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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Hydrostatic pressure measurement with piezoresistive level sensors

Hydrostatic pressure measurement with piezoresistive level sensors

by | Wednesday, 1 July, 2020 | Water

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.

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Level monitoring for pump control in rainwater and wastewater tanks

Level monitoring for pump control in rainwater and wastewater tanks

by | Tuesday, 30 June, 2020 | Water

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.

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