NBSIR 79-1708

Low Velocity Performance of a

Heated Foil Anemometer

J

L. P. Purtell

National Bureau of Standards

Fluid Engineering Division

Washington, DC 20234

March 1979

Task Report on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

4

Prepared for

qq

United States Department of the Interior

, _ Bureau of Mines

100

.1156

73-1708
DISCLAIMER NOTICE

The views and conclusions contained in this document should not be interpreted as necessarily representing the official policies or recommendations of the

Interior Department's Bureau of Mines.
Iteffnna! Bums of Standards MAY 1 4

t AC

NBSIR 79-1708

J 4 LOW VELOCITY PERFORMANCE OF A

HEATED FOIL ANEMOMETER

L. P. Purtell

Fluid Engineering Division

Center for Mechanical Engineering

and Process Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

March 1979

Task Report

Prepared for United States Department of the Interior

Bureau of Mines

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

J. Baruch. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Cl
- '

*
NBS-1 14A (REV. 0-7B)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-17083. R«clpteiit's Ae<*$sion No.

4. TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A HEATED

FOIL ANEMOMETER

5. Publication Date

7. AUTHOR(S)

L. P. Purtell

8. Performing Organ. Report No. NBSIR 79-1708

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. H0166198

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, st«te, 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 Oct. 15,1978-Nov.30,1978 _

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual aummary of moat aighificant information. If document includes a aigni ficant bibliography or

literature survey, mention it here.) Performance of a heated-foil anemometer is evaluated over the speed range of 16 to 140

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 (aix to twelve entries; alphabetical order; capitalize only the firat tetter of the first key word unless a proper name;

separated by semicolons

)

Airflow; anemometer; laser velocimeter; low velocity; mine ventilation; wind tunnel

18. AVAILABILITY Unlimited

fx] For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

Order From National Technical Information Service (NTIS), Springfield, VA, 22161

19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

UNCLASSIFIED

20. SECURITY CLASS 22. Price (THIS PAGE)

UNCLASSIFIED

1 USCOMM-DC
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 October 15, 1978 to November 30, 1978. This

report was submitted by the author March 1979.

2
LIST OF SYMBOLS

velocity measured by laser velocimeter

output indicated by anemometer under test, ya

line segments fitted to U, I data

group mean true velocity

group mean indicated output

standard deviation of I data from U... lf

standard deviation of I data expressed as true

velocity

a adjusted for known variance in laser

velocimeter measurements

resolution of the instrument, ya

resolution expressed as true velocity
LOW VELOCITY PERFORMANCE OF A

HEATED FOIL 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 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

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 anemometer tested for this report is a prototype instrument

(Thermogage, Inc., Air Velocity Meter) 1 intended for use as a portable

anemometer. It was supplied for test by the U. S. Bureau of Mines.

The instrument consists of a tube approximately 3/8 inch in diameter

and 4.0 inches in length aligned with the flow, and a 5/8 inch diameter

handle attached normal to the tube midway along its length (see

Figure 1). A part of the wall of the tube is composed of a heatconducting foil which has an electrically heated spot flanked upstream

and downstream by temperature sensors. As air passes through the

tube, the upstream sensor experiences a lower temperature than the

downstream sensor, and this difference, converted to an electrical

signal, is related to velocity. The instrument thus consists of this

thermal transducer and an electronics module containing a line-voltage

operated power supply, the necessary circuitry, and a micro-ammeter.

3.

THE TESTS

The NBS Low Velocity Airflow Facility [l] 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

T This particular instrument was selected as being representative of this

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

4
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. Since this anemometer presents a smaller disturbance to the flow than some previously tested

anemometers, e.g. [2], and since a distance upstream of 30 cm was found

adequate for those tests, this same distance was chosen for the tests

reported here. With no anemometer in the tunnel, variation in velocity

along the centerline is imperceptible over the distance traversed (30 cm).

The output of the anemometer was recorded during the time interval

required for the measurement by the laser velocimeter. Five separate

test runs at ten different velocities were made for each speed range.

The velocity was limited at the low end by instrument resolution to

above approximately 16 fpm and at the high end by full scale at approximately 140 fpm. The data are presented in chronological order in

Tables 1A to IE.

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 (fpm):

Since the groups of data are compact (small range of U within a group; see Figure 2) , a straight line segment is used to approximate the curve

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

group mean true velocity and the group mean indicated output. The

slope of the line segment is computed as the average of the slopes

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

4. TEST RESULTS

21 < U < 25

25 < U < 32

32 < U < 45

45 < U < 60

U < 21 60 < U < 80

80 < U < 90

90 < U < 110

110 < U < 130

130 < U <

5
one line passing through the (U,I) of th£ adjacent group higher in

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

lower in velocity. For the highest group (U > 130) there is only

one adjacent group^ and thus the line segment for this highest group

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

lowest group (U < 21 fpm) is similarly formed.

