How to make sensor it all

There is a bewildering choice of water-level sensors out there - from bubblers to ultrasonic transducers - with not many comparative reviews available to make things clearer. Here is a guide to sensors that could help you make that choice.

In the water and wastewater applications, reliable and accurate water-level sensing is critical to collection, conveyance, and treatment. Because level sensor failures can lead to system backup, loss of functionality, and wastewater overflows, selecting proper and reliable sensors and monitoring equipment is crucial.

There are many types of water-level sensors available in today's market. With few objective or comparative reviews available, selecting a level-sensing technology is challenging.

Stormwater, wastewater, and industrial effluents carry small and large particulates, minerals, debris, and other fouling agents that clog, corrode, and interrupt the functionality of water-level sensors. Fats, oils, and greases (FOG) contained in most wastewater streams are particularly problematic. Many liquid level-sensing technologies foul rapidly, and have significant limitations for wastewater applications.

The most common sensor types used in wastewater, stormwater and industrial effluents are ball floats, bubblers, hydrostatic pressure transducers, ultrasonic transducers, and conductive methods.

The main attraction of ball floats is simplicity. They hang from their cable at a predetermined level, and when the liquid level rises, it lifts up the float and tilts it, closing a contact. A control device then detects the contact closure.

Each ball float can be used for only one level-sensing point.

The ball float is one of the oldest technologies in use, and despite many drawbacks, is still the most widespread primary level measurement device in the US. In the UK, utilities have mostly switched away from floats.

Ball floats are by far the least reliable level-sensing devices. Tangled floats are common and often cause the switch mechanism to become seized open or closed, resulting in overflows, spills, or constant running of pumps. Also, FOG buildup can quickly foul the switch mechanism, and has the same result.

Bubblers are simple devices that force a gas through a hole that is located at a fixed point underwater, making bubbles. As water level rises, the amount of pressure required to produce bubbles increases, causing increased back pressure in the bubbler line. A guage monitors the back pressure of the gas in the line, and calculates the depth of water above the submerged bubbler.

One benefit of bubblers is that no electrical connection is made to the wetwell, and the device is therefore intrinsically safe. But bubbler accuracy can be reduced by leaks that form over time in the bubbler line, or by debris buildup.

To address debris buildup, many bubblers include a cleaning discharge valve. Strategic placement of the bubbler, depending on specific conditions and the required application, can also help reduce the frequency and severity of clogging. Generally speaking, a line leak will result in an underestimate of water level by the bubbler guage, potentially causing overflows.

Data logger
As a result of clogging and leaks, the UK and Australian wastewater industries moved away from bubblers years ago.

Pressure transducers typically consist of a sensing instrument housed inside a cylindrical, stainless-steel tube. The sensor end of the stainless steel tube contains a sensor or diaphragm made of ceramic, silicon, or stainless steel, or, less commonly, teflon, PVC, neoprene, or other material, while the other end is sealed to a cable that is connected to a data logger, switch, or guage.

In water and wastewater applications, the pressure transducer is mounted at a fixed level below the water's surface. As the water level increases, the amount of pressure experienced by the instrument also increases, as does the pressure on the sensor. The sensor then converts the amount of pressure into an electronic signal, which is relayed through the cable to a PLC/controller or other device.

Similar to bubblers, pressure transducers contain no moving parts. Also, like bubblers, these instruments can be subject to occasional fouling, especially when installed for use in untreated wastewater. Fouling occurs along the sensor or diaphragm, reducing its movement or sensitivity, and reducing sensitivity to changes in water level. Deterioration or fouling of the vent tube is a common cause of problems.

Ultrasonic sensors are installed above the highest water level, and provide level measurements using emitted sound waves. The sound waves are directed intermittently downward to the water surface, where they reflect off the water and return to the transducer. The transducer then measures the period of time that passed between emitting and detecting the reflected sound waves. It then translates this value into a distance of wave travel, and a corresponding measure of the water level. Ultrasonic sensors thereby produce a continuous measurement of level.

