Visibility and Present Weather Sensor VPF-750

The VPF-750

Like all VPF series sensors the VPF-750 enclosure is constructed from dip-brazed aluminium with a hard coat anodised finish. This combination results in a sensor that offers superb abrasion and corrosion resistance. It is not unusual for VPF series sensor to be still operating after 20 years’ service in harsh environments.

The 45° forward scatter angle and 850nm infrared light source used by the VPF-750 provide the best possible visibility measurement accuracy in all weather conditions. Visibility is output as Meteorological Optical Range (MOR) whilst World Meteorological Organisation (WMO) Table 4680 codes are used to report present weather. The backscatter receiver, unique to Biral present weather sensors, gives the VPF-750 the ability to discriminate between liquid and frozen precipitation with exceptional accuracy. Liquid precipitation scatters very little of the transmitted light towards the backscatter receiver, whilst frozen precipitation, be it ice pellets, hail or snow, scatters a significant amount in this direction. By observing the ratio of forward to backscatter for each precipitation particle the VPF-750 provides a much more accurate assessment of present weather than competitors’ sensors which rely on measuring the size and velocity of particles for classification. The reliance on size and velocity measurements for the determination of precipitation type is error prone as the velocity measurement is dependent on the local wind speed. The VPF-750 is fitted with additional sensors and employs advanced precipitation algorithms to enable the reliable identification of a wide range of precipitation types, including freezing precipitation.

In common with all Biral visibility and present weather sensors the VPF-750 uses 2kHz modulation of the light source combined with narrow pass band optical filters and synchronous detection at the receiver to ensure the measurement is not affected by bright or flashing lights. This design also ensures the sensor accurately reports visibility during precipitation. The horizontal hood design eliminates the problems caused by surface reflections that are a source of error in sensors employing look down hood geometries.

The optical windows of the sensor are fitted with low power heaters to prevent the formation of condensation. Window contamination monitoring and compensation allows the visibility output to maintain its accuracy even as dirt builds up on the optical surfaces. The sensor reports the window contamination in the output message using two stages of warning flags allowing cleaning to take place in a convenient manner.

Communication with the sensor is over a RS232 serial data link. ASCII encoded data is used and the sensor can operate in either a polled or broadcast modes. The data message can include a time stamp from the internal real time clock in addition to the standard visibility and self-test information.

The VPF-750 is low voltage DC powered. Optional hood heaters are available for use in environments where blowing snow may block the optical windows. The sensor is supplied with 6m cables for connection to the sensor using circular connectors. The standard length of all cables is 6m.

The ALS-2 ambient light sensor can be connected directly to the sensor using an optional interface. The ALS-2 takes its power form the VPF-730 and the light sensor output is integrated into the VPF-730 data message.

The VPF-750 is a single lightweight unit that can be easily transported and installed by one person. It is designed for use in all weather conditions and has a wide operating temperature.