LASER LEVEL TRANSMITTER MEETS WET WELL CHALLENGES

Problem:
Sewage pumping stations are used by waste water utility companies to transport raw sewage from residential and commercial areas to the central treatment works. The waste is collected in a "Wet well" which can be up to 80m deep in some instances. The measurement of the sewage level is critical in order to control the operation of the very large pumps, submerged at the bottom of the well and used to empty the well and transport the sewage to the next collection point. Until now the only options have been pressure transmitters which are notoriously unreliable and become less accurate as the level drops, or Ultrasonic transmitters. Ultrasonic devices emit a beam that quickly increases in diameter, which inevitably results in ‘false echoes’ from the wall of the well and are unable to measure the level once it drops below the pumps themselves. 

Solution:
LM80 LASERMETER level transmitters, supplied by level and flow specialists ALLISON ENGINEERING. The Laser is a very high frequency, narrow wavelength, optical device with a beam width of no more than 5cm at a range of 30m.This means the laser can be targeted at a specific area, avoiding the pumps & providing accurate & reliable level control. This beam angle versus wavelength is illustrated in Figure 1, which compares the three main level measurement technologies. For example, for Ultrasonic a beam angle of up to 1500mm over 10 metres and for Radar, up to 1000mm over 10 metres, both significantly more than the 35mm for laser.  The LM80 provides a 4-20mA analogue output together with 2 x relay outputs to start and stop the pumps. 

Conclusions:
LM80 LASERMETER transmitters can be mounted in any orientation and operate effectively even if a glass or plastic shield is required for protection against aggressive or pressurised applications. In addition to deep, wet well applications the benefits of the LM80 transmitters will be appreciated when installed on tanks, silos, hoppers, bins and mixing basins where media includes food and beverage ingredients such as, chocolate and sugar, and equally in other industrial applications involving media such as coal and plastic pellets.

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