Hydrostatic level measurement

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Hydrostatic level meaurement: what is it?

Hydrostatic level measurement is a simple and reliable method of measuring level. A submersible pressure sensor, regular pressure sensor or pressure gauge is lowered to or mounted at a specific depth (zero level). The sensor then measures the pressure caused by the weight of the liquid directly on top of it. Due to the hydrostatic paradox, the pressure sensor does not measure the complete volume in the tank above it, but only the liquid column vertically above it. The hydrostatic level measurement is thus completely unaffected by the shape of reservoir or vessel, just measuring the level by the weight of the liquid column above it. If the liquid and its specific gravity are known, the pressure measurement can be calculated as the distance from the zero level, where the sensor is located, to the surface of the liquid.

Hydrostatic level measurement in open geometries

For use in open geometries such as vented vessels, open reservoirs and tanks, but also wells and open bodies of water, WIKA offers submersible pressure sensors, also known as level probes, specifically developed for this purpose. Submersible pressure sensors are completely immersed in the liquid that is to be measured.

Hydrostatic level measurement in closed geometries

In closed geometries, such as enclosed tanks and vessels, generally, flush pressure sensors or pressure sensors are used, which can be screwed into the vessel from the outside. Pressure sensors are therefore particularly suitable in applications of free-standing tanks and vessels. A submersible pressure sensor (e.g. WIKA model LH-20) or a flush pressure sensor (e.g. WIKA model S-11) measures the height-dependent weight-force of the liquid column, also known as hydrostatic pressure. This hydrostatic level measurement is directly proportional to the liquid level respectively liquid column at the measuring point of the pressure sensor. It is therefore necessary, for the hydrostatic level measurement, that the pressure sensor is placed as close as possible to the level of the vessel’s bottom.

Setting up your hydrostatic level measurement

Once you have decided whether you will use a standard pressure sensor or a submersible one, for most applications you only need to define three to four simple things. The pressure connection (only if it is a standard sensor or pressure gauge for side-mounting), the measuring range, the accuracy and the electrical connection.

The pressure connection should fit to the threaded bore within your tank, while the measuring range should be chosen according to the level you want to measure in your application. If it is a stand-alone tank, you will know the level by the height of the tank, if it is an underground basin or reservoir, you need to decide how deep you want to submerse the level probe and choose a corresponding measuring range. Keep in mind, 10 metres of water column correspond to approximately 1.000 mbar (1 bar) or 14.5 psi.

The accuracy should be chosen according to the needs of your application, standard accuracy of <0.5 % will give you a possible error of max. 5 mm per metre of level, while highest accuracy variants with <0.1% will be as accurate as max. 1 mm per metre of level. Finally, you have to pick the electrical connection. If you have a standard pressure sensor, you are free to pick a connector (such as M12 or DIN A) or cable outlet, while you have to stick with a cable anyhow when choosing a submersible pressure sensor. If your sensor features a cable connection, you need to choose the appropriate length to allow wiring from the transmitter to the control system and to allow for the immersion length when using a submersible pressure sensor.

Increasing the accuracy of your hydrostatic level measurement

When you set up a hydrostatic level measurement system, you have to be aware that the level measurement is influenced by the specific gravity of the medium and the medium temperature, which is also affecting the specific gravity of the medium.

The specific gravity of the medium may not be stable, if the medium changes in its specific composition. Therefore your hydrostatic pressure will show a higher reading, thus you will calculate an increase in level, while the real level may actually be lower than your calculation. Yet, the specific gravity can also change due to varying medium temperature. If the temperature increases, the specific gravity of the medium will reduce and the level will increase. But the hydrostatic level measurement may not reflect this change in level accurately.

To increase the accuracy of your hydrostatic level measurement you have to compensate for the temperature effects and for the change of specific gravity of the medium.

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