NAT'L INST. OF STAND & TECH R.I.C.

AlllOM 3tlllS

NBSIR 78-1545

Low Velocity Performance of a

High Speed Vane Anemometer

II

i’*. ^

'

1’^'^

L. P. Purtell

If^ 'i

National Bureau of Standards

Fluid Engineering Division

Washington, D.C. 20234

September 1978

Task Report

on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

100

.056

NO. 78-1 545

1978

Prepared for United States Department of the Interior

Bureau of Mines

filAY i 4

NBSIR 78-1545

»*• LOW VELOCITY PERFORMANCE OF A

HIGH SPEED VANE ANEMOMETER

L P. Purtell

National Bureau of Standards

Fluid Engineering Division

Washington, D.C. 20234

September 1978

Task Report

on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

Prepared for United States Department of the Interior

Bureau of Mines

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, SecrMry

Dr. Sidney Herman, Under Seeretery

Jordan J. Baruch, Assistant Sacratary for Scianea and Taehnoiogy

NATIONAL BUREAU OF STANDARDS, Emaat Amblar. Director

NBS'lWA (REV. 5-7 »l

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-1545

2. Gov't Accession

No.

3. Recipient’s Accession No.

4- TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A HIGH SPEED

VANE ANEMOMETER

5. Publication Date

&• Performing Organization Code

7. AUTHOR(S)

L. P. Pur tell 8. Performing Organ. Report No. NBSIR 78-1545 . PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/Work Unit No. 7320483

11. Contract /Grant No. H0166198

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of the Assistant Director - Mining

Bureau of Mines

United States Department of the Interior

Washington, D. C. 20241

13. Type of Report & Period Covered

June 1 - July 31, 1977

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Perfoxmance of a high speed vane anemometer is evaluated over the speed range of

43.4 to 741 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 Unlimited 19, SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES ]

1

' For Official Distribution. Do Not Release to NTIS

UNCLASSIFIED

23

I

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20. SECURITY CLASS

(THIS page!

22. Price Rr Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161 UNCLASSIFIED $4.00

1 USCOMM-DC «fi6eO-P78
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. 11. R. Eveland was the contract administrator for

the Bureau of Mines.

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

contract during the period June 1, 1977 to July 31, 1977. This report

was submitted by the author September 1978.

2
LIST OF SYMBOLS

U velocity measured by laser velociraeter

velocity indicated by anemometer under test

line segments fitted to U, data

U group mean true velocity

group mean indicated velocity

a. standard deviation of U. data from U. ~ 1 1 if

a standard deviation of U. data expressed as true

velocity ,

a adjusted for known variance in laser

velocimeter measurements

3
LOW VELOCITY PERFORMANCE OF A HIGH SPEED

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., 4-Inch

High Speed Anemometer, S/N 31125 B)^ 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 4 inches in diameter and 1-3/4

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). This anemometer differs from that

reported on previously [1] in that it has only four vanes instead of

eight. 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)

^This particular instrument was selected as being representative of

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

4
3. THE TESTS

The NBS Low Velocity Airflow Facility [2] 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

siiape 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 ten such measurements over the range 43.4 to 741 fpm.

The lower velocity was limited by the starting and stopping speeds of

the instrument. The data are presented in chronological order in

Tables lA 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 47.8 fpm and a standard deviation of 2.4 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 has been reached. Ten such measurements are presented in Table 3 with an average of 31.7 fpm and a standard deviation

of 1.7 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. within a group. The groups are

U < 50 fpm

50 fpm < U < 60 fpm

60 fpm < U < 80 fpm

80 fpm < U < 100 fpm

100 fpm < U < 150 fpm

150 fpm < U < 250 fpm

250 fpm < U < 400 fpm

400 fpm < U < 500 fpm

500 fpm < U < 700 fpm

700 fpm < U

6
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 compijted as the average of the slopes of

two lines, both passing througl^ _(U,U. ) of the group being considered,

one line passing through the CU,U.) of the_adjacent group higher in

velocity, and one line passing through (U,U.) of the adjacent group lower

in velocity. For the highest group (U > 7oh fpm) there is only one adjacent £_roup, 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 < 56 fpm) is similarly formed.

Designating the above line segments as U._p, the standard deviation,

of the indicated velocity, U., about the fitted segments is determined

by squaring the differences between the data and i.e.,

lU. (U) - U^^CU)]^. 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 U. abou^U.. within that group

and specified at that group's mean true velocity, U. To convert this to_ a standard deviation in term.s of true velocity, designated a, each a. (U)

is divided by the slope (dU^^/dU) of the line segment associated witft the

a. (U) . Note that this a 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 unsteadiness in the velocity, and is estimated as O.OOlU for this

report. A standard deviation, a , corrected for the laser velocimeter

uncertainty may then be computed from

a2 = - (O.OOlU)^

for any given U. a and a _are presented in Figure 4 as velocity and

Figure 5 as percentage of U. Since ± 2a is extremely close to the 95

percent confidence interval for one measurement, curves of ± 2a 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.

