NBS

Puoli • cations

»bNBSIR 78-1485

Low Velocity Performance of a

Ball Bearing Vane

Anemometer

L. P. Purtell

National Bureau of Standards

Fluid Engineering Division

Washington, D.C. 20234

June 1978

Task Report on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

QC

100

,U56

73-1435

Prepared for United States Department of the Interior

Bureau of Mines

sWcnaT Bureau of Standards ,rR 17 197§

rat JLcc - (CQ.JK

NBSIR 78-1485

LOW VELOCITY PERFORMANCE OF A

BALL BEARING VANE

ANEMOMETER

L. P. Purtell

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

June 1978

Task Report

Prepared for United States Department of the Interior

Bureau of Mines

’i ©« XU XA Onus. DEPARTMENT OF COMMERCE, Juanita M. Kreps. Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

This report was prepared by the National Bureau of Standards, Fluid

Engineering Division, Washington, D. C. 20234, under USBM Contract

Number H0166198. The contract was initiated under the Coal Mine

Health and Safety Program. It was administered under the technical

direction of PM8SRC, with Dr. George H. Schnakenberg, Jr., acting

as the Technical Project Officer. Mr. H. R. Eveland was the con- tract administrator for the Bureau of Mines.

This report is a summary of the work recently completed as part of

this contract during the period February 1, 1977 to March 31, 1977.

This report was submitted by the author June 1978.

2
,
LIST OF SYMBOLS

U

i

U

if

U

u.

a

.

1

a

c

velocity measured by laser velocimeter

velocity indicated by anemometer under test

line segments fitted to U, IL data

group mean true velocity

group mean indicated velocity

standard deviation of U. data from U

.

1 if

standard deviation of U. data expressed as true

velocity

o adjusted for known variance in laser

velocimeter measurements

3
LOW VELOCITY PERFORMANCE OF A BALL BEARING

VANE ANEMOMETER

L. P. Purtell

1 . INTRODUCTION

The National Bureau of Standards in order to meet the need for a calibration capability with adequate accuracy at low air velocities,

i.e., below 500 feet per minute (fpm) undertook the development of a low-velocity calibration facility for wind speed measuring instruments

which would provide a capability down to 3 meters per minute (approximately 10 fpm) with an accuracy of plus or minus one percent. It was

a natural consequence therefore that when said facility became operational to undertake an evaluation of the state-of-the-art and to

provide the information needed as to the reliability and performance

of instrumentation for such measurement. Accordingly, a number of prototypes of various types of instruments for low velocity air measurements are undergoing test at NBS, and this report is concerned specifically with the results of one such test.

2 . THE INSTRUMENT

The rotary vane anemometer tested for this report is a commercially

available instrument (Davis Instrument Manufacturing Co., Inc., A-Inch

Low Speed Anemometer, S/N 2A323 B) 1 used in the mining industry and elsewhere as a portable anemometer. It was supplied for test by the U. S.

Mining Enforcement and Safety Administration (MESA) at the request of

the U. S. Bureau of Mines. The housing is A inches in diameter and

1-3/A inches deep (Figure 1) . Thin metal vanes without camber or twist

mounted on arms drive a rotor linked to a dial indicator by a gear

train. The bearings in this particular instrument are ball bearings

(as opposed to standard bronze sleeve bearings) . One revolution on

the dial represents an indicated passage of 100 feet of air through

the instrument. Thus an external timer (not a part of the anemometer)

is required to complete a measurement of velocity (an average velocity

for the duration of the measurement)

1 This particular instrument was selected as being representative of

this type of anemometer and its selection does not represent an endorsement

A
3. THE TESTS

The NBS Low Velocity Airflow Facility [1] used to test this

instrument generates a low velocity air stream having a low turbulence

intensity (less than 0.05%) and a large region of uniform flow (at

least 75 x 75 cm) . A laser velocimeter is employed as a primary

velocity standard. It is nonintrusive, has a linear response with

velocity, and has good spatial resolution. Adequate sensitivity is

obtained without the artificial seeding of scattering particles.

