Research project DeichSCHUTZ: Reliable measurements for safer waterfronts

Research project DeichSCHUTZ: Reliable measurements for safer waterfronts

In extreme flood situations, the hopes of those people affected lie solely with the dykes – will they hold or not? A dyke failure like the 2013 flood in Fischbeck (Saxony-Anhalt) caused immense damage to inland areas, which continue to have an impact to this day. The active research project DeichSCHUTZ (dyke protection) at the Bremen University of Applied Sciences is involved in an innovative dyke protection system that could prevent failures of this kind.

In Germany alone, river dykes safeguard many thousands of kilometers of waterfront lands. From today’s technological perspective, dykes consisting of three zones are being constructed. The individual zones, viewed from the water-side to the land-side, are built with steadily increasing porosity, thus affording good drainage of the dyke body during a flood event. In Germany, however, many older dykes of homogenous construction do still exist, such as the dyke breached during a flood of the River Elbe in June 2013 in Fischbeck. In contrast to the 3-zone dykes, older ones are particularly vulnerable to prolonged flood conditions. Water seeps into the dyke itself and its saturation line rises further within the dyke body over extended periods of high water. The further this saturation line rises, the more the ground material is subjected to buoyancy. The dyke thus loses its own essential self-mass, required to counteract against the pressure of high water.

Stabilization of a breach-prone dyke requires enormous resources in material and personnel, as well as time, which in acute flood situations is a scarce commodity.  Backup procedures are thus required, which, in terms of personnel, materials and time commitment, are more effective than layering sandbags on the landward side of the dyke.

Innovative mobile dyke protection system

Christopher Massolle of the Institute for Hydraulic and Coastal Engineering at the Bremen University of Applied Sciences is developing a solution that can considerably reduce the input of time and personnel. With the DeichSCHUTZ research project, sponsored by the Federal Ministry for Education and Research, an innovative, mobile dyke protection system is being tested for stabilizing dykes during flooding events. Measurement technology supplied by STS is also playing a role here.

To assess the mobile dyke protection system, a test-dyke has been built on the premises of the Agency for Technical Relief in Hoya. To this end, a U-shaped retention basin holding some 550 cubic meters of water has been constructed, at whose end sits a dyke.

As can be seen in the video, numerous pipes have been deployed at the left side of the dyke. Within these pipes rest ATM/N Level Sensors produced by STS. In the test arrangement, the retention basin is filled with groundwater. Under conditions approaching reality, the water should rise to a level of 3 meters over a period of 30 hours. The submersible level sensor ATM/N  now measure development of the saturation line over this time. With a pressure range from 1 to 250 mH2O and an accuracy of ≤ ± 0.30 %FS (-5 to 50 °C), this is recorded down to the very last centimeter. When the saturation line no longer continues to rise, the mobile dyke protection system is introduced to the water-side slope and should prevent the further penetration of seepage water. The dyke body now continues to drain and the extent of the resulting shift in the saturation line is to be measured by the level sensors employed. It is from these measured results that the functionality of the protection system can lastly be assessed.

Polluted sites: Groundwater decontamination requires robust level sensors

Polluted sites: Groundwater decontamination requires robust level sensors

Whether it be old landfills, coal tips, former military sites or refineries, what remains behind is contaminated ground, which is a danger to both humans and the environment. In the rehabilitation of these sites, level sensors are required which are resistant to the often aggressive hazardous substances encountered.

Contaminated sites are not only characterized by adverse health or environmental changes in the soil. In the absence of safety measures (as in old landfills) and depending on soil conditions, hazardous substances are flushed by rain into the groundwater. Depending on the type of usage, a number of different hazardous substances can be found, including, among others:

  • Heavy-metal compounds: Copper, lead, chromium, nickel, zinc and arsenic (a metalloid)
  • Organic materials: Phenols, mineral oil, benzenes, chlorinated hydrocarbons (CHC), aromatic hydrocarbons (PAH)
  • Salts: Chlorides, sulfates, carbonates

Decontamination of the groundwater supply

In the rehabilitation of contaminated sites, not only is cleansing of the soil of great importance, but also the control and purification of the groundwater. Without reliable level sensors that can withstand the adverse conditions, this would not be possible.

