A new high accuracy, low maintenance all weather precipitation gauge for meteorological, hydrological and climatological applications

Accurate measurement of precipitation is a challenge, especially in climatic conditions where both liquid and solid precipitation (snow, sleet) occur. In principle, weighing gauges are the most suitable instruments for these conditions due to the fact that the melting of snow is not a prerequisite for the measurement. However, the accuracy of conventional weighing precipitation gauges can be impaired by a multitude of errors. These include the error sources common to all point precipitation gauges - wind, evaporation and wetting errors - which all tend to cause systematic underestimation. In winter conditions the accumulation of snow and ice on rim and funnel parts of the gauge, as well as complete filling of the gauge with snow, may result in gross underestimation. With conventional designs these problems have been only partially solved by using antifreeze solution in the container and with rim heating. The new Vaisala all weather precipitation gauge VRG101 provides solutions to these measurement problems, resulting in higher accuracy, less maintenance and therefore lower life-cycle costs. Test data will be provided in this paper to support these claims.

The gauge utilizes the latest high-accuracy, temperature-compensated load cell technology. The load cell is designed for direct mounting of the weighing platform, allowing simple, robust and low cost mechanics. The load cell is insensitive to eccentric loading, so that unsymmetrical distribution of snow in the collecting bucket (typical for winter conditions) does not introduce measurement errors. In the design special emphasis has been put on easy maintenance and extended service intervals. The hinged upper part and detachable enclosure door allow smooth access for performing maintenance or adding antifreeze agent, as well as easy removal of the collector container. The electronics unit, including the load cell is field-removable. Data loss is kept to a minimum as there is no need to transport the whole gauge to the laboratory for calibration. If needed, field calibration can be done using weights.

The gauge software uses advanced algorithms to filter out noise, spurious signals (caused by i.e. vibration caused by wind, mechanical impacts and rubbish or other objects entering the collecting container), and to compensate evaporation. In addition to cumulative rainfall the gauge outputs also precipitation intensity, temperature (if the optional Pt100 sensor is connected), source voltage, gauge status and warning flags. Complete raw data (weight of the container) is also available and can be used i.e. for diagnostic or research purposes. The outputs include RS232 and RS485 serial lines with polled or automatic messaging, and a pulse output (tipping bucket emulation).

Whenever solid precipitation is measured, it is necessary to apply heating to prevent accumulation of snow and ice on the rim and collecting funnel. The common problems in conventional gauge heating systems - extraneous evaporation error and high power consumption - are solved by controlling the heater using an algorithm based on ambient temperature and precipitation conditions.

The options and accessories include: Alter and Tretyakov type wind shields; Pt100 sensor for air temperature measurement; screw pole foundation and pedestal for easy installation. Interfaced with the Vaisala MAWS-dataloggers and communication modules the gauge can be used both as a basic stand-alone hydrometeorological station, or as a component in larger observation networks. Due to it's versatile output options, the VRG101 can be interfaced with any data collection system with a RS232/485 or a pulse input, and has a wide application area in state and national meteorological, hydrological and climatalogical networks. The precipitation intensity output enables the gauge to be used also as a reference when weather radar signals are to be converted to quantitative rainfall amounts.

VRG101 has been field tested in co-operation with the Finnish Meteorological Institute, as well as with several other pilot customers. The results from tests in the FMI Jokioinen and Sodankylä Observatories demonstrate the good performance of VRG101 both in liquid and solid precipitation. The FMI test results together with the latest experience from other field trials will be presented in the conference.