SIST EN 416-2:2006
(Main)Single burner gas-fired overhead radiant tube heaters for non-domestic use - Part 2: Rational use of energy
Single burner gas-fired overhead radiant tube heaters for non-domestic use - Part 2: Rational use of energy
This European Standard specifies the requirements and test methods for the rational use of energy of non-domestic gas fired overhead radiant tube heaters incorporating a single burner system under the control of an automatic burner control system, referred to in the body of the text as 'appliances'.
This standard is applicable to Type A2, A3, B12, B13, B22, B23, B42, B43, B52, B53, C12, C13, C32 and C33 appliances intended for use in other than domestic dwellings, in which the supply of combustion air and/or the evacuation of the products of combustion is achieved by mechanical means located upstream of the draught diverter, if provided.
This standard is not applicable to:
a) appliances designed for use in a domestic dwelling;
b) outdoor appliances;
c) appliances of heat input in excess of 120 kW (based on the net calorific value of the appropriate reference test gas);
d) appliances having fully pre-mixed gas and air burners in which :
1) either the gas and all the combustion air are brought together just before the level of the combustion zone; or
2) pre-mixing of the gas and all combustion air is carried out in a part of the burner upstream of the combustion zone.
This standard is applicable to appliances which are intended to be type tested. Requirements for appliances which are not intended to be type tested need to be subject to further consideration.
Gasgeräte-Heizstrahler - Dunkelstrahler mit einem Brenner mit Gebläse für gewerbliche und industrielle Anwendung - Teil 2: Rationelle Energienutzung
Diese Europäische Norm enthält die Anforderungen und Prüfmethoden für die rationelle Energienutzung von Dunkelstrahlern mit einem einzigen Brenner mit Gebläse mit Überwachung durch einen Gasfeuerungs-automaten (im folgenden Text als 'Geräte' bezeichnet).
Diese Europäische Norm gilt für Geräte der Arten A2, A3, B12, B13, B22, B23, B42, B43, B52, B53, C12, C13, C32 und C33, die für gewerbliche und industrielle Anwendung vorgesehen sind und bei denen die Zufuhr der Verbrennungsluft und/oder die Abführung der Verbrennungsprodukte mit mechanischen Mitteln erfolgt, die vor der Strömungssicherung angeordnet sind, wenn eine solche vorgesehen ist.
Diese Europäische Norm gilt nicht für:
a) Geräte, die für eine Verwendung in Wohnungen gedacht sind;
b) Geräte, die für eine Verwendung im Freien gedacht sind;
c) Geräte mit einer Wärmebelastung über 120 kW (bezogen auf den Heizwert des jeweiligen Normprüfgases);
d) Geräte mit Brennern mit kompletter Gas- und Luftvormischung, bei denen
1) entweder das Gas und die gesamte Verbrennungsluft kurz vor der Verbrennungszone zusammengeführt werden; oder
2) das Vormischen des Gases mit der gesamten Verbrennungsluft in einem Teil des Brenners vor der Verbrennungszone erfolgt.
Diese Norm ist anwendbar für Geräte, die für eine Typprüfung vorgesehen sind. Anforderungen an Geräte, die nicht für eine Typprüfung vorgesehen sind, benötigen weitere Überlegungen.
