Hydrostatic level measurement

What is the principle of hydrostatics?

Hydrostatics is a science that deals with the properties of vacuum, liquid and vapor.

This discipline relies on the action of the pressure exerted by the fluid at its surface to determine the heights or volumes involved.

The principle of hydrostatics was developed in the 16th century by an Italian mathematician named Evangelista Torricelli (1608-1647). It is one of the fundamental bases in hydraulics.

 

What is a hydrostatic level sensor?

Hydrostatic level sensors measure the hydrostatic pressure which is proportional to the measured liquid height.
Consisting of a pressure measuring cell, the stainless steel or other material diaphragm of the sensor reacts to changes in level.
The force exerted by the diaphragm is converted by the level sensor into an analog electrical output signal.

What is the method for calculating the hydrostatic level measurement?

The relationship between the height h of the liquid and the hydrostatic pressure P in a liquid at rest and at atmospheric pressure, can be written as follows:

• g = 9.81 m/s 2 , constant, the Earth's acceleration of gravity (at sea level)
• ρ = density of the liquid
• h = height of the liquid

The pressure measured by the transmitter is proportional to the liquid column.

For example, for a water level measurement in a tank, the pressure varies by 100 mbar per meter of water column. For a tank height of 12 meters, the level probe measures a pressure on the sensor membrane of 1.2 bar relative.
 

How does the hydrostatic level measurement work: measurement principle?

Hydrostatic level measurement involves determining the height of a liquid using the hydrostatic pressure exerted by the liquid on its surface.
The higher the liquid level, the higher the measured pressure applied to the membrane. There are two different methods of measuring hydrostatic level.

The methods of measurement by relative pressure and by differential pressure.

LEVEL MEASUREMENT BY RELATIVE PRESSURE

Un capteur de pression manométrique assure la mesure de pression statique à une position basse du réservoir. Cette pression est ensuite convertie en hauteur en utilisant l'équation du niveau hydrostatique.

LEVEL MEASUREMENT BY THE DIFFERENTIAL PRESSURE

In closed tanks, a second measurement is required to measure the pressure of the gas atmosphere (air or gas above the liquid being measured). A differential pressure transmitter measures the differential pressure at two different points in the tank where the tank is subjected to a pressure different from the atmospheric pressure. The measuring cell calculates the pressure difference between these two points to determine the liquid level.

What are the applications of level measurement by hydrostatic pressure sensor?

For an accurate hydrostatic level measurement, the specific weight of the product in the tank or vessel as well as gravity must be taken into account. Hydrostatic pressure transducers are preferred for use where the density of the liquid is constant.

Common industrial applications of hydrostatic pressure transducer level measurement are as follows:

•   Level control in tanks or pressure vessels
•   Level measurement in industrial tanks open to the atmosphere or ventilated
•   Level-dependent stopping and starting of pumps in open pipes in the water and waste water sector
•   Dosing and distribution of liquids in processes
•   Automation of level control processes in industry and transmission of information

These pressure sensors are used in processes in the chemical and petrochemical industries (chemicals), the food industry (water, beverages, milk), the water industry (water and wastewater), the environment and the shipbuilding industry (sea water).
 

What are the advantages of hydrostatic level measurement?

Hydrostatic level probes use proven measurement technology, as they offer many advantages, including:

• Reliable and accurate measurement
• Insensitive to foam and viscosity
• Independent of the dielectric constant of the liquid
• Easy to install
• Works even in the presence of agitators
• Compatible with any internal tank structure
• Wide measuring range
• Long term stability

What are the disadvantages of hydrostatic level measurement?

•  The hydrostatic level sensor only works on liquids. The hydrostatic level transmitters offer a measurement by contact with the fluids to be measured.
•  The measurement is dependent on the density of the liquid to be measured and the pressure in the vessel.
•  Regular maintenance is required to clean the sensor membrane to prevent deposits and debris. Level measurement solutions with flush diaphragm or hygienic connection avoid creating retention zones.
•  Work on level transmitters may require emptying the tank. Level measurement systems with integrated ball valves avoid this problem. The level sensors can be removed without emptying the tank.
   

How to choose a hydrostatic level sensor?

To choose a hydrostatic pressure sensor, it is important to know the following parameters:

•     the nature of the liquid (water, milk, oil, chemicals,...)
•     the density of the product
•     the corrosiveness and abrasiveness of the process
•     the height of the liquid (measuring range)
•     the type of tank (atmospheric or pressurized)
•     pressure in the tank
•     temperature of the fluid
•     type of process connection (flanged, screwed, hygienic...)
•     local or remote display
•     a protection index

The choice of the level sensor will depend on these components.
The scale of the sensor's measuring cell, the materials of construction such as stainless steel, hastelloy C, monel must be adapted to guarantee a reliable and accurate measurement display over time.