TESTING METEOROLOGICAL SENSORS ON SHIPS SENSORS FOR SHIPBORNE AUTOMATIC WEATHER STATIONS. Simone Griesel

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1 P (4) TESTING METEOROLOGICAL SENSORS ON SHIPS SENSORS FOR SHIPBORNE AUTOMATIC WEATHER STATIONS Simone Griesel Deutscher Wetterdienst, Frahmredder 95, 9 Hamburg, Germany simone.griesel@dwd.de ABSTRACT Beside Voluntary Observing Ships (VOS) and data buoys an increased level of automation of weather observations are recommended by EUMETNET. In order to increase the availability and quality of data a common automatic weather station on ships (S-AWS) will be installed by several European Weather Services from 6 on. This work presents the sensors used by the German Meteorological Service (DWD) to be installed on these ships. Meteorological sensors, such as anemometers, temperature and humidity probes, as well as screens were tested on the German research vessel METEOR. Results of the measurement campaigns under the harsh conditions that occur in the marine environment will be shown.. INTRODUCTION A wide range of sensors and transmitters for marine applications is available on the market nowadays. Since the first E-SURFMAR design study 4 agreed by EUMETNET [] recommended increased levels of automation of weather observations, several ships have been equipped with shipborne automated weather station (S-AWS) systems and sensors of different types and capability. Because the German Meteorological Service (DWD) wants to equip vessels with a standardized infrastructure, tenders for anemometer, barometer, temperature and humidity sensors as well as screens have been organized during the last years. While barometer and temperature as well as humidity sensors were equal to the ones used in the surface network of the DWD, anemometers and screen had to achieve special requirements for ship installation. The first paragraph of this paper describes the selected sensors in detail. In the second paragraph results from a field test of these sensors are presented. Two research vessels are stuffed by DWD forecasters and observers. One of them, the FS METEOR operated by the University of Hamburg, is equipped with sensors from the surface network of DWD. For temperature, humidity and wind the possibility to replace the analogue sensors by the probes of a digital transmitter and an ultrasonic wind anemometer should be investigated. The impact of different size and ventilation of weather shields should also be examined.

2 . DESCRIPTION OF THE SENSORS USED BY DWD Table gives an overview of the technical details of the sensors to be installed with S-AWS specified by the manufacturers. Table : Technical details of the sensors used for EUCAWS Parameter Sensor name Manufacturer Sensor type unit / value uncertainty Dimension Material Housing protection class Wind Ventus VA G. Lufft Mess- und Regeltechnik unltrasound without heating ±. m/s or ± % RMS sampling rate s wind speed [m/s] wind direction angular dimension [ ] < (> m/s) RMS Ø 5 x 7 mm weight.6 kg Aluminum AlMgSi IP66 Air pressure PTB Vaisala Oyi BAROCAP sensor silicon membrane barometric pressure Pascal [hpa] ±.5 hpa weight kg AlSiMg IP 66 Temperatur EE E+E Elektronik Pt Grad Celsius [ C] DIN 675 class AA Probe : 89 mm, Ø 6 mm Probe: stainless steel IP 65; NEMA 4 Housing: 5 x 9 x 66,5 mm Housing: Polycarbonate Humidity EE E+E Elektronik heated monolithic measurement relative humidity [%RH] % RH for T > - C Probe : 6 mm, Ø mm Probe: stainless steel IP 65; NEMA 4 cell HMC, capacitive Housing: 5 x 9 x 66,5 mm Housing: Polycarbonate Radiation shield / screen LAM 6 Eigenbrodt GmbH&Co.KG non-aspirated naturally ventilated radiation protection round shaped multi-plate screen without value K 44 mm x 8 mm weight.9 kg PMMA /ABS IP 65. Anemometer Ventus VA For measuring the wind speed and direction, a compact version of an ultrasound sensor is used. The sensor Ventus VA of company Lufft Mess- und Regeltechnik GmbH, Fellbach, Germany is shown in Figure. This wind meter uses 4 ultrasound sensors which take cyclical measurements in all directions. The resulting wind speed and direction are calculated from the measured run-time sound differential. Calibrations were conducted in DWD laboratories by means of a wind tunnel. Figure : Anemometer VentusA [Lufft Mess- und Regeltechnik] In addition, the sensors are also capable of calculating virtual temperature and the air pressure is measured by an integrated air pressure sensor. Minimum, maximum and average values can be calculated within the sensor itself. The measured values can be requested over a variety of interfaces. For EUCAWS DWD the RS485 full duplex [resp. RS4] protocol is used. The supply voltage for the Ventus is V DC ± % for the unheated version which is used on ships. Consisting of aluminum the sensor should be seawater resistant. For installation the top of the mast with free field around the sensor is recommended. North alignment should be done by using the bow of the ship. Technical data are shown in Table.. Barometer PTB For measurements of the air pressure the PTB of company Vaisala Oyj, Helsinki, Finland is operated. It is shown in Figure. The PTB series barometers use a BAROCAP silicon capacitive absolute pressure sensor developed by Vaisala for barometric pressure measurement applications. The measurement principle of the PTB series digital barometers is based on an advanced RC oscillator and reference capacitors against which the capacitive pressure sensor is continuously measured.

