NBSIR 78-1544

Low Velocity Performance of a

Compact Impact-Deflection

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

-QC

100

.U56

1^19

Prepared for United States Department of the Interior

lureau of Mines

ttrw*

r^AY 1 ^

NBSIR 78-1544

• ;< LOW VELOCITY PERFORMANCE OF A

COMPACT IMPACT-DEFLECTION

ANEMOMETER

L. P. PurteM

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

V'i I.A U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technoiogy

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

NBS-114A (REV. 5-7»)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78- 1544

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A COMPACT

IMPACT-DEFLECTION ANEMOMETER

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

L. P. Purtell

B. Performing Orean. Report No. NBSIR 78-1544

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/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. 2Q241

13. Type of Report & Period Covered

Sept. 1 - Nov. 30, 1977

14. Sponsoring Agency Code

IS. SUPPLEMENTARY NOTES

1A. ABSTRACT (A 200-word or less factual sutnmary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Performance of a compact impact-deflection anemometer is evaluated over the speed

range of 50.8 to 716 feet per minute. 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 (he first letter of the first key word unless a proper name; separated by semicolons)

Airflow; anemometer; laser velocimeter; low velocity; performance; wind tunnel.

18. AVAILABILITY

^ Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

f For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

00

[ Order From Sup. of Doc., U.S. Government Printing Office Washington. D.C. 20402. SD Stock No. SN00J4)g3

20. SECURITY CLASS

(THIS PAGET

22. Price

pX Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161 UNCLASSIFIED

$4.50

1 USCOMM-DC 6«680*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 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 September 1, 1977 to November 30, 1977. This

report was submitted by the author September 1978.

2
LIST OF SYMBOLS

U

U.

1

U

u.

1

a.

1 a

a

c

R.

1 R

velocity measured by laser velocimeter

velocity indicated by anemometer under test

line segments fitted to U, 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

a adjusted for known variance in laser velocimeter

measurements

resolution of the instrument

resolution expressed as true velocity

3
LOW VELOCITY PERFORMANCE OF A COMPACT

IMPACT-DEFLECTION ANEMOMETER

L. P. Purtell

1. INTRODUCTION

The National Bureau of Standards in order to meet the need for a calibration capability vd.th adequate accuracy at low air velocities, i.e.,

below 500 feet per minute (fpm) undertook the development of a lowvelocity 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 con- sequence 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 anemometer tested for this report is a commercially available

instrument (Alnor Instrument Company, Velometer Jr., Type 8100)^ 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 at the request of the U. S. Bureau of Mines. The instrument is ap

approximately 3.25 x 4 x 1.5 inches in size (see Figure 1) and contains

entrance and exit ports for the air on the upstream and downstream sur- faces respectively. The air stream inside the instrument impacts and

deflects a small vane linked to a meter indicator. The entrance port is

adjustable (two sizes) providing two ranges of velocity measurement, 50

to 200 fpm and 100 to 800 fpm, referred to herein as the low and high

ranges respectively. The instrument is a one piece, self contained unit

not requiring power or other devices for operation.

^ 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 [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 unifonn 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 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 measurement (Figure 2) . These

measurements were performed at a free-stream velocity of 205 fpm. As

shown in a previous report [2] the variation of the ratio of the local

velocity to the free stream velocity with distance 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 anemometer in the tunnel, variation in

velocity along the centerline is imperceptible over the distance

traversed (30cm)

.

The air speed indicated by the anemometer was recorded during the

time interval required for the measurement by the laser velocimeter. If

fluctuations of the dial indicator were noticeable their magnitude was estimated and recorded. Five separate test runs were made for each

speed range, a run consisting of seven different velocities for the low

range and ten for the high range. The lowest velocities were limited by

instrument resolution and are 53.0 fpm for the low range and 50.8 fpm

for the high range. The data are presented chronological order in

Tables lA to IE and 2A to 2E for the low and high ranges respectively.

