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4.1 A NEW HIGH ACCURACY, LOW MAINTENANCE ALL WEATHER
PRECIPITATION GAUGEFOR METEOROLOGICAL, HYDROLOGICAL AND
CLIMATOLOGICAL APPLICATIONS
Heikki Turtiainen *, Pauli Nylander and Pekka PuuraVaisala Oyj,
Helsinki, Finland
Risto HölttäVaisala Inc, Boulder, Colorado
1. INTRODUCTION
Accurate measurement of precipitation isa challenge, especially
in climatic conditionswhere both liquid and solid precipitation
occur.In principle weighing gauges are the mostsuitable point
precipitation gauges for theseconditions. However, the accuracy
ofconventional weighing gauges is degraded by amultitude of errors.
Wind effects, evaporationand wetting error tend to cause
systematicunderestimation. In winter conditionsinstrumental errors
related to accumulation ofsnow and ice on rim and funnel parts of
thegauge - including complete filling of the gaugewith snow - may
result in gross errors. Theseproblems are only partially solved by
usingantifreeze solution in the container and with rimheating.
The new Vaisala all weather precipitationgauge VRG101 (fig. 1)
is a result of carefulstudy of the existing solutions and
newtechnical innovations. It provides higher qualityof measurement
and lower life-cycle cost in allweather conditions.
This paper describes the gauge designand presents field test
results obtained at theobservatories of the Finnish
MeteorologicalInstitute in Jokioinen and in Sodankylä.Preliminary
results from field tests by NADP(National Atmospheric Deposition
Program) arealso discussed.
* Corresponding author address: Heikki Turtiainen,Vaisala Oyj,
PB 26, FIN-00421 Helsinki, Finland;e-mail:
heikki.turtiainen(at)vaisala.com.
Fig. 1. Vaisala all weather precipitation gaugeVRG101 with wind
shield VRS111.
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2 VRG101 DESIGN
2.1 Design principles
VRG101 is designed as a reliable andaccurate all-weather
precipitation gauge, withspecial emphasis on easy maintenance
andextended service intervals. The electronics unitincludes a
processor with embeddedalgorithms for calculation of cumulative
rainfalland intensity. Versatile interfacing optionsguarantee easy
interfacing to various datacollecting systems.
The main features of VRG101 arepresented in table 1.
Gauge type Weighing precipitation gaugeParametersmeasured
- Cumulative precipitation(mm)- Precipitation intensity (mm/h)-
Temperature (°C), optional
Sensor element Single point load cellCollecting area 400 cm2 (62
in2)Capacity 650 mm (25 in)Resolution 0.1 mmAccuracy 0.2 mm
- precipitation event > 0.5 mmDimensions Height 950 mm (37
in)
Diameter 400 mm (16 in)Powerconsumption
- Without heating 30 mW- With heating 100 W max
Serial I/O - RS232 and RS485 lines forgauge output
andconfiguration.- Polled or automatic message
Data messageparameters
- Gauge status- Cumulative precipitation mm- Precipitation
intensity mm/h- Air temperature (option)- Container mass g-
Electronics temperature- Supply voltage
Options andaccessories
- Pulse output- Rim heating- Air temperature sensor- Wind shield
(types Alter,Double Alter and Tretyakovavailable)- Pedestal- Field
check kit
Table 1. VRG101 main features.
2.2 Mechanics
VRG101 utilizes the latest high-accuracy,temperature compensated
load cell technology.The single point - type load cell is designed
fordirect mounting of the weighing platform.Eliminating levers and
flexures, this allowssimple, robust and low cost mechanics.
The load cell is insensitive to eccentricloading unlike some
other types of weighinggauges. Thus unsymmetrical distribution
ofsnow in the collecting bucket (typical for winterconditions) does
not introduce measurementerrors.
The gauge volume and geometry havebeen optimized for good
performance in allweather. Its wide 400 cm2 collecting area
isadvantageous when measuring light rain andthe large 650 mm net
capacity decreases therisk of overflow. The deep container,
togetherwith the constriction formed by the inlet funneldecrease
the evaporation error and outblowingof collected snow.
