ASTM D5096-02(2017)

Standard Test Method for Determining the Performance of a Cup Anemometer or Propeller Anemometer

SIGNIFICANCE AND USE
5.1 This test method will provide a standard for comparison of rotating type anemometers, specifically cup anemometers and propeller anemometers, of different types. Specifications by regulatory agencies (4-7) and industrial societies have specified performance values. This standard provides an unambiguous method for measuring Starting Threshold, Distance Constant, Transfer Function, and Off-Axis Response.
SCOPE
1.1 This test method covers the determination of the Starting Threshold, Distance Constant, Transfer Function, and Off-Axis Response of a cup anemometer or propeller anemometer from direct measurement in a wind tunnel.  
1.2 This test method provides for a measurement of cup anemometer or propeller anemometer performance in the environment of wind tunnel flow. Transference of values determined by these methods to atmospheric flow must be done with an understanding that there is a difference between the two flow systems.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 28-Feb-2017
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: D22 - Air Quality
• Drafting Committee: D22.11 - Meteorology

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D5096 − 02 (Reapproved 2017)
Standard Test Method for
Determining the Performance of a Cup Anemometer or
Propeller Anemometer
This standard is issued under the fixed designation D5096; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2 Definitions of Terms Specific to This Standard:
3.2.1 starting threshold (U , m/s)—the lowest wind speed at
o
1.1 This test method covers the determination of the Start-
which a rotating anemometer starts and continues to turn and
ing Threshold, Distance Constant, Transfer Function, and
produce a measurable signal when mounted in its normal
Off-Axis Response of a cup anemometer or propeller anemom-
position. The normal position for cup anemometers is with the
eter from direct measurement in a wind tunnel.
axis of rotation vertical, and the normal position for propeller
1.2 This test method provides for a measurement of cup
anemometers is with the axis of rotation aligned with the
anemometer or propeller anemometer performance in the
direction of flow. Note that if the anemometer axis is not
environment of wind tunnel flow. Transference of values
aligned with the direction of flow, the calculated wind speed
determined by these methods to atmospheric flow must be done
component parallel to the anemometer axis is used to deter-
with an understanding that there is a difference between the
mine starting threshold.
two flow systems.
3.2.2 distance constant (L, m)—the distance the air flows
1.3 This standard does not purport to address all of the
past a rotating anemometer during the time it takes the cup
safety concerns, if any, associated with its use. It is the
wheel or propeller to reach (1 − 1 ⁄e) or 63 % of the equilibrium
responsibility of the user of this standard to establish appro- 3
speed after a step change in wind speed (1). The response of
priate safety and health practices and determine the applica-
a rotating anemometer to a step change in which wind speed
bility of regulatory limitations prior to use.
increases instantaneously from U = 0 to U = U is (2):
f
1.4 This international standard was developed in accor-
2t/τ
~ !
U 5 U 1 2 e (1)
~ !
t f
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
where:
Development of International Standards, Guides and Recom-
U = is the instantaneous indicated wind speed at time t in
t
mendations issued by the World Trade Organization Technical
m/s,
Barriers to Trade (TBT) Committee.
U = is the final indicated wind speed, or wind tunnel speed,
f
in m/s,
2. Referenced Documents
t = is the elapsed time in seconds after the step change
2.1 ASTM Standards:
occurs, and
D1356 Terminology Relating to Sampling and Analysis of
τ = is the time constant of the instrument.
Atmospheres
Distance Constant is:L 5 U τ (2)
f
D3631 Test Methods for Measuring Surface Atmospheric
3.2.3 transfer function (Û = a + bR, m/s)—the linear rela-
f
Pressure
tionship between wind speed and the rate of rotation of the
anemometer throughout the specified working range. Û is the
3. Terminology
f
predicted wind speed in m/s, a is a constant, commonly called
3.1 For definitions of terms used in this standard, refer to
zero offset, in m/s, b is a constant representing the wind
Terminology D1356.
passage in m/r for each revolution of the particular anemometer
cup wheel or propeller, and R is the rate of rotation in r/s. It
This test method is under the jurisdiction of ASTM Committee D22 on Air
should be noted that zero offset is not the same as starting
Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
threshold. In some very sensitive anemometers the constant a,
Current edition approved March 1, 2017. Published March 2017. Originally
zero offset, may not be significantly greater than zero. The
approved in 1990. Last previous edition approved in 2011 as D5096 – 02 (2011).
