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eLibrary >
ASHRAE Standard 41.2-2018 Standard Methods for Air Velocity and Airflow Measurement (ANSI Approved) >
ASHRAE Standard 41.2-2018 Standard Methods for Air Velocity and Airflow Measurement (ANSI Approved), 2018
- ANSI/ASHRAE Standard 41.2-2018 [Go to Page]
- CONTENTS
- FOREWORD
- 1. PURPOSE
- 2. SCOPE
- 3. DEFINITIONS AND SYMBOLS [Go to Page]
- 3.1 Definitions
- 3.2 Symbols and Subscripts. Table 3-1 lists symbols and subscripts that appear in this standard. [Go to Page]
- Table 3-1 Symbols and Subscripts Used in Standard 41.2-2018 (Continued)
- 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 those 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 those 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 (exact) (0.075 lbm /ft3). The conversion uncertainty associated with calculating air velocity or airflow measurement uncertainties in I-P units is ±0.000...
- 4.6 Test Apparatus. A test apparatus used to measure air velocity or airflow that includes instruments, airflow conditioning elements, and airflow control elements within a sealed conduit. These are classified as test ducts or test chambers as define...
- 5. REQUIREMENTS [Go to Page]
- 5.1 Test Plan. A test plan is required. The test plan shall specify the test points and the required measurement system accuracy at each test point. A test plan is a document or other form of communication that specifies the tests to be performed and...
- 5.2 Values to be Determined and Recorded
- 5.3 Test Requirements
- 5.4 Thermodynamic Properties of Air. The thermodynamic properties of the dry air and moist air shall be obtained from ASHRAE RP-1485 1.
- 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 Electrical Power Measurements. If electrical 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 3 geom...
- 7.2 Pitot-Static Tube Air Velocity Measurement Methods. The air velocity measurement methods in this section are based upon pitot-static tube measurement principles. [Go to Page]
- Figure 7-1 An example of pitot-static tube construction and connections.
- Figure 7-2 Pitot-static tube traverse measuring points for rectangular ducts and round ducts.
- Figure 7-3 Laser Doppler velocimeter (LDV).
- 7.3 Thermal Anemometer. The thermal anemometer incorporates one of the following velocity sensors at the sensing end of a probe: (a) a heated resistance temperature device, (b) a thermocouple junction, or (c) a thermistor sensor. Air movement past th...
- 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 Meters. Ultrasonic meters 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 7-11a (SI) or Equation 7-11b (I-P).
- 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 test ducts that are described in Section 9 include static pressure taps, piezometer rings, flow straighteners, transition pieces, and variable air supply or exhaust systems.
- 8.2 Static Pressure Taps. Unless otherwise specified, 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 located on each surface of a rect...
- 8.3 Piezometer Ring [Go to Page]
- Figure 8-1 Static pressure tap construction requirements.
- 8.4 Flow Straighteners. Cell-type flow straighteners shall conform to Figure 8-4, and the thickness dimension y shall not exceed 0.005Dh. 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 test ducts or chambers, or round UUTs to rectangular test ducts or chambers shall be made in accordance with Figure 8-6. [Go to Page]
- Figure 8-2 Piezometer ring connection alternatives.
- Figure 8-3 Piezometer leak test setup illustration.
- Figure 8-4 Cell-type flow straightener.
- Figure 8-5 Star-type flow straightener
- Figure 8-6 Transformation pieces.
- 9. AIRFLOW MEASUREMENT METHODS [Go to Page]
- 9.1 Constraint on All Airflow Measurement Methods. A selected airflow measurement plane shall exceed 7.5 geometrically equivalent diameters downstream of an obstruction or any change in the airflow direction and shall exceed 3 geometrically equivalen...
- 9.2 Pitot-Static Tube Airflow Measurement Methods. The airflow measurement methods in this section are based on pitot-static tube measurement principles. [Go to Page]
- Figure 9-1 An example of pitot-static tube construction and connections.
- Figure 9-2 Pitot-static tube traverse measuring points for rectangular ducts and round ducts.
- 9.3 Single- and Multiple-Nozzle Airflow Methods [Go to Page]
- Figure 9-3 Nozzle geometry.
- Figure 9-4 Single-nozzle duct test setup.
- Figure 9-5 Single- or multiple-nozzle chamber construction requirements.
- 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... [Go to Page]
- Figure 9-6 Thermal dispersion probe locations in rectangular and round ducts.
- Figure 9-7 Tracer gas airflow measurement schematic.
- 9.5 Vortex-Shedding Arrays. Review Section 9.1. Vortex- shedding arrays are used to determine gas velocities. Piezoelectric methods, strain-gage methods, or hot-film methods are used to sense dynamic pressure variations created by vortex shedding. Th...
