Ventilation for buildings - Air terminal devices - Aerodynamic testing and rating for mixed flow application

This European Standard specifies methods for the laboratory aerodynamic testing and rating of air terminal devices for mixed flow applications, including the specification of suitable test facilities and measurement techniques.
The standard gives only tests for the assessment of characteristics of the air terminal devices under isothermal conditions.
The testing of low velocity air terminal devices is specified in EN 12239.

Lüftung von Gebäuden - Luftdurchlässe - Aerodynamische Prüfung und Bewertung für Anwendung bei Mischströmung

Diese Europäische Norm legt Verfahren für die aerodynamische Laborprüfung und Bewertung von Luftdurchlässen für Anwendung bei Mischströmung sowie geeignete Prüfeinrichtungen und Meßverfahren fest. Die Europäische  Norm enthält nur Prüfungen zur Feststellung der Kenngrößen der Luftdurchlässe unter isothermischen Bedingungen. Die Prüfung von Luftdurchlässen niedriger Geschwindigkeiten ist in EN 12239 festgelegt.

Ventilation des bâtiments - Bouches d'air - Essais aérodynamiques et caractérisation pour applications en diffusion a mélange

La présente Norme européenne spécifie les méthodes pour les essais aérodynamiques en laboratoire et la caractérisation, des bouches d'air utilisées dans des applications en diffusion a mélange, y compris la spécification de dispositifs d'essai et de techniques de mesure appropriées.
La présente norme ne traite que des essais permettant d'évaluer les caractéristiques des bouches d'air dans des conditions isothermes.
L'essai des bouches d'air a basse vitesse est spécifiée dans l'EN 12239.

Prezračevanje stavb - Naprave za razvod zraka - Aerodinamično preskušanje in ocenitev aplikacij strujanja zraka

General Information

Status
Published
Publication Date
30-Nov-2001
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Dec-2001
Due Date
01-Dec-2001
Completion Date
01-Dec-2001

Relations

Buy Standard

Standard
EN 12238:2001
English language
36 pages
sale 10% off
Preview
sale 10% off
Preview
e-Library read for
1 day

Standards Content (Sample)

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Lüftung von Gebäuden - Luftdurchlässe - Aerodynamische Prüfung und Bewertung für Anwendung bei MischströmungVentilation des bâtiments - Bouches d'air - Essais aérodynamiques et caractérisation pour applications en diffusion a mélangeVentilation for buildings - Air terminal devices - Aerodynamic testing and rating for mixed flow application91.140.30VLVWHPLVentilation and air-conditioningICS:Ta slovenski standard je istoveten z:EN 12238:2001SIST EN 12238:2001en01-december-2001SIST EN 12238:2001SLOVENSKI
STANDARD



SIST EN 12238:2001



EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 12238August 2001ICS 91.140.30English versionVentilation for buildings - Air terminal devices - Aerodynamictesting and rating for mixed flow applicationVentilation des bâtiments - Bouches d'air - Essaisaérodynamiques et caractérisation pour applications endiffusion à mélangeLüftung von Gebäuden - Luftdurchlässe - AerodynamischePrüfung und Bewertung für Anwendung bei MischströmungThis European Standard was approved by CEN on 29 June 2001.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2001 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 12238:2001 ESIST EN 12238:2001



