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eLibrary >
ASHRAE Standard 41.2-2022 -- Standard Methods for Air Velocity and Airflow Measurements (ANSI Approved) >
ASHRAE Standard 41.2-2022 -- Standard Methods for Air Velocity and Airflow Measurements (ANSI Approved), 2022
- ANSI/ASHRAE Standard 41.2-2022 [Go to Page]
- CONTENTS
- FOREWORD
- 1. PURPOSE
- 2. SCOPE
- 3. DEFINITIONS
- 4. CLASSIFICATIONS [Go to Page]
- 4.1 Air Velocity and Airflow Measurement Applications. Air velocity and airflow measurement applications that are within the scope of this standard are classified as one of the following two types:
- 4.2 Airflow Meter Categories
- 4.3 Air Velocity Measurement Methods. Methods of air velocity measurement that are within the scope of this standard are listed below. These measurement methods are described in Section 7.
- 4.4 Airflow Measurement Methods. Methods of airflow measurement that are within the scope of this standard are the listed below. These measurement methods are described in Section 9.
- 4.5 Standard Air Density. For the purposes of this standard, standard air density = 1.202 kg/m3 (0.075 lbm/ft3) unless otherwise specified in the test plan in Section 5.1. The conversion uncertainty associated with calculating air velocity or airflow...
- 4.6 Test Apparatus. A test apparatus that is used to measure air velocity or airflow includes instruments, airflow conditioning elements, and airflow control elements within a sealed conduit. These are classified as single-nozzle ducts, or single- or...
- 5. REQUIREMENTS [Go to Page]
- 5.1 Test Plan. The test plan shall be one of the following documents:
- 5.2 Values to Be Determined and Recorded
- 5.3 Test Requirements
- 6. INSTRUMENTS [Go to Page]
- 6.1 Instrumentation Requirements for All Measurements
- 6.2 Temperature Measurements. If temperature measurements are required by test plan in Section 5.1, the temperature measurement system accuracy shall be within the following limits unless otherwise specified in the test plan:
- 6.3 Pressure Measurements
- 6.4 Power Measurements. If electrical power measurements or shaft power measurements are required by the test plan in Section 5.1, the measurement system accuracy shall be within ±1% of reading.
- 6.5 Steam-Flow Measurement. If steam flow rate measurements are required by the test plan in Section 5.1, the measurement system accuracy shall be within ±1% of reading.
- 6.6 Time Measurements. If time measurements are required by the test plan in Section 5.1, the measurement system accuracy shall be within ±0.5% of the elapsed time measured, including any uncertainty associated with starting and stopping the time me...
- 7. AIR VELOCITY MEASUREMENT METHODS [Go to Page]
- 7.1 Constraint on All Air Velocity Measurement Methods. A selected air velocity measurement plane shall be greater than 7.5 geometrically equivalent diameters downstream of an obstruction or any change in the airflow direction and shall exceed three ...
- 7.2 Pitot-Static Tube Air Velocity Measurement Methods. The air velocity measurement methods in this section are based on Pitot-static tube measurement principles.
- 7.3 Thermal Anemometer. The thermal anemometer incorporates one of the following velocity sensors at the sensing end of a probe: (a) heated resistance temperature device, (b) thermocouple junction, or (c) thermistor sensor. Air movement past the elec...
- 7.4 Rotary Vane Anemometers. Rotary vane anemometers provide a direct readout of air velocity based on the wheel revolution rate. Rotary vane anemometers shall be aligned with the airflow direction within ±10 degrees, and any misalignment shall be i...
- 7.5 Ultrasonic Velocity Flowmeters. Ultrasonic flowmeters measure air velocity. Clamp-on ultrasonic flowmeters measure air velocity within a pipe or tube without being inserted into the airflow stream.
- 7.6 Drag-Force Velocity Meters. Drag-force flowmeters determine air velocity. Piezoelectric or strain-gage methods are used to sense dynamic drag-force variations. Air velocity shall be obtained from Equation 7-15 in SI units or from Equation 7-16 in...
- 7.7 Laser Doppler Velocimeter. A laser Doppler velocimeter (LDV) is an optical measurement system that collects scattered light produced by particles that are seeded into the airstream that pass through two intersecting laser beams that have the same...
- 8. AIRFLOW MEASUREMENT DUCT FEATURES AND COMPONENTS [Go to Page]
- 8.1 Overview. Features and components used in the airflow measurement single-nozzle ducts and single- and multiple-nozzle chambers that are described in Section 9 include static pressure taps, piezometer rings, flow straighteners, transition pieces, ...
- 8.2 Static Pressure Taps. Unless otherwise specified in the test plan in Section 5.1, static pressure taps shall be constructed as defined in Figure 8-1 and shall be located around the duct perimeter in a measurement plane with (a) one pressure tap l...
- 8.3 Piezometer Ring
- 8.4 Flow Straighteners. Cell-type flow straighteners shall conform to Figure 8-4, and the thickness dimension y shall not exceed 0.005D. Star-type flow straighteners shall conform to Figure 8-5.