3 Figure : Barometer PTB [Vaisala Oyj] The microprocessor of the barometer performs compensation for pressure linearity and temperature dependence. The PTB barometer version for ships incorporates one BAROCAP sensor and is used without a display. Although the IP65 rated housing enables the sensor to be installed outdoors, the sensor will be incorporated inside the junction box of the EUCAWS system. To minimize the wind induced errors for the static pressure the pressure port of the barometer is connected to a static pressure head which comes together with the EUCAWS station itself on the top of the junction box. Data will be transferred via standard output RS and the voltage supply could range from 5 VDC. Further technical data are provided in Table. The calibration of the PTB is made by DWD itself as it is also operated in the operational network of DWD.. Temperature and humidity probe EE To determine the temperature and relative humidity the EE sensor system produced by E+E Elektronik Ges.m.b.h, Engerwitzdorf, Austria will be installed on ships. The sensor system consists of a heated humidity probe, a separated temperature probe and an electronic unit to be seen in Figure. Figure : Temperature and humidity sensor EE [E+E Elektronik] Data were transmitted via RS485 ASCII protocol. The housing of the electronic unit consists of polycarbonate and is rated IP65, NEMA 4. Supply voltage range from 8 5 VDC with a working temperature range from -4 C to +6 C. Calibration is made by DWD itself due to the EE being operated in the operational network of DWD. For temperature measurement the platinum (Pt) resistance thermometer with resistance value of Ω at C (Pt probe) of the EE was chosen. It is surrounded by stainless steel. For measuring relative humidity the monolithic capacitive measurement cell HMC of the EE is used. Due to the heating chemical contamination and condensation will evaporate from the sensor and will lead to faster return to normal condition. Additional coating and the PTFE filter cap protects the sensor in aggressive surroundings such as marine environments eg. seawater and corrosive fume. Further technical data are shown in Table..4 Screen LAM 6 A round-shaped multi-plate screen as generally used in automatic weather stations will be installed as weather shield. The screen LAM 6 by Eigenbrodt GmbH & Co.KG, Königsmoor, Germany consists of 9 plates in total, one larger top plate with diameter of 8 mm, closed upper shield plates with mm. The screen is non-aspirated naturally ventilated and consists of synthetic material with a length of 44 mm. Four threaded rods are countered on both sides. The sensor could be fixed by PG 9. A picture is shown in Figure 8b and more technically detailed in Table.

