Level loggers monitoring water levels in Venice

Level loggers monitoring water levels in Venice

by | Wednesday, 1 July, 2020 | Water

The Piazza San Marco will never flood: Level data loggers from STS are in use to continuously measure groundwater levels at the Piazza San Marco. These are particularly robust and are also suited for application in various scenarios.

In 2003, the company of S.P.G. began to install several groundwater dataloggers at the Piazza San Marco in Venice. These were designed for the specific demands and possess, above all, the attribute of withstanding several days submerged in saline waters, since on rising tides, Piazza San Marco is regularly flooded. The site operates in conjunction with efforts initiated by the water regulatory authority for protecting the lagoons and the city of Venice from flooding.

The appointed consortium of Venezia Nuova earmarked the wharfage opposite the Piazza San Marco with innovative technical features. The challenge consisted of monitoring the flow of groundwater, which was by degrees shifting from the site area to the buildings located behind. At the client’s request, level data loggers from STS were installed to continuously measure the fluctuations in groundwater levels.

The groundwater datalogger permits a simultaneous measurement of level, temperature and conductivity in ranges of 0…50 cmWS to 0…250 mWC, -5 to 50 °C and 0.020…20 mS/cm. When required, the end user can at any time retrofit a data transmission unit. The logger features a simple, user-friendly operation, an extended measurement memory for up to 1.5 million readings and a probe diameter of only 24 mm or 10 mm.

The plug-in units also allow for the possibility of cable extension. New software functions can also be updated without their requiring inconvenient return through the end user. The standard lithium batteries can be changed on site in no time. Data can be transferred in ASCII or XML format and further processed using standard software such as Excel. Variable data-saving intervals dependent on pressure or time allow for versatile measurements.

Through the use of various materials including stainless steel, titanium, PUR, PE or Teflon cable, a high medium tolerance is attained for the most varied of applications, such as landfills, contaminated sites, pump trials, high-water alarms and discharge/overflow logging in rain overflow basins.

Original publication: Konstruktion magazine

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Water in spite of drought

Water in spite of drought

by | Wednesday, 1 July, 2020 | Water

Water management experts at the Karlsruhe Institute of Technology (KIT) have constructed a subterranean dam with an integrated hydroelectric plant inside a karst cavern on the Indonesian island of Java. The power station located 100m below ground now provides plentiful water from the cavern during the dry season. Two data loggers installed there measure the water levels both in front of and behind the dam wall. The level of the upper water reaches 15 – 20m, while the lower level, where water discharges again from the turbine, attains a maximum of 2m.

The karst area of Gunung Kidul on the south coast of Java is one of the poorest regions in Indonesia. The ground is too barren for a bountiful supply and in the dry season the flowing waters actually run dry. Water from the rainy season peters out quite quickly, but does collect within an underground cave system. This natural water reservoir has now been harnessed with a cave power station. The fact that even in the dry season over 1,000 liters of water per second flows through the Bribin Cave speaks for the ideal location of this dam. Instead of complex turbines, the mechanical energy to drive the feed pumps is generated by reverse-driven circulation pumps. The five parallel-operating feed pumps are thus highly cost-effective, incurring only minor operating and maintenance costs. The supply pumps send part of the water 220m high to a lake named Kaligoro Reservoir situated upon a mountain. The key stumbling block to this project was successfully overcome during the test damming phase. The cave did effectively hold the water and a crucial dam height of 15m was indeed achieved.

In March 2010, the installation was then handed over to the Indonesian authorities. It can now provide 80,000 people with up to 70 liters of water per day. Previously, the populace had only 5 – 10 liters available per day during the dry season, for personal hygiene, household and livestock purposes. Incidentally, each German uses on average 120 liters per day, for comparison.

Function of the pressure data loggers

The pressure loggers measure the water levels in front of and behind the dam wall. The normal level amounts to 15m, but it can reach up to 20m during heavy rainfall. The other probes measure the water level whilst submerged, in particular where water discharges from the turbine. Levels of up to 2m are recorded in this area. The pressure loggers from STS were chosen due to their high overload capacity of 3x their full-scale range, the low characteristics deviation of maximal 0.1% and an enhanced long-term stability of between 0.1 % und 0.5 % FS per annum.

These level loggers cover pressure ranges between 0 – 100 mbar and 0 – 600 bar, thus permitting level measurements in the ranges of 0 – 100 cmAq to 0 – 6,000 mAq. The measurement interval itself is variable between 0.5s and 24h. The units are further distinguished by a measurement data memory of up to 1.5 million measured values and a narrow probe diameter. Additionally, their standard lithium batteries can be swapped out on site in no time at all.

Variable data-saving intervals dependent upon pressure or time permit for flexible measurements. With the use of various materials like stainless steel, titanium, PUR, PE or Teflon cable, a high medium tolerance is achieved, allowing for the most varied of applications. Besides the level recordings of groundwater, wells, boreholes, lakes and rivers, these level loggers are also suited to leak testing in gas, water and other pipeline projects, as well as pipeline analysis and pressure testing in gas, water and community heating pipeline networks. They have also proven themselves optimally in gas pressure control stations and in the verification of a constant supply pressure.

Sources: Karlsruhe Institute of Technology (KIT) – Institute for Water and River Basin Management (IWG)

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Hydrostatic level monitoring of tanks on piezoresistive basis

Hydrostatic level monitoring of tanks on piezoresistive basis

by | Wednesday, 1 July, 2020 | Water

Hydrostatic pressure measurement is one of the most reliable and simplest methods for fill level monitoring in liquid-carrying tanks. In the following, we explain how hydrostatic level monitoring works and what users should consider here.

