Paper submitted to ICAWS 2017: Topic 3 Sustainability of the measurements: Calibration, intercomparisons, laboratory and field performance tests, quality assurance and control assessment for traceable and nontraceable quantities (e.g. temperature and lightning). Quality assurance for sensors at the Deutscher Wetterdienst (DWD) Quality assurance on site / maintenance Holger Dörschel, Dr.-Ing. Tilman Holfelder 1 Introduction At the Deutscher Wetterdienst (DWD), the national meteorological service of Germany, the data for many meteorological parameters are collected automatically with data acquisition systems for decades, even if an observer has been present at the stations. In the nineties of the last century DWD renewed the technical infrastructure and the data acquisition systems at the international airports. In the following years the primary and secondary observing stations were harmonized and equipped with automated weather stations (AWS) and sensors in a project. During this period the maintenance department has been restructured with four service and logistic divisions in Hamburg, Offenbach, Potsdam and Munich and three minor service offices in Essen, Leipzig and Stuttgart. Each division and service office is responsible for regular preventive maintenance and troubleshooting of different types of manned and automated weather stations and several remote sensing systems. Figure 1: Service locations of DWD 1
Within the next years, by 2021, the DWD measuring network will be fully automated. As all Stations are already equipped with AWS only minor changes have to be done for automation, but quality assurance of the measurements will remain a very important task. One part of this quality assurance is an adequate maintenance concept including regular field checks and calibration of the sensors in the laboratory. To achieve the requested high availability of meteorological data from the measurement systems and to assure the requested high quality of the meteorological data several requirements need to be considered. Specific technical know-how for the meteorological measurement systems and the process of data acquisition has to be obtained. A short response time in case of technical failures, especially for the weather radar network and AWOS at international airports is needed, And of course the specific know-how for procurement and operation of the meteorological measurement systems has to be preserved. To fulfil these requirements DWD has its own technical staff for the maintenance of the meteorological observing systems, located at the before mentioned locations within Germany, to reduce travel costs and achieve short reaction times. DWD has the necessary specific facilities to support the technical staff like stock at each division of the department, electronical and mechanical workshops and calibration laboratories for meteorological sensors in Hamburg and Munich. 2 Actual maintenance concept The first step for the assurance of data quality is a regular technical maintenance at the sites. Depending on the type of weather station, the interval range varies from several months at primary observing stations (automated and manned) to at least once a year at some automated secondary observing stations with only a few sensors. In Fehler! Verweisquelle konnte nicht gefunden werden. the number of stations maintained by each service and logistic division and the actual interval range for maintenance or inspection for each station type is given. Table 1: Maintenance per year depending on the type of station To assure that the technical staff in each division is using the same procedures for maintenance, responsible engineers for every type of weather station and sensor type are nominated. They issue the necessary papers for handling, installation and maintenance of the technical equipment. 2
To assure a high quality of the measurements, most of the sensors are exchanged regularly with calibrated sensors (pressure, temperature, humidity, wind, radiation and precipitation). After refurbishing at DWD workshops the dismantled sensors are recalibrated in a laboratory and marked with a sticker with the date of calibration on it and then stocked. Additional field checks are performed at each station visit, for example with a pressure transfer standard. If an obvious malfunction or inaccuracy has been determined during maintenance the sensor will be exchanged. Via remote maintenance access all weather stations are checked regularly for technical issues. Any malfunction will be investigated and if needed an additional maintenance onsite will be initiated. Additional service is required if the post-processing with the interactive data control and monitoring system QualiMET indicates malfunctions or if a meteorological observer reported a problem. The technical staff is using a computer based database including mainly the primary and secondary observing stations with their specific instrumentation. Every maintenance work, either remote or on site, is documented in this database too. For each sensor the next date for calibration is calculated automatically, depending on the date of the last calibration and the calibration interval of the sensor type. So the database is indicating the sensors which have to be exchanged at the next inspection. For long time planning of maintenance trips a calendar overview is included too. 3 First definition and further development of calibration intervals Even if the DWD has been operating calibration facilities for several decades a regular and systematic exchange, refurbishing and calibration of sensors has started in 2006. To archive the requirements of the ISO9001 certification regarding to the control of monitoring and measuring devices (old chapter 7.6 of ISO 9000) DWD introduced a news