ASTM D6011-96(2022)

Standard Test Method for Determining the Performance of a Sonic Anemometer/Thermometer

SIGNIFICANCE AND USE
5.1 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 Practices D5527.  
5.2 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.  
5.3 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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 28-Feb-2022
• ICS: 17.200.10 - Heat. Calorimetry
• Technical Committee: D22 - Air Quality
• Drafting Committee: D22.11 - Meteorology

Relations

• Refers: ASTM D1356-20a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Sep-2020
• Refers: ASTM D1356-20 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Mar-2020
• Refers: ASTM D1356-15a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Oct-2015
• Refers: ASTM D1356-15 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Jul-2015
• Refers: ASTM D1356-14b - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Dec-2014
• Refers: ASTM D1356-14a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-May-2014
• Refers: ASTM D1356-14 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 15-Jan-2014
• Refers: ASTM C384-04(2011) - Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method
• Effective Date: 01-Nov-2011
• Refers: ASTM D5527-00(2011) - Standard Practices for Measuring Surface Wind and Temperature by Acoustic Means
• Effective Date: 01-Oct-2011
• Refers: ASTM D1356-05(2010) - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-Apr-2010
• Refers: ASTM D5527-00(2007) - Standard Practices for Measuring Surface Wind and Temperature by Acoustic Means
• Effective Date: 01-Oct-2007
• Refers: ASTM D1356-05 - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 01-May-2005
• Refers: ASTM C384-04 - Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method
• Effective Date: 01-Apr-2004
• Refers: ASTM C384-03 - Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method
• Effective Date: 10-Sep-2003
• Refers: ASTM D1356-00a - Standard Terminology Relating to Sampling and Analysis of Atmospheres
• Effective Date: 10-Nov-2000

Overview

ASTM D6011-96(2022): Standard Test Method for Determining the Performance of a Sonic Anemometer/Thermometer provides a standardized approach to assess the dynamic performance of sonic anemometers/thermometers that utilize the inverse time measurement technique. This method covers parameters like wind velocity components and speed of sound by using time-based acoustic measurement. The standard applies to manufacturers, test facilities, and end-users seeking clear criteria for measuring and specifying the performance of these high-precision meteorological instruments.

This test method ensures consistency in evaluating sonic anemometer/thermometer performance, aiding in both product development and operational calibration. It is particularly vital for applications involving atmospheric research, environmental monitoring, and industrial meteorology.

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Key Topics

• Performance Criteria: The standard details the essential criteria for evaluating sonic anemometers/thermometers, including:

  • Acceptance angle
  • Acoustic pathlength
  • System delay and delay mismatch
  • Thermal stability range
  • Shadow correction
  • Velocity calibration range
  • Velocity resolution

• Methodological Procedures: It offers procedures for:

  • Operating sonic anemometers/thermometers (referencing ASTM D5527)
  • Determining pathlength, system delays, and calibration using zero wind chambers and wind tunnels
  • Evaluating effects like transducer shadow and axial attenuation

• Instrument Requirements: The anemometer/thermometer arrays must have sufficient rigidity and minimal thermal expansion to maintain precise internal alignment during evaluation.

• Calibration and Traceability: Acoustic pathlength determination is integral for accurate calibration prior to data collection, ensuring traceability to SI units and international norms.

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Applications

Adhering to ASTM D6011-96(2022) adds practical value to a wide range of fields:

• Manufacturers: Describe and validate the performance capabilities of sonic anemometer/thermometer models, supporting product claims with standardized metrics.
• Testing Facilities: Verify instrument performance through repeatable laboratory or wind tunnel procedures, ensuring equipment meets required specifications.
• End Users: Specify and evaluate performance requirements for meteorological instruments used in atmospheric science, environmental compliance, wind energy assessments, and research applications.
• Calibration Laboratories: Utilize the acoustic pathlength and system delay procedures for instrument calibration, ensuring accuracy in wind and temperature measurements before deployment in the field.

By ensuring uniformity in measurement and reporting, the standard supports high-quality data collection and comparability across devices and studies.

