DIN ISO 17714-2008 Meteorology - Air temperature measurements - Test methods for comparing the performance of thermometer shields screens and defining important characteristics (IS0.pdf

1、June 2008DEUTSCHE NORM English price group 13No part of this standard may be reproduced without prior permission ofDIN Deutsches Institut fr Normung e. V., Berlin. Beuth Verlag GmbH, 10772 Berlin, Germany,has the exclusive right of sale for German Standards (DIN-Normen).ICS 07.060; 17.200.20!$OC:“14

2、43223www.din.deDDIN ISO 17714Meteorology Air temperature measurements Test methods for comparing the performance of thermometershields/screens and defining important characteristics (ISO 17714:2007)English version of DIN ISO 17714:2008-06Meteorologie Messungen der Lufttemperatur Prfverfahren zum Lei

3、stungsvergleich von Strahlungsschutzeinrichtungen beiThermometern und Festlegung der charakteristischen Eigenschaften (ISO 17714:2007)Englische Fassung DIN ISO 17714:2008-06www.beuth.deThis standard has been included in the VDI/DIN Handbook on air quality, Volume 1 B. Document comprises 23 pagesDIN

4、ISO 17714:2008-06 2 Contents Page National foreword 2 Introduction 3 1 Scope 5 2 Terms and definitions .5 3 Field test conditions6 3.1 Field test site6 3.2 Respective location of screens6 3.3 Screens.7 3.4 Thermometers7 3.5 Additional meteorological variables7 3.6 Data sampling 8 3.7 Reference value

5、s .8 3.8 Quality control8 3.9 Long-term intercomparison9 3.10 Typical conditions .9 3.11 Documentation 10 4 Field test methods 10 4.1 Comparability analysis. 10 4.2 Global analysis of air temperature 10 4.3 Analysis of extreme values 10 4.4 Statistical analysis of radiation error . 11 4.5 Influence

6、of surface albedo . 11 4.6 Selection of typical conditions 11 5 Measurement of screen characteristics.12 5.1 Aspiration rate 12 5.2 Internal ventilation factor. 12 5.3 Representative height and screen reference point. 12 5.4 System response time 12 Annex A (informative) Influence factors 14 Annex B

7、(informative) Examples of useful test-report graphs 17 Bibliography. 22 3 National foreword This standard has been prepared by Technical Committee ISO/TC 146 “Air quality”, Subcommittee SC 5 “Meteorology”. The responsible German body involved in its preparation was the Kommission Reinhaltung der Luf

8、t (KRdL) im VDI und DIN (VDI/DIN Air Quality Commission), Section II Umweltmeteorologie. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. DIN shall not be held responsible for identifying any or all such patent rights. The DIN Stan

9、dard corresponding to the International Standard referred to in this document is as follows: IEC 60751 DIN EN 60751 National Annex NA (informative) Bibliography DIN EN 60751, Industrial platinum resistance thermometer sensors DIN ISO 17714:2008-06 Introduction Commonly used air temperature sensors r

10、equire 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 perf

11、ormance 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 n

12、ighttime 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

13、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

14、 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 ai

15、r 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 di

16、ssipated 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

17、. 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 aro

18、und 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-measurin

19、g instruments.” Meteorology Air temperature measurements Test methods for comparing the performance of thermometer shields/screens and defining important characteristics 4 DIN ISO 17714:2008-06 1 Scope This International Standard defines characteristics of a thermometer shield/screen. It also define

20、s 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 consider

21、ed. 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 fa

22、ctor 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 screen

23、s, 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

24、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 5 DIN ISO 17714:2008-06 2.6 solar radiation errors overheating error of the measured air temperature, generated by solar radiation 2

25、.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 ms1NOTE 1 The system response time is a combination of the response times of the screen and the ther

26、mometer, 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 ms1 is used. 2.8 thermometer device used to measure the air temperature inside a screen NOTE Examples are platinum resistance thermom

27、eter 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

28、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 t

29、he 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 th

30、e 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

31、 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 in

32、teraction 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 represen

33、tative 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. 6 DIN ISO 17714:2008-06 3.3

34、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 interc

35、omparison, 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 charact

36、eristics, such as the time constant. WMO3recommends 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 unc

37、ertainty 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/

38、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 tha

39、t 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 ms1and above. Sonic anemometers may be good candidates for these measurements. Scalar average 1 m

40、in 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 humidit

41、y 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 l

42、ong-wave net radiation or cloud cover at night. The type of instruments used and their siting shall be documented. The instruments used shall be calibrated and regularly maintained, and should be installed and used following the recommendations of WMO3. 7 DIN ISO 17714:2008-06 3.6 Data sampling When

43、 making the screen intercomparison, a data base of all measurements should be constructed to represent averages from samples taken during 1 min periods. The data sampling rate should be at least 6 samples per minute. For wind measurements, the data sampling rate should be at least 1 Hz. 3.7 Referenc

44、e values There is no recognised reference system for measuring the true air temperature. Statistically, radiative errors of any screen lead to warmer (than the true air temperature) temperature measurement during the day, and cooler temperature measurement during the night. So when different screen

45、designs are compared, those that are cooler during the day and warmer during the night are likely to be giving measurements that are closest to the truth. By design, it is generally the case that there will be a fast response thermometer inside an artificially ventilated screen. However, not all des

46、igns of artificially ventilated screens that are available on the market are satisfactory for use as a reference. If an artificially ventilated screen is to be used as a reference, then its performance shall be fully tested. Check for potential psychrometric cooling effects after high humidity event

47、s or dew deposition. Check the effect of the ambient wind on the airflow in an artificially ventilated screen, as designs have been found where airflow across the thermometer is reduced or reversed in some wind conditions. Check if a large aspiration rate may cause heating of the air. Check if the a

48、ir exhausted by the fan may re-circulate and influence the incoming air. Check the sensor and screen for dirt and contamination that could obstruct the airflow or influence the temperature. A potential candidate for measuring the reference air temperature is the use of a very thin resistive wire (th

49、ickness 15 m,4, 5in the Bibliography), acting as a thermometer. If it is very thin, the wire may be exposed freely to the air, with no radiation screen. The problem with this technique is how to design a wire that has stable characteristics and will survive for long enough periods outside in all weather conditions; a suitable sensor has yet to be designed. If the purpose of the intercomparison is to check the behaviour of new designs against a specific screen model widely used (in a country and/or in the past), this screen may be