Tubes radiants suspendus a monobruleur a usage non domestique utilisant les combustibles gazeux - Partie 2 : Utilisation rationnelle de l'énergie
Stropna cevna sevala s plinskim gorilnikom za nestanovanjske prostore – 2. del: Smotrna raba energije
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Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Single burner gas-fired overhead radiant tube heaters for non-domestic use - Part 2: Rational use of energyStropna cevna sevala s plinskim gorilnikom za nestanovanjske prostore – 2. del: Smotrna raba energijeTubes radiants suspendus a monobruleur a usage non domestique utilisant les combustibles gazeux - Partie 2 : Utilisation rationnelle de l'énergieGasgeräte-Heizstrahler - Dunkelstrahler mit einem Brenner mit Gebläse für gewerbliche und industrielle Anwendung - Teil 2: Rationelle EnergienutzungTa slovenski standard je istoveten z:EN 416-2:2006SIST EN 416-2:2006en,fr,de97.100.20ICS:SIST ENV 1259-3:1996SIST ENV 1259-2:1996SIST ENV 1259-1:19961DGRPHãþDSLOVENSKI
STANDARDSIST EN 416-2:200601-september-2006
EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 416-2June 2006ICS 97.100.20Supersedes ENV 1259-1:1994, ENV 1259-2:1996, ENV1259-3:1996
English VersionSingle burner gas-fired overhead radiant tube heaters for non-domestic use - Part 2: Rational use of energyTubes radiants suspendus à monobrûleur à usage nondomestique utilisant les combustibles gazeux - Partie 2 :Utilisation rationnelle de l'énergieGasgeräte-Heizstrahler - Dunkelstrahler mit einem Brennermit Gebläse für gewerbliche und industrielle Anwendung -Teil 2: Rationelle EnergienutzungThis European Standard was approved by CEN on 16 March 2006.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 Central Secretariat 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 Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2006 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 416-2:2006: E
EN 416-2:2006 (E) 2 Contents
Page Foreword.4 1 Scope.5 2 Normative references.5 3 Terms and definitions.5 4 Classification of appliances.7 4.1 Classification according to the nature of the gases used.7 4.2 Classification according to the gases capable of being used.7 4.3 Classification according to the mode of evacuation of the combustion products.7 5 Symbols.7 6 Requirements for the rational use of energy.8 7 Test methods.9 7.1 General.9 7.2 Radiant factor.9 7.2.1 General.9 7.2.2 Method A.9 7.2.3 Method B.18 Annex A (informative)
Recording test data (Test method A).21 A.1 General information to be recorded.21 A.2 Measurement results.21 Annex B (informative)
Blank forms (Test method A).22 B.1 Model test result form - Quarter sphere burner end and opposite end.22 B.2 Model test result form - Quarter cylinder (Burner side and opposite side).23 B.3 Model test result form – Half sphere for appliances less than or equal to 1,3 m long.24 Annex C (informative)
Worked example (Test method A).25 C.1 Radiant factor - Recorded data and calculation.25 C.2 Radiant output - Recorded data and calculation.26 C.2.1 Quarter spheres (Burner end and opposite end).26 C.2.2 Quarter cylinders (Burner side and opposite side).27 Annex D (normative)
Procedure for measuring the window correction factor (Fw) (Test method A).28
Annex E (normative)
Correction of measured radiant output for absorption by air (Test methods A and B).29 E.1 General.29 E.2 Mean beam length (D).29 E.3 Absorption of radiation by water vapour.29 E.4 Absorption of radiation by carbon dioxide.30 E.5 Total radiation absorption.31 E.6 Calculation method.31 Annex F (informative)
Radiant heat output data - Recording of results (Test method B).32 F.1 General information to be recorded.32 F.1.1 Test and appliance data.32 F.1.2 Radiometer technical data.32 F.1.3 Measuring plane technical data.32 F.2 Measurement results.33
EN 416-2:2006 (E) 3 F.2.1 Test information.33 F.2.2 Test ambient conditions.33 F.2.3 Gas/heat input data.33 F.2.4 Flue gas data.33 F.2.5 Absorption of water vapour and CO2 data.34 F.2.6 Irradiation measurement data.34 Annex G (informative)
Worked example (Test method B).35 G.1 General information.35 G.2 Radiometer technical data.35 G.3 Measuring plane technical data.35 G.4 Measurement results.36 G.4.1 Test information.36 G.4.2 Test ambient conditions.36 G.4.3 Gas/heat input data.36 G.4.4 Flue gas data.36 G.4.5 Absorption of water vapour and CO2 data.37 G.4.6 Irradiation measurement data.37 Annex H (informative)
Radiometer design (Test method B).40 H.1 Principle radiometer design features.40 H.2 Radiometer technical design.41 H.3 Pyro-electric detector.41 Annex I (normative)