DOI: 10.1520/D5096-02R17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to the list of references at the end of
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5096 − 02 (2017)
constants a and b must be determined by wind tunnel measure-
ment for each type of anemometer (3).
3.2.4 off-axis response (U/(U cos θ))—the ratio of the
f
indicated wind speed (U ) at various angles of attack (θ) to the
indicated wind speed at zero angle of attack (U ) multiplied by
f
the cosine of the angle of attack. This ratio compares the actual
off-axis response to a cosine response.
3.3 Symbols:
a (m/s) = zero offset constant
b (m/r) = wind passage (apparent pitch) constant or
calibration constant
L (m) = distance constant
r (none) = a shaft revolution
R (r/s) = rate of rotation
τ(s) = time constant
t (s) = time
FIG. 1 Typical Anemometer Response Curve
U (m/s) = starting threshold
o
U (m/s) = indicated wind speed (used in off-axis test)
U (m/s) = final indicated wind speed or wind tunnel
f
speed
U (m/s) = anemometer application range
max
U (m/s) = instantaneous indicated wind speed at time t
t
Û (m/s) = predicted wind speed
f
θ (deg) = off-axis angle of attack
4. Summary of Test Method
4.1 This test method requires a wind tunnel described in
Section 6, Apparatus.
4.2 Starting Threshold (U , m/s) is determined by measur-
o
ing the lowest speed at which a rotating anemometer starts and
continues to turn and produce a measurable signal when
mounted in its normal position.
FIG. 2 Typical Anemometer Calibration Curve
4.3 Distance Constant (L, m) may be determined at a
number of wind speeds but must include 5 m/s, and 10 m/s. It
4.5 Off-Axis Response may be measured at a number of
is computed from the time required for the anemometer rotor to
wind speeds but must include 5 m/s, and 10 m/s.
accelerate (1 – 1/e) or 63 % of a step change in rotational speed
4.5.1 Cup Anemometers—A measurement is made of the
after release from a restrained, non-rotating condition. The
output signal when the anemometer is inclined into the wind
final response, U , is the wind tunnel speed as indicated by the
f
(representing a down-draft) and away from the wind (repre-
anemometer. In order to avoid the unrealistic effects of the
senting an updraft), while the wind tunnel is running at a steady
restrained condition, as shown in Fig. 1, the time measurement
speed. The output signal is measured with the anemometer axis
should be made from 0.30 of U to 0.74 of U . This interval in
f f
at 5° intervals from vertical to plus and minus 30° from
seconds is equal to one time constant (τ) and is converted to the
vertical. The measured signal is then converted to a ratio for
Distance Constant by multiplying by the wind tunnel speed in
each interval by dividing by the normal signal measured with
meters per second (m/s).
the anemometer axis in the normal, or vertical, position.
4.4 Transfer Function (Û = a + bR, m/s) is determined by 4.5.2 Vane Mounted Propeller Anemometers—A measure-
f
measuring the rate of rotation of the anemometer at a number ment is made of the output signal when the anemometer’s axis
of wind speeds throughout the specified working range. In the of rotation is inclined downward into the wind (representing a
range of wind speeds where the anemometer response is down-draft) and inclined upward into the wind (representing an
non-linear (near threshold) a minimum of five data points are updraft), while the wind tunnel is running at a steady speed.
recorded. A minimum of five additional data points are The output signal is measured at 5° intervals from a horizontal
recorded within the working range of the anemometer and axis of rotation to 630° from the horizontal. The measured
wind tunnel but above the non-linear threshold region (see Fig. signal is then converted to a ratio for each interval by dividing
2). Measurements are recorded for each data point with the by the normal signal with the anemometer in the normal, or
wind tunnel speed ascending and descending. The values of a horizontal position. This test may be conducted either with the
and b are determined by least-squares linear regression of the vane in place or with the vane removed and the axis of rotation
individual data points. fixed in the down-tunnel direction.