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Related Standards

Professionals using ASTM D6011-96(2022) should also be familiar with these related standards:

• ASTM D5527: Practices for Measuring Surface Wind and Temperature by Acoustic Means, referenced for operational procedures of sonic devices.
• ASTM D1356: Terminology Relating to Sampling and Analysis of Atmospheres, for definitions of relevant terms.
• ASTM C384: Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method, referenced for acoustic chamber specifications.
• IEEE/ASTM SI 10: American National Standard for Metric Practice, guiding the use of SI units throughout the test method.

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Summary

ASTM D6011-96(2022) serves as a comprehensive and practical international standard for determining the dynamic performance of sonic anemometers/thermometers. Its rigorous procedures ensure trusted, comparable, and scientifically valid results, supporting reliable environmental monitoring and research. By adopting this standard, stakeholders enhance the quality and traceability of wind and temperature data, pivotal for informed decision-making in meteorological and industrial applications.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6011 − 96 (Reapproved 2022)
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 IEEE/ASTM SI 10American National Standard for Metric
Practice
1.1 This test method covers the determination of the dy-
namicperformanceofasonicanemometer/thermometerwhich
3. Terminology
employs the inverse time measurement technique for velocity
3.1 Definitions—For definitions of terms related to this test
or speed of sound, or both. Performance criteria include: (a)
method, refer to Terminology D1356.
acceptanceangle,(b)acousticpathlength,(c)systemdelay,(d)
3.2 Definitions of Terms Specific to This Standard:
system delay mismatch, (e) thermal stability range, (f) shadow
3.2.1 axial attenuation coeffıcient—aratioofthefreestream
correction, (g) velocity calibration range, and (h) velocity
windvelocity(asdefinedinawindtunnel)tovelocityalongan
resolution.
acoustic propagation path (v /v ) (1).
t d
1.2 The values stated in SI units are to be regarded as
3.2.2 critical Reynolds number (R )—the Reynolds number
c
standard. No other units of measurement are included in this
at which an abrupt decrease in an object’s drag coefficient
standard.
occurs (2).
1.3 This standard does not purport to address all of the
3.2.2.1 Discussion—The transducer shadow corrections are
safety concerns, if any, associated with its use. It is the
no longer valid above the critical Reynolds number due to a
responsibility of the user of this standard to establish appro-
discontinuity in the axial attenuation coefficient.
priate safety, health, and environmental practices and deter-
3.2.3 Reynolds number (R )—the ratio of inertial to viscous
e
mine the applicability of regulatory limitations prior to use.
forces on an object immersed in a flowing fluid based on the
1.4 This international standard was developed in accor-
object’s characteristic dimension, the fluid velocity, and vis-
dance with internationally recognized principles on standard-
cosity.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- 3.2.4 shadow correction (v /v )—the ratio of the true
dm d
along-axis velocity v , as measured in a wind tunnel or by
mendations issued by the World Trade Organization Technical
dm
Barriers to Trade (TBT) Committee. another accepted method, to the instrument along-axis wind
measurement v .
d
3.2.4.1 Discussion—This correction compensates for flow
2. Referenced Documents
shadowing effects of transducers and their supporting struc-
2.1 ASTM Standards:
tures. The correction can take the form of an equation (3) or a
C384Test Method for Impedance andAbsorption ofAcous-
lookup table (4).
tical Materials by Impedance Tube Method
3.2.5 speed of sound (c, (m/s))—the propagation rate of an
D1356Terminology Relating to Sampling and Analysis of
adiabatic compression wave:
Atmospheres
0.5
D5527Practices for Measuring Surface Wind and Tempera-
c 5 ~γ]P/]ρ! (1)
s
ture by Acoustic Means
where:
P = pressure
ρ = density,
This test method is under the jurisdiction of ASTM Committee D22 on Air