Radiometer calibration (Test method B).42 I.1 Radiometer calibration.42 I.1.1 General.42 I.1.2 Black Body calibration method.42 I.2 Worked example.43 Annex ZA (informative)
Clauses of this European Standard addressing essential requirements or other provisions of EU directives.45
EN 416-2:2006 (E) 4 Foreword This European Standard (EN 416-2:2006) has been prepared by Technical Committee CEN/TC 180 “Non-domestic gas-fired overhead radiant heaters”, the secretariat of which is held by BSI. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by December 2006, and conflicting national standards shall be withdrawn at the latest by December 2006. This European Standard supersedes ENV 1259-1:1994, ENV 1259-2:1996 and ENV 1259-3:1996 This European Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive(s). For relationship with EU Directive(s), see informative Annex ZA, which is an integral part of this European Standard. This part of EN 416 complements EN 416-1: “Single burner gas-fired overhead radiant-tube heaters - Part 1: Safety”. It is intended that this standard is reviewed 3 years after publication. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EN 416-2:2006 (E) 5 1 Scope This European Standard specifies the requirements and test methods for the rational use of energy of non-domestic gas fired overhead radiant tube heaters incorporating a single burner system under the control of an automatic burner control system, referred to in the body of the text as 'appliances'. This standard is applicable to Type A2, A3, B12, B13, B22, B23, B42, B43, B52, B53, C12, C13, C32 and C33 appliances intended for use in other than domestic dwellings, in which the supply of combustion air and/or the evacuation of the products of combustion is achieved by mechanical means located upstream of the draught diverter, if provided. This standard is not applicable to: a) appliances designed for use in a domestic dwelling; b) outdoor appliances; c) appliances of heat input in excess of 120 kW (based on the net calorific value of the appropriate reference test gas); d) appliances having fully pre-mixed gas and air burners in which : 1) either the gas and all the combustion air are brought together just before the level of the combustion zone; or 2) pre-mixing of the gas and all combustion air is carried out in a part of the burner upstream of the combustion zone. This standard is applicable to appliances which are intended to be type tested. Requirements for appliances which are not intended to be type tested need to be subject to further consideration. 2 Normative references The following referenced documents are indispensable for the application of this European Standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 416-1: 1999, Single burner gas-fired overhead radiant tube heaters —
Part 1: Safety 3 Terms and definitions For the purposes of this European Standard, the terms and definitions given in Clause 3 of EN 416-1: 1999 apply together with the following. 3.1 radiation reference plane flat horizontal surface bounded by the lower edge of the reflector or, in the case where radiant parts project below this lower edge of the reflector, the flat horizontal surface touching the lowest radiant part (see Figure 1)
EN 416-2:2006 (E) 6 1222 Key
1 Reflector 2 Reference plane Figure 1 — Radiation reference plane 3.2 irradiance (E) radiant power per unit area (W/m2) incident upon a surface 3.3 radiant factor (Rf) heat emitted by the appliance through the radiation reference plane divided by the net heat input of the test gas 3.4 measuring plane (Test method B only) plane parallel to the radiation reference plane and 100 mm ± 3 mm below it 3.5 measuring grid (Test method B only) regular arrangement in the measuring plane of straight lines running parallel and perpendicular to the longitudinal axis of the appliance with sufficient precision (± 1 mm). The nodal points of the measuring grid are located at the points of intersection of these lines (see Figure 2) such that the distance between all adjacent nodes points on these lines is 100 mm ± 2 mm
EN 416-2:2006 (E) 7 (0,0)(n, k)132 Key
