HSS VF-500 Visibility / Fog Sensor
INTRODUCTION
 The HSS VF-500 sensor is configured for accurate measurement of visibility in the densest of fogs to very clear air conditions and is the refinement of 2 decades of experience and technology resulting in a compact, robust instrument with excellent performance.
The VF-500 is often deployed in extreme conditions and is able to withstand the rigours of many years in marine environments.
With the optional fog warning system the HSS VF-500 can be used for automatic switching of warning lights and alarms, such as lighthouse fog signals and roadside traffic warning systems,as well as for datalogging purposes.
The VF-500 is used for a variety of applications including:
- switching fog warnings on lighthouses and offshore platforms
- switching warnings and lights on windfarms
- military applications
- any application requiring switching of external devices when the visibility changes
Features
- measures visibility and fog density
- proven accuracy, reliability and repeatability
- optional fog warning system for switching alarms and lights
- suitable for portable or fixed installation
- low power requirements (mains, battery, solar)
- EEx zone I and II configuration available
Measurement Principle
The VF-500 sensor uses forward scatter meter technology to measure visibility in all weather conditions. The sensor measures the amount of light scattered by small suspended particulates (i.e. fog, haze and smoke aerosols) or larger particles (i.e. rain, snow, ice pellets, drizzle and mist) passing through the sample volume. The sensor calculates the atmospheric EXtinction COefficient (EXCO) from which the Meteorological Optical Range (MOR) is calculated. Using an infra-red light source and intelligent sensing technology the measurements are unaffected by other light-sources.
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Construction
The HSS VF-500 is lightweight and can easily be installed by one person and consists of a sensor head which makes the measurements and a control box housing the electronics. The sensor head is constructed of dip brazed high grade aluminium. All components that are exposed to weather are hard-coat anodised for protection against corrosion. The sensor head and control box can be sited up to 66 ft apart for safe and convenient access.
Maintenance and calibration
The sensor is fully calibrated at the time of manufacture. Routine maintenance, including a check on calibrations, can be performed easily by one person in a matter of a few minutes and a re-calibration (although this should never be required) takes only slightly longer.
Operation in temperature extremes
The sensor operates in temperatures ranging from - 58º F to + 140º F. For operation below - 27º F the heated version is recommended (please refer to the heated option below).
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Examples of data output
The EXCO values (which are related to visibility) are output in the form of an analogue 0-10 V DC logarithmic signal in the range according to your application needs. Optionally available is a 4-20 mA logarithmic output.
The VF-500 System
The system consists of two parts, a sensor head and a power and communications unit (mounting pole is not included)
The sensor head is constructed of high quality aluminium which is hard anodised to give a superior finish that lasts for years and does not require painting.
As standard the sensor has:
- 0-10 V analogue output
- window de-misters
- power line surge arrestors
- signal line surge arrestors
- electro magnetic interference protection
- 19 ft interconnect cable
- calibration reference certificate
- manual
In addition the following variations are available (described under options below)
- fog warning system
- heating
- output 0-20 mA or 4-20 mA as well as the standard 0-10 V
Options / Variants
Fog warning system - the fog warning system allows an external device such as a fog horn, road warning sign or ventilation fan to be switched automatically at a given visibility range, and the monitoring and adjustment of the intensity of warning lights.
The system features user adjustable activation and de-activation delays ensuring smooth and effective switching. For example, the delays could be set for 5 minutes so that a device would only be activated after 5 minutes of continuous reduced visibility. This allows for local transient events such as fumes from passing vehicles or vessels thus avoiding false alarms. Additionally power is used more effectively as the device is only turned on when necessary.
Heated version - the sensor comes with window de-misters as standard allowing operation of the sensor in temperatures down to 27º F. However, where the temperature drops below 27º F for more than a couple of hours a day the heated version is recommended. This version will provide error free operation in conditions down to - 58º.
Additional Analogue Outputs - in addition to the standard 0-10 V output the sensor can be ordered with either 0-20 mA or 4-20 mA output.
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Accessories
The following accessories are available:
- calibration kit
- stainless steel mounting kit
- transit case
Calibration reference kit - recommended for end-user confidence checks and recalibration. The kit contains a carrying case, zero plugs and a calibration plaque to a specific EXCO value. One kit can be used on any number of sensors of the same range.
Stainless steel mounting kit - includes U-bolts, fasteners and adapters to secure the sensor and power control unit to any pole with a diameter between 1.8 - 2.9 inch.
Transit case - scientific case lined with 3 inch foam to hold the sensor securely in place in extreme handling conditions.
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