ASTM D6011-96(2008)
(Test Method)Standard Test Method for Determining the Performance of a Sonic Anemometer/Thermometer
Standard Test Method for Determining the Performance of a Sonic Anemometer/Thermometer
SIGNIFICANCE AND USE
This test method provides a standard method for evaluating the performance of sonic anemometer/thermometers that use inverse time solutions to measure wind velocity components and the speed of sound. It provides an unambiguous determination of instrument performance criteria. The test method is applicable to manufacturers for the purpose of describing the performance of their products, to instrumentation test facilities for the purpose of verifying instrument performance, and to users for specifying performance requirements. The acoustic pathlength procedure is also applicable for calibration purposes prior to data collection. Procedures for operating a sonic anemometer/thermometer are described in Practice D 5527.
The sonic anemometer/thermometer array is assumed to have a sufficiently high structural rigidity and a sufficiently low coefficient of thermal expansion to maintain an internal alignment to within the manufacturer's specifications over its designed operating range. Consult with the manufacturer for an internal alignment verification procedure and verify the alignment before proceeding with this test method.
This test method is designed to characterize the performance of an array model or probe design. Transducer shadow data obtained from a single array is applicable for all instruments having the same array model or probe design. Some non-orthogonal arrays may not require specification of transducer shadow corrections or the velocity calibration range.
SCOPE
1.1 This test method covers the determination of the dynamic performance of a sonic anemometer/thermometer which employs the inverse time measurement technique for velocity or speed of sound, or both. Performance criteria include: (a) acceptance angle, (b) acoustic pathlength, (c) system delay, (d) system delay mismatch, (e) thermal stability range, (f) shadow correction, (g) velocity calibration range, and (h) velocity resolution.
1.2 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.
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Designation: D6011 − 96(Reapproved 2008)
Standard Test Method for
Determining the Performance of a Sonic Anemometer/
Thermometer
This standard is issued under the fixed designation D6011; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.1 axial attenuation coeffıcient—aratioofthefreestream
windvelocity(asdefinedinawindtunnel)tovelocityalongan
1.1 This test method covers the determination of the dy-
acoustic propagation path (v /v ) (1).
t d
namicperformanceofasonicanemometer/thermometerwhich
3.2.2 critical Reynolds number (R )—the Reynolds number
c
employs the inverse time measurement technique for velocity
at which an abrupt decrease in an object’s drag coefficient
or speed of sound, or both. Performance criteria include: (a)
occurs (2).
acceptanceangle,(b)acousticpathlength,(c)systemdelay,(d)
3.2.2.1 Discussion—The transducer shadow corrections are
system delay mismatch, (e) thermal stability range, (f) shadow
no longer valid above the critical Reynolds number due to a
correction, (g) velocity calibration range, and (h) velocity
discontinuity in the axial attenuation coefficient.
resolution.
3.2.3 Reynolds number (R )—the ratio of inertial to viscous
e
1.2 This standard does not purport to address all of the
forces on an object immersed in a flowing fluid based on the
safety concerns, if any, associated with its use. It is the object’s characteristic dimension, the fluid velocity, and vis-
responsibility of the user of this standard to establish appro- cosity.
priate safety and health practices and determine the applica-
3.2.4 shadow correction (v /v )—the ratio of the true
dm d
bility of regulatory limitations prior to use.
along-axis velocity v , as measured in a wind tunnel or by
dm
another accepted method, to the instrument along-axis wind
2. Referenced Documents
measurement v .
d
3.2.4.1 Discussion—This correction compensates for flow
2.1 ASTM Standards:
shadowing effects of transducers and their supporting struc-
C384Test Method for Impedance andAbsorption ofAcous-
tures. The correction can take the form of an equation (3) or a
tical Materials by Impedance Tube Method
lookup table (4).