In hydrostatic level measurement, the filling level of a liquid in a container is to be measured. In this case, the force of weight acting on the pressure transducer installed at the bottom of the container is measured. The weight force in this context is termed the liquid column. It increases in proportion to the filling level and acts as a hydrostatic pressure on the measuring instrument. The specific gravity of the fluid must always be considered in hydrostatic level monitoring. The filling height is thus calculated with the following formula:

h = p/sg

In this formula, h stands for the filling height, p for the hydrostatic pressure at the base of the tank and sg is the specific gravity of the liquid.

The actual quantity of fluid plays no role in hydrostatic level monitoring, since only the filling height is decisive. This means that the hydrostatic pressure is identical in a 200 liter tank narrowing towards its base and in a straight sided tank containing 150 liters of liquid, as long as the liquid and the fill height are identical (3 meters, for example).

The simplest application of hydrostatic pressure measurement is when the liquid concerned is water, since the specific gravity can be disregarded altogether here. When a fluid other than water is involved, the pressure transmitter has to be correspondingly scaled to compensate for the specific gravity of that liquid. Once this has been done, the fill level can be determined, as with water, via the hydrostatic pressure on the bottom of the tank. It becomes more complicated when different liquids are used in a single tank. In this case, not only the hydrostatic pressure at the bottom of the tank must be measured, but at the same time the specific gravity of the respective fluid also. We will leave aside the latter case at this point and instead consider hydrostatic pressure measurement in both closed and open tanks.

Hydrostatic pressure measurement in open and closed tanks

With open tanks, it does not matter whether they are above ground or set within it, as long as they have an opening that provides for a balanced air pressure inside and outside the tank. The measurement of the hydrostatic pressure can be carried out without further adjustments at the bottom of the tank. If measurement at the bottom of the tank is not possible, the filling level can also be found by means of a submersible probe, which is fed into the tank with a cable from above.

In closed tanks, higher gas pressures often prevail than in the atmosphere surrounding the tank. This gas layer above the liquid increases the pressure on the liquid itself. As a result, the liquid can flow off more quickly and there is also less loss due to evaporation. Tanks sealed from the ambient air are therefore frequently used in the oil and chemicals industries. The gas layer pushing down on the liquid also acts indirectly on the pressure transducer at the bottom of the tank and must therefore be taken into account in order to determine the correct filling level (a higher filling level than the actual would be indicated through this increased pressure). In closed containers, two pressures would therefore have to be measured: The gas pressure and the pressure at the bottom of the tank. The hydrostatic pressure of the fluid results from the difference between the measured gas pressure and the pressure measured at the base. This difference can then be converted into an indication of the fill level of the tank. For this type of application, a differential pressure sensor is generally used.

Concluding remarks

In hydrostatic level monitoring of tanks, two factors must always be considered: The type of fluid and the type of tank. The simplest application would be the monitoring of water levels in open tanks, since no adjustments have to be made for this constellation. If, however, a different liquid is involved, then the specific gravity of that liquid must also be taken into account. In addition, a measuring instrument is to be selected that can withstand the properties of the medium concerned. Whereas for most liquids stainless steel is sufficient as a housing material, highly corrosive media may also require different materials.

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Conductivity measurement in natural waters & other liquids

Conductivity measurement in natural waters & other liquids

by | Wednesday, 1 July, 2020 | Water

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

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Grounding level sensors for protection from surges

Grounding level sensors for protection from surges

by | Tuesday, 30 June, 2020 | Pressure Sensor Technology

When monitoring filling levels, make sure that the level sensors are sufficiently grounded in order to prevent serious damage. Should this be inadequate or absent altogether can lead to three serious effects.

  1. Because of insufficient potential equalization in conductive media such as water, corrosion can occur. This is a gradual process, which can be observed in long-term applications. The voltage differences between the sensor and its surrounding fluid lead to electrolytic corrosion. The metal housing becomes gradually perforated and liquid then penetrates into the housing itself. Damage to the electronics will then be the consequence here. This process can be observed both in open waters and in fill level monitoring within vessels, where the potential difference between the level sensor, medium and vessel wall can cause electrochemical corrosion.
  2. Filling level sensors are connected to the control system by cables or plugged into telemetric systems. Through these connections, atmospheric voltages can be passed on to the sensor. Overstrain to the electronics will be the end result in this case.
  3. If lightning strikes near the level probe, a very high voltage difference will exist over the shorter term. The increased voltage in the water will then seek the shortest path to earth here via the level sensor.

Grounding and lightning protection of level sensors

To protect level sensors from these effects, they can be equipped with lightning protection. For this purpose, a transient overvoltage protection is integrated into the level probe, which will react to rapidly rising voltage differences. Should a sudden voltage surge occur, the lightning arrester will trigger a short circuit within the electrical circuitry to channel that overvoltage to ground. This surge protector normally operates in a non-conductive state, but does conduct voltage transients so that they can flow to ground without causing any damage. It should be noted, however, that with a direct lightning strike to the immersion probe, even overvoltage protection cannot prevent damage.

Additionally, an earth connection that should have a resistance of less than 100 ohms is used for grounding. For fill level monitoring in liquid-carrying tanks made of metal or even plastic, care must be taken that all of the isolated metallic components are connected together to ground. In open waters, a greater effort is generally required to create a low resistance to ground. For this reason, an earthing grid is often set into the ground for these applications.

Users are generally advised to discuss a grounding concept with the manufacturers in regard to their respective application.

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