The decontamination process usually proceeds as follows: The contaminated groundwater is pumped to the surface and then treated. As filtered flush water, it is next returned to the source of contamination. To prevent this flush water from flowing to a margin away from the contamination source, active hydraulic methods are used for protective infiltration. Water is injected into the ground via several wells situated around the actual decontamination process. The pressure conditions arising here to some extent form a barrier wall and cause the flush water to flow towards the source of contamination. For controlling and monitoring this process, level sensors will be required.

Figure 1: Flow of a decontamination process

Level sensors are of course also used after the remediation work. Long after completion of this work, the affected sites will be monitored to check for any noticeable changes in the water level or the direction of flow.

Level sensors are also used when actively running applications potentially damaging to the environment. Newer landfills are now built like an impermeable basin. The groundwater level below the landfill is lowered, so that no water can flow into adjacent areas in the event of leakage. Here also, the respective water levels are to be monitored by level sensors.

Level sensors in contaminated waters: Highest demands 

Operators in the field of decontamination of polluted sites should be very careful in choosing suitable level sensors. Due to the large number of substances that can be dissolved in the water, there is no single solution that works reliably in every instance. There are several aspects to consider, which we next briefly outline.

Materials

Housing

In most applications, a high-quality stainless steel, as used by STS, is sufficient to protect the measuring cell from aggressive substances. If this were to come in contact with saltwater, then a titanium housing would be chosen, but where galvanic effects are to be expected, a level sensor made of PVDF should be the selection.

Figure 2: ATM/NC chemically resistant level sensor with PVDF housing

Probe cable

Far more critical than choosing a suitable housing, in our experience, is the choice of probe cable. Because of gradual diffusion processes, the progress of destruction is not immediately apparent. Often, this is not visible from the outside even when already damaged. Special caution is therefore required when consulting resistance tables, since these usually say little in particular about probe cables. In the middle of a probe cable is a small air tube, which serves for relative pressure equalization. If the cable material is not one hundred percent resistant, however, raw materials may diffuse through the cable sheath and travel across the air tube into the sensor chip.

Depending upon the substances anticipated, STS users can resort to PE, PUR or FEP cables. The latter can also be used at very high temperatures of up to 110 °C.

Installation

Cable routing

Old landfills and industrial sites are harsh environments, where not only hazardous substances can impair the functionality of the level sensors used. Care must be taken that the cable sheath is not damaged by mechanical burdens (such as debris). Chafing and kinking points should also be avoided. It is therefore recommended to use special protective tubes, such as those offered by STS, when routing cables.

Strain relief

The compression rating of level sensors varies from manufacturer to manufacturer. At STS, all level sensors are pressure-resistant up to 250 meters as standard and their cable is designed for normal tensile strains up to this depth. Nevertheless, operators should consider the use of strain relief in difficult installation conditions.

Mounting

If the sensor is used in flowing waters or tanks with agitators, it can be supplied either with a G 1/2” thread at the cable outlet (pipe mounting) or with a compression fitting (15 mm).

Explosion protection

In applications where a number of hazardous substances are to be expected, it is imperative to also pay attention to explosion protection. Information about this is given by the international standards-compliant ATEX directive.

Conductivity measurement in natural waters & other liquids

Conductivity measurement in natural waters & other liquids

When measuring conductivity, various factors have to be taken into account, depending upon the liquid being tested. Particular attention is to be paid to temperature as the major influencing factor.

Conductivity is expressed in microsiemens and indicates a substance’s ability to conduct electric current. The conductance is the inverse of resistance, as expressed in ohms. It thus follows that the higher the conductance, the lower the resistance.