D1356Terminology Relating to Sampling and Analysis of
3.2.5 speed of sound (c, (m/s))—the propagation rate of an
Atmospheres
adiabatic compression wave
D5527Practices for Measuring Surface Wind and Tempera-
0.5
c 5 ~γ]P/]ρ! (1)
ture by Acoustic Means
s
IEEE/ASTM SI-10Use of the International System of Units
where:
(SI): The Modern Metric System
P = pressure
ρ = density,
3. Terminology
γ = specific heat ratio, and
s = isentropic (adiabatic) process (6).
3.1 Definitions—For definitions of terms related to this test
method, refer to Terminology D1356.
3.2.5.1 Discussion—The velocity of the compression wave
3.2 Definitions of Terms Specific to This Standard:
defined along each axis of a Cartesian coordinate system is the
sum of propagation speed c plus the motion of the gas along
that axis. In a perfect gas (5):
1 0.5
This test method is under the jurisdiction of ASTM Committee D22 on Air
c 5 γR*T/M (2)
~ !
Qualityand is the direct responsibility of Subcommittee D22.11 on Meteorology.
Current edition approved Oct. 1, 2008. Published October 2008. Originally The approximation for propagation in air is:
approved in 1996. Last previous edition approved in 2003 as D6011-96(2003).
0.5 0.5
c 5 403 T 110.32 e/P 5 403 T (3)
@ ~ !# ~ !
DOI: 10.1520/D6011-96R08. air s
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6011 − 96 (2008)
3.2.6 system clock—the clock used for timing acoustic t 5 t 2 δ (9)
2 t2 t2
wavefront travel between a transducer pair. 3.2.11.2 Discussion—Proceduresinthistestmethodinclude
a test to determine whether separate determinations of δ t and
3.2.7 system delay (δt, µs)—the time delay through the
δt are needed, or whether an average δt can be used. The
transducer and electronic circuitry (7).
relationship of transit time to speed of sound is
3.2.7.1 Discussion—Each path through every sonic array
d 1 1
axis can have unique delay characteristics. Delay (on the order
2 2
c 5 1 1v (10)
F G
S D
n
2 t t
of 10 to 20 µs) can vary as a function of temperature and
1 2
directionofsignaltravelthroughthetransducersandelectronic
and the inverse transit time solution for sonic tempera-
circuitry.Theaveragesystemdelayforeachaxisinanacoustic
ture in air is as follows (6):
array is the average of the delays measured in each direction
2 2 2
d 1 1 v
n
along the axis
T 5 1 1 (11)
S D F G
s
1612 t t 403
1 2
δt 5 δt 1δt /2 (4)
~ !
1 2
3.2.12 velocity calibration range (U to U , (m/s))—the
c s
3.2.8 system delay mismatch (δt, µs )—the absolute differ-
t
range of velocity between creeping flow and the flow at which
ence in microseconds between total transit times t in each
t
a critical Reynolds number is reached.
direction (t , t ) through the system electronics and transduc-
t1 t2
3.2.12.1 Discussion—The shadow correction is valid over a
ers.
rangeofvelocitieswherenodiscontinuitiesareobservedinthe
3.2.8.1 Discussion—Due principally to slight differences in
axial attenuation coefficient.
transducerperformance,thetotaltransittimeobtainedwiththe
3.2.13 velocity resolution (δv, (m/s))—the largest change in
signal originating at one transducer can differ from the total
an along-axis wind component that would cause no change in
transit time obtained with the signal originating at its paired
the pulse arrival time count.
transducer. The manufacturer should specify the system delay
3.2.13.1 Discussion—Velocity resolution defines the small-
mismatch tolerance.
est resolvable wind velocity increment as determined from
δt 5 t 2 t (5)
t ? t1 t2 ?
systemclockrate.Forsomesystems,δvdefinedasthestandard
3.2.9 thermal stability range (°C)—a range of temperatures
deviation of system dither can also be reported.
over which the corrected velocity output in a zero wind
3.3 Symbols:
chamber remains at or below instrument resolution.