4 FIELD TEST ON FS METEOR On the research vessel FS METEOR stuffed with forecasters and observers the new sensor settings for EUCAWS were installed and compared to results from settings of the DWD surface network. The resistance to corrosion was investigated as well. Altogether the experiment compares different screens, different humidity and different temperature sensors, and different anemometers. Tests with screens and sensors for measuring humidity and temperature started in 4, the tests with different anemometers in 5.. Set up for temperature, humidity and screen tests Two ventilated LAM 6 screens [Eigenbrodt] were installed at FS METEOR on the star board and on the port side respectively. Each of them was equipped with four sensors in August 4.The temperature sensor LTS [Ketterer, Sölden, Germany] and the temperature probe of the EE, for humidity measurements the HMP 45D [Vaisala Oyi, Helsinki, Finland] and the humidity probe of the EE. Figure 4 shows the installation platform 9 m above sea level equipped with the screens and sensors. Additionally two unventilated smaller Young screens 4 [GWU Umwelttechnik, Erftstadt, Germany] were installed at the same platform at the bow of the ship. Inside the temperature and humidity probes of the EE transmitter were installed separately. Details can be seen in Figure 5 a-c. Figure 4: Set up of screens and sensors for temperature and humidity measurement on FS METEOR in 4 Figure 5 a, b, c: Details of the setup of temperature and humidity sensors in the screens. 4

5 .. Data acquisition Data were sampled and archived instantaneously every second. Wind data were sampled every.5 s. Averages for one and min were calculated. The resolution for temperature was. C, for humidity %RH, for wind speed. m/s and for wind direction. DEG respectively. One minute data were separated between port side and star board. Data with influences from exhausted fume or sun reflection noticed by the observer were eliminated from the data set. Wind was calculated according to GPS and compass data. After each cruise section (M to M5) the sensors were maintained, checked for damages and cleaned except for the EE at the bow. This temperature and humidity probes stayed uncleaned... Required measurement uncertainties The range, reported resolution and achievable and required uncertainties are detailed in Part I, Chapter, of WMO CIMO Guide, Annex.B. []. A comparison to DWD requirements can be seen in the annex of this paper... Results... Temperature... Comparison of sensors in the same screen The possibility to replace the analogue sensor LTS by the temperature probe of the EE digital transmitter was investigated using data of one year. Therefore the sensors were installed in the same ventilated screen LAM 6 on the star board and on the port side respectively. Approximately 5 data pairs could be compared. Calibrations of the sensors were conducted in DWD laboratories before installation and after one year. 8 8 Port side Data Count 6 4 Star board Data count Temperatur deviation port side [K] Temperatur deviation starbord [K] Figure 6 a and b: Temperature deviation for HMP 45D to EE As it can be seen in Figure 6 a and b only small differences occur for temperature measurements of the LTS (Pt) and EE (Pt) within one year. Separated between port side and star board data, the mean deviation for star board data is.6 K for port side data.7 K. The data distribution to the deviation ranges of. K,. K and. K is shown in Table. 5

6 Table : Temperature data distribution to deviation range Temperature deviation range [K]... star board 95 % 98 % 99 % port side < 68 % 95 % 98 % Due to a resolution of. C the resulting differences of the LTS and EE temperature sensors could be postulated to. K for port side and. K for star board site sensors. This means the deviations are within the declared achievable uncertainties by the WMO of. K. After months sensors were uninstalled and brought to the laboratory of DWD for recalibration. Recalibration data are shown in Table. All results are within required uncertainty range by DIN EN 675 Class AA of. K. Therefore, in 5 the analogue Pt LTS sensors were replaced by the digital Pt EE probes for further investigations on the FS METEOR. Table : Results of recalibration for temperature sensors after month on sea Reference [ C] Deviation [K] LTS LTS star port EE star EE port Comparison of sensors in different screens To compare sensor measurement accuracy influenced by different screens, the results from temperature probes installed in the ventilated LAM 6 were compared to those installed in the smaller unventilated Young 4 screen. In 6, the Young screens at the bow of the ship were replaced by the LAM 6 as shown in Figure 7 a and b for further investigations. Data were separately compared to star board (Tstar) and port side (Tport) data from the sensors in the ventilated LAM 6. Figure 7 a, b: Unventilated screens at the bow of the ship equipped with one sensor each. On the port side the temperature, on star board the humidity sensor is installed. A shows the Young 4, B the LAM6 screens. 6

