introduction rationale the test system method results conclusions

WMO TECHNICAL CONFERENCE ON METEOROLOGICAL AND ENVIRONMENTAL INSTRUMENTS AND METHODS OF OBSERVATION

TECO-2012Brussels, Belgium, 16-18 October 2012

ENVIRONMENTAL CHAMBER ASSESSMENT OF THE ACCURACY OF WEIGHING GAUGES UNDER LOW INTENSITY REFERENCE FLOW RATES

M. Colli, E. Vuerich and L.G. LanzaWMO Lead Centre on Precipitation Intensity (Italy)

I. Introduction

II. Rationale

III. The test system

IV.Method

V. Results

VI.Conclusions

OUTLINE

TECO Brussels (Belgium) - 16-18 October 2012

Introduction


The WMO/CIMO Lead Centre on Precipitation Intensity performs laboratory and in-field investigations and intercomparison campaigns in order to assess and improve the performance of precipitation intensity gauges.

Genoa labVigna di Valle test site

Genoa lab

Rationale


The Lead Centre was involved in the pre-SPICE session of measurements with the objective of performing laboratory tests on two potential candidate reference instruments, the Geonor T-200B (3-wires) and the OTT Pluvio2 weighing gauges.

Relevant issues within SPICE are the possibility of actually sampling snowfall data at a high resolution in time (one minute) and to ensure consistency between precipitation depth and intensity measurements.

Laboratory experiments at very low precipitation rates and controlled environmental testing conditions have been performed in order to demonstrate the good performance of the candidate reference instruments and to make available some reliable quantitative assessment of their accuracy and precision in measuring precipitation intensity.

This information, together with additional testing of the gauges under non-steady reference flow rate conditions and the assessment of the dynamic behavior of the reference instruments, will be useful to better interpret discrepancies between any instrument under test and the co-located reference instrument during the next SPICE winter campaign.

Rationale


The SPICE reference snowfall gauges

GEONOR T-200B

OTT PLUVIO2

Type: vibrating wires gaugeCollector area: 200 cm2

Resolution: 0.05 mm Range of measure: not provided Capacity 600 mm

Type: weighing gaugeCollector area: 200 cm2

Resolution: 0.1 mm Range of measure : not provided

Capacity: 1500 mm

The test system


• Constant head aspiration reservoir for the pumping device

• Flow rate generator based on a ISMATEC REGLO-CPF Digital piston pump.

Flow rate generator

range: 0.1 to 45 ml/minaccuracy: < 1%CV: <= 0.17 %

The test system


• Real time monitoring system of the environmental chamber conditions (temperature, RH, etc.)

• METTLER TOLEDO Densito 0PX Temperature and density meter for the testing fluid sample (range: 0 to 2 g/cm3, resolution: 0.0001 g/cm3)

• METTLER TOLEDOPB4002-S/FACT Precision scale for validation of generated flow rates

range: 0 to 4000 g resolution: 0.01g

Environmental chamber (Pratica di Mare – Rome)

The test system


Measured residual error of the flow rate generator

Qref avg(eres) st.dev(eres)

(ml/min) (%) (%)0.35 0.0 1.90.75 0.0 0.81.56 0.1 0.23.16 -0.1 0.36.38 -0.1 0.1

12.78 -0.3 0.118.11 0.4 0.1

1.1 mm/h

54 mm/h

Testing fluid: Ethylene glycol dimethyl ether density

(liquid state at snowing temperature)

Method


These generation conditions were realized to test the OTT PLUVIO 2 and the GEONOR T-200B gauges at the following environmental temperatures: -5, +0 and +5°C

avg(RIevap) avg(Tenv) avg(RHenv)

mm/h °C %-0.01 4.2 85-0.02 0.5 70.3-0.02 -5.8 71.7

d RI h W[sec] [mm/h] [mm] [g]

10500 1.20 3.5 706000 3.00 5 1003600 5.00 5 1001800 10.00 5 100

4 different combination of equivalent precipitation intensities (RI) and durations

Precise evaluation of the evaporation in terms of the negative contribution to precipitation intensity (RIevap) was also performed by weighing the content of a sample ethylene glycol dimethyl ether flask at the beginning and at the end of each test.

