NBSIrt 79-1586

Low Velocity Performance of a

Magnetic Pick-Up Vane

Anemometer

L P Purtell

National Bureau of Standards

Fluid Engineering Division

Washington, D.C. 2.0234

December 1978

Task Report on

Contract No. HOI 661 98

Evaluation of the Behavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility

Issued January 1 979

oc100

Prepared for United States Department of the Interior

Bureau of Mines
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.
®aflb'r3T Wrea® of Stanfords MAY 1 4 iqts

NBSIR 79-1566

LOW VELOCITY PERFORMANCE OF A

MAGNETIC PICK-UP 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

December 19 78

Task Report

Issued January 1979

Prepared for United States Department of the Interior

Bureau of Mines

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

Jordan J. Baruch, Assistant Secretary for Science and Technology

l

o

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
" ••
NBS-114A (REV. 5-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 79 - 1566

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

LOW VELOCITY PERFORMANCE OF A MAGNETIC PICK-UP

VANE ANEMOMETER

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

L. P. Purtell

8. Performing Organ. Report No. NBSIR 79 - 1566

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. 20241

13. Type of Report & Period

_ Covered „ _ _ _ Jan. 1, 1978 - April 30. 1978

14. Sponsoring Agency Code

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 magnetic pick-up vane anemometer is evaluated over the

speed range of 54.9 to 717 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; laser velocimeter; low velocity; mine ventilation; vane anemometers;

wind tunnel

18. AVAILABILITY ^

Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

1

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

Washington. D.C. 20402. SD Stock No. SN003-OQ3

20. SECURITY CLASS

(THIS PAGE)

22. Price

!T__ Order From National Technical Information Service (NTIS)

Springfield, Virginia 2216 1 UNCLASSIFIED

1

USCOMM-OC 66680-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 January 1, 1978 to April 30,

1978. This report was submitted by the author December 1978.

2
LIST OF SYMBOLS

U velocity measured by laser velocimeter

'LL velocity indicated by anemometer under test

line segments fitted to U, UL data

U group mean true velocity

tk group mean indicated velocity

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

1 lif a standard deviation of U. data expressed as

. . i velocity

a a adjusted for known variance in laser

velocimeter measurements

R. resolution of the instrument

1

R resolution expressed as true velocity

true

3
LOW VELOCITY PERFORMANCE OF A MAGNETIC PICK-UP

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 anemometer tested for this report is a commercially available

instrument (Abbirko Instruments, Ltd., Flowmaster, S/N 6184) 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 at the request of the U. S. Bureau of Mines. The instrument's

vane housing is approximately 2.75 inches in diameter (see Figure 1)

with an attached block for the magnetic pick-up, and a handle projecting

down from the block. The magnetic pick-up detects the passage of the

metal vanes, and the resulting electrical signal is converted to a deflection of a meter connected to the probe by a cable. The meter was located outside the tunnel during the tests and was oriented horizontally. Three ranges of operation are selectable, low - 0 to 300 fpm,

medium - 0 to 1000 fpm, and high - 0 to 3000 fpm. The anemometer

requires a 9 volt battery for operation.

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

This particular instrument was selected as being representative of

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

4
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 streamwise velocity upstream of anemometers previously tested - anemometers which offered greater blockage than the present one, e.g.,

[2]

,

was measured on the centerline of the test section 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. It was found that a distance of less than 30 cm upstream of the

anemometers was sufficient. Thus 30 cm was also chosen for the anemometer reported on here. With no anemometer in the tunnel, variation in

velocity along the centerline is imperceptible over this 30 cm distance.

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 ten different velocities for the low

range, nine for the medium range, and seven for the high range. The

data are presented in chronological order in Tables 1A to IE, 2A to 2E,

and 3A to 3E for the low, medium and high ranges, respectively.

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. Five such measurements are presented in

Table 4 and have an average of 59.6 fpm and a standard deviation of 0.6

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. Five such measurements are presented in Table 5 with an average of 39.5 fpm and a standard deviation of 0.6 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

5
curve, thus prohibiting computing the standard deviation of the data

from a simple average. Instead, deviations from a curve fitted 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) Medium Range (fpm) High Range (fpm)

U < 60.0 U < 70.0 U < 150

60.0 < u < 70.0 70.0 < u < 90.0 150 < u < 250

70.0 < u < 80.0 90.0 < u < 150 250 < u < 350

80.0 < u < 90.0 150 < u < 250 350 < u < 450

90.0 < u < 100 250 < u < 350 450 < u < 550

100 < u < 130 350 < u < 450 550 < u < 650

130 < u < 180 450 < u < 550 650 < u 180 < u < 220 550 < u < 650

220 < u < 280 650 < u 280 < U

Since the groups of data are compact (small range of U within a group; see Figures 2, 3, and 4), 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,U.) of the group being considered,

one line passing through the (U,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 (t > 280 and 650 fpm) there is

only one adjacent group, and thus the line segment for the lowest groups

(U < 60, 70, and 150 fpm) is similarly formed.