7 Comparing data from the temperature sensors in the artificial ventilated screens with results from the sensor in the unventilated smaller screen (Tstar resp. Tport to Tbow) show higher temperature measurements for the sensor at the bow of the ship. 99 % of the data were higher measured at the bow compared to those on port side and on star board, even 9 % of the data range from K to +.5 K. A histogram of the temperature deviation for the cruises M7 to M 5 ( month) is shown in Figure 8: Star board Data Count M7 M8 M9 M M M M4 M5 Temperature deviation [K] NO radiation Radiation > W/m Temperature deviation Tbow to Tstarb [K] Figure 8: Histogram of temperature deviation Tbow to Tstar [K] for month data Figure 9: Temperature deviation due to solar radiation measured with temperature sensors in different screens No temperature dependency was investigated up to now. An evaluation of the radiation error according to recommendation by the WMO Report No. 6 [] was performed for global radiation > W/m. Without radiation the mean deviation was.4 K for star board and.9 K for port side data. With solar radiation the mean deviation increases to.8 K for star board and. K for port side data. This significant difference can be seen in Figure 9. While 95 % of the data without radiation are within a range of.7 K to. K, the data with radiation are in the range of. K to. K. Because the uncertainty requirements of the WMO are. K, measurements in smaller unventilated screens tend to lie outside the rage and therefore should be further investigated. Laboratory tests should be performed according to ISO 774 [4].... Relative humidity... Comparison of sensors in the same screen Because the analogue sensor HMP 45D [Vaisala Oyi, Helsinki, Finland] is no longer produced there is a need to replace this humidity sensor. Prior to decision, the heated humidity probe of the EE digital transmitter was investigated for one year. To compare sensor measurement accuracy, the different humidity probes were installed next to another in the same ventilated screen LAM 6 on the star board and on the port side respectively. Differences for humidity measurements between HMP 45D and EE were observed within one year. The longer the time of installation the wider the differences up to 4 %RH at the end of the 5 journey. Results are shown in Figure as box-and-whisker plots for star board data (Stb). After each cruise section M to M6 the sensors were cleaned and investigated for damages. The bottom and top of the boxes are the first and third quartiles (meaning 5 % of the data are inside the boxes) with the band inside as median (second quartile) and dot as mean. The whiskers represent the 5 th and 95 th percentile. For humidity measurements differences between HMP 45D and EE first occurred after 4 month of installation on the ship. 7

8 Table 4: Results of recalibration of the humidity sensors HMP45D and EE after month of installation on a ship 5 4 Reference [%RH] Deviation [%RH] HMP45D star HMP45D port EE star EE port Deviation Stb HMP to EE M M M M M_ M_ M4_ M4_ M5 M6 Figure : Deviation of humidity sensors HMP 45D to EE in the same screen. Data from one year installation on a ship, separated by cruise sections In June 5 the sensors were uninstalled and brought back to the calibration laboratory of DWD. Results of the recalibration of the four sensors are shown in Table 4. All results of the EE sensors were within the required accuracy range for both EE humidity sensors. Results for both HMP 45D sensors showed differences according to the deviation found in the data from the ship installation. The starboard HMP 45D sensor exhibit up to - 4. %RH deviation to the reference the port side HMP 45D sensor up to - 6. %RH. By reason of these results it comes to the decision to replace the analogue sensors by the EE humidity transmitter.... Comparison of sensors in different screens EE temperature and humidity probes were installed in artificially ventilated LAM 6 screens and in naturally ventilated smaller Young 4 screens from 4 to 6. From 6 on the Young 4 screens were replaced by the naturally ventilated LAM 6 of the same size. Additionally the sensors in the Young 4 screens stayed uncleaned while the sensors on starboard and port side were cleaned after each cruise section. Differences between these sensors are demonstrated in Figures a and b. a shows the regularly cleaned sensors after months on sea. b shows the uncleaned sensors. It is known that stainless steel surfaces are influenced by the marine environment as well. However the light rust film could easily be removed and the filter caps could be changed. The data from the sensors at the bow were separately compared due to the different cleaning status. Data from the bow were compared to star board data and separately to the port side data. In Figure the results for the deviation from bow to star board data are presented as box-and-whisker plots. The bottom and top of the boxes are the first and third quartiles (meaning 5 % of the data are inside the boxes) with the band inside as median (second quartile) and dot as mean. The whiskers represent the 5 th and 95 th percentile. 8