4 . TEST RESULTS

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

reset from rion to run, scatter in the test data is distributed along a curve, thus prohibiting computing the standard deviation of the data

5
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

Low Range (fpm) High Range (fpm)

U < 60 U < 60

60 < u < 80 60 < u < 100

80 < u« 100 100 < u < 120

100 < u < 130 120 < u < 180

130 < u < 150 180 < u < 250

150 < u < 180 250 < u < 350

180 < u 350 < u < 450

450 < u < 550

550 < u < 650

650 < u

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

see Figures 3 and 4) , a straight line segment is used to approxima^e_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 com£u^ed as the average of the slopes

of two lines, both passing through (U,U.) of the group being considered,

one line passing through the (U,U.) of i^e_adjacent group higher in

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

lower iji velocity. For the highest groups tb > 180 and 650 fpm) there is

only one adjacen^ group, and thus the line segment for this highest group

passes through (U,U.) of that adjaceht group. The line segment for the

lowest groups (U < ^0 fpm) is similarly formed.

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

a. of the indicated velocity, U., about the fitted segments is determined by squaring the differences between the data and i.e.,

lU^(U) - U^^(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 within that

group and specified at that group's mean true velocity. O’. To convert

this to a_standard deviation in terms of true velocity, designated o, each (U) is divided by the slope (dU^^/dU) of the line segment

6
associated with 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, o , corrected

for the laser velocimeter uncertainty may thus be computed from

0^=0^- (O.OOlU)^

for any given U. a and a are presented_in Figures 5 and 6 as velocity

and in Figures 7 and 8 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 Figures 3 and 4 as dashed lines.

The actual differences between the true and indicated velocities,

U - U,, are presented in Figures 9 and 10 and as a percentage of U in

Figures 11 and 12. The curves shown in each figure have been drawn for

reference only.

5. DISCUSSION OF RESULTS

Computing o from measurements by an instrument having a scale with

a resolution, R. , much smaller than a. is a good procedure for determining repeatability of the instrument. If the resolution is large (poor)

compared to a. (where o. is presumed known by some means independent of

the scale being considered, say by a second scale with better resolution),

the indicated o may be much smaller than it should be. For a Gaussian

distribution of errors it is assumed rhat a. may be adequately computed

if the resolution is at most approximately twice a. . The following

values of resolution were judged to be the best that can be read on the

anemometer tested:

50 < U^ < 100 fpm,

100 < U^ < 200 fpm,

100 < U^ < 600 fpm,

600 < < 800 fpm.

Low Range

R^ = 1/5 division or 2 fpm

R^ = 1/10 division or 1 fpm

High Range

R^ = 1/5 division or 5 fpm

R^ = 1/10 division or 2.5 fpm

7
As with the computed values of a., these values of resolution, R. were converted to equivalent values, in terms of true velocity by

dividing by the slope (dU^^/dU) . These latter values, divided by two, were then included in Figures 5 and 6 in units of velocity and in

Figures 7 and 8 as percentage of U. As may be seen in Figures 5 and 6,

R/2 does indeed exceed a for several of the measurements. Thus these

particular values of a should be taken with reservation and perhaps re- placed by the values R/2. The performance of the instrument in these

instances in terms of repeatability may exceed the quality of its

resolution.

The instrument in general performed with no erratic behavior. Fluctuations of the dial indicator during a reading were generally below ±

1 or 2 fpm, but for the high range, the fluctuations at U = 715 fpm were

as large as ± 25 fpm diminishing to near zero below 600 fpm. As

evidenced by Figure 6, though, these fluctuations did not result in an exceedingly large a. 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 compact, low speed, impact-deflection

anemometer has been evaluated at air speeds up to 716 fpm. Evaluation

of the repeatability of measurements was found to involve consideration

of the resolution of the instrument. Figures are presented showing the

deviation of indicated velocity from true velocity and the standard

deviation of repeated runs about the mean curves. The lowest velocities

measurable were limited by resolution of the instrument and were 53.0

fpm for the low range and 50.8 fpm for the high range.

8
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.

9
Table lA

Alnor Velometer Jr.

Low Range

Indicated Air Speed, True Air Speed

fpm fpm

205 204.7

172 166.4

148 141.1

122 116.3

100 93.0

80 70.6

62 53.0

T = 26.9 “C

B = 747.5 nun Hg

Table IB

Alnor Velometer Jr.