γ = specific heat ratio, and
Quality and is the direct responsibility of Subcommittee D22.11 on Meteorology.
s = isentropic (adiabatic) process (5).
Current edition approved March 1, 2022. Published April 2022. Originally
approved in 1996. Last previous edition approved in 2015 as D6011–96 (2015).
3.2.5.1 Discussion—The velocity of the compression wave
DOI: 10.1520/D6011-96R22.
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 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 (2022)
defined along each axis of a Cartesian coordinate system is the actual transit times plus system delay through the electron-
sum of propagation speed c plus the motion of the gas along ics and transducers in each direction along an acoustic
that axis. In a perfect gas (6): path, δ and δ . System delay must be removed to calcu-
t1 t2
0.5 late v , that is:
d
c 5 γR*T/M (2)
~ !
t 5 t 2 δ (8)
1 t1 t1
The approximation for propagation in air is:
t 5 t 2 δ (9)
2 t2 t2
0.5 0.5
c 5 @403 T ~110.32 e/P!# 5 ~403 T ! (3)
air s
3.2.11.2 Discussion—Proceduresinthistestmethodinclude
3.2.6 system clock—the clock used for timing acoustic
a test to determine whether separate determinations of δ t and
wavefront travel between a transducer pair.
δt are needed, or whether an average δt can be used. The
3.2.7 system delay (δt, µs)—the time delay through the relationship of transit time to speed of sound is:
transducer and electronic circuitry (7).
d 1 1
2 2
c 5 1 1v (10)
3.2.7.1 Discussion—Each path through every sonic array F S DG
n
2 t t
1 2
axis can have unique delay characteristics. Delay (on the order
and the inverse transit time solution for sonic tempera-
of 10 to 20 µs) can vary as a function of temperature and
ture in air is as follows (5):
directionofsignaltravelthroughthetransducersandelectronic
2 2 2
circuitry.Theaveragesystemdelayforeachaxisinanacoustic
d 1 1 v
n
T 5 1 1 (11)
S D F G
s
array is the average of the delays measured in each direction
1612 t t 403
1 2
along the axis:
3.2.12 velocity calibration range (U to U , (m/s))—the
c s
δt 5 ~δt 1δt !/2 (4)
1 2 range of velocity between creeping flow and the flow at which
a critical Reynolds number is reached.
3.2.8 system delay mismatch (δt, µs )—the absolute differ-
t
ence in microseconds between total transit times t in each
3.2.12.1 Discussion—The shadow correction is valid over a
t
direction (t , t ) through the system electronics and transduc- rangeofvelocitieswherenodiscontinuitiesareobservedinthe
t1 t2
ers.
axial attenuation coefficient.
3.2.8.1 Discussion—Due principally to slight differences in
3.2.13 velocity resolution (δv, (m/s))—the largest change in
transducerperformance,thetotaltransittimeobtainedwiththe
an along-axis wind component that would cause no change in
signal originating at one transducer can differ from the total
the pulse arrival time count.
transit time obtained with the signal originating at its paired
3.2.13.1 Discussion—Velocity resolution defines the small-
transducer. The manufacturer should specify the system delay
est resolvable wind velocity increment as determined from
mismatch tolerance.
systemclockrate.Forsomesystems,δvdefinedasthestandard
δt 5 t 2 t (5)
t ? t1 t2 ? deviation of system dither can also be reported.
3.2.9 thermal stability range (°C)—a range of temperatures
3.3 Symbols:
over which the corrected velocity output in a zero wind
chamber remains at or below instrument resolution. c = speed of sound, m/s,
C = specific heat at constant pressure, J/(kg·K),
3.2.9.1 Discussion—Thermal stability range defines a range
p
C = specific heat at constant volume, J/(kg·K),
of temperatures over which there is no step change in system
v
e = vapor pressure, Pa,
delay.
d = acoustic pathlength, m,
3.2.10 time resolution (∆t, µs)—resolution of the internal
f = compressibility factor, dimensionless,
clock used to measure time.
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),
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):
d n
U = upper limit for creeping flow, m/s,
c
2 2 0.5 2 2 2
t 5 d@ c 2 v 6V #/@c 2 v 1v # (6)
~ ! ~ !
n d d n U = critical Reynolds number velocity, m/s,
s
v = velocity component along acoustic propagation path,
d
The transit time difference between acoustic wavefront