EN 12238:2001 (E)2ContentsPageForeword.41Scope.52Normative references.53Terms, definitions and symbols.53.1functional characteristics of air terminal devices.53.2Symbols.94Instrumentation.114.1Air flow rate measurement.114.2Pressure measurement.114.3Temperature measurements.124.4Velocity measurements.125Testing of pressure and air velocity vk (first test installation).125.1Measurement of pressure requirement for a supply air terminal device.125.1.1General.125.1.2Measurement of static gauge pressure ps with the first test installation A.135.1.3Direct measurement of total pressure pt with the first test installation A.135.1.4Measurement of static pressure ps with the first test installation B.145.1.5Presentation of total pressure pt.145.2Measurement of pressure requirement for an exhaust air terminal device.155.2.1Procedure.155.2.2Measurement of static pressure with the first test installation C for exhaust airterminal device (excluding air transfer devices).165.2.3Direct measurement of total pressure with the first test installation C, for exhaustair terminal device.165.2.4Measurement of static pressure with the first test installation D for exhaust airterminal device.175.2.5Presentation of total pressure pt.175.3Determination of air velocity vk and the corresponding area Ak for the air terminaldevice (not mandatory).185.3.1Principle.185.3.2Test installation.185.3.3Test procedure.185.3.4Correction of flow rates to standard air conditions.195.3.5Calculation of Ak.196Test to measure the isothermal air discharge characteristics of a supply airterminal device (second test installation).196.1Test measurements.196.2Test room.196.3Test room equipment and instrumentation.206.4Installation of the air terminal device.206.4.1Classification of air terminal devices.206.4.2Mounting of air terminal devices.206.4.3Test duct and flow rate.216.5Test procedure.216.6Determination of isothermal performance.216.6.1Test measurements.216.6.2Determination of the main air stream direction.226.6.3Measurement of air stream velocities.22SIST EN 12238:2001



EN 12238:2001 (E)36.6.4Determination of throw.236.6.5Determination of spread.246.6.6Determination of rise and drop.24Annex A (normative)
Alternative exploratory technique for determination of throw, spreadand drop.30A.1Scope.30A.2Determination of the point of maximum velocity.30A.3Determination of points at envelope velocity.31A.4Determination of spread.31A.5Determination of the rise and drop.31A.6Number of determinations.31SIST EN 12238:2001



EN 12238:2001 (E)4ForewordThis European Standard has been prepared by Technical Committee CEN/TC 156 "Ventilation forbuildings", the secretariat of which is held by BSI.This European Standard shall be given the status of a national standard, either by publication of anidentical text or by endorsement, at the latest by February 2002, and conflicting national standardsshall be withdrawn at the latest by February 2002.According to the CEN/CENELEC Internal Regulations, the national standards organizations of thefollowing countries are bound to implement this European Standard: Austria, Belgium, CzechRepublic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg,Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.SIST EN 12238:2001



EN 12238:2001 (E)51 ScopeThis European Standard specifies methods for the laboratory aerodynamic testing and rating of airterminal devices for mixed flow applications, including the specification of suitable test facilities andmeasurement techniques.The standard gives only tests for the assessment of characteristics of the air terminal devices underisothermal conditions.The testing of low velocity air terminal devices is specified in EN 12239.2 Normative referencesThis European Standard incorporates by dated or undated reference, provisions from other publications.These normative references are cited at the appropriate places in the text and the publications are listedhereafter. For dated references the subsequent amendments to or revisions of any of these publicationsapply to this European Standard only when incorporated in it by amendment or revision. For undatedreferences the latest edition of the publication referred to applies (including amendments).CR 12792, Ventilation for buildings — Symbols and terminologyEN 12239, Ventilation for buildings — Air terminal devices —- Aerodynamic testing and rating fordisplacement flow applicationsprEN 13182:1998, Ventilation for buildings — Instrumentation requirements for air velocity measurementsin ventilated spacesISO 3966, Measurement of fluid flow in closed conduits — Velocity area method using Pitot static tubesEN ISO 5167-1, Measurement of fluid flow by means of pressure differential devices
— Part 1: Orificeplates, nozzles and Venturi tubes inserted in circular cross-section conduits running full[ISO 5167-1: 1991]3 Terms, definitions and symbolsFor the purposes of this standard, the terms and definitions given in CR 12792, together with thefollowing, apply.3.1 functional characteristics of air terminal devicesNOTEThe acroynym ATD is used to signify an air terminal device.3.1.1nominal size of an air terminal devicenominal value of dimensions of the duct into which the air terminal device is to be fittedNOTEFor an air diffuser, the nominal size is generally known as neck size.SIST EN 12238:2001



