Visibility Sensors

The 8364 Dual Technology Visibility Sensor measures transparency of the atmosphere and calculates its extension coefficient and meteorological optical range (MOR) values.  Using both direct attenuation and forward scatter technologies the 8364 can measure airborne particle sizes once available only from a transmissometer while having the reliability and cost effectiveness of a forward scatter visibility sensor. Its precision makes it ideal for applications requiring the highest in performance and reliability, such as aviation and meteorological studies.

Accurate measurement of visibility in all weather conditions, including heavy precipitation, fog and snow is limited in other sensors. Two-headed forward scatter visibility sensors that rely solely on light scattering techniques are blind to certain size airborne particles that do not reflect light. By measuring both the light attenuation and light scattering at the same time the 8364 computes ratio-metric values to derive the most accurate answer. This measuring process cancels several variables during calculation and ensures that the visibility measurement is not affected by contaminants on the lenses, or by temperature effects on the emitters and electronics.

By having two direct attenuation and two scatter values for every measurement the 8364 does not have to depend upon absolute measurements to be the most accurate sensor available today. This advantage means that measurements are independent of the effects of the environment, thereby maximizing accuracy, reducing recurring calibration, and minimizing maintenance requirements.

Every 8364 is calibrated through a scientifically valid chain of reference. The response of the calibration device can be clearly traced to the “FAA golden standard transmissometer” at the FAA testing facility. This standard was established in direct comparison during certification of the sensor for aviation quality measurements.

If one of the heads should fail the four-head configuration allows for continued operating with only three sensor heads. Built-in test (BIT) functions report the sensor head failure so that appropriate maintenance can be scheduled. The BIT functions also monitor power supply voltages, heater status, and indications of abnormal operation.

The 8365 Dual Technology Visibility Sensor measures transparency of the atmosphere and calculates its extension coefficient and meteorological optical range (MOR) values. Using both direct attenuation and forward scatter technologies, the 8365 can measure airborne particle sizes once available only from a transmissometer, while having the reliability and cost-effectiveness of a forward scatter visibility sensor. Its precision makes it ideal for applications requiring the highest in performance and reliability, such as aviation and meteorological studies.

The Model 8365 determines visibility by measuring the optical extinction coefficient of a beam of light as it passes through a known volume of air. Any particles in the air such as fog, rain, or snow will affect the extinction coefficient. This value can then be transmitted to an external computer in its unaltered form or translated into an equivalent MOR value in miles or kilometers covering a range of 33 feet to 20 miles (10 meters to 32 kilometers).

In AWOS systems, the extinction coefficient value is sent to the Model 1190 Data Collection Platform (DCP), which passes the value along with sensor status information to the Model 2090 Central Data Processor (CDP). The CDP then calculates visibility, variable visibility, and RVR (international systems only) over a visibility range of 33 feet to more than 20 miles.

When used in AWOS systems, the 8365 sensor does not require the optional Handheld Terminal; all setup, test, and calibration functions are accessible from the AWOS DCP's built-in keypad and display. Other AWOS features include support for a Day/Night sensor and an ambient light sensor (ALS), which is required for RVR applications. For 220VAC applications, a transformer kit is available to allow the sensor (which normally operates at 110V, 60 Hz) to operate from a 220V, 50 Hz AC supply.

The Model 8365 utilizes a unique two-sensor design that eliminates measurement errors and simplifies calibration. The sensing portion of the sensor (optical emitters and detectors) operates in conjunction with a visibility controller, which performs control functions for the emitters and detectors, performs built-in test and calibration functions, processes data, and calculates the extinction coefficient product.