Another error source that is eliminated bythe advanced mechanics
is theunderestimation caused by water and snowsticking to the inner
surfaces of the gauge inletfunnel. In conventional designs this
mass is notmeasured and eventually evaporates. InVaisala's design
the funnel element rests onthe collector container. All water and
snow onit’s surface is therefore included in themeasured mass.
The hinged upper part (rim and collectingfunnel) and detachable
enclosure door alloweasy access for maintenance or addingantifreeze
solution, as well as easy removal ofthe collector container (see
fig. 2).
The electronics unit, including the load cellis field-removable
(fig. 3). Replacement of theelectronics is straightforward and
quick. Dataloss is kept to a minimum as there is no needto
transport the whole gauge to the laboratoryfor calibration.
On-field checking of the gaugeaccuracy can be done using a
dedicated fieldcheck kit.
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Fig. 2. The gauge with enclosure opened.
Fig. 3. Electronics unit and load cell are situatedunder the
weighing platform.
2.3 Software and interfacing
The gauge software uses advancedalgorithms to filter out noise,
spurious signals(e.g. vibration by wind, mechanical impacts
andrubbish or other objects entering the collectingcontainer) and
to compensate for evaporation.
The outputs include RS232 and RS485serial lines with polled or
automatic messaging.Configuration of the gauge is also donethrough
the serial lines. An optional pulseoutput (tipping bucket
emulation) withprogrammable tip size is available.
In addition to cumulative rainfall the gaugedata message also
includes precipitationintensity, supply voltage, electronics
internaltemperature, gauge status with error flags andair
temperature (if the optional Pt100 sensor isconnected). Complete
raw data (weight of thecontainer) is also available to be used
fordiagnostic or research purposes.
2.4 Heating option
Optional rim heating is recommendedwhenever solid precipitation
needs to bemeasured. Heating prevents accumulation ofsnow and ice
on the rim and the collectingfunnel. To prevent extraneous
evaporationerror caused by heating and to minimize
powerconsumption, the heating is controlled by thegauge's software.
The intelligent controlalgorithm is based on ambient temperature
andprecipitation conditions.
2.5 Wind shields
Use of a wind shield is essential for highaccuracy, especially
when measuring snow orsleet. There are two types of basic
shieldsavailable for the VRG101: Tretyakov and Alter.For
climatological stations and otherapplications requiring the highest
accuracy, aDouble Alter shield is recommended.
2.6 Other options and accessories
Other options and accessories includepulse output, air
temperature sensing kit (aPt100 temperature sensor with a
radiationshield), field check weight, gauge pedestal andscrew pole
foundation.
Automatic draining option will be availablein spring 2006.
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3. TEST RESULTS
The gauge has undergone extensivetesting at Vaisala's own test
field, in the FinnishMeteorological Institute's (FMI) observatories
inJokioinen and Sodankylä, and by various pilotcustomers. Here the
results obtained from FMIfield test facilities and the National
AtmosphericDeposition Program (NADP) test field arepresented.
In the FMI Jokioinen Observatory,Southwestern Finland trial,
VRG101 wascompared with the FMI's double fenceintercomparison
reference (DFIR): a highaccuracy weighing gauge with it's orifice
heightset at 3 m and surrounded by an octagonalvertical double
fence.
The comparison was started in August2004 and still continues.
The results presentedhere are from the period August - November2004
(Turtiainen et. al. 2005). Figure 5 showsthe observed relation
between VRG101 andthe FMI reference gauge. Table 1 shows dailytotal
values divided into three categories (lightrain, rain and heavy
rain).
0,1 1,0 10,0
FMI reference gauge (mm)
0,1
1,0
10,0
Vai
sala
gau
ge (m
m)
R Sq Linear = 0,993
Fig. 5. Daily totals, Vaisala gauge versus FMIreference gauge
(DFIR)
Overall, the Vaisala gauge caught slightlymore precipitation
compared to the FMI gauge,the difference being highest for light
rainevents. The difference is believed to be causedat least
partially by the designs of the the inletsections of the gauges.