m/s,
propagation in one direction (t , computed for+ v ) and
1 d
v = tunnel velocity component parallel to the array axis
dm
the other (t , computed for− v ) for each transducer pair
2 d
(v, cos θ), m/s,
t
determines the magnitude of a velocity component. The
v = velocitycomponentnormaltoanacousticpropagation
n
inverse transit time solution for the along-axis velocity is
path, m/s,
(9):
v = free stream wind velocity component (unaffected by
t
d 1 1
thepresenceofanobstaclesuchastheacousticarray),
v 5 2 (7)
F G
d
2 t t
m/s,
1 2
δt = system delay, µs,
The total transit times t and t , include the sum of
t1 t2
D6011 − 96 (2022)
δt = system delay mismatch, µs,
t
∆t = clock pulse resolution, s,
α = acceptance angle, degree,
γ = specific heat ratio (C /C ), dimensionless,
p v
δv = velocity resolution, m/s,
θ = array angle of attack, degree, and
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.
FIG. 1 Sonic Anemometer Array in a Zero Wind Chamber
5. Significance and Use
6.2 Pathlength Chamber—See Fig. 2.
5.1 This test method provides a standard method for evalu-
6.2.1 Design the pathlength chamber to fit and seal an axis
ating the performance of sonic anemometer/thermometers that
of the array for acoustic pathlength determination. Construct
use inverse time solutions to measure wind velocity compo-
thechambercomponentsusingnon-expanding,non-outgassing
nents and the speed of sound. It provides an unambiguous
materials. Employ O-ring seals made of non-outgassing mate-
determination of instrument performance criteria. The test
rials to prevent pressure loss and contamination. Design the
method is applicable to manufacturers for the purpose of
chamber for quick and thorough purging.The basic pathlength
describing the performance of their products, to instrumenta-
chamber components are illustrated in Fig. 2.
tion test facilities for the purpose of verifying instrument
6.2.2 Gas Source and Plumbing, to connect the pathlength
performance, and to users for specifying performance require-
chamber to one of two pressurized gas sources (nitrogen or
ments.Theacousticpathlengthprocedureisalsoapplicablefor
argon).Employapurgepumptodrawoffusedgases.Required
calibration purposes prior to data collection. Procedures for
purity of the gas is 99.999%.
operating a sonic anemometer/thermometer are described in
6.3 Temperature Transducer (two required), with minimum
Practices D5527.
temperature measurement precision and accuracy of 60.1°C
5.2 Thesonicanemometer/thermometerarrayisassumedto
and 60.2°C, respectively, and with recording readout. One is
haveasufficientlyhighstructuralrigidityandasufficientlylow
required for the zero wind chamber and one for the pathlength
coefficient of thermal expansion to maintain an internal align-
chamber.
ment to within the manufacturer’s specifications over its
6.4 Wind Tunnel:
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
data obtained from a single array is applicable for all instru-
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.
6. Apparatus
6.1 Zero Wind Chamber, sized to fit the array and accom-
modate a temperature probe (Fig. 1) used to calibrate the sonic
anemometer/thermometer. Line the chamber with acoustic
foam with a sound absorption coefficient of 0.8 or better (Test
Method C384) to minimize internal air motions caused by
thermal gradients and to minimize acoustic reflections. Install
asmallfanwithinthechambertoestablishthermalequilibrium
FIG. 2 Pathlength Chamber for Acoustic Pathlength Determina-
before a zero wind calibration is made. tion
D6011 − 96 (2022)
NOTE 3—Array support should not protrude into the wind tunnel.
6.4.1 Size, large enough to fit the entire instrument array
withinthetestsectionatallrequiredorientationangles.Design
8. Sampling
the tunnel so that the maximum projected area of the sonic
array is less than 5% of tunnel cross-sectional area.
8.1 Acoustic Pathlength, System Delay, and System Delay
6.4.2 Speed Control, to vary the flow rate over a range of at
Mismatch—If the dual gas procedure is used, repeat the
least1.0to10m/swithin 60.1m/sorbetterthroughoutthetest
procedures used to determine d and δ in argon and nitrogen
t
section.
gasesforaminimumoftentimes,oruntilconsistentresultsare
