NBSIR 78-1524

Low Velocity Performance of a

Jewel Bearing Vane Anemometer

by

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

repared for ‘QC inited States Department of the Interior

100 lureau of Mines

,U56

NBSIR 78-1524

LOW VELOCITY PERFORMANCE OF A

JEWEL 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

September 1978

Task Report

Prepared for United States Department of the Interior

Bureau of Mines

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

Dr. Sidnay Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Emast Amblar, Director

I
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NHS I I4A ' I / II I.

U OI b( COMM

BI8U0CRAPHIC DATA

SHEET

1. pi hi |( A 1 ION OK K 1 POR I NO. NBSIR 78-1524

2. (»<>v ^ A( f r s s lofi No.

3, ( ipiciK's Acccs5sion No.

4. TITLE AND SUH I I I 1.1 LOW VELOCITY PERFORMANCE OF A JEWEL BEARING

VANE ANEMOMETER

5. PubI ic at ion Date

6. Performing Organization ( ode

7. AUTHOR(S) D D ^ 1 1 L. P. Purtell8. Performing Organ. Report No. NBSIR 78-1524

9. PEREORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Protect /Task /Work Unit No. 7320483

11. Contract/Grant No. H0166198

-1 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

Apr. 1,1977 -May 31,1977

14. Sponsoring Agency ( ode

15. SUPPLEMENTARY NOTES

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

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

of 60.6 to 752 feet per minute including starting 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; mine ventilation; performance;

vane anemometer; wind tunnel.

18. AVAILABILITY pOf Unlimited I 19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

i Eor Official Distribution. Do Not Release to N1 IS UNCL ASSIEIED

[ (i>rder 1- rom Sup. of Doc-, U.S. Government Printing Office 20. SECURI TY CLASS 22. Price Washington. D.C . 20402. SD Cat. No. Cl? (THIS PAGE)

Order E rom National Technical Information Service (NTIS)

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1 USCOMM-DC 29042-P74
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 PM&SRC, with Dr. George H. Schnakenberg, Jr., acting as the Technical Project Officer. Mr. H. 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 April 1, 1977 to May 31, 1977. This

report was submitted by the author September 1978.

2
LIST OF SYMBOLS

U velocity measured by laser velocimeter

velocity indicated by anemometer under test

U.^ line segments fitted to U, U, data

U group mean true velocity

group mean indicated velocity

a. standard deviation of U. data from U.^

1 Ilf a standard deviation of U, data expressed as velocity

a a adjusted for known variance in laser

velocimeter measurements

true

3
LOW VLLOCITY PERFORMANCE OF A JEWEL 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 (Sybron/Taylor Corporation, 4-Inch Anemometer,

S/N H873)^ 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 with jewel

bearings. One revolution on the dial represents an indicated passage

of 100 feet of air through the instriament. 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)

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

This particular instriament was selected as being representative of

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

4
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. 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 984 fpm (Figure 2). As shown in [2] 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.

A distance of 30 cm upstream of the anemometer was chosen as the position for velocity measurement by the laser velocimeter. With no anemo- meter 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 60.6 to

752 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.

To determine the starting speed 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 66.6 fpm and a standard deviation

of 1.8 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 60.6 fpm and a standard

deviation of 0.6 fpm.

5
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

U < 70 fpm

70 fpm < U < 80 fpm

80 fpm < u < 120 fpm

120 fpm < u < 180 fpm

180 fpm < u < 300 fpm

300 fpm < u < 400 fpm

400 fpm < u < 550 fpm

550 fpm < u < 650 fpm

650 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 Figure 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 throu^h_(U,U- ) of the group being considered, one line passing

through the (U,Uj^) of the adjacent group higher in velocity, and one

line passing through (U,Ui) of the adjacent group lower in velocity.

For the highest group (U > 650 fpm) there is only one adjacent group^,

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

of that adjacent group. The line segment for the lowest group (U < 70 fpm)

is similarly formed.

Designating the above line segments as Uj^f, the standard deviation,

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

by squaring the differences between the Ui data and Uj^f, i.e., [U-(U) - Uif(U)]^. 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^f 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 oi(U) is divided

by the slope (dUj^^/dU) of the line segment associated with the Oj^(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 un- steadiness in the velocity, and is estimated as 0.002U for this report.