D5096 − 02 (2017)
4.5.3 Fixed Axis Propeller Anemometer—A measurement is 6.3 Wind Tunnel (8):
made of the output signal when the anemometer is rotated in 6.3.1 Size—The wind tunnel must be large enough so that
the air stream throughout the complete 360° angle of attack. the projection of the cup wheel or propeller, sensor, and
The signal is measured at a number of angles but must include support apparatus, is less than 5 % of the cross sectional area
10° intervals with additional measurements at 85, 95, 265, and of the tunnel test section.
275°. The measured signal for each angle of attack is then 6.3.2 Speed Range—The wind tunnel must have a speed
converted to a ratio by dividing by the signal measured at 0° control which will allow the flow rate to be varied from 0 to a
angle of attack (axial flow). Additionally, the stall angle of the minimum of 50 % of the application range of the anemometer
propeller is measured by orienting the anemometer at 90° and under test. The speed control should maintain the flow rate
slowly rotating into and away from the air flow until the within 60.2 m/s.
propeller starts rotating continuously. Stall angle is the total 6.3.3 Calibration—The mean flow rate must be verified at
contained angle within which the propeller does not continu- the mandatory speeds by use of transfer standards which have
ously rotate. The procedure is repeated at 270°. been calibrated at the National Institute of Standards and
Technology or by a fundamental physical method. Speeds
5. Significance and Use
below 2 m/s for the threshold determination must be verified by
5.1 This test method will provide a standard for comparison a sensitive anemometer or by some fundamental time and
distance technique, such as measuring the transition time of
of rotating type anemometers, specifically cup anemometers
and propeller anemometers, of different types. Specifications smoke puffs, soap bubbles, or heat puffs between two points
separated by known distance. A table of wind tunnel blower
by regulatory agencies (4-7) and industrial societies have
specified performance values. This standard provides an un- rpm or some other index relating method of control to flow rate
should be established by this technique for speeds of 2 m/s and
ambiguous method for measuring Starting Threshold, Distance
Constant, Transfer Function, and Off-Axis Response. below.
6.3.4 The wind tunnel must have a relatively constant
6. Apparatus
profile (known to within 1 %) and a turbulence level of less
than 1 % throughout the test section.
6.1 Measuring System:
6.3.5 Environment (9-11). Differences of greater than 3 % in
6.1.1 Rotation—The relationship between the rate of rota-
the density of the air within the test environment may result in
tion of the anemometer shaft and the transducer output must be
poor intercomparability of independent measurements of start-
determined. The resolution of the anemometer transducer
ing threshold (U ) and distance constant (L) since these values
limits the measurement. The resolution of the measuring or
o
are density dependent. The temperature and pressure of the
recording system must represent the indicated wind speed with
environment within the wind tunnel test section, and the
a resolution of 0.02 m/s.
ambient air pressure (Test Methods D3631) shall be reported
6.1.2 Time—The resolution of time must be consistent with
for each independent measurement.
the distance accuracy required. For this reason the time
resolution may be changed as the wind tunnel speed is
7. Sampling
changed. If one wants a distance constant measurement to 0.1
meter resolution one must have a time resolution of 0.05 s at 2
7.1 Starting Threshold—The arithmetic mean on ten con-
m/s and 0.01 s at 10 m/s. If timing accuracy is based on 50 Hz
secutive tests is required for a valid starting threshold mea-
or 60 Hz power frequency it will be at least an order of
surement.
magnitude better than the resolution suggested above.
7.2 Distance Constant—The arithmetic mean of ten tests is
6.1.3 Angle of Attack—The resolution of the angle of attack
required for a valid measurement at each speed. The results of
(θ) must be within 0.5°. An ordinary protractor of adequate size
the measurements at two or more speeds are averaged to a
with 0.5° markings will permit measurements with sufficient
single value for distance constant.
resolution. A fixture should be constructed to permit alignment
of the anemometer to the off-axis angles while the wind tunnel 7.3 Transfer Function—Two measurements of U and R are
f
is running at a steady speed. recorded for each data point, one with the wind tunnel speed
ascending and one with the wind tunnel speed descending. The
6.2 Recording Techniques:
values are then tabulated for each data point.