3.2.9.1 Discussion—Thermal stability range defines a range c = speed of sound, m/s,
of temperatures over which there is no step change in system C = specific heat at constant pressure, J/(kg·K),
p
C = specific heat at constant volume, J/(kg·K),
delay. v
e = vapor pressure, Pa,
3.2.10 time resolution (∆t, µs)—resolution of the internal
d = acoustic pathlength, m,
clock used to measure time.
f = compressibility factor, dimensionless,
M = molecular weight of a gas, g/mol,
3.2.11 transit time (t, µs)—the time required for an acoustic
P = pressure, Pa,
wavefront to travel from the transducer of origin to the
R* = universal gas constant, 8.31436 J/(mol·K),
receiving transducer.
RH = relative humidity, %,
3.2.11.1 Discussion—Transit time (also known as time of
t = transit time, µs,
flight) is determined by acoustic pathlength d, the speed of
t = total transit time, µs,
t
sound c, the velocity component along the acoustic propaga-
T = absolute temperature, K,
tion path v , and cross-path velocity components) v (8)
d n T = sonic absolute temperature, K,
s
2 2 0.5 2 2 2
U = upper limit for creeping flow, m/s,
t 5 d@~c 2 v ! 6V #/@c 2 ~v 1v !# (6) c
n d d n
U = critical Reynolds number velocity, m/s,
s
The transit time difference between acoustic wavefront
v = velocity component along acoustic propagation path,
d
propagation in one direction (t , computed for+ v ) and
1 d
m/s,
the other (t , computed for− v ) for each transducer pair
2 d v = tunnel velocity component parallel to the array axis
dm
determines the magnitude of a velocity component. The
(v, cos θ), m/s,
t
inverse transit time solution for the along-axis velocity is
v = velocitycomponentnormaltoanacousticpropagation
n
(9)
path, m/s,
v = free stream wind velocity component (unaffected by
t
d 1 1
v 5 2 (7)
F G thepresenceofanobstaclesuchastheacousticarray),
d
2 t t
1 2
m/s,
The total transit times t and t , include the sum of
t1 t2 δt = system delay, µs,
actual transit times plus system delay through the electron-
δt = system delay mismatch, µs,
t
ics and transducers in each direction along an acoustic ∆t = clock pulse resolution, s,
path, δ and δ . System delay must be removed to calcu- α = acceptance angle, degree,
t1 t2
γ = specific heat ratio (C /C ), dimensionless,
late v , that is,
p v
d
δv = velocity resolution, m/s,
t 5 t 2 δ (8)
1 t1 t1
θ = array angle of attack, degree, and
D6011 − 96 (2008)
ρ = gas density, kg/m .
3.4 Units—Units of measurement are in accordance with
IEEE/ASTM SI-10.
4. Summary of Test Method
4.1 Acoustic pathlength, system delay, and system delay
mismatch are determined using the dual gas or zero wind
chamber method. The acoustic pathlength and system clock
rate are used to calculate the velocity resolution. Thermal
sensitivity range is defined using a zero wind chamber. The
axial attenuation coefficient, velocity calibration range, and
transducer shadow effects are defined in a wind tunnel. Wind
tunnel results are used to compute shadow corrections and to
define acceptance angles.
5. Significance and Use
5.1 This test method provides a standard method for evalu-
ating the performance of sonic anemometer/thermometers that
use inverse time solutions to measure wind velocity compo-
FIG. 1 Sonic Anemometer Array in a Zero Wind Chamber
nents and the speed of sound. It provides an unambiguous
determination of instrument performance criteria. The test
method is applicable to manufacturers for the purpose of
describing the performance of their products, to instrumenta-
tion test facilities for the purpose of verifying instrument
performance, and to users for specifying performance require-
ments.Theacousticpathlengthprocedureisalsoapplicablefor
calibration purposes prior to data collection. Procedures for
operating a sonic anemometer/thermometer are described in
Practice D5527.