1 Heater 2 Nodal point 3 Measuring cell Fij Figure 2 — Measuring grid (Test method B) 4 Classification of appliances 4.1 Classification according to the nature of the gases used The requirements of 4.1 of EN 416-1:1999 apply. 4.2 Classification according to the gases capable of being used The requirements of 4.2 of EN 416-1:1999 apply. 4.3 Classification according to the mode of evacuation of the combustion products The requirements of 4.3 of EN 416-1:1999 apply. 5 Symbols The symbols shown in Table 1 are used in this standard. Table 1 — Symbols Symbol Title Unit 2COα Coefficient in equation for kco2 kPa-1 m-1 2HOα Coefficient in equation for kmo kPa-1 m-1 2COA Absorption factor of carbon dioxide — 2HOA Absorption factor of water vapour — a Reflector length mm ATOT Radiant correction factor for water vapour and CO2 in air (see Annex E) — b Reflector width mm c Distance between two nodal points parallel with the longitudinal axis mm
EN 416-2:2006 (E) 8 Symbol Title Unit Cαβ Surface area correction factor — D Average thickness of radiating gas layer (i.e. from measurement point to radiation reference plane) m εCO2 Emission factor of carbon dioxide — εOH2 Emission factor of water vapour
— E Actual irradiance from overhead radiant heater W/m2 Ea Actual irradiance output by appliance in air W/m2 Eij Irradiance of the appliance measured at the nodal points of the measurement W/m2 ifE Average irradiance over the measurement grid Fij W/m2 Fw Window correction factor — Hi Net calorific value of the test gas (at 15 °C, 1013,25 mbar, dry gas)
W h/m3 2COk Coefficient in equation for emission factor of carbon dioxide kPa-1 m-1 2HOk Coefficient in equation for emission factor of water vapour kPa-1 m-1 L Length of reference surface cylinder m N Number of arc positions along the half cylinder (see Figure 2) — n Coefficient in equations for 2COk and 2HOk — P2CO Partial pressure of carbon dioxide in ambient air kPa 2HOP Partial pressure of water vapour in ambient air kPa pmax,H20 Saturated vapour pressure mbar p Gas supply pressure mbar pa Atmospheric pressure mbar pW Saturation vapour pressure of fuel gas at temperature tg mbar Qm Measured heat input based on the net calorific value of the test gas W Q(R)C Radiant output after correction for absorption of radiation in air W Q(R)M Measured radiant output W R Radius to radiometer from centre of reference plane m Rf Radiant factor — S Radiometer sensitivity µV/(W/m2)tA Ambient air temperature °C tg Gas temperature at measuring point °C ts Sensor temperature °C U Sensor voltage v V Gas volume input at test conditions m3/h Vb Sensor voltage recorded with radiation shield in place µV Vt Sensor voltage recorded without radiation shield in place µV Vo Gas volume rate under reference conditions (at 15 °C, 1013,25 mbar, dry gas) m3/h OH2ε Emissivity of water — 6 Requirements for the rational use of energy When mounted horizontally in accordance with the manufacturer’s instructions and measured by one of the methods given in 7.2 the radiant factor of the appliance, adjusted to the nominal heat input, shall be in accordance with the values given in Table 2.
Table 2 — Radiant factor for appliances mounted horizontally Class Radiant Factor 1 > 0,4
to
≤ 0,5 2 > 0, 5
EN 416-2:2006 (E) 9 7 Test methods 7.1 General The test shall be carried out with the appliance mounted horizontally in accordance with the manufacturer’s instructions. The requirements of 7.1 of EN 416-1: 1999 apply unless otherwise specified. 7.2 Radiant factor 7.2.1 General 7.2.1.1 Working area (requirements applicable to all methods of test) The working area shall be of a size to allow installation of the appliance and shall: a) provide sufficient ventilation to remove the combustion products and heat generated by the appliance; b) have an ambient air temperature of 20 °C ± 5 °C; c) allow the sensors to be positioned free from draughts; The sensor temperature shall be checked before and after measurements are taken and: d) for air cooled sensors shall be 20 °C ± 5 °C; and e) for water cooled sensors, the temperature of the cooling water shall not change by more than ± 5 °C during the test. 7.2.1.2 Choice of test method The radiant factor of the appliance may be determined either by the method described in 7.2.2 or by the method described in 7.2.3. 7.2.2 Method A 7.2.2.1 Installation and adjustment of the appliance The appliance shall be installed at a height of between 2 m and 2,5 m and initially adjusted in accordance with the requirements of 7.1. The test shall be carried out with the appliance adjusted to its nominal heat input or, in the case of a range-rated appliance, to its minimum and maximum nominal heat inputs 1(see 7.1.3.2.3 of EN 416-1: 1999) and supplied with one of the reference gases for the category to which the appliance belongs (see 7.1.1 of EN 416-1: 1999).
