Optical level switches are specialty devices which are employed for measurement of level in some special areas. Their operation basically depends upon the light transmission, reflection or refraction characteristics of the substance whose level is to be measured. They employ visible, infrared or laser light for level measurement. Their working is based upon the principle of total internal reflection.
A typical optical level sensor consists of
- Light emitting diodes i.e. LEDs,
- prisms and
LEDs emit infrared radiations whereas phototransistors act as a trigger by generating a digital output depending upon the existence or absence of fluid. “When a translucent liquid meets the point where the light contacts the prism, the light is reflected into the water and does not allow the photo transistor to energize.”1
Both the LED and phototransistor are housed enclosed inside a plastic dome at the top of the device. In case of absence of liquid, infrared light from the LED gets internally reflected from the dome to the photocell.
These switches can detect levels of all types of liquids such as clear, translucent, reflective, and opaque. Sometimes, they can measure levels in case of solids too. With the help of manifold photocells, a sensor becomes competent enough to measure numerous levels. They are available in following two major designs:
- Contacting Type
- Non-contacting Type
Following are the major features of optical level switches:
- These types of level sensors are capable of handling pressure upto 500 psig.
- Their temperature range extends up to 125°C or 260°F.
- They provide very fast response.
- Besides, their accuracy is very good (within 1 mm).
- Since they are made up of fiber optics, they are extremely resistant to electrical interferences.
- They provide economical and convenient solution for level detection.
- They exist in small and compact sizes and hence can be easily installed.
- They are available in numerous designs and can be applied for variety of applications. “For instance, PFA optical level switches are available for sensing the level of ultra-pure fluids. Other unique designs include a level switch that combines an optical with a conductivity-type level sensor to detect the presence of both water (conductive) and hydrocarbons (nonconductive).”2
- In general, they are applied in areas where ordinary level measurement devices like floats and displacers can not be conveniently employed and use of complicated devices such as ultrasonic, nuclear, radar or microwave designs proves to be costly or inappropriate for the job.
- They can be used to measure either fluid levels or interfaces between fluids.
- They are considered suitable to work with high foam levels if they are spray washed subsequent to every measurement.
- Optical level switches based on laser technology are the only sensors which are found suitable for intricate level measurements such as in case of molten metals, molten glass, glass plate, or any other type of process material which contains a reflecting surface.
- These sensors are usually provided with automatic washers to get rid of the coating deposited after every high level measurement.