9 Deviation EEbow to EEstar [percent rel. humidity] M M M M M_M_ M4 M5 M6 M7 M8 M9 M Figure a, b : EE humidity and temperature probes. A cleaned after each cruise section. B sensor stayed uncleaned for months on sea. Figure : No differences for EE humidity probes installed in different screens and with different cleaning status. Data from 5 months. For the humidity probes of the EE transmitter no differences occur due to different screen and cleaning status. The mean deviation is within the resolution of %RH. The same results occur for port side data comparison. No drift or other changes in the data appeared during 5 month on sea (M- M). Because of the observers on board, all sensors could be regularly cleaned and checked for damages and inconsistencies. Changes on the surface of the sensors had no influence on measurement uncertainty.. Set up for anemometer tests On ships usually a cup anemometer is installed. Due to the special tasks on FS METEOR a D ultrasonic anemometer of Thies [Adolph Thies GmbH & Co.KG, Göttingen, Germany] is installed on the top mast at star board. For investigating the Ventus VA a setup is chosen demonstrated in Figure. On the top mast on the port side the cup anemometer, in the middle the Ventus and on the star board the D ultrasonic anemometer. Sensors were installed at 4 m above sea level. First wind tunnel tests show uncertainties in direction up to 4 degrees and in velocity up to. m/s. These results imply the uncertainty of velocity depending on the angle of incidence and with lower velocity the uncertainty rises. Presently the sensor is installed on the research vessel to yield results under sea weather conditions. First results are demonstrated in this section. Figure : Set up of anemometers at the top mast. Port side the cup anemometer, middle the Ventus, star board the D USA 9

10 .. Wind speed By comparing the data from the cup anemometer and the D USA differences in the range from -.5 m/s up to.5 m/s were found for wind speed measurements. No angle dependency was investigated. But comparing the Ventus and the D USA, an angle dependency in measurement deviation could be investigated pointed out in Figure 4. The Ventus shows higher wind speeds in the area of the bridges of the anemometer (NE, SE, SW and NW). Figure 5 shows the comparison of the cup anemometer with the D USA. NNW N NNE NNW N NNE Wind speed deviation [m/s] WNW W WSW NW SW NE SE ENE E ESE Wind speed deviation [m/s] WNW W WSW NW SW NE SE ENE E ESE SSW S SSE SSW S SSE Figure 4: Deviation between Ventus and D USA shows wind direction dependency of wind speed measurements Figure 5: Deviation between cup anemometer and D USA for wind speed measurements.. Wind direction All measured data for wind direction depend on the movement of the ship. Therefore, the data are calculated to the real wind with the help of a GPS compass. What can be seen in Figure 6 is an influence of the installation situation on the measurement. The cup anemometer shows different results for wind direction compared to the D USA, depending on the direction the wind comes from. Because requirements for measurement uncertainties varying only from to.5, further investigation is needed. 6 NNW N NNE 4 NW NE Wind direction [DEG ] WNW W WSW ENE E ESE 4 SW SE 6 SSW S SSE Figure 6: Differences between cup anemometer and D USA for wind direction measurements