1 A test at the maximum nominal heat input need not be applied if it is known that the lowest radiant output is achieved at the minimum nominal heat input.
EN 416-2:2006 (E) 10 7.2.2.2 Apparatus 7.2.2.2.1 Mechanical apparatus In order to move the sensor positions, in an imaginary envelope around the appliance, a mobile, rigid test rig having a graduated, circular metal arc, with sensors attached, pivoted on its vertical axis is required.
The radius of the metal arc shall be within the range given in Figure 3. NOTE It is important to check that the maximum irradiance does not exceed the maximum value allowed for the instrument. 20º40º60º80º100º120º140º160º180º90º0º70º50º30º10ºR12432 Key
1 Removable radiation shield 2 Radiometer 3 Parallel 4 Meridian R Radius measured from the arc centre to the surface of the radiometer.
The radius shall be in the range 1,54 m to 1,88 m.
For any one measurement, the radius shall not vary by more than ± 20 mm Figure 3 — Test rig (Test method A) Test equipment shall: a) for an appliance with a length of more than 1,3 m, provide adjustment to allow the arc centre to coincide with either end of the reference plane; b) for an appliance with a length of 1,3 m or less, make the arc centre coincident with the centre of the reference plane (see Figure 1);
EN 416-2:2006 (E) 11 c) be installed in a test area with sufficient floor area to allow marking on the floor of the measurement positions; d) have a detachable or retractable radiation shield in front of each sensor to mask it from the appliance. The radiation shield shall be designed and arranged so that the surface of the shield facing the sensor is at thermal equilibrium under the ambient conditions of the working area (see 7.2.1.1). The general arrangement and construction of the radiation shields is given in Figure 4; e) provide an individual radiation shield for each sensor which does not reflect radiation towards any other sensor(s);
f) if appropriate, provide a guide rail to position the arc along the length of the appliance. 215 + - 5 mm3451> 270 mm> 270 mm Key
1 Radiation shield 2 Sensor 3 Reflective aluminium sheet 4 15 mm insulation (e.g. Rockwool or Polystyrene) 5 Matt black non-reflective surface Figure 4 — Radiation shield (Test method A) 7.2.2.2.2 Measurement apparatus 7.2.2.2.2.1 Sensor characteristics Sensors used shall : a) have a sensitivity factor that does not change by more than ± 3 % in an ambient temperature range of 15 °C to 30 °C; b) have a sensitivity that is constant either in the wavelength range 0,8 µm to 40 µm, or in another wavelength range which shall be stated in the test report (see 7.2.2.5)2); c) have a span angle ≥ 170 °C.
There shall not be a large variation in sensitivity with a change in the radiation angle of incidence;
2) This may be necessary for the purposes of calibration.
EN 416-2:2006 (E) 12 d) have a sensitivity which is constant within an irradiance range of 10 W/m2 to 1 100 W/m2; e) in order to eliminate the influence of draughts on the radiometer, a suitable window shall be installed and : 1) have a viewing angle ≥ 170°; 2) maximise radiation transmission in the range 2 µm to 9 µm. f) window correction factor (Fw) shall be calculated for each window (see Annex D). 7.2.2.2.2.2 Sensor positions The sensors shall be positioned (see Figures 3, 5a) and 5b)) such that: a) where a single sensor is used, it shall be capable of being moved along the length of the metal arc and of being positioned every 20° ± 1° (between 10° and 90°); b) where multiple sensors are used, they shall be positioned along the length of the arc every 20°± 1° (between 10° and 90°); c) measuring surface shall be tangential to the surface generated by movement of the metal arc. NOTE It is recommended that the face in front of the sensor thermopiles should be protected from irradiance and dust when not being used for taking measurements. Care should be taken to prevent accidental re-radiation from reflecting surfaces (e.g. white clothes and equipment not necessary for the test) within the 180° view of the radiometer.