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

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

[U (U) -U

if (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 U. about U . _ within that

i ill —

group and specified at that group’s mean true velocity, U. To convert

this to a. standard deviation in terms of true velocity, designated CT, each G_. (U) is dividend by the slope (dlh^/dU) of the line segment associated with the CT. (U) . Note that this CT 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, CT , corrected

for the laser velocimeter uncertainty may thus be computed from

2

CT = a

2 - (0.001U) 2 c

6
for any given U. G and O are presented in Figures 5 ,_6 , and 7 as velocity and in Figures 8, 9, and 10 as percentage of U. Since ± 2o

Q is extremely close to the 95 percent confidence interval for one measurement, curves of ± 2o are also included in Figures 2, 3, and 4 as dashed

lines.

The actual differences between the true and indicated velocities,

U - U. , are presented in Figures 11, 12, and 13 and as a percentage of U

in Figures 14, 15 and 16. 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 O. is a good procedure for determining repeatability of the instrument. 1

If the resolution is large (poor)

compared to G. (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 that O. may be adequately computed

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

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

anemometer tested:

Low Range

U_^ < 300 fpm, R^ = 1/10 division or 2 fpm

Medium Range

U_^ < 1000 fpm, R^ = 1/10 division or 5 fpm

High Range

U_^ < 3000 fpm, R_^ = 1/5 division or 20 fpm

As with the computed values of G., 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

Figures 5, 6, and 7 in units of velocity and in Figures 8, 9, and 10 as percentage

of U. As may be seen in Figures 5, 6, and 7, R/2 does indeed exceed G for several

of the measurements. Thus these particular values of G 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. Fluctuations

of the dial indicator during a reading were generally below ± 1 or 2 fpm.

Some general comments concerning application of the instrument follow. With

any measurement problem the instrument's capabilities should be matched to the

required measurement.

7
This anemometer is intrusive, i.e., it must be placed in the flow.

This anemometer requires a battery 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 en- vironmental 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 magnetic pick-up vane anemometer has been

evaluated at air speeds up to 717 fpm. Evaluation of the repeatability

was found to involve considering 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. Starting and stopping speed measurements are presented and