Ultrasonic level sensors have the substantial advantage of measuring water level without ever physically contacting the water. Given that the primary problems of almost all other level sensors commonly used in wastewater relate to contact with wastewater, this is a significant advantage over other technologies.

Temperature sensor
Because the speed of sound varies with air temperature, most ultrasonic instruments include a built-in temperature sensor to correct for air temperature fluctuations. A built-in temperature sensor will only correct for temperature at the level of the sensor; it will not account for any additional variation in air temperature between the sensor and the water surface.

However, in most cases, changes in air temperature would result in a less than 5% reduction in accuracy, which would not significantly affect most wastewater applications.

Ultrasonic level sensor accuracy may also be reduced by excessive debris, turbulence of waves, and agitators or other objects. Some sensors include electronic filters that reduce the noise generated by such obstructions. High concentrations of suspended sediment can dampen the reflected signal and prevent detection of the water surface level. Also, build-up of films or debris on the sensor head, even condensation, can reduce sensor efficacy. Problems also occur with wildlife - spider webs and frogs causing false readings.

While ultrasonic sensors have improved significantly over recent years, many problems can occur during or immediately following installation.

Conductive rods were very common some years ago, but are rarely found in new installations. The control device for the conductive rod applies a low voltage signal to a conductive sensor. As soon as the liquid contacts the sensor, an electrical connection to ground is formed, triggering an on-off switch, alarm, or other device. Like ball floats, each conductive rod provides a single point of measurement.

With no moving parts, conductive rods should be more reliable than ball floats. But in practice this is not the case. Conductive rods are connected to the side of the wetwell with a variety of structures. These structures inevitably become fouled with debris. These buildups often cause inaccurate level sensing.

More than 20 years ago, MultiTrode developed a solution to address the drawbacks of ball floats, conductive rods and other level-sensing technologies. Developed specifically for wastewater, and other harsh liquid environments, the MultiTrode Conductive Probe consists of a series of conductive sensors on a uPVC probe. The standard probe is available in a range of lengths, and has either three or ten equally spaced sensors. The ten sensor probe provides 10% resolution along its length, rather than the single measurement point provided by ball floats and conductive rods.

To reduce fouling, the probe hangs under its own weight and is mounted close to the inflow. Turbulence associated with the inflow, as well as the slim profile of the probe, keep the probe relatively clean. With no moving parts and no structures for debris to get tangled up in, the probe provides the simplicity of the ball floats and conductive rods, but with much less maintenance and much greater reliability.

Control device
Of course, buildup inevitably occurs on the MultiTrode probe. But, in most applications, the buildup is conductive enough that the probe continues to provide readings. With the right conductivity setting for the application, the buildup becomes invisible to the control device, and accuracy is not compromised. One of the most common problems for the probe is that it can be incorrectly installed in the quietest part of the well - not near the inflow. This can result in substantially increased levels of buildup, since the turbulence around the intake has a cleaning effect. The additional buildup on an incorrectly installed probe can produce false readings. But this problem can be easily remedied by re-installing the probe in its proper location.

No electronics
With no moving parts, and no electronics, the product typically has a 20-plus year life and requires no specialist knowledge to install and commission. Surveys of a large number of end-users indicate that there is a large diversity of opinion about the reliability of different level-measurement technologies.

Some end-users threw out ball floats 20 years ago, while others feel that ball floats are the most reliable system ever. Some users refuse to try another ultrasonic because of perceived reliability issues, while the UK water industry has adopted them as standard.

And 15 years ago, when conductivity methods seemed as though they would be superseded by newer technologies, large numbers of end-users today claim that the MultiTrode probe is the most reliable level device that they have used.

Clearly, level measurement in wastewater, stormwater, and industrial effluent applications is not a simple exercise.

As of yet, there is no perfect solution.



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