Conductivity in natural waters

Pure water is practically non-conductive (0.055 µS/cm, compared to drinking water at 500 µS/cm). It becomes conductive only through dissolved substances such as chlorides, sulfates and others. The purity of a water body can thus be determined via a conductivity measurement, where the higher the conductivity, the more substances are dissolved in the water. Typical applications for conductivity measurement include, for example, landfills for testing groundwater contamination and the monitoring of salt water ingress into groundwater sources. This makes conductivity an important factor for monitoring tasks in environmental technology in order to draw conclusions about possible impurities. Although conductivity is only an indicator of pollution, the composition of the substances entering the water must subsequently be chemically analyzed. In addition, not all substances that can be dissolved in water are also conductive (e.g. hormones or fungicides).

Another common application is the determination of flow direction and flow velocity. For this purpose, salt is added to the water and its conductivity accordingly increased. Flow velocity and direction can be precisely determined by measuring the conductance at specific points.

As already mentioned, the conductivity of a substance is highly temperature-dependent. Two samples of a substance can therefore produce different conductance values at different temperatures. Without temperature compensation, there is practically no possibility of comparing two substances if they cannot be examined at exactly the same temperature. For this reason, conductivity measurement and temperature measurement go hand in hand. Usually, therefore, both the conductivity and the temperature are measured in a single conductivity measurement. Temperature compensation is then used to calculate the conductance at a reference temperature, which is normally set at 25 °C.

Temperature compensation function: The substance decides

Which temperature compensation function is used to determine the conductivity at reference temperature depends entirely on the liquid being examined. For natural waters, the non-linear function according to the DIN EN 27888 standard  for water quality is used.

Linear functions are used for salt solutions, acids and alkalis. To calculate the percentage change in conductivity (K) per °C temperature change (∆T), we use the following formula:

α = (∆K(T)/∆T)/K(25°C)*100

K(T) = Conductivity change from the selected temperature range
T = Temperature change from the selected temperature range
K(25°C)= Conductivity at 25°C

Finally, let us consider an example calculation for determining the conductivity of a quick descaler. To obtain the necessary figures for the calculation, three measurements must be carried out:

122.37 mS/cm at 20°C
133.10 mS/cm at 25°C
135.20 mS/cm at 26°C

K(T) = 135.20 mS/cm -122.37 mS/cm = 12.83 mS/cm
T = 26°C – 20°C = 6°C
K(25°C)= 133.10 mS/cm

α = ((135.20 – 122.37)/(26 – 20))/133.10*100 = 1.60 %/°C

Hydrostatic pressure measurement with piezoresistive level sensors

Hydrostatic pressure measurement with piezoresistive level sensors

Whether as a life-giver, a danger to life or simply a refreshment in summertime, the element of water determines daily life on earth in many ways. Because of its sheer importance, a reliable monitoring of this element becomes essential.

What cannot be measured can also not be managed efficiently. From fresh water supply, drinking water treatment, storage and consumption measurement, to waste water treatment and hydrometry, it will not be possible to work and plan efficiently without the correct input parameters. A range of devices and processes are now available to capture today’s complex hydrometric infrastructure. The classic in water level measurement is without doubt the level gauge, for which an accuracy of +/- 1 cm must be applied and which, of course, still functions completely “analog” – having to be inspected visually and doing without electronic data transmission. Today, far more advanced and precise instruments provide remote transmission of the measured data, including piezoresistive pressure sensors for water level measurement in both groundwater and surface waters.

Level measurement with pressure sensors

Pressure sensors for level measurement are installed at the bottom of the water body to be monitored. In contrast to level gauges, it is generally not possible to read them without getting wet. This is not necessary either, since piezoresistive level sensors were developed to meet today’s requirements for process automation and control. It goes without saying that water levels can thus be measured without human intervention, which makes continuous monitoring at difficult-to-access locations possible in the first place.

