LEVEL SENSOR

FUNCTION

 

The purpose of remote level sensors in sewer collection systems

is straightforward: measure and monitor the water levels at

manholes, pipes or lift station wet or dry wells. There are a wide

variety of level sensors and level measuring techniques.  The list

here is meant to be descriptive of the most commonly used level

sensors and is not intended to be exhaustive.

 

There are two classes of level sensor: contact and non-contact.  While both classes of sensors are widely used, contact sensors have the disadvantage of requiring more frequent service due to the accumulation of debris common in wastewater.  In some cases, this accumulation can go unnoticed, leading to erroneous readings or false alarms.

 

Contact sensors include the following general types:

 

  • Mechanical sensors, such as floats. There are two basic classes of floats.

    • Suspended floats are usually suspended by cables and are triggered when the water level causes the float read-head to tilt.  Suspended floats use no power and may be arranged in a vertical series to provide an indication that water levels have reached various fixed levels.  Suspended floats are useful for cycling lift station pumps, for example, between high and low water levels. Suspended floats provide no level data beyond the position of the floats, but they respond immediately to a fixed water level.

    • Constrained floats  are made of two components: the buoyant head that floats at the water level, and a pole or rod on which the buoyant head rises and falls. This type of float system provides continuous level data and uses little or no power.  The water level is read via capacitive or conductive means.

 

  • Pressure sensors operate through direct contact of the water with a pressure transducer.  Since the pressure at the transducer is the sum of the atmospheric pressure and the level of water above the sensor, a measurement of the atmospheric pressure is needed to properly calibrate the water level above the pressure measurement.  This is typically accomplished with a vent tube.  Maintaining an unobstructed vent tube can be a challenge in sewer environments. Pressure sensors must be powered, and may be cycled on and off to lower power requirements.  Communication to/from pressure sensors in situ is provided via cable, although in some configurations wireless means, such as Bluetooth, could be used to transmit data locally. Pressure sensors are commonly used for lift station applications, although they may be used in pipes where water levels are high enough for reliable pressure measurement.

 

  • Bubblers are devices that force a stream of air bubbles through a tube placed in the water.  These sensors measure the force required to inject bubbles into the water, a direct function of the pressure at that location.  Bubbler sensor heads are located above and out of the water and these heads provide ambient atmospheric pressure measurements, allowing calibration of the force and hence water level. Bubblers are powered and provide continuous level measurements.

 

Non-contact sensors include the following general types:

 

  • Ultrasonic sensors are the most common non-contact sensor for sewer water level measurements. Ultrasonic sensors measure time-of-flight of ultrasonic pulses reflected from a water surface. Level is calculated by knowing this distance, ambient temperature, and the vertical position of the sensor head.  There are a wide variety of styles and specifications of ultrasonic sensors. short range (up to 2 meters) to long range (up to 10 meters). Important operating characteristics of ultrasonic sensors are: range (2 meters to 10 meters maximum range is typical); “dead zone”, the distance from the sensor that is effectively unreadable; level resolution and accuracy; and beam spread, the dispersion of the ultrasonic beam as it leaves the read head. Wide beams can cause problems in tight spaces like manholes, where ladders or walls may interfere with attempts to gain high dynamic range. Well-known problems with ultrasonic sensors include difficulties measuring in mist, foam covered surfaces, or highly turbulent surfaces.  Many of these historical shortcomings can be overcome with signal processing techniques. For example, ultrasonic sensors require power, and may be cycled to minimise power consumption for battery-operated systems.

  • Radar is sometimes used to measure water level in sewers. Radar operates by measuring the time-of-flight of radio frequency pulses bouncing from a water surface.  Radar uses more power than ultrasonic sensing, and due to its complexity, tends to be more expensive than ultrasonics.  However, radar can be used in conditions where ultrasonic sensing has had troubles, for example misting.  Most radar applications are associated with lift station level measurement.

 

  • Optical ranging, using for example LED light sources, has been suggested as a level sensing method in sewers.  Successful in many industrial and commercial applications, the fundamental issue with optical ranging is keeping the optical elements clean in a sewer environment, characterised with condensing humidity, high levels of dust and dirt, and moisture. Due to its high maintenance requirements in sewers, optical ranging is not recommended.

 

BENEFITS

 

Sensors are the foundation of any good network monitoring system. They provide the data to allow engineers to make informed decisions on the setting and management of their network. Without consistent, accurate and reliable data, it is difficult to manage and adjust network conditions over time. Level sensors enable the intelligent monitoring of water levels in the wastewater network over time. 

 

 

SYSTEM REQUIREMENTS

 

Level measurement systems can be used as data loggers, or they can be used to transmit real-time data to a central server where it can be analysed and used to make operational decisions.

 

For data logging level systems, the following are required for operations:

 

  • Power, may be battery power or local line power;

  • A data storage module, allowing a time series of level measurements to be stored for later manual retrieval;

  • A user interface, such as a USB port, to download the data;

  • A control module providing intelligence for the operations of the ultrasonic sensor.

 

For real-time data transmission level systems, the following are required for operations. There are two types of real-time data transmission level systems, one-way - the data come from the sensor but there is no back channel communications, and two-way - the data come from the sensor, and data or commands can be sent to the system, allowing operational changes such as timings or signal processing filter parameters.

 

  • Power, may be battery power or local line power;

  • A control and communications module that directs the sensor operations and provides external communication, either wired or wireless.  Wired communications generally require a more complex and expensive installation, but they can be more reliable and secure;

  • In the case of wireless communications, an antenna.