7
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 2.4 fpm (5.0%) and 1.7 fpm

(5.4%), 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.

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 high speed vane anemometer

with ball bearings has been evaluated, including starting speed and

stopping speed, at air speeds up to 741 fpm.

The starting and stopping speed measurements are presented and give an average starting speed of 47.8 fpm and an average stopping speed of

31.7 fpm.

7.

REFERENCES

1. L. P. Purtell, Low Velocity Performance of a Ball Bearing

Anemometer, NBSIR 78-1485, 1978.

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

Facility in preparation.

8
Table lA

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

f pm

True Air Speed,

fpm

785

639

496

359

219

736

606

478

347

217

136.4

80.2

53.7

38.9

28.6

T = B =

141.2

92.1

68.2

56.6

47.7

23.6 °C

753.8

mm Hg

Table IB

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

786

640

497

358

219

137.3.

81.8

56.1

41.8

26.4

T =

B =

True Air Speed,

fpm

741

608

477

348

218

141.7

93.1

70.2

58.8

47.3

.8 °C

3.8

mm Hg

9
Table 1C

Davis Vane Anemometer

Serial No. 31125B

Indicated Air speed,

f pm

True Air Speed,

fpm

786

641

498

358

218

738

608

477

347

216

135.5

81.1

53.7

38.9

24.5

139.5

91.5

68.0

55.7

43.4

T = 24.2 °C

B = 753.8 mm Hg

Table ID

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

True Air Speed,

fpm

787

641

498

359

220

738

608

477

34 7 217

135.6

80.6

53.0

37.8

24.8

140.5

91.4

67.7

55.4

44.6

T = 24.6 °C

B = 753.8 mm Hg

10
Table IE

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

f pm

True Air Speed,

fpm

786 740

641 608

498 476

357 34 7 219 217

134.6 140.0

81.0 91.4

52.6 67.1

38.3 55.3

27.1 46.3

T = 24.7 °C

B = 753.2 mm Hg

Starting Speed,

f pm

Table 2 Davis Vane Anemometer

Serial No. 31125B

51.4

48.2

Average Starting Speed, 47.8 fpm

47.8

46.8

50.2

46.6

50.0

45.2

48.3

43.6

Standard Deviation, 2.4 fpm

11
Stopping Speed,

f pm

Table 3

Davis Vane Anemometer

Serial No. 31125B

33.5

31.

34.0

29.9

33.0

30.9

33.6

30.9

30.5

29.3

Average Stopping Speed, 31.7 fpm

Standard Deviation, 1.7 fpm

Table 4A

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

True Air Speed,

fpm

782 722

1052 974

3260 3000

5460 4960

7700 6960

9840 9000

T = 25.8 to 26.0 °C

B=745.6to753.8mmHg

12
Table 4B

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

f pm

True Air Speed,

fpm

783 735

1057 988

3260 3000

5450 4950

7690 6960

9620 8970

T = 26.1 to 26.4 °C

B = 745.0 to 753.8 mm Hg

Table 4C

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

True Air Speed,

fpm

781 730

1058 978

3260 3000

5450 4960

7740 6960

9830 8980

T = 26.4 to 26.5 °C

B = 745.0 to 753.3 mm Hg

13
Table 4D

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

780

1053

3260

5450

7590

10,150

T = 26.6

B = 744.7

True Air Speed,

fpm

736

988

3000

4940

6950

8960

to 26.8 °C

to 753.3 mm Hg

Table 4E

Davis Vane Anemometer

Serial No. 31125B

Indicated Air Speed,

fpm

781

1056

3260

5460

7780

10,310

\

T

B

True Air Speed,

fpm

731

978

3000

4960

6960

8970

= 26.8 °C

= 744.6 to 753.0 mm Hg

14
FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL SHOWING

METHOD OF SUPPORT.

15
pi

txj HOs<o

Pi

PiH

to04ZuuzcH

toMo , 8 zlz>

16

FIGURE

2.

VARIATION

IN

VELOCITY

WITH

DISTANCE

UPSTREAM

FROM

ANEMOMETER •
800

700

600

500

400

300

200

100

0

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

17
18

U

(fpm)

FIGURE

4.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION

OF

INDICATED

VELOCITY

IN

TERMS

OF

TRUE

VELOCITY.
in<

u D

ol

t> I 1^:3

o I 19

U

(fpni)

figure

5.

STANDARD

DEVIATION,

o

,

AND

CORRECTED

STANDARD

DEVIATION

PERCENT

OF

GROUP

MEAN

VELOCITY.
20

FIGURE

6.

DEVIATION

OF

INDICATED

VEI.OCITY

FROM

TRUE

VELOCITY.
21

FIGURE

7.

PERCENT

DEVIATION

OF

INDICATED

VELULITY

FROM

TRUE

VEEUCHY.