ASTM D6011-96(2003)
(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’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: D 6011 – 96 (Reapproved 2003)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.2 critical Reynolds number (R )—the Reynolds number
c
at which an abrupt decrease in an object’s drag coefficient
1.1 This test method covers the determination of the dy-
occurs (2).
namicperformanceofasonicanemometer/thermometerwhich
3.2.2.1 Discussion—The transducer shadow corrections are
employs the inverse time measurement technique for velocity
no longer valid above the critical Reynolds number due to a
or speed of sound, or both. Performance criteria include: (a)
discontinuity in the axial attenuation coefficient.
acceptance angle, (b) acoustic pathlength, (c) system delay, (d)
3.2.3 Reynolds number (R )—the ratio of inertial to viscous
e
system delay mismatch, (e) thermal stability range, (f) shadow
forces on an object immersed in a flowing fluid based on the
correction, (g) velocity calibration range, and (h) velocity
object’s characteristic dimension, the fluid velocity, and vis-
resolution.
cosity.
1.2 This standard does not purport to address all of the
3.2.4 shadow correction (v /v )—the ratio of the true
dm d
safety concerns, if any, associated with its use. It is the
along-axis velocity v , as measured in a wind tunnel or by
responsibility of the user of this standard to establish appro- dm
another accepted method, to the instrument along-axis wind
priate safety and health practices and determine the applica-
measurement v .
bility of regulatory limitations prior to use. d
3.2.4.1 Discussion—This correction compensates for flow
2. Referenced Documents shadowing effects of transducers and their supporting struc-
tures. The correction can take the form of an equation (3) or a
2.1 ASTM Standards:
lookup table (4).
C384 Test Method for Impedance and Absorption of
3.2.5 speed of sound (c, (m/s))—the propagation rate of an
Acoustical Materials by the Impedance Tube Method
adiabatic compression wave
D1356 Terminology Relating to Sampling andAnalysis of
0.5
Atmospheres
c 5 ~g]P/]r! (1)
s
D5527 Practice for Measuring SurfaceWind andTempera-
where:
ture by Acoustic Means
P = pressure
E380 Practice for Use of the International System of Units
r = density,
(SI) (the Modernized Metric System)
g = specific heat ratio, and
s = isentropic (adiabatic) process (6).
3. Terminology
3.2.5.1 Discussion—The velocity of the compression wave
3.1 Definitions—For definitions of terms related to this test
defined along each axis of a Cartesian coordinate system is the
method, refer to Terminology D1356.
sum of propagation speed c plus the motion of the gas along
3.2 Definitions of Terms Specific to This Standard:
that axis. In a perfect gas (5):
3.2.1 axial attenuation coeffıcient—aratioofthefreestream
0.5
windvelocity(asdefinedinawindtunnel)tovelocityalongan
c 5 ~gR* T/M! (2)
acoustic propagation path (v/v ) (1).
t d
The approximation for propagation in air is:
0.5 0.5
c 5[403 T ~1 10.32 e/P!# 5 ~403 T ! (3)
air s
This test method is under the jurisdiction of ASTM Committee D22 on
Sampling andAnalysis ofAtmospheres and is the direct responsibility of Subcom-
mittee D22.11 on Meteorology.
Current edition approved April 10, 2003. Published June 2003. Originally
approved in 1996. Last previous edition approved in 1996 as D6011-96.
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
the ASTM website.
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6011 – 96 (2003)
3.2.6 system clock—the clock used for timing acoustic dt are needed, or whether an average dt can be used. The
wavefront travel between a transducer pair. relationship of transit time to speed of sound is
3.2.7 system delay (dt, µs)—the time delay through the
d 1 1
2 2
c 5 1 1 v (10)
F S DG
transducer and electronic circuitry (7). n
2 t t
1 2
3.2.7.1 Discussion—Each path through every sonic array
andtheinversetransittimesolutionforsonictemperatureinairisas
axis can have unique delay characteristics. Delay (on the order
follows (6):
of 10 to 20 µs) can vary as a function of temperature and
2 2 2
directionofsignaltravelthroughthetransducersandelectronic d 1 1 v
n
T 5S DF 1 G 1 (11)
s
circuitry.Theaveragesystemdelayforeachaxisinanacoustic 1612 t t 403
1 2
array is the average of the delays measured in each direction
3.2.12 velocity calibration range (U to U , (m/s))—the
c s
along the axis
range of velocity between creeping flow and the flow at which
dt 5 ~dt 1dt !/2 (4) a critical Reynolds number is reached.
1 2
3.2.12.1 Discussion—The shadow correction is valid over a
3.2.8 system delay mismatch (dt, µs)—the absolute differ-
t
rangeofvelocitieswherenodiscontinuitiesareobservedinthe
ence in microseconds between total transit times tin each
t
axial attenuation coefficient.
direction (t , t ) through the system electronics and transduc-
t1 t2
3.2.13 velocity resolution (dv, (m/s))—the largest change in
ers.
an along-axis wind component that would cause no change in
3.2.8.1 Discussion—Due principally to slight differences in
the pulse arrival time count.
transducerperformance,thetotaltransittimeobtainedwiththe
3.2.13.1 Discussion—Velocity resolution defines the small-
signal originating at one transducer can differ from the total
est resolvable wind velocity increment as determined from
transit time obtained with the signal originating at its paired
systemclockrate.Forsomesystems, dvdefinedasthestandard
transducer. The manufacturer should specify the system delay
deviation of system dither can also be reported.
mismatch tolerance.