Thus the difficulties and inconvenience associated with seeding and

the possible effect of such seeding on the performance of the device

under test are avoided.

The vane anemometer was mounted on the centerline of the tunnel

test section one meter downstream of the entrance to the test section

in a manner to minimize the effect of the support on the air stream

around the anemometer (Figure 1) . Since the anemometer itself modifies

the airflow in the tunnel, the velocity should be measured at a location

in the flow which has the same velocity in the presence of the anemometer

as it does in the absence of the anemometer. Since this anemometer is

identical in shape to a bronze bearing anemometer tested previously, the

results are used from the tests on that instrument wherein the velocity

upstream of the anemometer on the centerline was measured to find the

position where deceleration of the flow due to the presence of the

anemometer was no longer detectable within the scatter of the measurements. These measurements were performed at two free-stream speeds,

700 and 72 fpm, and as predicted by ideal flow theory the variation

of the ratio of the local velocity to the free-stream velocity with

distance upstream of the anemometer is independent of free-stream

velocity (Figure 2) . A distance of 30 cm upstream of the anemometer

was chosen as the position for velocity measurement by the laser

velocimeter. With no anemometer in the tunnel, variation in velocity

along the centerline is imperceptible over the distance traversed

(30 cm)

.

The air speed indicated by the vane anemometer was computed from

initial and final readings of the dial and of the associated time

interval (around two minutes) . The anemometer runs continuously in the

tunnel since it cannot be accessed while the tunnel is in operation

without disturbing the flow. Thus the readings of the anemometer were

performed with the anemometer in operation. The laser velocimeter

measurement of the air velocity was performed during the time interval

for reading the vane anemometer. Five separate test runs were made,

each consisting of about ten such measurements over the range 23 to

724 fpm. The lower velocity was limited by the starting and stopping

speeds of the instrument. The data are presented in chronological order

in Tables 1A to IE.

5
To determine the starting speeds of the instrument, the velocity

in the tunnel was increased from below the starting speed at a smooth

acceleration of approximately 30 fpm/min until movement of the vanes

could be detected by eye. At that moment the air velocity would be

fixed and the laser velocimeter measurements initiated. If the

anemometer continued rotating for at least thirty seconds and did not

decelerate, the measurement of velocity by the laser velocimeter was recorded as the starting speed. Ten such measurements are presented

in Table 2 and have an average of 23.3 fpm and a standard deviation

of 1.3 fpm.

Because of the anemometer's angular momentum, stopping speed is

more difficult to determine than starting speed. Some preliminary runs indicated that a two minute interval between reductions in air velocity

of approximately 2 fpm was sufficient for the anemometer to come to

rest if the stopping speed had been reached. Ten such measurements

are presented in Table 3 with an average of 18.2 fpm and a standard

deviation of 0.4 fpm.

4 . TEST RESULTS

Since a particular air speed in the wind tunnel cannot be exactly

reset from run to run, scatter in the test data is distributed along

a curve, thus prohibiting computing the standard deviation of the data

from a simple average. Instead, deviations from a curve fit to the

data were computed and the standard deviation approximated by the r.m.s. value of these deviations within a group. The groups are

U < 28 fpm

28 fpm < U < 35 fpm

35 fpm < u < 45 fpm

45 fpm < u < 60 fpm

60 fpm < u < 80 fpm

80 fpm < u < 130 fpm

130 fpm < u < 180 fpm

180 fpm < u < 300 fpm

300 fpm < u < 600 fpm

600 fpm < u

Since a curve fit to the data would have very little curvature and since

the groups of data are compact (small range of U within a group; see

Fig. 3), a straight line segment is used to approximate_the curve within

a group. The line segment passes through the point (U,U^), the group

mean true velocity and the group mean indicated velocity. The slope of

the line segment is computed as the average of the slopes of two lines,

both passing through (U,Ui) of the group being considered, one line passing

6
through the (U,U^) of Uie adjacent group higher in velocity, and one line passing through (U,U^) of the adjacent group lower in velocity.

