

Table 5 shows that wind speed does affect measurements of flow from the end of 

a pipe. In general, tests on an 8-in 

(20.3-cm) pipe show that the measured 

velocity tends to be less when a 15 mi/h 

(24 km/h) wind blows across the pipe compared with velocity measurements in a 

quiescent environment. As the velocity 

of pipe flow increases, the effect 

of wind on measurement of velocity 

TABLE 4. - Effects of an elbow on 

downstream velocity measurements 

1 I 4-in PIPE 

Velocity 

at 7.5 D, 

Difference, pct 

8-in PIPE 

1,000.. ............ 1,500.............. 

2,000.............. 4 

D = Pipe diameter. 

1- 1D ( 3D 1 5D 

500.............. 2 

1,000.............. 2 

1,500.............. 2 3 

2,000.. ............ 2 3 

2,500.............. 1 3 

diminishes. The effects of wind can be 

eliminated by mounting the anemometer inside the pipe (fig. 7). Table 6 shows 

that except for very low velocities [I00 

ft/min (0.51 m/s)], wind has a negligible 

effect on velocity measurements when the 

anemometer is mounted inside the pipe a 

short distance. 

500.............. 

1,000.............. 

1,500.............. 

2,000.. ............ 

TABLE 5. - Effects of a 15-mi/h wind 

speed on flow measurements at pipe 

outlet 

Anemometer measurement, 1 

6-in PIPE 

4 

4 

7 

7 

TABLE 6. - Effects of a 15-mi/h wind 

speed on an anemometer mounted 

inside the pipe 

Anemometer measurement, I 

Change, pct 

0 

13 

14 

7 

7 

8 

6 

4 

6 

2 

3 

1 

(ftlmin), at wind speed of-- 

1 

2 

2 

2 

(ftlmin), at wind 

0 mi/h 1 15 

0 mi/h 

180 

240 

360 

560 

7 50 

1,000 

1,280 

1,490 

1,750 

1,950 

2,260 

2,900 

1 

1 

0 

0 

15 mi/h 

180 

2 10 

310 

520 

700 

920 

1,200 

1,430 

1,650 

1,910 

2,180 

2,860 
SUMMARY AND CONCLUSIONS 

Method factors for 4-, 6-, and $-in 

(10.2-, 15.2-, and 20.3-cm) pipe are 

0.68, 0.71, and 0.78 respectively, when 

the flow velocity at the discharge end 

ranges from 30 to 5,000 ft/min (0.15 to 

25.4 m/s). The method factors for larger 

pipe sizes approach 0.85, which is also 

the value of the method factor for a mine 

entry . Although the anemometer is calibrated 

and used with the plane of vane rotation 

in the vertical, no significant difference in method factors is observed 

when the anenometer is used to measure 

vertical flows (plane of vane rotation in 

the horizontal). 

Tests on an $-in (20.3-cm) pipe show 

that the measured velocity tends to be 

less when a 15 mi/h (24 km/h) wind blows 

across the pipe compared with velocity 

measurements in a quiescent envircnment. 

Wind effects tend to dim-inish as pipe 

flow velocity increases. Wind effects on 

anemometer measurements can be eliminated 

by mounting the anemometer a short 

distance inside the pipe. 

REFEKENCES 

1. Ower, E. The Measurement of Air 

Flow. Chapman and Hall Ltd, London, 

1949, pp. 151-153, 183-184. 

2. American Conference of Governmental 

Industrial Hygienists. Industrial Ventilation. Edward Brothers Inc., Ann Arbor, 

MI, 11th ed., 1970, p. 9-3. 

3. McElroy, G. E. Engineering Factors 

in the Ventilation of Metal Mines. 

BuMines B 385, 1935, pp. 7-8, 11-12. 

4. Thimons, E. D., and J. L. Kohler. 

Measurement oE Air Velocity in Mines. 

BuMines RI 8971, 1985, 26 pp. 

5. Northover, E. W. Vane Anemometer 

Measurements of the Quantity of Air 

Entering and Leaving Auxiliary Sys tems. 

National Coal Board, M.R.E. Rep. 2061, 

Mar. 1957, 13 pp. 

6. American Society of Mechanical Engineering. Fluid Meters. 5th ed., 1959, 

pp. 125-141. 

7. Haney, R. A., and T. C. Hlausa. 

Methods Factors for Anemometer Keadings 

at Tubing Inlets. MESA Invest. Rep. 

1041, 1976, 9 pp. 

U.S. GOVERNMENT PRINTING OFFICE. 1986 605-01 71401 10