6.4.3 Calibration—Calibratethemeanflowrateusingtrans-
achieved. If the caliper method is used, measure and verify the
fer standards traceable to the National Institute of Standards
transducer spacing to a tolerance of 0.1 mm. Independently
and Technology (NIST), or by an equivalent fundamental
determine d and δ for each axis of the acoustic array for each
t
physical method.
instrument.
6.4.4 Turbulence, with a uniform velocity profile with a
8.2 Thermal Stability Range—Obtain a zero velocity read-
minimumofswirlatallspeeds,andknownuniformturbulence
ing over a period of at least one minute at room temperature.
scale and intensity throughout the test section.
Repeat the procedure over the instrument’s expected tempera-
6.4.5 Rotating Plate, to hold the sonic transducer array in
ture operating range. Repeat the test for each transducer axis
varying orientations to achieve angular exposures up to 360°,
for each instrument.
as needed.The minimum plate rotation requirements are 660°
in the horizontal and 615° in the vertical, with an angular
8.3 Axial Attenuation and Angular Shadow Effects—After
alignment resolution of 0.5°.
the wind tunnel test section velocity has stabilized, obtain the
velocity readings at each position for a measurement period of
NOTE 1—Design the plate to hold the array at chosen angles without
disturbing the test section wind velocity profile or changing its turbulence 30 s. Obtain at least three consecutive measurements at each
level.
angle and tunnel velocity settings. Calculate the average and
range of each of these readings.
6.5 Measuring System:
6.5.1 Counter, to log the anemometer velocity component
8.4 Shadow Correction—Select a low velocity setting (at or
readings, with a count resolution equaling or exceeding the
below2.0m/s)andtakeonehead-on(0°)reading,followedby
clock rate of the sonic anemometer/thermometer.
one reading at each 10° interval to+60° or beyond, as the
6.5.2 Recorder, with at least a 10 Hz rate and a resolution
apparatus permits. Reverse the process, going back through 0°
comparable to instrument resolution, for recording onto mag-
to−60°, and return to 0°.Average the results to a single value
netic or optical media the anemometer velocity component
for each angular position. Use a measurement period of 30 s at
readings.
each angle, and begin measurements only when the tunnel
velocity is stable at the selected velocity. Repeat the procedure
6.6 Calipers, for transducer separation distance
measurements, with minimum tolerance of 0.1 mm. for an intermediate velocity (5 to 6 m/s) and high velocity (10
m/s or greater), but not exceeding U . Repeat the sequence for
s
6.7 Ancillary Measurements—Ancillary pressure (60.5
vertical angle orientations over a range of at least 615°.
hPa) and relative humidity measurements (610%) are needed
for sonic temperature and acoustic pathlength determination if
NOTE 4—Positions may be found where the flow across the array is not
unambiguously defined, or where consistent results cannot be obtained
the ambient vapor pressure is greater than 20 Pa. These
duetoflowblockage.Thelocationsofthesepositionsshouldbenoted.For
measurements can be obtained from on-site instruments or
non-orthogonal axis sonic anemometers, refer to procedures described in
estimated from nearby data sources.
(4) and (10).
7. Precautions
9. Procedure
7.1 Exercise care while using gas pressurized containers.
9.1 Velocity Resolution (δv)—The zero wind chamber pro-
Procedures for handling pressurized gas cylinders shall be
cedure and the clock rate procedure are available to compute
postedandobserved.Performalltestingwithpressurizedgases
the velocity resolution.
in a well-ventilated room. Use of the buddy system is recom-
mended.
NOTE5—Theclockrateprocedureisapplicabletoallsystems.Thezero
wind chamber procedure may also be applicable for systems that use
7.2 Maintain chamber temperatures and pressures close to
synchronous phase angle detection or similar methods.
laboratorytemperatureandpressuretominimizegradientsthat
9.1.1 Velocity Resolution by the Zero Wind Chamber
could cause convection within the chamber, but use sufficient
Procedure—Place the array in a zero wind chamber and wait
o
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