For the highest group (U > 600 fpm) there is only one adjacent grcnrp,

and thus the line segment for this highest group passes through (U,U-^)

of that adjacent group. The line segment for the lowest group

(U < 28 fpm) is similarly formed.

Designating the above line segments as U^, the standard deviation,

Oi of the indicated velocity, U-^, about the fitted segments is determined

by squaring the differences between the data and U^f, i.e., [U^(U) - U if (U)] 2

. Since the data within the specified groups are reasonably compact, the mean of the squared differences within a group is taken as an estimate of the variance of about_U^^ within that group and specified

at that group’s mean true velocity, U. To convert this to a standard

deviation in terms of true velocity, designated a, each a^(U) is divided

by the slope (dU-j^f/dU) of the line segment associated with the a-^(U).

Note that this o does not include the "scatter" in the U measurements

(due to the inability to exactly reset the wind tunnel to a specified

speed), but does include the uncertainty in a particular laser velocimeter

measurement. This uncertainty may be estimated from repeated measurements

of velocity at a particular fan setting, thus also including any un- steadiness in the velocity, and is estimated as 0.005U for this report.

A standard deviation, a , corrected for the laser velocimeter uncertainty

may thus be computed from

a = a

2 - (0.005U) 2 c for any given U. a and o c_are presented in Figure 4 as velocity and

Figure 5 as percentage of U. Since ± 2a c is extremely close to the

95 percent confidence interval for one measurement, curves of ± 2o

c are also included in Figure 3 as dashed lines.

The actual differences between the true and indicated velocities,

U - U-[, are presented in Figure 6 and as a percentage of U in Figure 7.

The curves shown in each figure have been drawn for reference only.

5. DISCUSSION OF RESULTS

The instrument performed over the speed range tested with no erratic

behavior. The repeatability of the starting and stopping speeds was

quite good having standard deviations of 1.3 fpm (5.6%) and 0.4 fpm

(2 . 2%), respectively . Some general comments concerning application of

the instrument follow. With any measurement problem the instrument’s

capabilities should be matched to the required measurement.

This anemometer is intrusive, i.e., it must be placed in the flow.

This anemometer is entirely mechanical and does not require an outside source of power.

7
Many other factors that can affect the suitability of an instrument

for a particular application, such as turbulence or unsteadiness of

the air stream, rough handling (shock and vibration), dirt and other

environmental factors, time, orientation to the velocity and gravity

vectors, etc., have not been tested herein but should be considered.

6. SUMMARY

The performance of a 4-inch diameter low speed vane anemometer

with ball bearings has been evaluated, including starting speed and

stopping speed, at air speeds up to 724 fpm.

The starting and stopping speed measurements are presented and give

an average starting speed of 23.3 fpm and an average stopping speed of

18.2 fpm.

7. REFERENCES

1. L. P. Purtell and P. S. Klebanoff, The NBS Low Velocity Airflow

Facility, in preparation.

8
Table 1 A

Davis Vane Anemometer

Serial No. 24323 B

Indicated Air Speed,

f pm

True Air Speed,

f pm

712

205

466

80.4

56.5

692

211

455

93.0

72.0

33.2

12.4

23.1

5.8

50.4

30.3

40.8

24.0

T = 20.9 to 21.0 °C

B = 745.7 mm Hg

Table 1 B Davis Vane Anemometer

Serial No. 24323 B

Indicated Air Speed,

f pm

True Air Speed,

f pm

752

218

491

85.6

60.8

724

224

484

99.3

73.1

37.1

14.3

24.8

6.8

151

51.1

29.6

40.6

23.0

158

T = 21.7 to 21.9 °C

B = 744.8 mm Hg

9
Table 1 C

Davis Vane Anemometer

Serial No. 24323 B

Indicated Air Speed,

f pm

True Air Speed,

f pm

717

206

468

80.5

57.5

690

209

458

94.4

71.1

35.0

12.2

22.7

8.1

144

51.5

30.7

41.7

26.4

150

T = 21.3 °C

B = 747.1 mm Hg

Table 1 D

Davis Vane Anemometer

Serial No. 24323 B

Indicated Air Speed,

fpm

True Air Speed,

fpm

715

206

469

80.3

56.8

33.1

12.3

22.4

6.4

142

T = 22.1

B = 750.

