WATER & WASTEWATER
Laser-based water level measurement technology By Nicolas Ho
L
aser-based level measurement is gaining more and more popularity for water and wastewater applications. Up to now, it was severely limited for such uses because of its inability to detect clear liquids reliably. However, recent technological advances allow laser level measurement for these applications. The advantages of laser level measurement can now be relied upon for a number of challenging level applications. The LLT100, ABB’s new laser level measurement product, uses laser time of flight to measure the level of any solids or liquids. By the use of improved laser pulse control, detection performance, and powerful algorithms, it can reliably measure any liquid regardless of color, transparency, or turbidity. Liquids can be highly agitated without impact on measurement. By using a laser beam, the transmitter is not affected by the presence of gases. It can work under vacuum, and its accuracy is not affected by the type of liquids or its composition. The measurement of clear liquids was extensively tested in a number of different conditions. LLT100 performed well on all tested liquids, from colored liquids to transparent ones such as pure water or transparent mineral oil. Reliable measurements can be done at distances up to 30 m. LLT100 also tracks the liquid surface level even in the presence of waves and surface agitation. In Figure 1 waves are created in a clear water tank. There is no damping, so fast changes can be observed. As can be seen, LLT100 always detects the surface even in strong agitation. When no agitation is present, on the right of the plot, the standard deviation of the measured level is ± 1 mm, showing the repeatability of the measurement. In the presence of dense foam, the LLT100 will return to the level of the top of the foam. However, it is not confused by foam and the measurement is not lost or erratic. The laser beam of the LLT100 has a very small beam divergence. After traveling about 6 m, it has a width of only 50 mm since it has a divergence of only 26 | June 2019
LLT100 tank installation.
Figure 1. LLT100 measurement of agitated water.
0.3 degrees. Performing the level measurement with such a narrow beam has many advantages since the narrow beam doesn’t interfere with local structures and fitments. Therefore, no echo mapping is necessary. This means that the LLT100 will automatically track the water surface. Even if the space is limited, the laser beam will find its way to the water surface and the transmitter will detect it without the need to be configured for a specific application. This is very advantageous when there are a lot of internal structures, when an agitator is used in the vessel, or when the local area changes. Since the LLT100 laser beam doesn’t interfere with surrounding objects, it is immune to their presence. Typical applications are lift
stations, deep wet wells, pumping stations, slurry tanks, and liquid reservoirs. CUTTING STILLING WELL MEASURING COSTS Over the course of a few days, dirty water in a stilling well will coat everything. As such, measurement products in contact with the water require maintenance every week. When using laser measurement, it is possible to measure up to when the buildup becomes so severe it almost completely blocks the stilling well. Often this can take about six weeks. This leads to significant maintenance cost reductions. Nicolas Ho is with ABB. For more information, email: ken.a.burnett@ca.abb.com
Environmental Science & Engineering Magazine