EN-61207-2-1994.pdf

EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM EN 61207-2 June 1994 UDC 621.317.79:543.27:543.25 Descriptors: Gaseous mixtures, oxygen in gaseous mixtures, gas analyzers, performance of gas analyzers, high temperature electrochemical sensors English version Expression of performance of gas analyzers Part 2: Oxygen in gas (utilizing high-temperature electrochemical sensors) (IEC 1207-2:1994 + corrigendum 1994) Expression des qualités de fonctionnement des analyseurs de gaz Partie 2: Oxygène contenu dans le gaz (utilisant des capteurs électrochimiques à haute température) (CEI 1207-2:1994) Angabe zum Betriebsverhalten von Gasanalysatoren Teil 2: Sauerstoff in Gas (unter Verwendung von elektrochemischen Hochtemperatur-Sensoren) (IEC 1207-2:1994) This European Standard was approved by CENELEC on 1994-05-15. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member. This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions. CENELEC members are the national electrotechnical committees of Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom. CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung Central Secretariat: rue de Stassart 35, B-1050 Brussels © 1994 Copyright reserved to CENELEC members Ref. No. EN 61207-2:1994 E EN 61207-2:1994 2 Foreword The text of document 65D(CO)3, as prepared by Subcommittee 65D: Analyzing equipment, of IEC Technical Committee 65: Industrial-process measurement and control, was submitted to the IEC-CENELEC parallel vote in October 1993. The reference document was approved by CENELEC as EN 61207-2 on 15 May 1994. The following dates were fixed: For products which have complied with the relevant national standard before 1995-05-15, as shown by the manufacturer or by a certification body, this previous standard may continue to apply for production until 2000-05-15. Annexes designated “normative” are part of the body of the standard. Annexes designated “informative” are given only for information. In this standard, Annex ZA is normative. Contents Page Foreword2 Introduction3 1Scope3 2Normative references3 3Definitions3 4Procedures for specification5 4.1Specification of essential units and ancillary services5 4.2Additional terms related to the specification of performance5 4.3Important terms related to the specification of performance5 5Procedures for compliance testing6 5.1General6 5.2Testing procedures6 5.3Output fluctuation6 5.4Delay time, rise time and fall time6 Annex ZA (normative) Other international publications quoted in this standard with the references of the relevant European publications10 Figure 1 General test arrangement, in situ analyser8 Figure 2 General test arrangement, extractive analyzer9 latest date of publication of an identical national standard(dop) 1995-05-15 latest date of withdrawal of conflicting national standards(dow) 1995-05-15 EN 61207-2:1994 3 Introduction This part of IEC 1207 includes the terminology, definitions, statements and tests that are specific to oxygen analyzers, which utilise high-temperature electrochemical sensors. Oxygen analyzers employing high-temperature electrochemical sensors operating at temperatures usually in excess of 600 °C, have a wide range of applications for the measurement of oxygen in gas samples. Such samples are typically the result of a combustion process. Two main types of analyzer exist, the in situ analyzer, where the sensor is positioned within the process duct work, and the “extractive” analyzer, where the sample is drawn from the duct via a simple sample system and presented to the sensor. An analyzer will typically comprise a sensor head, mounted on the process duct, and a control unit remotely mounted, with interconnecting cable. 1 Scope This part of IEC 1207 applies to all aspects of analyzers using high-temperature electrochemical sensors for the measurement of oxygen in gas. It should be used in conjunction with IEC 1207-1. It applies to in-situ and extractive analyzers and to analyzers installed indoors and outdoors. The object of this part is: to specify the terminology and definitions related to the functional performance of gas analyzers, utilizing a high-temperature electrochemical sensor, for the continuous measurement of oxygen concentration in a sample of gas; to unify methods used in making and verifying statements on the functional performance of such analyzers; to specify what tests should be performed to determine the functional performance and how such tests should be carried out; to provide basic documents to support the application of standards of quality assurance ISO 9001, ISO 9002 and ISO 9003. 2 Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of this part of IEC 1207. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this part of IEC 1207 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. IEC 654, Operating conditions for industrial-process measurement and control equipment. IEC 1207-1:1994, Expression of performance of gas analyzers Part 1: General. 3 Definitions 3.1 High-temperature electrochemical sensor The high-temperature electrochemical sensor can be constructed in two basic forms: a) Galvanic concentration cell. b) Ion pump cell. 3.1.1 galvanic concentration cell most commercially available analyzers employ the galvanic concentration cell consisting of two gas chambers, separated by an oxygen ion conducting solid electrolyte, and provided with a porous electrode on each side NOTE 1Platinum is frequently used for the electrodes, and the ceramic electrolyte is usually zirconium oxide, fully or partially stabilized with yttrium oxide, calcium oxide or thorium oxide, which when heated above 600 °C, allows the charge transfer mechanism to be predominantly oxygen ion conduction. NOTE 2When the sensor is brought to a temperature at which the solid electrolyte conducts oxygen ions and the e.m.f. between the two electrodes is measured, the output will be related to the logarithm of the ratio of the partial pressures of oxygen at each of the electrodes in accordance with the Nernst equation: (1) (2) (3) where P1is the partial pressure of oxygen in the reference gas; P2is the partial pressure of oxygen in the sample gas; Eis the electromotive force output from the cell in V; Ris the gas constant (8,3144 J K1 mol1); Tis the absolute temperature (K); Fis the Faraday constant (96,484 56 × 103 C mol1); kis the Nernstian coefficient (slope factor). EN 61207-2:1994 4 Thus, provided the oxygen partial pressure is known at one electrode (P1), then the potential difference between the two electrodes will enable the unknown oxygen pressure to be determined at the other electrode (P2). The Nernstian response of the high-temperature electrochemical ceramic sensor holds over a very wide range of oxygen partial pressures differences, and the sensor output increases logarithmically with linear reduction of the oxygen partial pressure at a given temperature. The sensor output is directly proportional to temperature, and hence for quantitative analysis, the temperature of the cell should be closely controlled or measured, and the necessary corrections applied in equation (1). NOTE 3Zero offset Theoretically the output e.m.f. of the sensor, when the partial pressures of the sample gas and reference gas are equal, is zero volts. In some sensors a zero offset is measured and is considered largely due to thermoelectric effects, and thermal gradients across the electrodes. This offset can be considered theoretically as an extra constant (asymmetry potential). Non-ideal oxygen ion conduction can also be compensated for by introducing modifications to the slope factor k. In practice, manufacturers whose sensors exhibit zero offset may supply practical average values of U to help in calibration. Modern equipment will automatically compensate the asymmetry potential during air point calibration (i.e. air in both chambers). 