- 8.5 Transformation Pieces. Transformation pieces used to connect rectangular units under test (UUTs) to round single-nozzle ducts or single- or multiple-nozzle chambers, or round UUTs to rectangular single-nozzle ducts or single-or multiple-nozzle ch...
- 9. AIRFLOW MEASUREMENT METHODS [Go to Page]
- 9.1 Constraint on All Airflow Measurement Methods Except Nozzle Chambers. Except for single- and multiple-nozzle chambers, a selected airflow measurement plane shall exceed 7.5 geometrically equivalent diameters downstream of an obstruction or any ch...
- 9.2 Pitot-Static Tube Airflow Measurement Methods. The airflow measurement methods in this section are based on Pitot-static tube measurement principles.
- 9.3 Single- and Multiple-Nozzle Airflow Methods
- 9.4 Thermal Dispersion Arrays. Review Section 9.1. A thermal dispersion sensor measures air velocity at a single point in an airstream by measuring the heat dispersed from the heated sensor into the airstream. Commercial thermal dispersion arrays inc...
- 9.5 Vortex-Shedding Arrays. Review Section 9.1. Vortex-shedding arrays are used to determine air velocities. Piezoelectric methods, strain-gage methods, or hot-film methods are used to sense dynamic pressure variations created by vortex shedding. The...
- 9.6 Capture Hoods. Review Section 9.1. Flow capture hoods are portable instruments designed to measure the airflow from diffusers and grilles. A capture hood system consists of a fabric hood and a rigid base assembly that contains the flow sensing eq...
- 9.7 Tracer Gas Airflow Measurement. Review Section 9.1. Figure 9-7 is a schematic of the tracer gas airflow measurement method. This method uses a tracer gas dilution technique that is based on the principle of mass conservation. Users shall first ch...
- 10. MEASUREMENT UNCERTAINTY [Go to Page]
- 10.1 Post-Test Uncertainty Analysis. A post-test analysis of the measurement uncertainty, performed in compliance with ASME PTC 19.1 1, shall accompany each air velocity measurement, volumetric airflow measurement, standard volumetric measurement, an...
- 10.2 Method to Express Uncertainty. All assumptions, parameters, and calculations used in estimating uncertainty shall be clearly documented prior to expressing any uncertainty values. Uncertainty shall be expressed as shown in Equation 10-1:
- 11. TEST REPORT [Go to Page]
- 11.1 Test Identification
- 11.2 Unit Under Test (UUT) Description
- 11.3 Instrument Description
- 11.4 Measurement System Description
- 11.5 Ambient Conditions
- 11.6 Test Conditions
- 11.7 Test Results. Test results, if required by the test plan in Section 5.1, are as follows:
- 12. NORMATIVE REFERENCES
- INFORMATIVE APPENDIX A: INFORMATIVE REFERENCES AND BIBLIOGRAPHY
- INFORMATIVE APPENDIX B: MULTIPLE-NOZZLE UNCERTAINTY ANALYSIS EXAMPLE [Go to Page]
- B1. BACKGROUND
- B2. UNCERTAINTY IN AREAS (Ax)
- B3. UNCERTAINTY IN CONVERSION FACTOR OF 1097.8 (I-P ONLY)
- B4. UNCERTAINTY IN e
- B5. UNCERTAINTY IN DENSITY (r1)
- B6. UNCERTAINTY IN VISCOSITY
- B7. UNCERTAINTY IN ReX AND VX
- B8. UNCERTAINTY IN C
- B9. UNCERTAINTY IN S(CA)
- B10. UNCERTAINTY IN Q
- INFORMATIVE APPENDIX C: VELOCITY UNCERTAINTY ANALYSIS EXAMPLE USING PITOT-STATIC TUBE
- INFORMATIVE APPENDIX D: SUPPLEMENTARY UNCERTAINTY CALCULATION PROCEDURES [Go to Page]
- D1. EXACT CONVERSION of 1.202 kg/m3 to lbm/ft3 [Go to Page]
- D1.1 Exact Conversions. When conversions between different systems of units are made, uncertainty needs to be accounted for if exact conversions are not used. Exact conversions can often be obtained with no added uncertainty, as one system of units i...
- D1.2 Exact Conversion of m to ft. The conversion from metres to feet can be found on NIST Special Publication 1038, pp. 8 and is reproduced below.
- D1.3 Exact Conversion of kg to lbm. The conversion from kg to lbm can be found on NIST Special Publication 1038, page 11 and is reproduced below.
- D1.4 Exact Conversion of 1.202 kg/m3 to lbm/ft3. The conversion from 1.202 kg/m3 to lbm/ft3 can be found by mathematical functions from the conversions in Equations D-3, D-4, and D-5.