EN 12238:2001 (E)63.1.2
core and specific areas3.1.2.1core of an air terminal devicethat part of an air terminal device located within a convex closed surface of minimum area, inside whichare all the openings of the air terminal device through which the air can pass3.1.2.2free area (of an air terminal device)sum of the smallest areas of the cross-section of all openings of the air terminal device3.1.2.3core of a grillethat part of a grille located inside a convex closed plane curve of minimum length of contour, inside whichare all the openings of the grille3.1.2.4core area (of a grille)area limited by the plane curve defined in 3.1.2.33.1.2.5free area (of a grille)sum of the minimum measured areas of each opening through which the air can pass3.1.2.6free area ratio (of a grille)ratio of the free area to the core area3.1.2.7effective area of an air terminal device Akquotient resultant from measured air flow rate and measured air velocity as determined in a specifiedmanner with a specified instrument3.1.3 aspect and vane ratios3.1.3.1aspect ratio (of a rectangular air terminal device)ratio of the larger side to the smaller side of the rectangular core3.1.3.2vane ratio (of a grille)ratio of the chord length to the vane pitch3.1.4Special terms relating to air3.1.4.1standard airatmospheric air having a density of 1,2 kgm-3 at 20 C, 101 325 Pa (1 013,25 mbar) and 65 % RelativeHumidity3.1.4.2supply airair entering a supply air terminal device from an upstream duct3.1.4.3induced airairflow induced by the supply air from a supply air terminal deviceSIST EN 12238:2001



EN 12238:2001 (E)73.1.4.4exhaust airair leaving an exhaust air terminal device into a downstream duct3.1.5 specific terms relating to air diffusion rating3.1.5.1mean measured air temperature of the occupied zonearithmetical average of the measured values of air temperature within the occupied zone3.1.5.2supply temperature differentialalgebraic difference between the supply air temperature and the mean measured air temperature of theoccupied zone3.1.5.3exhaust temperature differentialalgebraic difference of the exhaust air temperature and the mean measured air temperature of theoccupied zone3.1.5.4temperature differential within the occupied zonelargest value of the difference between measured air temperatures within the occupied zone3.1.5.5primary air flow ratevolume of air entering a supply air terminal device in unit time from an upstream duct3.1.5.6exhaust air flow ratevolume of air leaving an exhaust air terminal device in unit time3.1.5.7local air velocitymagnitude of the time-averaged vector of velocity at a point of an air streamThe velocity vector (and therefore its three mutually perpendicular components u,v,w) in any point of aturbulent stream is submitted to fluctuations with respect to time. The time-averaged vector of velocity is avector for which each component is averaged with respect to time. The components being:TTT;wdtTw;vdtTv;udtTu000111the local air velocity is therefore: w + v + u2223.1.5.8local measured air velocitymeasured value of local air velocity3.1.5.9treated spaceenclosure served by an air distribution systemNOTEIn this standard the treated space is the test room described in 6.2.3.1.5.10envelopegeometrical surface in a treated space where the local measured air velocity has the same value and isthe reference velocity associated with this envelopeSIST EN 12238:2001



EN 12238:2001 (E)83.1.5.11room air velocityvalue of velocity conventionally derived from the various local measured air velocities within theoccupied zone3.1.5.12free area velocityprimary air flow rate divided by the free area of a supply air terminal device or exhaust air flow divided bythe free area of an exhaust air terminal device3.1.5.13throw (for a supply air terminal device)maximum distance between the centre of the core and a plane which is tangent to a specified envelope,such as 0,25 ms-1, 0,5 ms-1, etc., and the centre of the core3.1.5.14drop (for a supply air terminal device)vertical distance between the lowest horizontal plane tangent to a specified envelope, such as 0,25 ms-1,0,5 ms-1, etc., and the centre of the core3.1.5.15rise (for a supply air terminal device)vertical distance between the highest horizontal plane tangent to a specified envelope, such as0,25 ms-1, 0,5 ms-1, etc., and the centre of the core3.1.5.16spread (for a supply air terminal device)maximum distance between two vertical planes tangent to a specified envelope, such as 0,25 ms-1, 0,5 ms-1,etc., and perpendicular to a plane through the centre of the coreThere can be two different spreads, which are not always equal: one for the left side, the other for theright side (considered when looking at the treated space from the supply air terminal device).3.1.5.17occupied zone (for laboratory purposes)portion of the treated space geometrically limited to be no closer than 0,15 m to all walls and within aheight of 1,8 m above the floor3.1.5.18slot air terminal devicedevice with single or multiple components with aspect ratio [the ratio of length to width of the enclosedrectangular opening] of 10:1 or greater for each slotSIST EN 12238:2001