3.1.2 ion pump cell If a direct current is made to flow between the electrodes of a cell, with air in one chamber and an inert gas in the other chamber, the current flow will cause a pumping of oxygen molecules from one side to the other. The action obeys Faradays laws and the quantity of oxygen pumped by diffusion into the inert gas is given by: This is used generally in two basic configurations. 3.1.2.1 limiting current a diffusion pinhole limits the rate of arrival of oxygen molecules at the measuring electrode, and a constant voltage across the electrodes ensures that all the oxygen arriving at the measuring electrode is pumped to the other side. The current generated is quantitatively related to the number of oxygen molecules transferred 3.1.2.2 fixed volume this configuration consists of two sets of electrodes arranged across a small fixed volume. The first set comprises a concentration cell, the second set the ion pump. The volume is initially swept of oxygen molecules to a predetermined low level. Pump action is then initiated until the concentration cell reading shows that the oxygen concentration in the volume and that outside at the sample side, are the same. The current and time required to achieve this are related to the oxygen concentration of the sample gas 3.2 reference gas all analyzers using the high-temperature electrochemical concentration cell require a reference sample of known and constant composition usually air is employed NOTEThe sensor output is a function of the partial pressure of oxygen in the sample, provided the reference has a constant partial pressure of oxygen. 3.3 in situ analyzer the in situ analyzer has the high-temperature electrochemical sensor situated in the sample; however the sensor may require a filter to remove particulates one version of the in situ analyzer controls the temperature of the sensor in the range 600 °C to 800 °C. In this case the sample temperature cannot exceed the control temperature. The second version relies on the temperature of the sample to attain the operating temperature. It is then necessary to measure the sensor temperature to enable the oxygen value to be calculated 3.4 extractive analyzer in the “extractive” analyzer the sensor head is installed outside the gas stream to be measured, and the sample is drawn through a sample probe and presented to the sensor which is maintained at a controlled temperature to ensure ionic conduction (typically in the range 600 °C to 800 °C) the extractive analyzer may require a filter to remove particulates, and a driving force (often an aspirator) to move the sample. The pipework involved should be minimized and maintained above the dew-point of any condensible species to prevent formation of any condensation (4) (5) where UTis the asymmetry potential (mV). (6) where Qis the quantity of oxygen pumped in mol s1; Iis the current (A); Fis the Faraday constant (96,484 56 × 103 C mol1). EN 61207-2:1994 5 3.5 hazardous area an area where there is a possibility of release of potential flammable gases, vapours or dusts 3.6 flametrap a device used to prevent a flame, resulting from the ignition of a flammable gas mixture, from propagating 3.7 essential ancillary units essential ancillary units are those without which the analyzer will not operate (e.g. pumps for aspirators, calibration systems, etc.) 4 Procedures for specification The procedures for specification are detailed in IEC 1207-1. This covers: operation and storage requirements; specification of ranges of measurement and output signals; limits of errors; recommended reference values and rated ranges of influence quantities. In this part of IEC 1207, specifications of ranges for ancillary equipment are given. Additional terms for specification of performance, and important aspects of performance relevant to high-temperature electrochemical sensors are also detailed. 4.1 Specification of essential units and ancillary services All oxygen analyzers utilizing high-temperature electrochemical concentration cells require a reference gas supply. This is usually air, filtered to remove moisture and oil. Analyzers require facilities for calibration after installation. Bottled calibration gases and pressure regulation facilities are generally required. 4.1.1 Rated range of reference gas pressure Reference gas pressure in practice may have small effects on error. Also the reference gas pressure will affect reference gas flow. High flows can cause cooling of electrodes and subsequent errors. 4.1.2 Rated range of calibration gas pressure Calibration gas pressure may have small effects on error. Also calibration gas pressure will affect calibration gas flow in a similar manner as described in 4.1.1. 4.1.3 Rated range of aspirator gas pressure For analyzers employing aspirators, the rated range of aspirator gas pressure is required to ensure correct sample flow (and sometimes reference air flow). 4.2 Additional terms related to the specification of performance The following additional statements may be required to define the performance of the analyzer. Dependent on the design details, some of these additional terms may be omitted. 4.2.1 Hazardous classification of the area in which the sensor head and electronic unit are to be located. General purpose analyzers will not be suitable for location in hazardous areas. 4.2.2 As the high-temperature electrochemical sensor is a potential ignition source, the additional statement on the permissible level of flammable gas in the sample is required. NOTEMany analyzers are designed to prevent ignition of the sample gas, for example by using flametraps. 4.2.3 Sensor life expectancy The high-temperature electrochemical sensor has a finite life expectancy and will require occasional replacement. The actual cell life will be dependent on the sample. 4.3 Important terms related to the specification of performance Although covered in IEC 1207-1, the following terms are particularly relevant. 4.3.