Two Heads are Better than One: The advantages of the Biral back scatter head
When it comes to automatic visibility and present weather sensors, most manufacturers opt to detect the forward scattering of infrared light via a single receiver to estimate visibility and to classify precipitation. However, the intensity of light scattering is not the same in all directions, since this is strongly dependent on the size, shape and composition of these particles.
Whilst this single measurement approach is quite effective and slightly lower cost, Biral sensors employ a pair of receiver heads to detect light scattered both forwards and backwards from hydrometeors and particles in the air.
So what does this really mean?
We sat down with Dr Alec Bennett, Meteorological Products Manager at Biral and a Chartered Meteorologist, to talk about the technology behind the company’s back scatter head for precipitation classification, and its importance. Dr. Bennett also worked at the UK Met Office and is a visiting Research Fellow at the University of Bath.
What issues can companies encounter if they don’t use products with back scatter technology, and how have they been solved?
AB: Using forward-scatter only technology increases the risk that snow and hail are falsely classified as rain, which increases the danger to personnel who rely on the equipment for their safety.
How has Biral's back scatter head been benefiting projects? Can you name an industry where it has worked well?
AB: Biral's proven combined forward and back scatter technology has benefited industries such as transportation. For example, our combined sensors consistently outperform traditional forward scatter only technology for alerting airports when rain turns to snow. This allows efficient de-icing of aircraft, keeping the airport operating safely and efficiently.
How do they form part of Biral's sensors?
AB: The sensor consists of a transmitter which emits a cone of invisible near infrared light that scatters on all the particles in the air in front of it. The scattered light is then detected by two receivers, one measuring scatter in the forward direction away from the transmitter, with another measuring scatter in the backward direction, back towards the transmitter. This extra information on scattered light direction allows Biral sensors to make a more accurate classification of what is falling from the sky.
How does the technology work with different weather conditions?
AB: Raindrops scatter most of the light shining on them in the forward direction, away from the light source. This is why we can easily see through clear water droplets. Ice particles such as snowflakes and hailstones scatter much of the light back towards the source, which is why we cannot see through them and they appear white. The Biral sensor uses this difference to more accurately decide whether a falling particle is liquid or ice.
Why is this technology important in this field and what impact can it have going forward?
AB: With weather becoming so unpredictable due to climate change, what type of weather is affecting a site is important since it can have a significant impact on activities occurring there. For example, rainfall is likely to have a different impact on driving conditions compared to snowfall. A roadside present weather sensor must therefore be able to accurately and reliably tell the difference between these different weather conditions, and Biral’s back scatter head is key to achieving this.
Is there anything else about Biral's back scatter sensors that you'd like to add, that you haven't mentioned?
AB: Biral has been unique in its use of this combined forward and back scatter technology for several decades and is therefore the leading expert on this technology. It uses its extensive experience, customer feedback and long-term monitoring to ensure its products offer the best in dependable present weather discrimination.
One of its products is the SWS-250 weather sensor, which is optimised for use in aviation applications where both visibility and extended present and past weather information is required. With its ultimate visibility performance, the visual range can extend to 99.99km, allowing for use in meteorological observation networks and research applications.
It incorporates a back scatter receiver to form a visibility and present weather sensor. Designed for both runway use and ‘met gardens’, the SWS-250 has a visibility performance up to 99.99km and reports 39 WMO 4680 codes. The sensor measures visibility, fog, haze, drizzle, snow, hail and other non-frozen precipitation according to WMO-4680.
Another advanced product is the VPF-750 Visibility and Present Weather sensor. Its enhanced performance allows the reporting of freezing precipitation in addition to the weather codes typically available from a forward scatter sensor.
The sensor is designed for use in aviation applications where extended weather information, including past weather, is required for inclusion in METAR reports. The sensor is also well suited to research applications and use in meteorological observation networks.
The unique back scatter receiver combined with accurate external temperature, humidity and precipitation sensors gives the VPF-750 unparalleled abilities in distinguishing frozen from liquid precipitation, with the added advantage of identifying freezing precipitation. The sensor reports both WMO table 4680 and METAR codes to simplify integration to host systems. A wealth of other data is provided including a transmissometer equivalent EXCO.
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