EN 12238:2001 (E)93.2 SymbolsThe symbols used
in this standard are given in Table 1.Table 1 — SymbolsSymbolQuantitySI unitAAream2AdArea corresponding to the cross section of the nominal sizeof the duct to which the device is fitted (neck area)m2AfFree aream2AkEffective area (k-factor area)
kvvqm2bRWidth of test room or installationmDeEquivalent diameter
dA4mDhHydraulic diameter
perimeterA4dmdDiametermhDFace height of linear grille of diffusermhRHeight of test room or installationmlRLength of test room or installationmpsaAbsolute static pressurePapaAtmospheric pressurePapsStatic pressure or static gauge pressure (psa - pa)PaptaStagnation (or absolute total) pressurePaptTotal pressure (pta - pa)Papt1,2Total pressure corresponding to a density of 1,2 kgm-3PaptDTotal pressure requirement of the devicePaSIST EN 12238:2001



EN 12238:2001 (E)10Table 1
Symbols [concluded]SymbolQuantitySI unitps1,2Static gauge pressure corresponding to a density of1,2 kgm-3PapsDStatic pressure requirement of the devicePapd1,2Dynamic pressure (Velocity pressure)
2v2PapPressure difference (for a pressure difference device)PaqvVolume rate of flowm3s-1vVelocityms-1Velocity referred to the effective area (k-factor velocity)vk
Aqkvms-1vxMaximum velocity at distance x from centre of supply airterminal devicems-1xDistance from supply air terminal device along the centrelineof the jetmXThrowmYSpreadmZDropmLoss coefficient-Thermodynamic temperatureKDensity of airkgm-3RArea parameter that relates to the effective size of the airterminal devicem2SLinear parameter that relates to the effective size of the airterminal devicemXCCoordinate (see annex A)mYCCoordinate (see annex A)mZCCoordinate (see annex A)mSIST EN 12238:2001



EN 12238:2001 (E)114 Instrumentation4.1 Air flow rate measurement4.1.1The air flow rate shall be measured using instruments in accordance with EN ISO 5167-1.4.1.2Air flow meters shall have a minimum accuracy according to Table 2 ranges:Table 2 — Ranges and accuracies of air flow metersRangeAccuracy of measurementm3s-1%> 0,07

2,5> 0,007
0,07± 5,0NOTEFlow meters can be calibrated in situ by means of the Pitot static tube traverse techniques described inISO 3966.4.1.3Flow meters shall be checked at intervals as appropriate but not exceeding 12 months. Thischeck may take the form of one of the following:a) a dimensional check for all flow meters not requiring calibration;b) a check calibration over their full range using the original method employed for the initialcalibration of meters calibrated in situ;c) a check against a flow meter which meets EN ISO 5167-1.4.2 Pressure measurement4.2.1Pressure in the duct shall be measured with a calibrated manometer.4.2.2The maximum scale interval shall not be greater than the characteristics listed for theaccompanying range of manometer, shown in Table 3.Table 3 — Maximum scale intervals for the ranges of manometersRangeMaximum scale intervalPaPa
251,0> 25
2502,5> 250
5005,0> 50025SIST EN 12238:2001