system. New relevant documents have been added: a document with rules for calibration and traceability a document with rules for the achievement of valid calibration a document with a table of the valid time after calibration Regular calibration intervals for a variety of sensors were defined. The decision for the individual calibration interval depends on the type of sensor, the experiences of its stability, manufacturer s instructions and the capacity of the calibration laboratories. Some examples are shown in Fehler! Verweisquelle konnte nicht gefunden werden.. Type calibration interval in month value in 2006 actual value Pt100 air thermometer 60 60 Pt100 soil thermometer 120 not regularly Cup anemometer 24 24 Ultrasonic anemometer 24 36 Pressure gauge 18 24 Humidity transmitter 18 18 Weighing rain gauge 48 60 Table 2: Defined calibration interval for different sensors in 2006 and 2017 3
Comparing the two columns in Fehler! Verweisquelle konnte nicht gefunden werden. it can be seen, that only minor changes were made to these intervals. In all cases, the intervals have been slightly increased due to the analysis of the sensor stability or special requirements of service have to be respected. For example in 2013 the calibration interval for the soil thermometers was investigated, because an exchange of soil thermometers has an impact on the surrounding soil and in consequence on the homogeneity of the measurement data. This effect might be larger than the possible drift of the sensor itself after the originally defined calibration interval of 10 years. As high class Pt 100 sensors have in general a very good long time stability, the actual drift of some sensors has been determined. Seven soil thermometers were recalibrated after an operation period of ten years. Figure 2: Result of recalibration of soil thermometers after 10 years of operation After ten years all soil thermometers were within a tolerance of ±0.1 Kelvin. In addition to that 22 other soil thermometers, which have been stored in the warehouse, were recalibrated also. Only two sensors had a tolerance up to 0,2K, the other were within a tolerance of ±0,1 Kelvin too. In consequence it was decided that soil thermometers will not have to be recalibrated after installation. They will only be replaced, if they have a malfunction. Another example is the calibration interval of pressure gauges. DWDs first calibration interval was set to 18 month. In 2014 a bigger sample was investigated. With a few exceptions all pressure gauges were within the accepted tolerance of ±0.15 hpa in the relevant pressure range (700-1050 hpa) so DWD decided to increase the interval to 24 month. 4
Figure 3: Result of recalibration of pressure gauges after 24 month of operation After improving the equipment in the laboratories a review of the calibration interval was carried out in September 2017. All pressure gauges of Vaisala PTB220 type were within the accepted tolerance. Figure 4: Result of recalibration of pressure gauges after 24 month of operation 5
DWD is using weighing rain gauges for more than 10 years. In 2013 the calibration interval has been enhanced from 4 to 5 years, because of a good long term stability. In 2017 DWD will replace this rain gauge with a new sensor type. Based on the manufacturers proposal, the calibration interval has been defined to 48 month. This proposal will be investigated in the future, because DWD has only limited experiences with the stability of this rain gauge. The installation of the sensors started in September 2017. To prove this interval the calibration laboratories will calibrate a sample from different locations after an operation period of 12, 24, 36 and of course 48 month. Hopefully the calibration interval of 48 month can be confirmed. 4 Development of the quality system at the calibration laboratory To achieve best results and traceability of the measurements, the calibration laboratory has been subjected the accreditation process according to DIN EN ISO/IEC 17025:2005 with the status as a testing laboratory (2007-2016). In 2015 DWD decided to become accredited as a calibration laboratory according to DIN EN ISO/IEC 17025:2005. This accreditation process has been finalized now, so the status has been changed to an accredited calibration laboratory in September 2017 (s. Fehler! Verweisquelle konnte nicht gefunden werden.). The accredited parameters are temperature, relative humidity, absolute pressure, wind speed and direction. 6
Figure 5: The accreditation certificate issued by the German accreditation body DAkkS. All instruments in the laboratory are regularly calibrated and the measurements are traceable to national standards provided by the National Metrological Institute of Germany (PTB). The following table gives an overview of the capabilities of the calibration laboratory with its two locations in Hamburg and Oberschleißheim (near Munich). Instrument Undergoing Calibration Calibration Range Reference wind speed sensors 0,5 50 ms -1 anemometer 2D laser doppler burst spectrum analyzer Calibration and Measurement Capability (CMC) 1,2 % of reference but not smaller than 0,1 ms -1 wind direction sensors 0 360 precision turntable 0,9 Pt100-30 +40 C special selected IPRT or SPRT 50 mk temperature transmitter -30 +40 C IPRT with electronic thermometer 60 mk 7
heated and unheated humidity transmitters and probes 15 95 % of relative humidity chilled mirror dewpoint meter 0,1 + 0.006 x U in % of reference but not smaller than 0,4 % absolute pressure transmitters 100 2500 hpa piston gauge, vacuum gauge Table 3: Capabilities of the accredited DWD calibration laboratories 0,0025 hpa + 0,00002 x P + uncertainty of the reference vacuum measurement It is verified whether the calibration of rain gauges could be accredited in future, too. 8