- D1.5 Approximation of 1.202 kg/m3 to lbm/ft3. The conversion from 1.202 kg/m3 to lbm/ft3can be approximated as 0.075 lbm/ft3 with an associated error not more than +0.00004 lbm/ft3. The convention in uncertainty analysis is to lump errors in with all...
- INFORMATIVE APPENDIX E: USER INFORMATION [Go to Page]
- E1. INTRODUCTION
- E2. COMPARISON OF AIRFLOW MEASUREMENTS
- E3. COMPARISON OF AIR VELOCITY MEASUREMENT METHODS
- E4. SELF-AVERAGING ARRAYS
- E5. THERMAL ANEMOMETERS
- E6. THERMAL DISPERSION ARRAYS
- E7. AIRFLOW HOODS
- NORMATIVE APPENDIX F: LEGACY SINGLE- AND MULTIPLE-NOZZLE CHAMBER REQUIREMENTS [Go to Page]
- F1. Construction Requirements for Single- and Multiple-Nozzle Chambers [Go to Page]
- F1.1 Cross Sections of Single- and Multiple-Nozzle Chambers. The cross section of single- or multiple-nozzle chambers shall be round or rectangular. Transformation pieces described in Section 8.5 shall be used to connect rectangular units under test ...
- F1.2 Nozzle Throat Velocity. The throat velocity of each nozzle shall exceed 3000 fpm (15 m/s).
- F1.3 Longitudinal Spacing Requirements. In single- and multiple-nozzle chambers, the minimum distance between the upstream screens and the nozzle inlets shall be the greater of 0.5m or 1.5dL, where dL is the largest nozzle throat diameter.
- F1.4 Airflow Settling Means Requirements for Single- and Multiple-Nozzle Chambers. An airflow settling means, consisting of screens or perforated sheets having open areas of 50% to 60%, shall be installed in single- and multiple-nozzle chambers as in...
- F2. Single- and Multiple-Nozzle Chamber Design [Go to Page]
- F2.1 Single- or Multiple-Nozzle Chamber Diameter. The single- or multiple-nozzle chamber geometrically equivalent diameter m shall be sized so that the maximum average air velocity is 2 m/s (400 fpm).
- F2.2 Upstream Settling Means Verification Test. The maximum velocity at a distance of 0.1m downstream of the upstream settling means shall be measured and shall not exceed the average velocity by more than 20%.
- F3. Nozzle Airflow Calculations [Go to Page]
- F3.1 Measurements. Measurements required for nozzle airflow calculations are as follows:
- F3.2 Thermodynamic Properties of Air. Nozzle inlet density for dry and moist air shall be obtained from ASHRAE RP-1485 2 using nozzle inlet absolute pressure, temperature, and humidity.
- F3.3 Dynamic Viscosity. Calculate the dynamic viscosity of air behaving as an ideal gas at moderate pressures and temperatures using Equation F-2 for SI units or Equation F-3 for I-P units.
- F4. Single- and Multiple-Nozzle Airflow Calculations [Go to Page]
- F4.1 Nozzle Throat Diameter for Single- or Multiple-Nozzle Chambers. If airflow operating temperatures are not within ±6°C (±10°F) of the temperature when the nozzle dimensional measurements were obtained, the nozzle throat diameter d for each no...
- F4.2 Reynolds Number for Single- and Multiple-Nozzle Chambers. The Reynolds number Red for each nozzle in use shall be obtained from Equation F-4.
- F4.3 Single- and Multiple-Nozzle Beta Ratio. b = 0 for single- and multiple-nozzle chambers 7.
- F4.4 Nozzle Limits of Use for Single- and Multiple- Nozzle Chambers. The nozzle geometry in Figure 9-3 fits into ASME’s long-radius nozzle type, and the throat velocity requirement in Section 9.3.3.2 confirms that each nozzle in use will be operati...
- F4.5 Expansibility Factor for Single- and Multiple-Nozzle Chamber Nozzles. The dimensionless expansibility factor ε for a long-radius nozzle is shown in Equation F-5.
- F4.6 Discharge Coefficient for Nozzle Single- and Multiple-Nozzle Chamber Nozzles. Nozzle discharge coefficients shall be calculated for each nozzle in use from Equation F-7 using the Reynolds number from Equation F-4.
- F4.7 Volumetric Airflow Rate for Single- and Multiple-Nozzle Chambers. The volumetric airflow rate for single- or multiple-nozzle chambers shall be obtained from Equation F-8 in SI units or from Equation F-9 in I-P units, where the area is measured a...
- F4.8 Multiple-Nozzle Standard Airflow Rate. The standard airflow rate shall be calculated in compliance with Section 4.5 using Equation F-12 in SI units or Equation F-13 in SI units.
- F4.9 Mass Airflow Rate for Single- and Multiple-Nozzle Chambers. The mass airflow rate for multiple-nozzle chambers shall be obtained from Equation F-14, where r1 is the nozzle inlet air density in SI units or I-P units, and Q is the volumetric airfl... [Go to Page]
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