11 4 CONCLUSION AND OUTLOOK Technical details of the sensors to be installed by DWD together with the European automatic weather station for ships are described: ultrasound anemometer Ventus A barometer PTB Pt temperature and heated humidity probes of the EE transmitter screen LAM 6. Results from a field trial on a ship are presented. Data from months of intercomparison temperature and humidity sensors show good agreements with the postulated requirements by WMO for both temperature sensors and one humidity sensor. Therefore, the analog sensors for humidity and temperature (LTS and HMP 45D) could be replaced by digital EE transmitter. Although strong changes on the surface of the EE were investigated after 5 months, no alteration in data was observed. Comparing data from an unventilated screen equipped with one sensor to data from the ventilated screens with 4 sensors showed no differences in humidity. But a temperature dependency for data from the smaller unventilated screen was observed. Higher temperatures were exhibit in the smaller passive screen in case of high solar radiation. By comparing data from cup and different ultrasound anemometers, construction and setup seems to influence the wind measurements. These differences of measurement of anemometers need further investigation. We are grateful to thank E. Knuth and A. Reake for the support with FS METEOR data. 5 REFERENCES [] [] WMO No.8 CIMO Guide 7 th Edition 8 Guide to Meteorological Instruments and Methods of Observation WMO-No. 8 ISBN [] WMO Instruments and Observing Methods Report No. 6 WMO FIELD INTERCOMPARISON OF THERMOMETER SCREENS/SHIELDS AND HUMIDITY MEASURING INSTRUMENTS ; Ghardaïa, Algeria, November 8 October 9, M. Lacombe (France), D. Bousri (Algeria), M. Leroy (France), M. Mezred (Algeria), WMO/TD-No. 579 in

12 [4] ISO 774:7 Meteorology -- Air temperature measurements -- Test methods for comparing the performance of thermometer shields/screens and defining important characteristics 6 ANNEX Details of measurement uncertainties 6. Temperature For air temperature the reported resolution should be. K. Required measurement uncertainties by WMO are. K for > - 4 C and + 4 C but achievable are only. K. Output time should be min. On request of DWD temperature measurement uncertainties were established according to DIN 675 class AA for the surface network. For VOS ships the requirements were established according to class B. DIN 675 class AA -5 C +5 C, C +,7*TT [ C = ±,4 C] DIN 675 class B -96 C +6 C, C +,5*TT [ C = ±,4 C] Table 5 gives an overview of the required temperature measurement uncertainties listed by temperature. Table 5: Temperature measurement uncertainties required by DWD and WMO Measurement uncertainties [K] Temperature DWD WMO C DIN class AA DIN class B required achievable Humidity Table 6 gives an overview of the required and achievable measurement uncertainties and resolution for humidity measurements. Table 6: Resolution and Measurement uncertainties for relative humidity of WMO and DWD Resolution Measurement uncertainties [%RH] required achievable WMO %RH %RH %RH DWD. %RH C TT 45 C for %RH<RH<%RH ± %RH - C TT C for %RH<RH<%RH ± 5 %RH - C TT C for %RH<RH<85%RH ± 8 %RH

13 6. Wind Table 7 gives an overview of the required measurement uncertainties and resolution for wind speed measurements. The required output averaging time should be and/or min. The DWD measurement uncertainty requirements for wind speed depend on the wind direction. Due to the anemometer design requirements in the area of the sensors head and bridges are different for the whole temperature range. Table 7: Resolution and measurement uncertainties for wind speed WMO Resolution.5 m/s Measurement uncertainties [m/s].5 m/s for 5 m/s % for > 5 m/s DWD. m/s.5 m/s for m/s % for > m/s m/s or % Table 8 gives an overview of the required measurement uncertainties and resolution for wind direction measurements. The required output averaging time should be and/or min. Table 8: Resolution and measurement uncertainties for wind direction Resolution Measurement required achievable uncertainties [ deg] WMO 5 %RH DWD.5. 5