RWS, SWS and VPF Visibility & Present Weather Sensor FAQs

Home / Service Support / FAQs / RWS, SWS and VPF Visibility & Present Weather Sensor FAQs

Below are a range of our RWS, SWS and VPF Sensor FAQs.

For more sensor information please get in contact with Biral using the contact form.

For regular product and service updates be sure to follow us on TwitterLinkedInFacebook and YouTube.

If you have a technical problem with your sensor, please contact the Biral service department.

The procedures for checking the sensor’s calibration and re-calibrating (only necessary if the calibration checks fail) are to be found in the user manuals, which can be downloaded from the Biral website (click here). It is important to use the correct calibration plaque – if it doesn’t fit onto the sensor easily then please contact the Biral service department. When you call please have the serial numbers of your sensors and the calibration kit available.


Determination of visibility is based on measurements of the atmospheric extinction coefficient (EXCO), which is then converted into Meteorological Optical Range (MOR). To measure the extinction coefficient a beam of IR light is emitted at an angle to a sensitive receiver and the amount of light scattered, by particles in the air, into the receiver is measured. It is these particles in the air that are the dominant factor in determining visibility so a relationship can be formed between the number of particles, the amount of light scattered into the receiver and the corresponding extinction coefficient, such that more particles = more scattered light = lower visibility.

Firstly, contact our service department to inform us that you need to return the sensor.


Once that is done, simply place the sensor in suitable packaging and send it to

For the attention of the Service Department
Unit 8
Harbour Road Trading Estate
BS20 7BL

The length of the warranty on your sensor depends on which sensor you have and whether, or not, an extended warranty was purchased at the time of order placement.

All warranty periods start from the day of despatch from our factory. If you need confirmation of whether your sensor is still in warranty then please contact our service department. When you call please have the serial numbers of your sensors available.

Extended warranties are only available if purchased with the sensor. Please contact the sales department for details.

Yes, but we recommend the VPF series of sensors for off-shore and marine environments due to their rugged construction and hard anodised finish. You can also specify an SWS sensor with a hard anodised coating for off-shore and marine applications by adding the suffix “A” to the sensor model type (e.g. SWS-100A).

Yes, but for additional anti-corrosion protection we recommend you specify the “A” model (e.g. SWS-050A) as these have a hard anodised treatment underneath the paint finish. The “A” model also comes with a 5 year warranty as standard.

Sensors are supplied with two stainless steel U-bolts to enable mounting to a round pole or mast. This is the recommended method of mounting the sensor as it provides the least disturbance to the free flow of air around and through the sample volume.

This depends on where the sensor is going to be installed and the environmental conditions there. For most applications our standard self-test, which monitors and compensates for contamination on the transmitter window, is sufficient. At sites that are liable to have dust, sand, snow or other particles blown on the wind we recommend the advanced self-test, which additionally monitors and compensates for contamination on the detector windows.

In many instances the mounting height of the sensor is determined by the end customer or by Regulation and you should check the requirements for the country you are in. As a general guide we suggest the following heights – 1.5-2.0m for road-side applications, 2.5m for airport applications and 1.8m for general meteorological applications.

There are effectively two reasons why Biral sensors have level arms. Firstly, it maximises the overlap between the forward scatter and backscatter detectors in our present weather sensors thereby improving our ability to differentiate between frozen and liquid precipitation. Secondly, sensors with arms that are angled downwards can suffer from incorrect measurements due to light being reflected off objects, such as foliage or a wet roadway, beneath the sensor. In wet conditions, when reflections are greater than normal, this can lead to a significant increase in the amount of light entering the detector window resulting in an under-reporting of visibility.  Further information can be found in our technical note The effects of Surface Reflections and extraneous light of Forward Scatter Visibility Sensors”.

This is very much application and location dependent but in general terms the sensor should be orientated so that neither the rising nor setting sun appears in the receiver lenses. For visibility sensors installed in the Northern hemisphere the ideal orientation is for the receiver head to be facing North. In the case of present weather sensors (which have an added backscatter receiver) the optimum orientation it is to have the backscatter receiver off-set by 34o East of North. For sensors installed in the Southern hemisphere simply add 180o to the angles above.

The VPF-750 present weather sensor is supplied with a high accuracy Hygro-thermo transmitter and the sensor electronics have been calibrated to work with that probe. Substituting another probe is not possible without adversely affecting the performance of the sensor.

It is a common misconception that fog, mist and haze are actually measured whereas, in actual fact, they are determined by the visibility measurement plus, in the case of mist, relative humidity. For example, ICAO defines fog as visibility of less than 1km and haze as visibility of between 1 and 5km. Mist is similarly defined as visibility of between 1 and 5km but with humidity greater than 80%. Other definitions are applicable in certain countries and our sales team would be pleased to advise you on that.


The determination of mist requires the sensor to be equipped with a Hygro-thermo transmitter and is, therefore, limited to the VPF-750, VPF-730 and SWS-250. Whilst the VPF-750 is supplied with a Hygro-thermo transmitter as standard this is a costed extra on the VPF-730 and SWS-250. For more information please consult our sales team

The performance of visibility meters in the presence of dust or sand will be largely dependent on the physical size and scattering properties of the particles. As non-hydrometeors, sand and dust do not scatter light forwards in the same way as liquid meteors, so if the sensor registers a reduction in visibility with a different scatter signal to that associated with liquid particles then the particles must be solids. In-house testing has provided Biral with a set of parameters which enable us to determine the type of particles present and customer led trials have shown that our sensors will reliably report visibility in the presence of sand or dust. Further information can be found in our white paper The use of Biral Forward Scatter Visibility Meters in the presence of haze, smoke and sandstorms.

This can be done but as the definitions are embedded into the sensor firmware it requires an update to the parameter file which is then applied using the Biral sensor interface software. There may be a cost involved in preparing a custom parameter file. For more information please contact the sales team.


This is possible on some but not all sensors and where it is possible the method depends on the type of sensor. RWS-30 sensors have a fixed range to conform to International Regulations for use in tunnels and cannot be altered. With the RWS-20 you can change the high end of the range between 600 and 7500m via the serial communications port. For an SWS sensor you will need a copy of the Biral Range Update software and a suitable Windows PC to run it on. The visibility range of all VPF sensors is fixed as 10m to 75km. For more details please contact the service department.


Yes, but only one country code can be present in the sensor firmware at a time. For more information please contact the sales team.


There are several published papers in the public domain dealing with the fundamentals of visibility measurement using forward scatter meters. Further information can be found in our white paper “Introduction to atmospheric visibility estimation”.

Yes, there have been a number of independent trials undertaken to assess the ability of forward scatter meters to accurately report visibility in the present of haze, smoke and sand. Further information can be found in our white paper “Introduction to atmospheric visibility estimation”.

Sensor visibility, in relation to forward scatter meters, is an assessment of the visibility within the sample volume which is extrapolated assuming that the atmosphere around the sensor is homogenous. Prevailing visibility uses different techniques, including cameras, AI, known object distances and visual interpretation to determine the visibility in a complete 360o horizontal circle around the sensor.