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

Oj of the indicated output, I, about the fitted segments is determined

by squaring the differences between the I data and U. f

, i.e.,

[I(U) - U^(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 I 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.^/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 unsteadiness in the velocity,

and is estimated as 0.001U for this report. A standard deviation,

a , corrected for the laser velocimeter uncertainty may thus be

computed from

a

2 = a

2 - (0.001U) 2

for any given U. is presented in Figure 3 as current and in

Figure 4 as percentage of I . o and a are_presented in Figure 5 as velocity and in Figure 6 as percentage of U.

5. DISCUSSION OF RESULTS

Computing a 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 (where a is presumed known by some means independent of the scale being considered, say by a second scale with

better resolution), the indicated a may be much smaller than it should be.

For a Gaussian distribution of errors it is assumed that may be adequately

computed if the resolution is at most approximately twice o^. The resolution

judged to be the best that can be read on the anemometer is ± 1 ya over the

entire range.

6
As with the computed values of o^, these values of resolution, R. were

converted to equivalent values, R, in terms of true velocity by dividing by

the slope (dU.^/dU). These latter values, divided by two, were then included

in Figure 5 in units of velocity and in Figure 6 as percentage of U. As may

be seen in Figure 5, R/2 does indeed exceed a for several of the measurements.

Thus these particular values of a should be taken with reservation and perhaps

replaced 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. 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 requires AC power for operation.

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 heated foil anemometer has been evaluated at

air speeds up to 140 fpm. Evaluation of the repeatability was found to

involve considering the resolution of the instrument. Figures are presented showing the response of the instrument to air velocity and the

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

velocities measurable were limited by resolution of the instrument to

about 16 fpm.

7. REFERENCES

1. L. P. Purtell and P. S. Klebanoff, A Low Velocity Airflow Research and

Calibration Facility, in preparation.

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

Anemometer, NBSIR 78-1433.

7
Table 1A

Thermogage Air Velocity Meter

Instrument Output, True Air Speed,

pa fpm

99 138.7

89 119.2

80 101.8

70 85.4

58 70.0

44 54.8

27 39.2

16 27.9

9 22.1

8 17.7

T = 22.2 °C

B = 752.8 mm Hg
Table IB

Thermogage Air Velocity Meter

Instrument Output,

pa

True Air Speed,

fpm

100

91

81

70

58

138.7

119.1

102.7

86.

69.8

A5

26

18

11

7

5A.7

37.5

30.

23.

15.8

T = 22.2 °C

B = 752.8 mm Hg
Table 1C

Thermogage Air Velocity Meter

Instrument Output,

pa

99

90

81

70

58

44

26

18

11

9

True Air Speed,

f pm

137.9

119.1

102.7

86.3

70.0

54.3

38.2

29.8

23.0

20.1

T = 22.2 °C

B = 752.8 mm Hg
Table ID

Thermogage Air Velocity Meter

Instrument Output,

pa

True Air Speed,

f pm

100

90

81

71

58

138.6

118.9

101.8

86.5

69.5

43

24

17

12

8

53.6

35.6

28.7

23.6

19.4

T = 22.2 °C

B = 752.8 mm Hg
ya

99

91

82

69

57

43

27

18

11

9

Table IE

Thermogage Air Velocity Meter

Output, True Air Speed,

f piQ

140.0

119.1

102.2

85.0

68.4

52.9

35.7

29.9

21.7

18,6

T = 22.0 °C

B = 752.8 mm Hg
Figure 1. The anemometer mounted in the tunnel showing method of support.
110

100

90

80

70

60

50

40

30

20

10

0

J f

! ?

• ? 1 ! f * ! f ? f

-g—__ 4—.— 4 S

1 3 1— 1 1 1 * *

20 40 60 80 100 120 140

U (fpm)

Figure 2. Output of instrument against velocity.
U (fpm)

Figure 3. Standard deviation of instrument output against group mean velocity.
10

9

8

7

6

5

4

3

2

1

0

!

20 40 60 80

U (fpm)

100 120 140

gure 4. Standard deviation of instrument output as percent of

instrument output.
1.6 * —j _» -?

1.4

1.2

1.0

0.8

n,o ,R/2

c

(f pm)

0.6

0.4

0.2 ..

J

1 J f 1 1 g 1 1 1 } f ! K

0 20 40 60 80 100 120 140

U (fpm)

Figure 5. Standard deviation and corrected standard deviation in

terms of velocity. R/2 noted.
a R_

9 U 2U

(%)

U (fpm)

Standard deviation in terms of velocity as percent of

group mean velocity. R/2U noted.

Figure 6.