The inlet funnel of thethe Vaisala gauge is a part of the
instrumentweighing system, whereas this is not the casefor the
reference gauge. Therefore waterdroplets caught on the surface of
the inlet ofthe FMI gauge are likely to cause wetting error,
and consequently an underestimationespecially in light rain.
Daily rainfall Nbr ofdays
FMIreferencegauge(mm)
VRG101(mm)
< 1 mm 10 4.5 6.21 - 4.4 mm 15 38.5 43.0> 4.4 mm 12 133.9
138.9TOTAL 37 176.9 188.1
Table 2. Summary of liquid precipitationmeasurements at FMI
Jokioinen Observatory,August - November 2004.
Daily rainfall Nbr ofdays
FMIgauge(mm)
VRG101(mm)
< 1 mm 4 2.0 1.21 - 4.4 mm 7 14.1 14.1> 4.4 mm 4 35.4
34.3TOTAL 15 51.5 49.6
Table 3. Summary of snowfall measurementsat FMI Sodankylä
Observatory, February - April2005.
Another, identical gauge was installed inthe FMI Sodankylä
Observatory in NorthernFinland. Snowfalls are more abundant
inSodankylä and extend further into spring thanthey do in
Jokioinen. The gauge was equippedwith rim heating and a Tretyakov
type windshield. The reference used in Sodankylä was astandard FMI
precipitation station: a manualgauge and a Tretyakov type wind
shield.
Table 3 shows the summary of the dailyresults. The Vaisala and
the reference gaugeswere in good agreement. The difference in
thetotal values was only 1.9 mm or 4 %.
VRG101 was also tested by several pilotcustomers. An example of
interim results oftests performed at the test field of the
NationalAtmospheric Deposition Program (NADP),Champaigne, IL. are
presented here. Thesummary of the results is shown in fig. 6.
Ingeneral the two gauges were in goodagreement, the difference in
the totalcumulative rainfalls being only 2 %, withVRG101 showing
slightly more.
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4. CONCLUSIONS
The VRG101 belong to a new generationof weighing precipitation
gauges. Simple androbust mechanics, optimized gauge geometry,the
latest high-accuracy load cell technologycombined with advanced
measurement andheating control algorithms ensure highperformance in
all weather conditions.
The gauge has been field tested in co-operation with the Finnish
MeteorologicalInstitute, as well as with several pilotcustomers.
The results demonstrate the goodperformance of VRG101 both in
liquid andsolid precipitation.
When interfaced with the Vaisala MAWS-dataloggers and
communication modules thegauge can be used both as a basic
stand-alonehydrometeorological station, or as acomponent in larger
observation networks. Dueto it's versatile output options, the
VRG101 canbe interfaced with any data collection systemwith a
RS232/485 or a pulse input, and has awide application area in state
and national
meteorological, hydrological and climatalogicalnetworks. The
precipitation intensity outputalso enables the gauge to be used as
areference when weather radar signals are to beconverted to
quantitative rainfall amounts.
Acknowledgements. The authors thank Ms AnuPetäjä from the
Finnish Meteorological Instituteand Roger Claybrooke from
NationalAtmospheric Deposition Program for their kindco-operation
in organizing the field tests andpermission to publish the
results.
REFERENCES
Turtiainen, H., S. Räisänen and A. Petäjä2005: Next generation
all weather precipitationgauge. WMO Technical Conference
onMeteorological and Environmental Instrumentsand Methods of
Observation (TECO-2005).Bucharest, Romania, 4-7 May 2005.
0.94
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/200
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NWS stick gage VRG101
Fig. 6. Comparison of daily rainfall measured by VRG101 and NWS
stick gage at the NADP test field,Champaigne, IL, from 19 August to
20 September 2005.