696

215

466

96.0

72.0

50.4

31.2

41.4

25.2

152

°C mm Hg

10
Table 1 E

Davis Vane Anemometer

Serial No. 24323 B

Indicated Air Speed,

f pm

True Air Speed,

f pm

718

207

471

78.8

56.0

682

212

461

97.2

70.8

33.8

13.6

26.9

11.5

114

50.4

32.2

43.4

29.5

153

T = 22.5 °C

B = 750.5 mm Hg

11
Table 2 Davis Vane Anemometer

Serial No. 24323 B

Starting Speed,

f pm

21.8

24.6

23.0

21.9

22.9

Average starting speed,

23.3 f pm

Standard Deviation,

1.3 f pm

26.0

22.4

23.3

24.3

22.9

Table 3

Davis Vane Anemometer

Serial No. 24323 B

Stopping Speed,

fpm

17.9

19.0

17.6

18.6

17.8

Average stopping speed,

18.2 fpm

Standard Deviation

0.4 fpm

18.4

18.6

17.9

18.3

18.0

12
FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL,

SHOWING METHOD OF SUPPORT.

t

*

13
m

CMoCNI mmo e

s-/ p<wHZ<o

ptS Pm

14

FIGURE

2.

VARIATION

IN

VELOCITY

WITH

DISTANCE

UPSTREAM

FROM

ANEMOMETER .
800

700

600

500

i

400

300

200

100

r

--

U (fpm)

FIGURE 3. INDICATED VERSUS TRUE VELOCITY WITH ± 2 a CURVES.

15
16

FIGURE

4.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION

OF

INDICATED

VELOCITY

IN

TERMS

OF

TRUE

VELOCITY.
CJ t>

<N Ol

17

U

(fpm)

FIGURE

5.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION

AS

PERCENT

OF

GROUP

MEAN

VELOCITY.
•H

CD

I CD

E

U-i

18

FIGURE

6.

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY.
80

70

60

50

40-

30

20 .

10

0 -

h + o

o

1 1

1 1 1 11 100 200 300 400 500 600 700

U Cfpm)

FIGURE 7. PERCENT DEVIATION OF INDICATED VELOCITY FROM TRUE VELOCITY.

800

19
NBS-1 14A (R E V 7-7 )1

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR REPORT NO. NBSIR 78-1485

2. Gov’t Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A BALL BEARING

VANE ANEMOMETER

5. Publication Date June 1978

6. Performing Organization ( ode

8. Perf. eming Organ. Report No. NBSIR 78-1485

10. Project/Task/Work Unit No. 7320483

11. Contract/Grant No. H0166198

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Office of the Assistant Director - Mining Covr red Bureau of Mines Feb . 1 , 1977-Mar. 31, 1977

United States Department of the Interior 14. Sponsoring Agency C ode Washington, D. C. 20241

15. SUPPLEMENTARY NOTES

16.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Performance of a ball bearing vane anemometer is evaluated over the speed range of

23 to 724 feet per minute including starting speed and stopping speed. The tests were performed in the NBS Low Velocity Airflow Facility which provides a uniform

flow of low turbulence and utilizes a laser velocimeter as the velocity standard.

17.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name, separated by semicolons

)

Airflow; anemometer; laser velocimeter; low velocity; performance; vane

anemometer; wind tunnel.

18.

AVAILABILITY [XX Unlimited

[ For Official Distribution. Do Not Release to NT1S

r ‘^der From Sup. of Doc., U.S. Government Printing Office Washington, D.G. 20402, SD Cat. No. (13

XX Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

UNCL ASSIF1F.D 20

20. SECURITY CLAS.1 22. Price (THIS PAGE)

UNC LASSIFTED $4.00

7. AUTHOR(S) D D . , , L. P. Purtell

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

USCOMM.DC 29042-P74
I

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