FAQ

What is meant by hydrostatic level measurement?

Hydrostatic level measurement is a method of measuring the level of a liquid in a tank, basin or underground resource by using pressure sensors. Basically, the pressure sensor is positioned at a specific depth in direct contact to the media, measuring the weight force of the liquid column above it all the way down from the surface to the sensor. By measuring this weight force as pressure, the sensor is able to accurately calculate the current level and monitor any change in level.

Have a look at our blog article "What is meant by hydrostatic level measurement or hydrostatic pressure?"


How to do hydrostatic level measurement?


What is a level probe or submersible pressure transmitter?

A level probe, also called submersible pressure transmitter, is a special design of a pressure transmitter. It is specifically designed for level measurement applications in open waters, basins, deep wells, monitoring wells, groundwater and boreholes. These sensors are submersible up to several hundreds of metres and feature an ingress protection of IP68. They are easily suspended by their cable into the media and provide a long-life, reliable level measurement.

Have a look at our blog article "What exactly is a level probe / submersible pressure transmitter?"


How is a level probe or submersible pressure transmitter built up?

A level probe, also called submersible pressure transmitter, is a pressure sensor optimised for level measurement applications especially in open geometries (tanks, basins or reservoirs). It features special design characteristics to guarantee highest media resistance, a long lifetime and it is hermetically sealed against water ingress, even when exposed to the force of the surrounding pressure in applications of several hundred metres of depth. The design of a level probe is therefore optimised in terms of its sensor element, the integrated electronics, the housing, the cable inlet into the sensor and the cable itself.

The sensor element is specifically designed to be capable of measuring media with high loads of contamination such as waste water. The integrated electronics are featuring components to better withstand harsh conditions in the surrounding application such as lightning strikes into the vicinity of the media. The housing, all its welding seams, gaskets and the cable inlet design have to ensure that the level probe is hermetically sealed and chemically resistant against the media to be measured. Last but not least, the cable itself is manufactured following a uniquely developed design to provide highest degree of protection against water ingress and chemical resistance against the media.

Have a look at our blog articles:
"The design of a level probe - Part I"
"The design of a level probe - Part II"


How to measure the level in an open vessel?

If a hydrostatic level sensor is used to measure the level within an open vessel, an open basin or an underground reservoir, a gauge pressure sensor design should be chosen. This ensures that the changes in ambient pressure due to weather influence are automatically compensated. The level is then calculated using this formula:

h = p / (ρ * g)

p = hydrostatic pressure [bar (gauge)]

ρ = specific gravity of the liquid [kg/m³]

g = gravitational force or gravitational acceleration [m/s²]

h = height of the liquid column [m] = level


Have a look at our blog article
"Hydrostatic level measurement in open geometries and vessels – calculation of the filling height"


How to measure the level in a closed tank?

If a hydrostatic level sensor is used to measure the level within a closed tank, an overpressure could build up above the liquid media in the tank due to the evaporaton of gas from within the media, very common e.g. in fuel tank farms or chemical processes. This overpressure disturbs the measurement of the hydrostatic level, as the overpressure works as an additional pressure or weight force on the liquid. Thus, two sensors need to be used in these applications to measure the liquid level and the pressure of the gas via two separate measurements. Both sensors can then be used to compensate for the effects of overpressure and used for the calculation of the level by the following formula:

h = (p2 - p1) / ((ρ * g)

p2 = hydrostatic pressure [bar]

p1= pressure of the enclosed gas in the vessel [bar]

ρ = specific gravity of the fluid [kg/m³]

g = gravitational force or gravitational acceleration [m/s²]

h = height of the liquid column [m] = level


Have a look at our blog article
"Hydrostatic level measurement in closed geometries – calculation of the filling height"


How to properly ground level sensors?

Hydrostatic level sensors are always in direct contact to the media, media which are often aqueous and electrically conductive. Due to improper or missing grounding or a faulty potential equalisation, electro-chemical corrosion and ultimately a premature failure of the level sensor may occur.

Most hydrostatic level sensors, especially level probes, feature a cable connection with an integrated braid for grounding, potential equalisation and EMC protection. The potential equalisation is the most important part of the grounding concept for level sensors. The hydrostatic level sensors should be on the same electric potential as the tank and surrounding media and should be protected against any interfering currents caused by surrounding equipment such as pumps and drives.

Have a look at our blog articles
"Grounding of hydrostatic level sensors"
"Grounding of hydrostatic level sensors prevents corrosion"


How to prevent moisture entering the level probe / submersible pressure transmitter?

Level probes / submersible pressure transmitters are, due to their characteristic applications, always surrounded by liquid and high ambient moisture in the field. Therefore measures have to be taken to prevent any moisture from entering through the cable itself or through the cable connection into the pressure sensor.

As level probes are completely surrounded by media, it has to be ensured that a product is chosen which is specifically designed for this application. It must feature materials guaranteeing a high chemical resistance against the medium that is to be measured. Also it has to be ensured that the submersible pressure transmitter is able to withstand the high pressure load of the surrounding media in up to a few hundred metres depth.

Have a look at our blog article "Selection criteria for the prevention of moisture-related failures of level probes"

Level probes always feature a cable outlet which has to be routed and connected properly to a control system or PLC. As the cable is also used to compensate for the ambient pressure, it features a ventilation tube which transports ambient pressure from the cable outlet into the pressure sensor. Therefore, the cable connection has to be connected in a moisture-free cable connection box or control cabinet, so no moisture can enter the ventilation all the way down into the electronics of the pressure sensor.

Have a look at our blog article "How to prevent the ingress of moisture through the cable of level probes?"


How to clean a pressure sensor's diaphragm?

Hydrostatic pressure sensors are often used in highly contaminated wastewater. These applications may require maintenance of the pressure sensors from time to time, to clean them of any buildup, sludge, fibres and other typical contaminations caused by the media.

As pressure sensor diaphragms are extremely thin with a thickness of only a few micrometres, a mechanical cleaning may damage the diaphragm and reduce the accuracy and stability of the pressure measurement or even destroy the sensor. Refrain from using any sharp tools, such as screwdrivers or pens in order to avoid damage to the sensor. Pressure sensor diaphragms should be cleaned very carefully by using a soft cloth, rinsing with water or blow-cleaning with pressurised-air.

Have a look at our blog article "Cleaning of submersible pressure transmitters or level probes"


Products

WIKA offers a comprehensive range of level measuring instruments. Have a look at our comparison overviews.

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