Fog

It is a large density of small water droplets which are small enough to ‘float’ in the air. The size of fog particles is 5 to 50 μm.

The major optical methods used for detecting fog and in measuring visibility are-

Transmission method:-

Its is a classical method to measure visibility by measuring how much light is transmitted from light source to a receiver located at a distance for instance 100 meters away in foggy weather . Light will not reach the receiver because of the scattering of light by particles.

The scattered light will not be collected by the receiver. The instrument calculates that reduction which is used as raw data for further calculation.

Fog

Forward scatter or Backward scatter method :-

The main concept for building optical sensors for detecting particles in the air is by collecting and measuring the light scattered by the particles in air. The most common concept is to collect the forward scattered light.

The backward scatter concept is the most compact way for detecting particles in the air since the transmitter and receiver can be located close to each other in the same box.

The main disadvantage of the backscatter concept is that the optical power reaching the receiver is lower than that in the forward scatter case. This causes lower signal to noise ratio when using the backscatter method but can be compensated by using high performance electro optic solutions.

Fog

The Instruments for measuring fog are:- Free Space Optics (FSO):- It is a device using modulate visible or

infrared beam for optical communication through atmospheric line medium.

FSO system sends optical beam from one device to other using laser diodes with small output power. The receiver of the beam is

equipped with highly sensitive photoreceptors. Fog sensor is a device that measures data about density,

temperature and relative humidity. These are important parameters for subsequent static and statistical evaluation of the quality of free space optical communication environment . In order to estimate the attenuation due to fog, content of liquid water is measured in unit (g/m3). This task usually requires relatively complex and expensive measuring instruments.

Principle of fog density measurement :- Outdoor unit regularly sends and receives short infrared optical pulses. Amplitude of the reflected optical pulses is influenced by the reflectance of the elements in the environment.

Fog

This information may be used for a good estimate of liquid water content in the environment. In addition the optical sensor is also equipped with temperature and relative humidity sensors. These parameters provide important information and have characteristic values in foggy weather. The sensor is built for the rainy conditions and is shielded from direct sunlight, but allows free movement of fog droplets into the sensor.

Fog

Mist

1. Sigrist-Photometer AG

Scattering of light by condensed acid mist is used to measure acid mist content.  Sample is cooled to 120°C at which acid mist will condense by water will still be as a vapour.

2. Ankersmid B.V.

The SO3 or H2SO4 in the gas sample is absorbed as sulphate ions (SO4

2-) in an aqueous solution of propan-2-ol in water. The solution is passed through a bed of barium chloranilate. The acid chloranilate ions are measured in a continuous flow photometer. By maintaining a constant ratio of flow rates for the gas and the propan-2-ol absorbing solution, the concentration of acid chloranilate ions can be directly related to the sulphate ion concentration in the absorbing solution, and hence the SO3 concentration in the gas, which causes mist.

Mist

3. Hand-Held Electronic MonitorsContinuous hand-held monitors come in a variety of shapes and sizes. They can vary from a relatively simple single-gas monitor to a complex data logging four-gas monitor. They all make a quantitative analysis that is displayed on a digital or analog readout. Some also have the capabilities to store information, which can be downloaded to a computer.

Advantages: The readout is immediate. There is no waiting for a color change or waiting for a lab to analyze the results of a badge. The monitors also give real-time readout. What is being read on the display is what the gas concentration is at that time. Most hand-held meters also have a visible or audible alarm or both that will alert the user if a gas is above a safe level.

Disadvantages: This type of meter is usually calibrated with one type of gas and interference from similar gases could alter the readings. The operator must have adequate training to interpret the information that the monitor is providing, and also be familiar with calibration and limitations of the device. In comparison with other types of monitors, they are often more expensive.

Mist

Dust

Dust

1.Optical Opacity/Dust monitor The device operates using the double pass method according to the auto collimation principle. The light beam traverse the measuring distance twice .The attenuation of the light beam by the dust .The attenuation of the light beam by the dust content is measured and evaluated.ApplicationsFurnace plants, converter plants, cement manufacture etc.

2.Dust monitor Measuring device used to monitor small to medium dust emission using laser technology. The device works according to the principle of forward scattering. The concentrated and modulated light of a laser diode penetrates the measuring volume. The forward-scattered light largely reflected from the dust particles is measured and assessed.

Dust

Dust

3.Combined probe sensor Single rod measurement probe for simultaneous measurement of -dust concentration -volume flow -temperature -absolute pressureThe probe measures the electric charge of the incident particles. The measurement of volume flow is based on the mechanical action principle.The probe has two separate chambers between which a differential pressure builds up under flow .Dust concentration is calculated from the tribo electric measuring signal and the volume of flow.

Dust

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