Method


GEONOR T-200B

OTT PLUVIO2

Level 0 indication:Vibrating wires average frequency values converted into net water depth by means ofa manufacturer provided function

Level 0 indication:“Real Time Bucket” output string,

the unfiltered gross water depth measured by the weighing gauge

(the bucket contribution is included)

The water depth measurements were retrieved with a 6-seconds time resolution.

Method


𝑒= ൫𝑅𝐼2𝑚𝑖𝑛 − 𝑅𝐼𝑟𝑒𝑓൯𝑅𝐼𝑟𝑒𝑓 ∗100

GEONOR T-200B OTT PLUVIO2

Precipitation intensity values used for the rain gauge performance evaluation are averaged over a 2-minute time span, providing the RI2min measurement.

𝑅𝐼2𝑚𝑖𝑛 = 𝐸ሾ𝑅𝐼ሿ2𝑚𝑖𝑛

The data analysis is carried out by calculating the percentage relative errors e between RI2min measurements and the reference value

previously validated by weighing the actual fluid depth generated by the rainfall simulation system.

Results


OTT GEONOR

RIref

10 mm/h

5 mm/h

3 mm/h

1.2 mm/h

10 mm/h

5 mm/h

3 mm/h

1.2 mm/h

T eavg eavg eavg eavg eavg eavg eavg eavg

°C % % % % % % % %-5 -1.3 -2.0 -1.4 -3.5 -1.1 -1.4 -1.2 1.80 -3.5 -3.1 -2.3 -0.9 -2.0 -1.2 -1.8 -0.15 -1.2 -3.8 -3.9 -2.2 -2.3 -3.2 -2.0 -0.5

OTT GEONORRIref 10 mm/h 5 mm/h 3 mm/h 1.2 mm/h 10 mm/h 5 mm/h 3 mm/h 1.2 mm/h

T dev.st.(e) dev.st.(e) dev.st.(e) dev.st.(e) dev.st.(e) dev.st.(e) dev.st.(e) dev.st.(e)°C % % % % % % % %-5 1.7 5.5 3.6 8.2 5.2 9.0 16.0 40.10 1.9 2.5 3.8 7.2 4.8 8.4 16.4 38.55 2.3 2.5 3.6 7.6 4.4 10.1 16.2 42.7

Measurement accuracy in terms of average percentage relative errors (eavg) with varying the environmental temperature (T) and the simulated precipitation intensity (RIref)

Measurement precision in terms of standard deviation of the percentage relative errors (dev.st(eavg)) when varying the environmental temperature (T) and the simulated precipitation intensity (RIref)

Results


RIref [mm/h]

0 2 4 6 8 10 12 14

e[%

]

-100

-50

0

50

100

GEONORPLUVIO2

Rain gauges performance at T = -5 °C

Results


RIref [mm/h]

0 2 4 6 8 10 12 14e[%

]

-100

-50

0

50

100

GEONORPLUVIO2

Rain gauges performance at T = 0 °C

Results


RIref [mm/h]

0 2 4 6 8 10 12 14e [

%]

-100

-50

0

50

100

GEONORPLUVIO2

Rain gauges performance at T = +5 °C

Conclusions


• Steady-state tests demonstrate that precision and accuracy (trueness) deteriorate with the decrease of the precipitation intensity for both instruments. Precision, in particular, decreases with reducing the intensity, with relevant differences between the two gauges.

• The inherent resolution of the instruments in measuring precipitation intensity also affects the performance at low precipitation rates.

• Dynamic tests performed at ambient temperature (not reported here) show that both gauges are able to reproduce likely real world events with quite comparable performance.

• The results from laboratory tests open the way for possible improvements of their accuracy and precision.

• The Lead Centre is continuing to perform laboratory tests under both static and dynamic conditions and with varying temperature as well.

introduction rationale the test system method results conclusions

Documents

environmental instruments

precipitation depth

intensity measurements

b gauges

low precipitation rates

flow rate generator1

flow rate generatorrange

accuracy of weighing