Low Range

Indicated Air Speed, True Air Speed

fpm fpm

205 204.6

170 165.8

146 141.6

122 118.0

100 94.9

78 70.3

60 53.2

T = 26.9 ®C

B = 747.5 mm Hg

10
Table IC

Alnor Velometer Jr.

Low Range

Indicated Air Speed, True Air Speed,

fpm fpm

200 204.2

170 166.6

148 142.1

120 117.0

100 92.7

80 70.7

62 53.3

T = 26.9 °C

B = 747.5 ram Hg

Table ID

Alnor Velometer Jr.

Low Range

Indicated Air Speed, True Air Speed

fpm fpm

205 204.7

170 166.3

146 141.1

120 116.4

100 92.9

80 69.5

62 53.4

T = 26.9 °C

B = 747 . 5 mm Hg

11
Table IE

Alnor Velometer Jr

Low Range

Indicated Air Speed, True Air Speed

fpm fpm

205 204.6

170 166.2

146 141.3

122 117.5

102 93.6

80 70.1

62 53.4

T * 27.0 "C

B * 747.5 nm Hg

12
Table 2A

Alnor Velometer Jr.

High Range

Indicated Air Speed, True Air Speed

fpm fpm

775 712.8

600 608.5

525 503.1

480 399.0

420 296.1

300 192.2

230 142.0

195 117.2

160 93.5

100 50.8

T = 26.9 °C

B = 747.5 mm Hg

Table 2B

Alnor Velometer Jr.

High Range

Indicated Air Speed, True Air Speed

fpm fpm

800 714.7

610 608.8

525 504.2

485 400.0

420 296.5

300 192.4

230 141.7

195 117.4

160 93.3

105 52.5

T = 26.9 °C

B = 747.5 mmHg

13
Table 2C

Alnor Velometer Jr.

High Range

Indicated Air Speed, True Air Speed,

fpm fpm

800 714.2

610 609.7

530 504.9

480 399.3

420 296.9

300 191.8

210 140.3

180 117.4

150 93.0

100 53.3

T = 26.9 °C

B = 747.5 mm Hg

Table 2D

Alnor Velometer Jr.

High Range

Indicated Air Speed,

f pm

True Air Speed,

fpm

800

610

530

480

415

715.9

609.4

504.4

400.2

296.9

300

230

195

160

100

192.5

141.5

116.9

93.4

52.8

T = 26.9 °C

B = 747. 5 mm Hg

14
Table 2E

Alnor Velometer Jr.

High Range

Indicated Air Speed, True Air Speed,

f pm fpm

800 715.7

625 610.6

530 504.6

480 399.0

420 296.3

300 191.5

225 141.0

195 115.4

160 92.3

100 54.4

T = 27.0 °C

B = 747.5 mm Hg

15
FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL SHOWING

METHOD OF SUPPORT (VIEWED LOOKING DOWNSTREAM).

16
8

17

FIGURE

2.

VARIATION

IN

VELOCITY

WITH

DISTANCE

UPSTREAM

OF

ANEMOMETER.

U

=

205

fpm.

oo

^
220

200

180

160

140

120

100

80

60

40

20

0

U (fpm)

3. INDICATED VERSUS TRUE VELOCITY WITH ± 2 o CURVES, LOW RANGE.

18
FIGURE 4. INDICATED VERSUS TRUE VELOCITY WITH ± 2 a CURVES, HIGH RANGE.

19
20
CM

DS

cj e D a<

>4-1 D

21
FIGURE

7.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION,

a^,

AS

PERCENT

OF

GROUP

VELOCITY,

FOR

LOW

RANGE.

R/2

U

NOTED.
CO CM

^ I O I 1^ 6^

D I \tD

23
I r-i ,-H

I I

•H ^ B

a.

I

24

0

20

40

60

80

100

120

140

160

180

200

220

(fpm)

FIGURE

9.

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY,

LOW

RANGE.
•H E

oI >+-i

S3

25

0

100

200

300

400

500

600

700

(fpm)

figure

10.

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY,

HIGH

RANGE.
wu

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27