EN 12238:2001 (E)124.2.3For air flow rate measurements, the minimum pressure differential shall be:a) 25 Pa with an inclined tube manometer or micromanometer;b) 500 Pa with a vertical tube manometer.4.2.4Calibration standards shall be:a) for instruments with the range up to 25 Pa, a micromanometer accurate to ± 0,5 Pa;b) for instruments with the range up to 100 Pa, a micromanometer accurate to ± 1 Pa;c) for instruments with the range over 100 Pa, a micromanometer accurate to ± 1 % of reading.4.3 Temperature measurementsMeasurements of temperature shall be made by means of mercury-in-glass thermometers, resistancethermometers, thermocouples or other suitable instruments. Instruments shall be graduated or givereadings in intervals not greater than 0,5 K and calibrated to an accuracy of ± 0,25 K.4.4 Velocity measurements4.4.1The measurements of low velocities within treated spaces, to determine air terminal deviceperformance characteristics, shall be made with a measuring device in accordance withprEN 13182:19984.4.2The measurements of air terminal device velocities to determine air terminal device vk velocitycharacteristics shall be made with a measuring device in accordance with prEN 13182:19985 Testing of pressure and air velocity vk (first test installation)5.1 Measurement of pressure requirement for a supply air terminal device5.1.1 General5.1.1.1 The-pressure requirement of an air terminal device is, for a given value of flow rate,dependent on the type and size of the device and on the velocity profile upstream of the device. Astandard test duct immediately upstream of the air terminal device shall be employed. If an inlet ductarrangement, or flow equalizing and/or damping device, is an integral part of an air terminal device,then the standard test duct shall be employed immediately upstream of the integral inlet duct oraccessory.5.1.1.2The test system shall comprise at least a fan, a means for controlling the air flow rate, aflow rate measuring device and a standard test duct for the air terminal device. Tests shall be carriedout under isothermal conditions.5.1.1.3Pressure tests on the air terminal device alone, or the air terminal device in combinationwith a flow equalizing and/or damping device, shall be conducted to establish a pressure for a givenair flow rate. The air terminal device shall be mounted in one of the two test installations described in5.1.2 (see Figure 1) or 5.1.4 (see Figure 3). To determine minimum pressure, measurements shall bemade with flow equalizing and/or damping devices in the normally open position. Pressure tests onthe air terminal device shall be clearly referenced to any position of adjustment.Two methods can be used for determining pressure requirements on test installation A: one bymeasuring static pressure (see 5.1.2), the other by directly measuring total pressure (see 5.1.3).SIST EN 12238:2001



EN 12238:2001 (E)135.1.2 Measurement of static gauge pressure ps with the first test installation A5.1.2.1This test installation shall be used for air terminal devices normally duct mounted.The air terminal device shall be mounted in a test duct with cross-sectional dimensions equal to thenominal size of the device or to the duct dimensions normally recommended by the manufacturer. Thisduct shall be straight and at least 20 De long to guarantee a uniform velocity profile or shall include anefficient flow straightener located at a position at least 3 De from any part of the air terminal device. It isrecommended that straightener cells have an axial length at least equal to six times the hydraulicdiameter of their cross-section.5.1.2.2The test installation shall be generally constructed as shown in Figure 1. The plane ofmeasurement shall be at 1,5 De upstream of the air terminal device. A static pressure traverse shallbe taken on two orthogonal diameters in order to obtain the maximum and minimum values. Themeasured pressure at the selected point of test in the plane of measurement shall not differ by morethan 10 % from both the maximum and the minimum value within the pressure measurement plane.As an alternative method a piezometer ring can be used.5.1.2.3 Record the results for a minimum of four air flow rates regularly distributed over theworking range for each air terminal device tested.5.1.2.4The total pressure pt in the plane of measurement is equal to the sum of the measuredstatic gauge pressure ps and the dynamic pressure pd according to the following formula.22dvstAqpp5.1.3 Direct measurement of total pressure pt with the first test installation A5.1.3.1This installation shall be used for air terminal devices normally installed duct mounted.The test installation and the plane of measurement shall be the same as described in 5.1.2. A pitot tubeshall be used for successively measuring the total pressure at five points in this plane. These five pointsshall be distributed as shown in Figure 2. One point is on the duct axis, the other four points are locatedon two orthogonal diameters at a distance from the duct axis equal to 0,4 times the diameter of the cross-section. The total pressure is the mean arithmetic value of the five total pressure recordedmeasurements.For rectangular cross-section, measurements shall be made on diagonals with their length used as thereferenced dimensions to locate the four supplementary points as shown on Figure 2.5.1.3.2Record the results for a minimum of four air flow rates regularly distributed over theworking range for each air terminal device tested.SIST EN 12238:2001