6.2.1 Digital recording systems and appropriate reduction
programs will be satisfactory if the sampling rate is at least 100 7.4 Off-Axis Response—The results of the measurement at
samples/s. Exercise care to avoid electronic circuits with time two or more speeds are averaged to a single value for each
constants which limit the proper recording of anemometer angle of attack. The averaged values are tabulated for each
performance. Oscilloscopes with memory and hard copy capa- angle of attack.
bility may also be used. Another simple technique is to use a
fast-response strip chart recorder (flat to 10 Hz or better) with
8. Procedure
enough gain so that the signal produced by the anemometer
8.1 Starting Threshold (U ):
o
when the wind tunnel is running at 2 m/s is sufficient to provide
8.1.1 Provide a mechanical method for holding the an-
full scale pen deflection on the recorder. The recorder chart
emometer in its normal position (see 3.1) and for releasing the
drive must have a fast speed of 50 mm/s or more.
anemometer from a restrained, or non-rotating condition, while
the wind tunnel is running at the test speed. Test the release
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D5096 − 02 (Reapproved 2011) D5096 − 02 (Reapproved 2017)
Standard Test Method for
Determining the Performance of a Cup Anemometer or
Propeller Anemometer
This standard is issued under the fixed designation D5096; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of the Starting Threshold,Distance Constant,Transfer Function, and Off-Axis
Response of a cup anemometer or propeller anemometer from direct measurement in a wind tunnel.
1.2 This test method provides for a measurement of cup anemometer or propeller anemometer performance in the environment
of wind tunnel flow. Transference of values determined by these methods to atmospheric flow must be done with an understanding
that there is a difference between the two flow systems.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
D3631 Test Methods for Measuring Surface Atmospheric Pressure
3. Terminology
3.1 For definitions of terms used in this standard, refer to Terminology D1356.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 starting threshold (U , m/s)—the lowest wind speed at which a rotating anemometer starts and continues to turn and
o
produce a measurable signal when mounted in its normal position. The normal position for cup anemometers is with the axis of
rotation vertical, and the normal position for propeller anemometers is with the axis of rotation aligned with the direction of flow.
Note that if the anemometer axis is not aligned with the direction of flow, the calculated wind speed component parallel to the
anemometer axis is used to determine starting threshold.
3.2.2 distance constant (L, m)—the distance the air flows past a rotating anemometer during the time it takes the cup wheel or
propeller to reach (1 − 1 ⁄e) or 63 % of the equilibrium speed after a step change in wind speed (1). The response of a rotating
anemometer to a step change in which wind speed increases instantaneously from U = 0 to U = U is (2):
f
2t/τ
~ !
U 5 U 12 e (1)
~ !
t f
where:
U = is the instantaneous indicated wind speed at time t in m/s,
t
U = is the final indicated wind speed, or wind tunnel speed, in m/s,
f
t = is the elapsed time in seconds after the step change occurs, and
τ = is the time constant of the instrument.
Distance Constant is:L 5 U τ (2)
f
This test method is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved Oct. 1, 2011March 1, 2017. Published October 2011. Originally approved in 1990. Last previous edition approved in 20072011 as
D5096 - 02(2007).D5096 DOI: 10.1520/D5096-02R11.– 02 (2011). DOI: 10.1520/D5096-02R17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5096 − 02 (2017)
3.2.3 transfer function (Û = a + bR, m/s)—the linear relationship between wind speed and the rate of rotation of the
f
anemometer throughout the specified working range. Û is the predicted wind speed in m/s, a is a constant, commonly called zero
f
offset, in m/s, b is a constant representing the wind passage in m/r for each revolution of the particular anemometer cup wheel or
propeller, and R is the rate of rotation in r/s. It should be noted that zero offset is not the same as starting threshold. In some very
sensitive anemometers the constant a, zero offset, may not be significantly greater than zero. The constants a and b must be
determined by wind tunnel measurement for each type of anemometer (3).
3.2.4 off-axis response (U/(U cos θ))—the ratio of the indicated wind speed (U ) at various angles of attack (θ) to the indicated
f
wind speed at zero angle of attack (U ) multiplied by the cosine of the angle of attack. This ratio compares the actual off-axis
f
response to a cosine response.