- 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 shows 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...
- 10. MEASUREMENT UNCERTAINTY [Go to Page]
- 10.1 Uncertainty Requirements. An estimate of the measurement uncertainty, performed in accordance with ASME PTC 19.1 10, shall accompany each air velocity and airflow measurement. Installation effects on the accuracy of the instrument shall be inclu...
- 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 Measurement System Description
- 11.3 Ambient Conditions
- 11.4 Test Conditions
- 11.5 Test Results
- 12. REFERENCES
- INFORMATIVE ANNEX A—INFORMATIVE REFERENCES AND BIBLIOGRAPHY
- INFORMATIVE ANNEX B—MULTIPLE-NOZZLE UNCERTAINTY ANALYSIS EXAMPLE [Go to Page]
- B1. Background
- B2. UNCERTAINTY IN AREAS Ax [Go to Page]
- Table B2-1 Nozzle Throat Area
- Table B2-2 Dnxt2 Computations
- Table B2-3 Result of ¶Ax /¶DNx Computations
- Table B2-4 Result of DAx Computations
- B3. Uncertainty in Conversion Factor of 1097.8 (for I-P only)
- B4. Uncertainty in e
- B5. Uncertainty in Density r1
- B6. Uncertainty in Viscosity
- B7. UNCERTAINTY IN Rex AND Vx [Go to Page]
- Table B7-1 Reynolds Number Computations
- Table B7-3 Results of ¶Redx / ¶rx Computations
- Table B7-5 Results of ¶Redx / ¶m1 Computations
- Table B7-7 Results of ¶Vx / ¶Redx Computations
- Table B7-9 Results of ¶Vx / ¶Dp Computations
- Table B7-2 Results of ¶Cx /¶Redx Computations
- Table B7-4 Results of ¶Redx / ¶dx Computations
- Table B7-6 Results of ¶Redx / ¶Vx Computations
- Table B7-8 Results of ¶Vx / ¶e Computations
- Table B7-10 Results of ¶Vx / ¶r1 Computations
- Table B7-11 Results of DRedx and DVx Computations
- B8. Uncertainty in C [Go to Page]
- Table B8-1 Nozzle Discharge Coefficients
- Table B8-2 Results of DCx Computations
- B9. Uncertainty in S (CA)
- B.10 Uncertainty in Q
- INFORMATIVE ANNEX C—VELOCITY UNCERTAINTY ANALYSIS EXAMPLE USING PITOT-STATIC TUBE [Go to Page]
- Table C-1 Assumed Parameter Values (SI)
- Table C-2 Assumed Parameter Values (I-P)
- Table C-3 Uncertainty Associated with Each Measured Parameter (SI)
- Table C-4 Uncertainty Associated with Each Measured Parameter (I-P)
- Figure C-1 Experimental test setup (left) to determine the dependence of the angle of attack on the coefficient of pressure (right).
- INFORMATIVE ANNEX 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 often can be obtained with no added uncertainty, as one system of units i...
- D1.2 Exact Conversion of m to ft. The conversion from m to ft can be found on NIST Special Publication 1038 A13, p. 8, and is reproduced in Equation D-1.
- D1.3 Exact Conversion of kg to lbm. The conversion from kg to lbm can be found on page 11 of NIST Special Publication 1038 A13 and is reproduced in Equation D-2.
- D1.4 Exact Conversion of 1.202 kg/m3 to lbm/ft3. The conversion from 1.202 kg/m3 to lbm/ft3can 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 /ft3 can 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...
- INFORMATIVE ANNEX E—USER INFORMATION [Go to Page]
- E1. Introduction
- E2. Comparison of Airflow Measurements
- E3. Comparison of Air Velocity Measurement Methods
- E4. Commentary Regarding Default Steady-State Criteria for Air Velocity and Airflow Measurements [Go to Page]
- Table E-1 Summary of Airflow Measurement Methods
- Table E-2 Summary of Air Velocity Measurement Methods
- E5. Self-Averaging Arrays [Go to Page]
- Figure E-1 Targeted steady-state conditions.
- Figure E-2 The approach to steady-state conditions.
- E6. Thermal Anemometers
- E7. Thermal Dispersion Arrays. Energy dissipated from a heated thermistor is directly related to velocity and mass velocity. This is one application of the definition of “thermal dispersion” based on the known physical relationship between power ...
- E8. Airflow Hoods. Flow-measuring hoods are portable instruments designed to measure supply or exhaust airflow through diffusers and grilles in HVAC&R systems. The assembly typically consists of a fabric hood section, a plastic or metal base, an airf... [Go to Page]
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