Hydrostatic Level Measurement

There are many different methods for measuring the level of liquids, and the choice of the appropriate method depends on the specifics of the application. One of the most popular and universal methods is the measurement of the hydrostatic pressure of a liquid column. The hydrostatic pressure exerted by a column of liquid is proportional to its height. Knowing the density of the liquid, one can calculate the height of the column, and thus the level of the liquid, based on the measured pressure. In other words, the higher the column of liquid, the greater the hydrostatic pressure it exerts on the bottom of the tank. Measuring this pressure allows us to precisely determine the liquid level, regardless of its type.

The formula for hydrostatic pressure is as follows:

Hydrostatic pressure (ph) = liquid density (ρ) * height of liquid column (h) * gravitational acceleration (g)

It can also be written in the following form:

ph = ρgh

Where:

ph - hydrostatic pressure (in Pa)
ρ - liquid density (in kg/m³)
h - height of the liquid column (in m)
g - gravitational acceleration (in m/s²; a standard value of 9.81 m/s² is typically assumed)

Example:

Let's calculate the hydrostatic pressure at the bottom of a tank filled with water to a height of 10 meters. The density of water is 1000 kg/m³.

ph = 1000 kg/m³ * 10 m * 9.81 m/s² = 98100 Pa = 0.981 bar

The hydrostatic pressure at the bottom of the tank is 0.981 bar.

Hydrostatic Probes vs. Pressure Transducers in Hydrostatic Level Measurement

For hydrostatic level measurement, hydrostatic probes or pressure transducers are used. Level probes are particularly useful in the case of ventilated tanks or open waters. Pressure transducers are a universal and very accurate solution for measuring liquid levels in various conditions, especially in closed tanks, in extreme conditions such as explosion hazard areas, and for viscous liquids. In this blog post, we will focus on pressure transducers and discuss possible measurement solutions using them.

Pressure Transducers in Level Measurement – Application Examples

Pressure transducers are widely used in liquid level measurement across various industries. Their versatility and accuracy make them an ideal solution for many applications.

Application examples:

Chemical Industry:

• Measurement of the level of raw materials, chemicals, and products
• Resistance to corrosion and aggressive chemicals

Food Industry:

• Control of filling levels
• Ease of cleaning and sterilization

Pharmaceutical Industry:

• Accurate measurement of drug levels
• Ease of cleaning and sterilization

Energy Sector:

• Monitoring water levels in steam boilers
• Resistance to high temperatures and pressures

Water and Sewage:

• For continuous measurement of water level in wells
• Resistance to weather conditions

Relative pressure transducers vs. absolute pressure transducers - which to choose for measuring liquids?

Relative pressure transducers can be used to measure liquid levels, but they have some limitations. Fluctuations in atmospheric pressure, such as due to weather, affect liquid level measurements. A change in pressure of 1 hPa corresponds to a change in water level of 1 cm. As a result, fluctuations in atmospheric pressure can cause measurement errors of ±20 cm.

In addition, the relative pressure transducer must measure the hydrostatic pressure of the liquid and the atmospheric pressure. As a result, a larger measurement range is required, which can lead to a larger measurement error. In conclusion, in many cases, absolute or differential pressure transducers will be a better choice for measuring liquid levels than relative pressure transducers.

Level Measurement in Pressurized Tanks

In the case of closed tanks, where the pressure is not equal to atmospheric pressure, level measurement requires the use of special techniques.

Differential Pressure Measurement

For level measurement in closed tanks, two pressure tapping points are used:

The lower tapping point measures the level and total pressure.
The upper tapping point measures only the internal pressure of the tank.
The difference between these two values allows for the calculation of the liquid level.

There are two ways to measure differential pressure:

Measurement using 2 separate transducers:

Pressure sensors can be absolute or relative.
The pressure difference (p2 - p1) is calculated mathematically in a computing unit.

Measurement using a differential pressure transducer:

The transducer measures the pressure difference directly.
In the case of using this type of transducer, diaphragm separators are often used.

Example 1: Differential Pressure Measurement with a Diaphragm Separator

The differential pressure transducer is connected to diaphragm separators with capillary tubes, creating a closed system. The diaphragm separators are mounted on the tank using a flange.

The hydrostatic pressure of the liquid filling the capillaries causes a "pre-pressure" that must be considered when scaling the output signal.

Example 2: Differential Pressure Measurement with a Diaphragm Separator, Pressure Transducer Below

In this case, the pressure transducer is installed below the lower pressure tapping point.

The height of the transducer installation does not affect the measurement, as the hydrostatic pressure of the liquid column is taken into account.

Scaling the Measurement Range

Pressure transducers offer the possibility of adjusting the output signal to the measurement range. There are several ways to scale:

Teach-In function:

Setting the lower value of the range when the tank is empty.
Setting the upper value of the range when the tank is full.

Calculation of scaling:

Using formulas that consider the density of the liquid, the height of the liquid column, and the gravity constant.
In summary, level measurement in closed tanks requires the use of special techniques, such as differential pressure measurement. There are several methods of measurement and scaling that allow for accurate measurement under various conditions.

About the author

My name is Ewelina Szmit and I have been working in the field of content marketing for several years, combining my professional skills with my passion for writing. I believe that even the most technical topics can be presented in an interesting and accessible way for everyone. Outside of work, I develop my creativity by creating newspaper collages. I like to spend my free time most actively, walking my dog or running.

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