3.3 Symbols:
a (m/s) = zero offset constant
b (m/r) = wind passage (apparent pitch) constant or calibration constant
L (m) = distance constant
r (none) = a shaft revolution
R (r/s) = rate of rotation
τ(s) = time constant
t (s) = time
U (m/s) = starting threshold
o
U (m/s) = indicated wind speed (used in off-axis test)
U (m/s) = final indicated wind speed or wind tunnel speed
f
U (m/s) = anemometer application range
max
U (m/s) = instantaneous indicated wind speed at time t
t
Û (m/s) = predicted wind speed
f
θ (deg) = off-axis angle of attack
4. Summary of Test Method
4.1 This test method requires a wind tunnel described in Section 6, Apparatus.
4.2 Starting Threshold (U , m/s) is determined by measuring the lowest speed at which a rotating anemometer starts and
o
continues to turn and produce a measurable signal when mounted in its normal position.
4.3 Distance Constant (L, m) may be determined at a number of wind speeds but must include 5 m/s, and 10 m/s. It is computed
from the time required for the anemometer rotor to accelerate (1 – 1/e) or 63 % of a step change in rotational speed after release
from a restrained, non-rotating condition. The final response, U , is the wind tunnel speed as indicated by the anemometer. In order
f
to avoid the unrealistic effects of the restrained condition, as shown in Fig. 1, the time measurement should be made from 0.30
of U to 0.74 of U . This interval in seconds is equal to one time constant (τ) and is converted to the Distance Constant by
f f
multiplying by the wind tunnel speed in meters per second (m/s).
4.4 Transfer Function (Û = a + bR, m/s) is determined by measuring the rate of rotation of the anemometer at a number of wind
f
speeds throughout the specified working range. In the range of wind speeds where the anemometer response is non-linear (near
threshold) a minimum of five data points are recorded. A minimum of five additional data points are recorded within the working
FIG. 1 Typical Anemometer Response Curve
D5096 − 02 (2017)
range of the anemometer and wind tunnel but above the non-linear threshold region (see Fig. 2). Measurements are recorded for
each data point with the wind tunnel speed ascending and descending. The values of a and b are determined by least-squares linear
regression of the individual data points.
4.5 Off-Axis Response may be measured at a number of wind speeds but must include 5 m/s, and 10 m/s.
4.5.1 Cup Anemometers—A measurement is made of the output signal when the anemometer is inclined into the wind
(representing a down-draft) and away from the wind (representing an updraft), while the wind tunnel is running at a steady speed.
The output signal is measured with the anemometer axis at 5° intervals from vertical to plus and minus 30° from vertical. The
measured signal is then converted to a ratio for each interval by dividing by the normal signal measured with the anemometer axis
in the normal, or vertical, position.
4.5.2 Vane Mounted Propeller Anemometers—A measurement is made of the output signal when the anemometer’s axis of
rotation is inclined downward into the wind (representing a down-draft) and inclined upward into the wind (representing an
updraft), while the wind tunnel is running at a steady speed. The output signal is measured at 5° intervals from a horizontal axis
of rotation to 630° from the horizontal. The measured signal is then converted to a ratio for each interval by dividing by the normal
signal with the anemometer in the normal, or horizontal position. This test may be conducted either with the vane in place or with
the vane removed and the axis of rotation fixed in the down-tunnel direction.
4.5.3 Fixed Axis Propeller Anemometer—A measurement is made of the output signal when the anemometer is rotated in the
air stream throughout the complete 360° angle of attack. The signal is measured at a number of angles but must include 10°
intervals with additional measurements at 85, 95, 265, and 275°. The measured signal for each angle of attack is then converted
to a ratio by dividing by the signal measured at 0° angle of attack (axial flow). Additionally, the stall angle of the propeller is
measured by orienting the anemometer at 90° and slowly rotating into and away from the air flow until the propeller starts rotating
continuously. Stall angle is the total contained angle within which the propeller does not continuously rotate. The procedure is
repeated at 270°.
5. Significance and Use
5.1 This test method will provide a standard for comparison of rotating type anemometers, specifically cup anemometers and
propeller anemometers, of different types. Specifications by regulatory agencies (4-7) and industrial societies have specified
performance values. This standard provides an unambiguous method for measuring Starting Threshold,Distance Constant,Transfer
Function, and Off-Axis Response.