A standard deviation, o^., corrected for the laser velocimeter uncertainty

may thus be computed from

6
0^

= - (0.002U)^

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

Figure 5 as percentage of U. Since ± 2o^ is extremely close to the

95 percent confidence interval for one measurement, curves of ± 2o^ are also included in Figure 5 as dashed lines.

The actual differences between the true and indicated velocities,

U - Uj^, 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 1.8 fpm (2.7%) and 0.6 fpm

(1.0%), 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 low speed vane anemometer

with jewel bearings has been evaluated, including starting speed and

stopping speed, at air speeds up to 752 fpm.

The starting and stopping speed measurements are presented and give

an average starting speed of 66.6 fpm and an average stopping speed of

60.6 fpm.

7.

REFERENCES

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

Facility, in preparation.

2. L. P. Purtell, Low Velocity Performance of a Bronze Bearing Vane

Anemometer, NBSIR 78-1433, 1978.

7
Table lA

Taylor Vane Anemometer

Serial No. H873

Indicated Air Speed,

f pm

True Air Speed,

fpm

722

589

457

309

190

752

619

494

322

221

110

56.8

26.7

7.1

27.1

145

100

76.2

60.6

76.2

T = 22.8 °C

B = 751 . 2 mm Hg

Table IB

Taylor Vane Anemometer

Serial No. H873

Indicated Air Speed,

fpm

True Air Speed,

fpm

719

589

454

324

190

746

623

498

360

222

109

55.8

25.1

8.4

25.7

142

98.0

74.6

64.8

76.0

T = 22.6 ®C

B = 753. 5 mm Hg

8
Table 1C

Taylor Vane Anemometer

Serial No. H873

Indicated Air Speed,

f pm

True Air Speed,

fpm

720

588

456

323

190

750

622

495

363

222

110

54.7

23.4

3.8

23.8

143

96.9

74.2

63.0

74.3

T = 22.6 °C

B = 753.1 mm Hg

Table ID

Taylor Vane Anemometer

Serial No. H873

Indicated Air Speed,

fpm

True Air Speed,

fpm

723

590

458

324

190

752

622

499

361

221

110

54.9

22.4

23.0

142

97.1

73.6

72.4

T = 22.7 °C

B = 752.9 mm Hg

9
Table IE

Taylor Vane Anemometer

Serial No. H873

Indicated Air Speed,

fpm

True Air Speed,

fpm

721

588

A57

323

190

7A2

62A

A96

360

220

109

5A.5

20.0

8.5

20.8

lAl

96.

A

72.6

65.5

72.5

9.8 66.1

T = 22.8 °C

B = 753.0 mm Hg

10
Table 2 Taylor Vane Anemometer

Serial No. H873

Starting Speed,

f pm

66.0

67.2

68.4

65.4

64.2

66.0

69.0

67.2

63.6

66.6

Average starting speed,

66.6 fpm

Standard deviation,

1.8 fpm

11
Table 3

Taylor Vane Anemometer

Serial No. H873

Stopping Speed,

f pm

61.2

60.0

60.0

60.6

60.0

61.2

60.6

61.8

61.2

61.2

Average stopping speed,

60.6 fpm

Standard deviation,

0.6 fpm

12
FIGURE 1. The anemometer mounted in the tunnel, showing

method of support.

13
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00

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00

CSJ

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vO

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00

vO

-<r

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14

DISTANCE

UPSTREAM

FROM

ANEMOMETER

(CM)

FIGURE

2.

Variation

in

velocity

with

distance

upstream

from

anemometer
800

700

600

500

400

300

200

1000 i

i! j

800

15
ea

16

(fpm)

FIGURE

6.

Deviation

of

indicated

velocity

from

true

velocity.
100 oo

00oooo

MOoooo

<roomooCNI ooo y o o o o CTn 00 ^ o o m <f o o o CN iH O

•H

piI p 6^

19

U

(fpm)

FIGURE

7.

Percent

deviation

of

indicated

velocity

from

true

velocity.