3.3 Symbols:
dt 5?t 2 t ? (5)
t t1 t2
3.2.9 thermal stability range (°C)—a range of temperatures
c = speed of sound, m/s,
over which the corrected velocity output in a zero wind
C = specific heat at constant pressure, J/(kg·K),
p
chamber remains at or below instrument resolution.
C = specific heat at constant volume, J/(kg·K),
v
3.2.9.1 Discussion—Thermalstabilityrangedefinesarange
e = vapor pressure, Pa,
of temperatures over which there is no step change in system
d = acoustic pathlength, m,
delay.
f = compressibility factor, dimensionless,
3.2.10 time resolution (Dt, µs)—resolution of the internal
M = molecular weight of a gas, g/mol,
clock used to measure time.
P = pressure, Pa,
3.2.11 transit time (t, µs)—the time required for an acoustic
R* = universal gas constant, 8.31436 J/(mol·K),
wavefront to travel from the transducer of origin to the
RH = relative humidity, %,
receiving transducer.
t = transit time, µs,
3.2.11.1 Discussion—Transit time (also known as time of
t = total transit time, µs,
t
flight) is determined by acoustic pathlength d, the speed of
T = absolute temperature, K,
sound c, the velocity component along the acoustic propaga- T = sonic absolute temperature, K,
s
tion path v , and cross-path velocity components) v (8) U = upper limit for creeping flow, m/s,
d n c
U = critical Reynolds number velocity, m/s,
2 2 0.5 2 2 2 s
t 5 d[~c 2 v ! 6 V #/[c 2 ~v 1 v !# (6)
n d d n
v = velocity component along acoustic propagation
d
The transit time difference between acoustic wavefront propagation path, m/s,
in one direction (t , computed for+ v ) and the other (t , computed v = tunnel velocity component parallel to the array axis
1 d 2
dm
for− v ) for each transducer pair determines the magnitude of a
d (v, cos u), m/s,
t
velocitycomponent.Theinversetransittimesolutionforthealong-axis
v = velocity component normal to an acoustic propaga-
n
velocity is (9)
tion path, m/s,
d 1 1
v = freestreamwindvelocitycomponent(unaffectedby
t
v 5 2 (7)
F G
d
2 t t
1 2 the presence of an obstacle such as the acoustic
The total transit times t and t , include the sum of actual transit
t1 t2
array), m/s,
timesplussystemdelaythroughtheelectronicsandtransducersineach
dt = system delay, µs,
direction along an acoustic path, d and d . System delay must be
t1 t2
dt = system delay mismatch, µs,
t
removed to calculate v , that is,
d
Dt = clock pulse resolution, s,
t 5 t 2d (8)
1 t1 t1
a = acceptance angle, degree,
g = specific heat ratio (C /C ), dimensionless,
p v
t 5 t 2d (9) dv = velocity resolution, m/s,
2 t2 t2
u = array angle of attack, degree, and
3.2.11.2 Discussion—Proceduresinthistestmethodinclude
r = gas density, kg/m .
a test to determine whether separate determinations of d t and
D 6011 – 96 (2003)
3.4 Abbreviations:Units—Units of measurement are in ac-
cordance with Practice E380.
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-
nents and the speed of sound. It provides an unambiguous
determination of instrument performance criteria. The test
FIG. 1 Sonic Anemometer Array in a Zero Wind Chamber
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 The sonic anemometer/thermometer array is assumed to
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.
5.3 This test method is designed to characterize the perfor-
mance of an array model or probe design. Transducer shadow
FIG. 2 Pathlength Chamber for Acoustic Pathlength
data obtained from a single array is applicable for all instru-
Determination
ments having the same array model or probe design. Some
non-orthogonal arrays may not require specification of trans-
ducer shadow corrections or the velocity calibration range.
chamber for quick and thorough purging.The basic pathlength
chamber components are illustrated in Fig. 2.
6. Apparatus
6.2.2 Gas Source and Plumbing, to connect the pathlength
6.1 Zero Wind Chamber, sized to fit the array and accom-
chamber to one of two pressurized gas sources (nitrogen or
modate a temperature probe (Fig. 1) used to calibrate the sonic
argon).Employapurgepumptodrawoffusedgases.Required
anemometer/thermometer. Line the chamber with acoustic
purity of the gas is 99.999%.
foam with a sound absorption coefficient of 0.8 or better (Test
6.3 Temperature Transducer (tworequired),withminimum
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
of the array for acoustic pathlength determination. Construct 6.4.1 Size, large enough to fit the entire instrument array
thechambercomponentsusingnon-expanding,non-outgassing withinthetestsectionatallrequiredorientationangles.Design
materials. Employ O-ring seals made of non-outgassing mate- the tunnel so that the maximum projected area of the sonic
rials to prevent pressure loss and contamination. Design the array is less than 5% of tunnel cross-sectional area.
D 6011 – 96 (2003)
6.4.2 Speed Control, to vary the flow rate over a range of at procedures used to determine d and d 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—Calibrate the mean flow rate using transducer spacing to a tolerance of 0.1 mm. Independently
transfer standards traceable to the National Institute of Stan- determine d and d for each axis of the acoustic array for each
t
dardsandTechnology(NIST),orbyanequivalentfundamental 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
angle and tunnel velocity settings. Calculate the average and
NOTE 1—Design the plate to hold the array at chosen angles without
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 setting (at or
6.
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