BS-ISO-17714-2007.pdf

BRITISH STANDARD BS ISO 17714:2007 Meteorology Air temperature measurements Test methods for comparing the performance of thermometer shields/screens and defining important characteristics ICS 07.060 ? Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI BS ISO 17714:2007 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 July 2007 © BSI 2007 ISBN 978 0 580 54322 7 National foreword This British Standard is the UK implementation of ISO 17714:2007. The UK participation in its preparation was entrusted to Technical Committee EH/2, Air quality. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity from legal obligations. Amendments issued since publication Amd. No. DateComments Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI Reference number ISO 17714:2007(E) INTERNATIONAL STANDARD ISO 17714 First edition 2007-07-01 Meteorology Air temperature measurements Test methods for comparing the performance of thermometer shields/screens and defining important characteristics Météorologie Mesurage de la température de l'air Méthodes d'essai pour comparer les performances d'abris/d'écrans pour thermomètres et définir les caractéristiques importantes BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI ii Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Terms and definitions. 1 3 Field test conditions. 2 3.1 Field test site. 2 3.2 Respective location of screens. 2 3.3 Screens 3 3.4 Thermometers. 3 3.5 Additional meteorological variables. 3 3.6 Data sampling . 4 3.7 Reference values 4 3.8 Quality control. 4 3.9 Long-term intercomparison. 5 3.10 Typical conditions 5 3.11 Documentation 6 4 Field test methods 6 4.1 Comparability analysis. 6 4.2 Global analysis of air temperature 6 4.3 Analysis of extreme values 6 4.4 Statistical analysis of radiation error 7 4.5 Influence of surface albedo . 7 4.6 Selection of typical conditions 7 5 Measurement of screen characteristics. 8 5.1 Aspiration rate. 8 5.2 Internal ventilation factor. 8 5.3 Representative height and screen reference point. 8 5.4 System response time 8 Annex A (informative) Influence factors 10 Annex B (informative) Examples of useful test-report graphs 13 Bibliography. 18 BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI iv Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 17714 was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5, Meteorology. BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI v Introduction Commonly used air temperature sensors require protection from influences such as solar and terrestrial radiation, rain and snow. Screens (also known as shields) protect the thermometers from these influences. At present, there is no commonly accepted reference screen design, nor are there generally accepted test methods to determine performance characteristics of screens. Screens that protect the temperature sensors from daytime heating and nighttime cooling due to radiation transfer are necessary for proper air temperature measurements. In very general terms, a poor design of the screen will tend to give higher daytime and lower nighttime temperatures. This International Standard was developed to define the most relevant screen characteristics and to provide the methods to determine or compare screen performances. Air temperature is a basic meteorological variable. Temperature sensors are widely used in all human activities and are well known and controlled. For the measurement of the outside air temperature, the sensor must be protected against external influence, mainly radiation and hydrometeors (e.g. precipitation, fog). The sensor is usually protected by a screen, but even then, measurement errors of up to 5 K may be encountered. The general function of a screen for operational temperature measurements used in meteorological applications is given in WMO No. 8 3. The following text is an extract from this document. “Radiation from the sun, clouds, the ground and other surrounding objects passes through the air without appreciably changing its temperature, but a thermometer exposed freely in the open can absorb considerable radiation. As a consequence, its temperature may differ from the true air temperature, the difference depending on the radiation intensity and on the ratio of absorbed radiation to dissipated heat. For some thermometer elements such as the very fine wire used in an open-wire resistance thermometer, the difference may be very small or even negligible, but with the more usual operational thermometers the temperature difference may reach 25 K under extremely unfavourable conditions. Therefore, in order to ensure that the thermometer is at true air temperature it is necessary to protect the thermometer from radiation by a screen or shield which also serves to support the thermometer. This screen also shelters it from precipitation while allowing the free circulation of air around it, and prevents accidental damage. Maintaining a free circulation may, however, be difficult to achieve under conditions of rime ice accretion. Practices for reducing observation errors under such conditions will vary and may involve the use of special designs of screens or temperature- measuring instruments.” BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI blank Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 1 Meteorology Air temperature measurements Test methods for comparing the performance of thermometer shields/screens and defining important characteristics 1 Scope This International Standard defines characteristics of a thermometer shield/screen. It also defines test methods to inter-compare the behaviour of different screen designs. Although screens are usually used for both air temperature and humidity measurements, this International Standard is applicable only to temperature measurements. Both naturally and artificially ventilated screens are considered. 2 Terms and definitions For the purposes of this document, the following terms and definitions apply. 2.1 aspiration rate rate of air flow passing the thermometer. NOTE This term is only used for an artificially aspirated screen, and is expressed in metres per second. 