5.2 Thesonicanemometer/thermometerarrayisassumedto
haveasufficientlyhighstructuralrigidityandasufficientlylow
coefficient of thermal expansion to maintain an internal align-
ment to within the manufacturer’s specifications over its
designedoperatingrange.Consultwiththemanufacturerforan
internal alignment verification procedure and verify the align-
ment before proceeding with this test method.
FIG. 2 Pathlength Chamber for Acoustic Pathlength Determina-
tion
5.3 This test method is designed to characterize the perfor-
mance of an array model or probe design. Transducer shadow
data obtained from a single array is applicable for all instru-
ments having the same array model or probe design. Some
rials to prevent pressure loss and contamination. Design the
non-orthogonal arrays may not require specification of trans-
chamber for quick and thorough purging.The basic pathlength
ducer shadow corrections or the velocity calibration range.
chamber components are illustrated in Fig. 2.
6. Apparatus 6.2.2 Gas Source and Plumbing, to connect the pathlength
chamber to one of two pressurized gas sources (nitrogen or
6.1 Zero Wind Chamber, sized to fit the array and accom-
argon).Employapurgepumptodrawoffusedgases.Required
modate a temperature probe (Fig. 1) used to calibrate the sonic
purity of the gas is 99.999%.
anemometer/thermometer. Line the chamber with acoustic
foam with a sound absorption coefficient of 0.8 or better (Test 6.3 Temperature Transducer (two required), with minimum
Method C384) to minimize internal air motions caused by temperature measurement precision and accuracy of 60.1°C
thermal gradients and to minimize acoustic reflections. Install and 60.2°C, respectively, and with recording readout. One is
asmallfanwithinthechambertoestablishthermalequilibrium required for the zero wind chamber and one for the pathlength
before a zero wind calibration is made. chamber.
6.2 Pathlength Chamber—See Fig. 2. 6.4 Wind Tunnel:
6.2.1 Design the pathlength chamber to fit and seal an axis 6.4.1 Size, large enough to fit the entire instrument array
of the array for acoustic pathlength determination. Construct withinthetestsectionatallrequiredorientationangles.Design
thechambercomponentsusingnon-expanding,non-outgassing the tunnel so that the maximum projected area of the sonic
materials. Employ O-ring seals made of non-outgassing mate- array is less than 5% of tunnel cross-sectional area.
D6011 − 96 (2008)
6.4.2 Speed Control, to vary the flow rate over a range of at procedures used to determine d and δ in argon and nitrogen
t
least1.0to10m/swithin 60.1m/sorbetterthroughoutthetest gasesforaminimumoftentimes,oruntilconsistentresultsare
section. achieved. If the caliper method is used, measure and verify the
6.4.3 Calibration—Calibratethemeanflowrateusingtrans- transducer spacing to a tolerance of 0.1 mm. Independently
fer standards traceable to the National Institute of Standards determine d and δ for each axis of the acoustic array for each
t
and Technology (NIST), or by an equivalent fundamental instrument.
physical method.
8.2 Thermal Stability Range—Obtain a zero velocity read-
6.4.4 Turbulence, with a uniform velocity profile with a
ing over a period of at least one minute at room temperature.
minimumofswirlatallspeeds,andknownuniformturbulence
Repeat the procedure over the instrument’s expected tempera-
scale and intensity throughout the test section.
ture operating range. Repeat the test for each transducer axis
6.4.5 Rotating Plate, to hold the sonic transducer array in
for each instrument.
varying orientations to achieve angular exposures up to 360°,
8.3 Axial Attenuation and Angular Shadow Effects—After
as needed.The minimum plate rotation requirements are 660°
the wind tunnel test section velocity has stabilized, obtain the
in the horizontal and 615° in the vertical, with an angular
velocity readings at each position for a measurement period of
alignment resolution of 0.5°.
30 s. Obtain at least three consecutive measurements at each
NOTE 1—Design the plate to hold the array at chosen angles without
angle and tunnel velocity settings. Calculate the average and
disturbing the test section wind velocity profile or changing its turbulence
range of each of these readings.
level.
8.4 Shadow Correction—Select a low velocity sett
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