give an average starting speed of 59.6 fpm and an average stopping speed

of 39.5 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.
Table 1A

Abbirko Flow-master

S/N 6184

Low Range

Indicated Air Speed, fpm True Air Speed, fpm

50 59.9

52 64.1

64 74.5

74 86.1

86 97.2

96 108.4

144 157

184 200

228 248

272 300

T = 20 . 8°C

B = 758.4 mm Hg

Table IB

Abbirko Flowmaster

S/N 6184

Low Range

Indicated Air Speed, fpm True Air Speed, fpm

50 59.6

54 64.0

64 74.0

76 86.4

86 97.1

98 108.8

144 157

186 200

228 250

274 299

T = 20. 8°C

B = 758.4 mm Hg
Table 1C

Abbirko Flowmaster

S/N 6184

Low Range

Indicated Air Speed, fpm True Air Speed, fpm

52 59.8

54 64.1

64 74.3

76 86.2

86 96.7

98 107.8

144 157

184 199

230 249

274 298

T « 20 . 9°C

B = 758.4 mm Hg

Table ID

Abbirko Flowmaster

S/N 6184

Low Range

Indicated Air Speed, fpm True Air Speed, fpm

48 54.9

50 60.0

60 72.1

74 82.8

84 94.4

98 106,8

142 155

184 198

228 248

272 299

T = 22 . 2°C

B = 750.3 mm Hg
Table IE

Abbirko Flowmaster

S/N 6184

Low Range

Indicated Air Speed, fpm True Air Speed, fpm

50 56.6

52 60.7

60 72.1

76 84.2

84 95.3

98 106.4

142 155

182 199

228 249

272 298

T = 22 . 3°C

B = 751.1 mm Hg
Table 2A

Abbirko Flowmaster

S/N 6184

Medium Range

Indicated Air Speed, fpm True Air Speed, fpm

30

55

75

170

265

63.4

86.1

108.1

199

299

370

475

585

685

402

507

611

716

T = 20. 8°C

B = 758.4 mm Hg

Table 2B

Abbirko Flowmaster

S/N 6184

Medium Range

Indicated Air Speed, fpm

25

55

80

170

270

375

480

585

685

True Air Speed, fpm

64.4

86.1

108.2

200

298

401

507

612

716

T = 20 . 9°C

B = 758.4 mm Hg
Table 2C

Abbirko Flowmaster

S/N 6184

Medium Range

Indicated Air Speed, fpm True Air Speed, fpm

25 59.9

50 82.8

75 106.2

165 199

265 298

370 402

475 506

580 611

685 714

T = 22 . 2°C

B = 750.0 mm Hg

Table 2D

Abbirko Flowmaster

S/N 6184

Medium Range

Indicated Air Speed, fpm True Air Speed, fpm

25 61.1

50 83.6

80 107.5

170 199

265 299

375 402

485 506

585 611

690 716

T = 22 . 3°C

B = 750.8 mm Hg
Table 2E

Abbirko Flowmaster

S/N 6184

Medium Range

Indicated Air Speed, fpm True Air Speed, fpm

25 61.2

50 83.0

75 106.8

170 199

265 299

375 402

480 506

585 611

685 717

T = 22 . 4°C

B = 751.2 mm Hg
Table 3A

Abbirko Flowmaster

S/N 6184

High Range

Indicated Air Speed, fpm True Air Speed, fpm

80 107.4

170 199

270 298

370 401

480 506

580 612

690 711

T = 20 . 6 °C

B = 758.4 mm Hg

Table 3B

Abbirko Flowmaster

S/N 6184

High Range

Indicated Air Speed, fpm True Air Speed, fpm

90 108.6

170 199

270 299

380 403

480 507

590 613

690 716

T = 20. 9°C

B = 758.4 mm Hg
Table 3C

Abbirko Flowmaster

S/N 6184

High Range

Indicated Air Speed, fpm True Air Speed, fpm

90 106.3

180 198

280 299

380 401

490 505

590 612

695 712

T = 22. 1°C

B = 750.0 mm Hg

Table 3D

Abbirko Flowmaster

S/N 6184

High Range

Indicated Air Speed, fpm True Air Speed, fpm

90 106.7

180 199

280 299

380 403

490 507

590 611

695 715

T = 22 . 3°C

B = 751.1 mm Hg
Table 3E

Abblrko Flowmaster

S/N 6184

High Range

Indicated Air Speed, fpm True Air Speed, fpm

80 107.9

180 200

280 299

390 403

490 506

590 612

695 715

T = 22 . 4°C

B = 751.2 mm Hg
Table 4

Abbirko Flowmaster

S/N 6184

Starting Speed, fpm

59.8 Average: 59.

59.2

58.9

59.7 Standard

60.5 Deviation: 0.

Table 5 Abbirko Flowmaster

S/N 6184

Stopping Speed, fpm

39.0 Average

:

38. 9 39.5

40.4 Standard

39.5 Deviation

fpm

fpm

fpm

6 fpm
FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL SHOWING

METHOD OF SUPPORT.
u.

1(fpm)

U (fpm)

FIGURE 2. INDICATED VERSUS TRUE VELOCITY WITH ± la CURVES,

LOW RANGE

.
700

600

500

i

400

300

200

100

0

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

MEDIUM RANGE.
700

600

500

400

300

200

100

0

4 —4. -4 -4— -4 4 » « I « • —*

4 4 4 4 * 4 —4 4— 4 I 4 I 100 200 300 400 500 600 700

U (fpm)

4. INDICATED VERSUS TRUE VELOCITY WITH ± 2a CURVES,

HIGH RANGE.
FIGURE

5.

a

AND

a

IN

TERMS

OF

TRUE

VELOCITY,

LOW

RANGE.

R/2

NOTED.


!

3

100

200

300

U

(fpm)

FIGURE

8.

a

AND

a

AS

PERCENT

OF

GROUP

MEAN

VELOCITY,

LOW

RANGE.

R/2U

NOTED.
D | IE3

U

(fpm)

FIGURE

9.

a

AND

a

AS

PERCENT

OF

GROUP

MEAN

VELOCITY,

MEDIUM

RANGE.

R/2U

NOTED

r*

p ea I >4-1 3 ^
•H

I

Ea

M-J

0

100

200

300

400

500

600

700

IK

(fpm)

FIGURE

12.

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY,

MEDIUM

RANGE.

sa

4-1

wazsoG

>-1 HM

CJoGW>wG

OSHSO

OSGGWH

<3UMasGozoMH

<3M>GQHZGu

PigG<i—I GuMG

GI G

S'?
S'? 3

0

100

200

300

400

500

600

700

U

(

f

pm)

FIGURE

15.

PERCENT

DEVIATION

OF

INDICATED

FROM

TRUE

VELOCITY,

MEDIUM

RANGE.