6. Apparatus
6.1 Measuring System:
6.1.1 Rotation—The relationship between the rate of rotation of the anemometer shaft and the transducer output must be
determined. The resolution of the anemometer transducer limits the measurement. The resolution of the measuring or recording
system must represent the indicated wind speed with a resolution of 0.02 m/s.
6.1.2 Time—The resolution of time must be consistent with the distance accuracy required. For this reason the time resolution
may be changed as the wind tunnel speed is changed. If one wants a distance constant measurement to 0.1 meter resolution one
must have a time resolution of 0.05 s at 2 m/s and 0.01 s at 10 m/s. If timing accuracy is based on 50 Hz or 60 Hz power frequency
it will be at least an order of magnitude better than the resolution suggested above.
6.1.3 Angle of Attack—The resolution of the angle of attack (θ) must be within 0.5°. An ordinary protractor of adequate size
with 0.5° markings will permit measurements with sufficient resolution. A fixture should be constructed to permit alignment of the
anemometer to the off-axis angles while the wind tunnel is running at a steady speed.
FIG. 2 Typical Anemometer Calibration Curve
D5096 − 02 (2017)
6.2 Recording Techniques:
6.2.1 Digital recording systems and appropriate reduction programs will be satisfactory if the sampling rate is at least 100
samples/s. Exercise care to avoid electronic circuits with time constants which limit the proper recording of anemometer
performance. Oscilloscopes with memory and hard copy capability may also be used. Another simple technique is to use a
fast-response strip chart recorder (flat to 10 Hz or better) with enough gain so that the signal produced by the anemometer when
the wind tunnel is running at 2 m/s is sufficient to provide full scale pen deflection on the recorder. The recorder chart drive must
have a fast speed of 50 mm/s or more.
6.3 Wind Tunnel (8):
6.3.1 Size—The wind tunnel must be large enough so that the projection of the cup wheel or propeller, sensor, and support
apparatus, is less than 5 % of the cross sectional area of the tunnel test section.
6.3.2 Speed Range—The wind tunnel must have a speed control which will allow the flow rate to be varied from 0 to a minimum
of 50 % of the application range of the anemometer under test. The speed control should maintain the flow rate within 60.2 m/s.
6.3.3 Calibration—The mean flow rate must be verified at the mandatory speeds by use of transfer standards which have been
calibrated at the National Institute of Standards and Technology or by a fundamental physical method. Speeds below 2 m/s for the
threshold determination must be verified by a sensitive anemometer or by some fundamental time and distance technique, such as
measuring the transition time of smoke puffs, soap bubbles, or heat puffs between two points separated by known distance. A table
of wind tunnel blower rpm or some other index relating method of control to flow rate should be established by this technique for
speeds of 2 m/s and below.
6.3.4 The wind tunnel must have a relatively constant profile (known to within 1 %) and a turbulence level of less than 1 %
throughout the test section.
6.3.5 Environment (9-11). Differences of greater than 3 % in the density of the air within the test environment may result in poor
intercomparability of independent measurements of starting threshold (U ) and distance constant (L) since these values are density
o
dependent. The temperature and pressure of the environment within the wind tunnel test section, and the ambient air pressure (Test
Methods D3631) shall be reported for each independent measurement.
7. Sampling
7.1 Starting Threshold—The arithmetic mean on ten consecutive tests is required for a valid starting threshold measurement.
7.2 Distance Constant—The arithmetic mean of ten tests is required for a valid measurement at each speed. The results of the
measurements at two or more speeds are averaged to a single value for distance constant.
7.3 Transfer Function—Two measurements of U and R are recorded for each data point, one with the wind tunnel speed
f
ascending and one with the wind tunnel speed descending. The values are then tabulated for each data point.
7.4 Off-Axis Response—The results of the measurement at two or more speeds are averaged to a single value for each angle of
attack. The averaged values are tabulated for each angle of attack.
8. Procedure
8.1 Starting Threshold (U ):
o
8.1.1 Provide a mechanical method for holding the anemometer in its normal position (see 3.1) and for releasing the
anemometer from a restrained, or non-rotating condition, while the wind tunnel is running at the test speed. Test the release
mechani
...