Level measurement in a steel tank: float, hydrostatic probe or radar?

Knowing that a tank is "about half full" is sufficient only for the simplest operations. When the level controls a pump, prevents overfilling or supports delivery planning, the measurement method becomes part of the installation. A float, a hydrostatic probe and radar can indicate the same quantity, but they use different principles and respond differently to foam, deposits, density changes and equipment inside the tank.

The device should therefore not be selected by capacity alone. The medium, temperature, pressure, required accuracy and intended use of the signal must be defined. When buying a used tank, the construction of the available steel tank is also important: its height, bottom geometry, nozzles, manway, agitator and areas where deposits may form.

Start with the measurement task and tank conditions

A local reading for an operator requires a different solution from a continuous signal sent to a controller. Independent high-level or low-level detection is another separate function. A sensor used for routine indication should not automatically be treated as the only overfill protection. The required functions, reliability and safe response after signal loss must follow from the risk assessment for the specific process.

Before discussing instrumentation with a supplier, collect at least the following information:

• the medium, its density and possible density changes with temperature or concentration,
• viscosity and the tendency to foam, crystallize, coat probes or form deposits,
• measurement height, working capacity and tank shape,
• operating and cleaning pressure and temperature, including vapour or condensation,
• the position of the agitator, coils, ladders, spray devices and filling stream,
• the expected output: level, volume, limit alarm or data for stock records.

Measuring liquid height does not automatically provide an accurate volume. In a tank with an irregular bottom or a horizontal layout, the relationship between height and litres is not linear. The controller then needs a correct conversion table or geometric characteristic, which should be confirmed by tank measurements or documentation.

Float: simple indication with direct contact

A float follows the liquid surface, and its movement is transferred mechanically or converted into an electrical signal. In its simplest form it can operate an indicator or a limit switch. Its advantages are a clear operating principle and the option to build a system without complex electronics. This may be sufficient for water and other uniform liquids when the operator needs an approximate reading or an on-off signal.

The main limitation is direct contact with the medium. Deposits, sticky product, surface layers or crystallisation can restrict free movement. A density change affects float immersion, while strong waves caused by filling or mixing can destabilise the reading. The material of the float, guide and seals must be checked, as well as possible interference with internal equipment.

A float is therefore practical when conditions are calm and well understood. However, a mechanical design should not be treated as maintenance-free. Access for inspection, cleaning and periodic functional testing should be planned just as it is for an electronic sensor.

Hydrostatic probe: level calculated from liquid pressure

A hydrostatic probe measures the pressure exerted by the liquid column. In simplified terms, this pressure depends on height, density and gravitational acceleration. This explains both the advantage and the main limitation of the method: stable density allows a continuous signal that is easy to transmit, but an uncompensated density change may be interpreted as a level change.

In an open or vented tank, the reading must be correctly referenced to atmospheric pressure. In a closed tank, pressure above the liquid may require differential measurement or another compensation method. A submersible probe should be suspended so that it does not rest in deposits or receive a direct impact from the filling stream. With installation in a bottom nozzle, isolation, venting and service access without uncontrolled draining are important.

The diaphragm, cable, seals and all wetted parts must be compatible with the medium and temperature. For viscous products or liquids that form hard deposits, the risk of coating the diaphragm must be assessed. Hydrostatic measurement works well in many storage tanks, but its parameters should cover the actual density range rather than one assumed value.

Radar: non-contact measurement with installation requirements

A radar sensor installed above the medium transmits a microwave signal, receives the reflection from the surface and calculates the distance. Non-contact operation reduces problems with corrosion and coating of immersed parts. Radar measurement is not directly dependent on process temperature or pressure. This does not mean that any radar can be installed in any nozzle and immediately provide a reliable result.

The reflective properties of the medium, foam, condensation, antenna geometry, dead zone and obstacles in the beam must be checked. Coils, an agitator, a ladder or the wall of a narrow nozzle can generate false echoes. Correct location, range configuration and false-echo mapping can help, but their effectiveness depends on the specific instrument and tank. If the risk assessment identifies an explosive atmosphere zone, the device must match its classification and the installation and manufacturer documentation in line with Directive 1999/92/EC. Correct grounding and wiring are also required.

Radar can be the best option for aggressive, variable or fouling liquids, especially when inserting a probe into the medium is undesirable. It is usually more expensive than a simple float, but it can reduce service contact with the product. If a tank selected from the Agrotex range requires a suitable mounting point, paid tank modernization can be agreed before the purchase is finalized. The scope may include a nozzle, platform access, cable routing and the required clearance from obstacles. Agrotex carries out the agreed work on tanks selected from its range.

The best method is therefore a compromise. A float is clear and simple, hydrostatic measurement provides a convenient continuous signal when density is known, and radar works without contact but requires informed selection and commissioning. In critical applications, the control and alarm functions should be separated and a safe installation state should be defined for sensor failure.

Summary

When selecting a measurement method, define the medium, its density, tendency to foam or form deposits, temperature, pressure and expected output. If you are considering a specific tank from the Agrotex range, provide its offer number and process requirements in the contact form. Agrotex can then select a float, hydrostatic probe or radar and define the instrumentation and paid modernization scope before the purchase is finalized.