EN 12238:2001 (E)145.1.4 Measurement of static pressure ps with the first test installation B5.1.4.1This test installation shall be used for air terminal devices normally installed in a plenum.The test installation shall be constructed as shown in Figure 3 such that the following formula issatisfied:5svpAqwhereqv is the volume flow rate;A is the area of the internal cross-section of the chamber; (A = W H)ps is the required pressure; is the density of the air.The air terminal device to be tested shall be mounted in a short test duct equal to the nominal size of theair terminal device and having a length equal to D or 0,15 m, whichever is greater. The test duct shallhave a conical entrance.The required pressure shall be measured with at least a single wall static tapping located within 0,05 m ofthe inside surface of the air terminal device mounting plate.Equalizing sections shall be provided within the chamber to ensure that a relatively uniform flow, free fromswirl, exists in the test chamber with the air terminal device mounting plate removed.5.1.4.2Record the results for a minimum of four air flow rates regularly distributed over theworking range for each air terminal device tested.5.1.4.3 The measured pressure ps is equal to the total pressure pt.5.1.5 Presentation of total pressure pt5.1.5.1Correct the data to standard air conditions according to the following formula:1,2 p = pt2,t1Present the data in a graph showing the total pressure versus the air flow rate.5.1.5.2Alternative method: Presentation of static pressure ps. Correct the data to standard airconditions according to the following formula:1,2 p = ps2,s1Present the data in a graph showing the static pressure versus the air flow rate.SIST EN 12238:2001



EN 12238:2001 (E)155.1.5.3Alternative method: Presentation of the loss coefficient . The loss coefficient
may bycalculated from the following appropriate relationships, based upon the pressures measured under5.1.2, 5.1.3 and 5.1.4.5.1.4) and 5.1.3 (see
pp = dt5.1.2)(see
1 + pp = dsWhere pt and ps are the measured quantities and pd is calculated as follows:dvdAqp2and where both
and qv are at the same test conditions.5.2 Measurement of pressure requirement for an exhaust air terminal device5.2.1 Procedure5.2.1.1 The pressure requirement of an exhaust air terminal device is for a given value of flow ratedependent on the type and size of the device and on the velocity profile upstream and downstream ofthe device. A standard test duct immediately downstream of the air terminal device shall be employed.If a connecting duct arrangement, flow equalizing and/or damping device is an integral part of an airterminal device, then the standard test duct shall be employed immediately downstream of theintegral connecting duct or accessory.5.2.1.2 The test system shall comprise at least a fan, a means for controlling the air flow rate, aflow rate measuring device and a standard test duct for the air terminal device. Tests shall be carriedout under isothermal conditions.5.2.1.3 The device under test shall be mounted in a simulated wall or ceiling surface using themethod of fixing recommended by the manufacturer. For circular and rectangular air terminal devicesthis surface shall extend on all sides of the air terminal device to at least 2 De from the boundaries ofthe air terminal device.For slots or similar air terminal devices, the surface shall extend by at least twice the width of the slot oneach side of the device.For special exhaust air terminal devices (for example, heat removal luminaries), where the velocity doesnot exceed 1 ms-1 in the plane of the ceiling surface, no extended surface is necessary.5.2.1.4 Pressure tests on the exhaust air terminal device alone or in combination with connectingducts, flow equalizing and/or damping devices shall be conducted to establish a pressure for a givenair flow rate. The air terminal device shall be mounted in one of the test installations described in 5.2.2(see Figure 4) or 5.2.4 (see Figure 5).To determine the minimum pressure, measurements shall be made with the damping device in thenormally open position. Pressure tests on the exhaust air terminal devi
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.