2.2 internal ventilation factor ratio between the internal air speed and the external wind speed, at the thermometer height NOTE This term is only used for a naturally ventilated screen. 2.3 representative height height above ground at which the air temperature is supposed to be measured NOTE 1 For naturally ventilated screens, the representative measurement height is usually the height of the temperature sensor. NOTE 2 For aspirated screens, the representative height can be different from the height of the temperature sensor. It is design dependent, however it is usually the height of the air intake. 2.4 screen shield or shelter used to protect the thermometer from radiation, precipitation and accidental damage 2.5 screen reference point location of the thermometer within the screen BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 2 2.6 solar radiation errors overheating error of the measured air temperature, generated by solar radiation 2.7 system response time time needed for the temperature recorded by the thermometer within the screen to reach 63 % of a step change in the external temperature, with a given external wind speed of 1 m·s1 NOTE 1 The system response time is a combination of the response times of the screen and the thermometer, and depends on the thermometer time constant. NOTE 2 The response time of the system is also dependent on wind speed. For this reason, a given air speed of 1 m·s1 is used. 2.8 thermometer device used to measure the air temperature inside a screen NOTE Examples are platinum resistance thermometer sensors (IEC 60751 1) and thermistor sensors (ASTM E 644-04 2). 3 Field test conditions 3.1 Field test site The test site should experience the range of meteorological conditions that are expected at the sites where the screens will be installed. Important influence factors are: radiation, low wind speed, periods with snow cover, falling and blowing snow, blowing dust or sand, wet fog, strong winds and wind-driven precipitation. It may be necessary to perform testing at more than one site, to address the full range of meteorological conditions. The meteorological conditions occurring at the site during the intercomparison shall be described, with at least the temperature, wind and daily insolation distribution. 3.2 Respective location of screens For the field test, all the screens shall be installed above a level area covered by homogeneous natural ground cover. The vegetation at the test site should completely cover the surface and the type of vegetation shall be defined. It should be kept at a height below 10 cm. All screens should be freely exposed to sunshine and wind and should not be shielded by, or close to, trees, buildings and other obstructions (see A.8). The screens should be installed at a minimum distance of 30 m from any heat source or other construction that could artificially influence the air temperature, such as concrete, asphalt, buildings, standing water, etc. The separation between screens is a compromise, and should be large enough to ensure that interaction between screens is insignificant, while being small enough to minimise temperature variations across the site. The distance between each screen should be at least 3 m. A representative height between 1,25 m and 2 m should be chosen for the test to meet the WMO recommendations. The representative height shall be the same for each screen, with a maximum tolerance of ± 5 % of the height. When testing artificially ventilated screens, the probe orientation and the inlet/outlet orientations shall be documented, as wind direction may influence the aspiration rate. BS ISO 17714:2007 Licensed Copy: London South Bank University, London South Bank University, Wed Aug 08 03:51:12 GMT+00:00 2007, Uncontrolled Copy, (c) BSI 3 3.3 Screens Two or more screens of each design should be included in the test. This allows assessment of measurement repeatability of a given screen design and also for measurement of homogeneity of the test site. At least two reference screens with identical thermometers shall be used. During an intercomparison, observations of the screens are required to determine if they are wet, covered with ice, clogged with snow, dirty, if the aspirator (if any) is working, etc. 3.4 Thermometers The thermometers and associated data acquisition system(s) should be matched to ensure equivalent response characteristics, such as the time constant. WMO 3 recommends the use of thermometers with a time constant of about 30 s. The sensors and measuring system shall be calibrated and used in such conditions that there is no significant self heating of the thermometer due to an excessive measuring current. The uncertainty of the temperature measurements shall be 0,2 K or better. 3.5 Additional meteorological variables In addition to the air temperature measurements, the following parameters should be measured and recorded during an intercomparison. Solar global radiation on a horizontal plane. Sunshine (yes/no). Scalar average 1 min or 2 min wind speed and direction. This should be measured at a position that is as close as possible to the representative height of the temperature measurement. Where the test array of screens can shelter the anemometer in certain wind directions, then it is preferred that the anemometer is raised slightly above the level of the top of the screens and this measurement height recorded. The anemometer used should be capable of calibrated measurements of wind speeds at 0,5 m·s1 and above. Sonic anemometers may be good candidates for these measurements. Scalar average 1 min or 2 min wind speed and direction at 10 m height (standardized meteorological height). Wind measurements at 10 m allow a comparison between meteorological conditions during the test and climatological conditions (as 10 m measurements are currently used in climatological records). Relative humidity of air. Cloud cover. Precipitation occurrence, type and intensity. Manual observations may be required to determine precipitation occurrence and type. Sun azimuth and elevation angle. Surface albedo (ground conditions, snow cover, etc.) It is also desirable to measure direct solar radiation, and long-wave net radiation or cloud cover at night. The type of instruments used and their siting shall be documented. The ins