Presentation on theme: "Comparison between temperature sensors in plastic, aluminium shields and “Stevenson” screen Franco Stravisi, Stefano Cirilli (University of Trieste, Dept."— Presentation transcript:
Comparison between temperature sensors in plastic, aluminium shields and “Stevenson” screen Franco Stravisi, Stefano Cirilli (University of Trieste, Dept. Mathematics and Geosciences)
The meteorological station is near the sea; University is on the hills.
The meteorological station is on the roof of the Nautical Institute, close to the sea. The place is the same since 1819 (two centuries).
The meteorological instruments are on an airy wooden terrace over the roof of the Nautical Institute. The large conventional screen and the test screens are showed.
TRIESTE station; large wood shield (LWS) with double walls and conventional instruments. In 1992 a Pt100 was located inside LWS for comparison with the small PVC screen outside. LWS
Micros datalogger (1992-2011) and Pt100 (Class B) screen NESA datalogger (since 2011)
Air temperature data at TRIESTE: 1.1802 – 1883: thermometer readings, 3/day; MIN / MAX 2.1884 – 2000: thermographs & MIN / MAX thermometers, conventional wood shield 3.1992 – 2011: Pt100 (*), small PVC screen, 10 min averages. MICROS datalogger, modem link. 4.2011 – present: Pt100 (**), small aluminum screen, 10 min averages. NESA datalogger, web link. (*) Pt100: Class B, 4-wire cable, calibrated in situ (*) Pt100: 1/3 DIN, 4-wire cable, calibrated in situ Necessary to: A) check and ensure the continuity between conventional and “modern” thermometers/screens; B) test the performance of different current screens
(1) Monthly average temperatures measured by means of Pt100 sensors (in situ calibrated) inside the large wood shield and in the small PVC screen (SR) are the same. (10 min data from 1996 to 2010) (A)Conventional wood screen vs. small PVC screen 1996 – 2010 LWS vs. SR
(2) Monthly means of daily minimum (*) temperatures measured by means of Pt100 sensors (in situ calibrated) inside the large wood shield and in the small PVC screen (SR) are the same. (*) the minimum of 144 daily 10 min averages Conventional wood screen vs. small PVC screen LWS vs. SR
Conventional wood screen vs. small PVC screen (3) Monthly means of daily maximum (*) temperatures measured by means of Pt100 sensors (in situ calibrated) inside the large wood shield and in the small PVC screen (SR) are the same. (*) the maximum of 144 daily 10 min averages LWS vs. SR
INITIAL CONSIDERATION: After a long comparison between the conventional large wood screen LWS in use at the meteorological station of Trieste and a small PVC screen (SR) during 15 years (1996 to 2010), with Pt100 probe on site calibrated and adapted,we can consider: these particular shelters equivalent; 10-min averaged daily extreme Pt100 temperatures are equivalent to readings of standard MIN / MAX thermometers. = MAX MIN LWS = SR LWS shows a time delay ≤ 10 min with respect to SR
(B) Comparison of different screens SR reference screen, PVC, 20 cm cones, teflon stem S1 all aluminium, 16 cm cones S1’ one more open bottom cone S4 6 aluminium cones, large PVC top, teflon ring between sensor and stem S2 aluminium, PVC top S3 same with 7 cones REFERENCE SCREEN: our choice does not mean that screen SR is the “best one” in absolute, but just that it assures continuity in our climatic temperature series, performing like LWS. Similar study on commercial screens of different types have been made e.g. by M. Lacombe, D. Bousri, M. Leroy, M. Mezred, “WMO field intercomparison of thermometer screens/ shields and humidity measuring instruments”, in: WMO Instruments And Observing Methods Report No. 106. Our purpose here, is to test some different screens customized and made available to us, by a company (Nesa S.r.l.) who produces meteorological equipment and datalogger.. In the following, attention is limited for simplicity to screens S1 and S4; S1’, S2 and S3 behave in an intermediate way.
Test screens; the shadowed courtyard below prevents direct ascent of hot air. From left: SR, S3, S4.
S1 0.39 ± 0.59 °C Departure from SR reference temperature (overheating) S4 0.15 ± 0.24 °C
Mean overheating (i.e. departure from the reference screen) diurnal cycle, screens S1 and S4, Screens S1’ to S3 present an intermediate behaviour.
Examples of infrared pictures of different screens (see under the dish on the top of sensor) S1 S4 S3
RAIN PROTECTION Screen S4 seems to be more sensitive than the larger SR to intense rainfalls. Water could produce a temperature drop (psychrometric effect). Example (16 July 2014):
Air temperature shelters/screens: final results (A)Air temperature data measured in LWS with mechanical thermographs and MIN/MAX reference thermometers are equivalent to 10 min averages data logged by means of Pt100 in a small PVC screen calibrated and adapted (SR). (B)Different sensor screens can show different degrees of overheating, increasing with sun irradiance in still air. In order to minimize that, the sensor should be white painted, the inner stem thermally insulated from the supporting arm, the air should flow freely also along the vertical; cones should cover a good part of the stem and a flat top dish should shadow the whole screen. Our results (comparing SR and S4, of similar shape) indicate that SR is affected by just a little bit smaller overheating (0.2 °C in average) than S4, substantially equivalents; further investigation could explain whether this depends on the different material used or the size. Larger screens give a better protection from intense rainfalls. Acknowledgements: special thanks to NESA s.r.l. that patiently supplied us many screens specially assembled according to our requests...