EN 416-2:2006 (E) 13
170º150º130º110º90º70º50º30º10º90º70º50º30º10ºL/N½L/NL/NLL/NL/NL/N12ADetail A3½
Key
1 Meridian 2 Parallel 3 Identification of sensor positions A Sensor position L Reference surface cylinder length N Number of arc positions along the cylinder length a)
Integrating surface (Test method A) – Appliance greater than 1,3 m in length
EN 416-2:2006 (E) 14 90º70º50º30º10º12ADetail A380º120º140º160º180º100º200º220º240º260º280º300º320º340º60º40º20º0º Key
1 Meridian 2 Parallel 3 Sensor position notation A Sensor position b)
Integrating surface (Test method A) – Appliance less than 1,3 m in length Figure 5 — Appliance integrating surface (Test method A) 7.2.2.3 Working area Working areas shall: a) have walls and ceilings that are isolated from exterior influences (e.g. sunlight through windows and other heating equipment); b) have interior surfaces treated so as to reduce spurious radiation reflection (e.g. matt non-reflective surfaces);
EN 416-2:2006 (E) 15 c) be arranged such that the wall and ceiling temperatures do not change by more than ± 5 °C during the measurement phase of testing. 7.2.2.4 Procedure 7.2.2.4.1 Integration surface The integration surface (see Figures 5a) and 5b)) shall be the envelope generated by moving the arc such that: a) for an appliance less than or equal to 1,3 m long, the centre of the hemisphere shall be on the centre of the radiating reference surface; b) for an appliance of length greater than 1,3,m, it describes a half cylinder of length equal to the effective length of the emitter of which the axis coincides with the reference surface. This surface shall be terminated at its extremities by two half hemispheres; c) where the emitter is symmetrical (e.g. a linear tube) examination of the radiation shall be limited to : 1) in the case of an appliance of less than or equal to 1,3 m long, a quarter hemisphere (the result shall be multiplied by two); or 2)
in the case of an appliance of length greater than 1,3 m to a quarter cylinder plus two quarter hemispheres (the result shall be multiplied by two). 7.2.2.4.2 Measurement Connect each sensor to a millivoltmeter of the potentiometric type, electronic type or electronic device with an input impedance of at least 1 MΩ and a sensitivity of 1 µV. Make the measurements in a still atmosphere with the appliance in thermal equilibrium when operating under the adjustment conditions described in 7.2.2.1. NOTE It is important to measure the outside temperature of the instrument to ensure it is not being overheated. The measurement points shall be situated at the intersection of the parallels and the meridians (see Figures 5a) and 5b)) such that: a) for an appliance less than or equal to 1,3 m long, the measurement point shall be on the hemisphere and the intersections shall be at meridians 0°, 20°, 40° etc, up to 180°, with parallels 10°, 30°, 50° etc. up to 90° (See Figure 5b)); b) for an appliance of length greater than 1,3 m, the measurement point shall be on the half hemisphere (see Figure 5a)), the intersections at the extremities shall be at meridians 10°, 30°, 50° etc. up to 170°, with parallels 10°, 30°, 50° etc. up to 90°.
On the half cylinder required for a reference surface of length L for a number of measurements N, the intersections shall be at the points given by Expression (1). 35(21).2'2'2'2LLLNLNNNN−
(1) where : L is the reference surface length;
EN 416-2:2006 (E) 16 N is the number of measurements taken. with parallels 1
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