Hydrostatic level sensors measure the hydrostatic pressure at the bottom of the water body, where the hydrostatic pressure remains proportional to the height of the liquid column. Additionally, it is dependent upon the density of the liquid and gravitational force. According to Pascal’s law, this results in the following calculation formula:

p(h) = ρ * g * h + p0

p(h) = hydrostatic pressure
ρ = density of the liquid
g = gravitational force
h = height of the liquid column

Important considerations for trouble-free level monitoring

Because piezoresistive level sensors are placed at the bottom of the water body, they are then protected from surface influences. Neither foam nor flotsam can now influence the measurements. But, of course, they do have to be adapted to the expected underwater conditions. For salt water, for example, a level sensor with a titanium housing is to be preferred. Should galvanic effects be expected, however, then a measuring device of PVDF would be the best choice. In most freshwaters, high-quality stainless steel will be sufficient. And lastly, a sufficient grounding of the level sensors is essential to prevent damage from lightning strikes, for example (read more on this topic here).

Modern level sensors: All data from just one device

Piezoresistive level sensors can be used for level monitoring in open waters such as lakes, in groundwater occurrences and also in closed tanks. In open waters, relative pressure sensors will be used. With these devices, air pressure compensation is provided by a capillary inside the pressure sensor cable. A differential pressure sensor is normally used in tanks, since the gas overlay pressing down on the liquid must also be taken into account (read more on this topic here).

Because piezoresistive level sensors are largely self-sufficient and can also be optimized for very high pressures, measurements at great depths now become a possibility. Theoretically, there are hardly any limits to this depth, only that the pressure sensor cable has to be long enough.

Figure 1: Examples of level sensors for hydrostatic pressure measurement

Apart from the fact that there are hardly any depth limits, these modern measuring instruments are also extremely versatile. After all, it is not only the level of a water body that is of interest to us. Water quality is also of great importance for groundwater monitoring. The purity of a groundwater reservoir, for example, can also be determined by its conductivity, where the lower the conductivity, the purer the water will be (read more about conductivity here). In addition to conductivity sensors, level probes today are now also available with integrated temperature measurement. Piezoresistive level sensors provide a wide range of monitoring tasks and are without question preferable to the level gauge in most cases.

Level monitoring for pump control in rainwater and wastewater tanks

Level monitoring for pump control in rainwater and wastewater tanks

Water supply and wastewater disposal vary according to local conditions. In Belgian buildings, many cellars are situated deeper than the sewage system. Wastewater disposal here must therefore be regulated by pumps.

The Belgian company Pumptech provides home owners and caretakers with powerful industrial pumps, through which water circulation within the buildings is partly regulated. This is essential in various regions of Belgium, because the cellars in the buildings there are often located beneath the sewage system.

Since this wastewater cannot flow directly into the sewage system, however, it is temporarily stored inside tanks. Rainwater is also often collected in these buildings and then used for sanitary facilities. The rainwater hitting the roof is fed into underground tanks where it remains available for further use. As wastewater, it finally flows into the separate wastewater tanks, from where it is then pumped into the sewage system.

Whether in these wastewater or rainwater tanks, monitoring of the levels is essential for a regulated operation of the pumps. For this purpose, Pumptech has been using ATM.ECO/N submersible probes for 15 years now. Originally, level monitoring was performed here by float switches. As it turned out over time, this was an unsatisfactory solution – especially in regards the wastewater tanks. The big disadvantage of float switches in comparison to immersion probes is that they quickly become dirty due to impurities floating on the water surface and will then no longer work properly. This can have far-reaching consequences, since the pumps themselves are controlled by measurement of the filling level. Usually there are two to three pumps inside the tanks. When a predetermined level is exceeded, the first pump starts operation, with the second pump cutting in at the next fixed level. Alarms can also be triggered should certain limits be reached

Submersible probes, which are usually installed at the bottom of the tank, are not particularly susceptible to waterborne contamination. Once Pumptech had tested various suppliers, their choice eventually fell on the analogue level probe ATM.ECO/N from STS, since these best met their requirements when compared to competitors in regards their required long-term stability. Since then, these pump controls have been working away without incident.

The ATM.ECO/N immersion probes boast a fully sealed membrane made of high-quality stainless steel. A moisture filter on the pressure connection cable also prevents water or other contaminants from entering its measuring cell. A further advantage is the far better reaction time when compared to the previous float switch solution, which now allows users to see immediately what is happening inside the tanks.

You can find the data sheet for the ATM.ECO/N level probe here.