Choosing the Right Level Sensor

VEGA level sensors

In order to choose the technology that will best suit your process, it is important to know the nature of the product that will be measured. Is it a liquid, paste or loose solids? If it is a liquid, is there foam on the surface of the liquid?

Level measurement technologies can be divided into two groups: contact and non-contact. Non-contact technologies are more appropriate for use in extreme environments or with aggressive products. Non-contact level measurement also has the advantage that there is no risk of contaminating the product.

As with any sensor, the measuring range is very import and care must be taken to consider the position of the sensor in relation to the product. For example, if you install a sensor at the top of a silo and there is 1 meter in height between the sensor and the maximum height of the product in the silo, you must take this meter into account when evaluating the level range to be measured.

It is important to choose a level sensor that is resistant to the  pressure and temperature of the process conditions and has an output signal that is compatible with the measuring system: analog or digital output; with or without display; relay output; etc.

WIKA float level sensor

The principle of a float level sensor is relatively simple: a float slides on a vertical axis as the level of a liquid changes. To measure the liquid level, simply measure the position of the float on the vertical axis.

It is a precise type of sensor, using a simple and proven process, and the measurement is unaffected by the viscosity of the liquid or the presence of foam on its surface. Nevertheless, the measurement is sensitive to disturbances on the liquid’s surface, such as waves. These sensors are inexpensive and do not require regular maintenance, but are not suitable for corrosive products.

The measuring range is limited by the length of the indicator tube and is usually less than one meter.

BinMaster hydrostatic level sensor

Hydrostatic level sensors are submersible pressure sensors. They measure the hydrostatic pressure which is proportional to the height of the liquid above the sensor. This technology is only suitable for liquids.

Hydrostatic level sensors offer a wide measuring range and are unaffected by foam formation. However, the measurement depends on the density of the liquid and the pressure in the tank. Both accurate and simple to install (the sensor is often mounted at the bottom of the tank), this technology is widely used in the food industry and in water treatment plants. This kind of sensor must be maintained, however, as deposits can form on the diaphragm and falsify the measurement. It is also necessary to empty the tank beforehand for any intervention on a sensor installed on the tank bottom (installation, removal, etc.).

AMETEK Drexelbrook radar level sensor

The radar level sensor is installed on top of the tank, above the product. It sends microwaves to the product surface, which in turn reflects them back to the sensor. By measuring the travel time between the emitted and reflected wave, the distance between the sensor and the product surface can be deduced. This makes it possible to calculate the level of the product.

The main advantage of this type of sensor is that it can measure the level of any type of product (liquids, pastes, solids, etc.) as long as it is not electrically conductive. Radar sensors are extremely accurate and insensitive to product properties (temperature, pressure, density, conductivity, etc.).

As this is a completely contactless technology, these sensors can be used in harsh environments or with products that are abrasive, corrosive, etc.

Another advantage is that these sensors are installed above the product level, usually at the top of the tank, so they can easily be removed without needing to empty the tank.

There is an interesting alternative to radar technology: the guided radar sensor or guided wave sensor. This type of sensor works on the same principle, but the microwaves are guided along a rod or a cable which is immersed in the product. By guiding the wave in both directions, the sensor is less sensitive to disturbances, in particular to disturbances on the product surface (i.e. an agitated surface, foam generation, drain cones for loose solids, etc.) and by elements in the tank (internal structural elements or equipment with wave-reflecting surfaces).

Microsonic ultrasonic level sensors

Like radar level sensors, ultrasonic level sensors are installed above the product. The principle is similar, they emit ultrasonic pulses that are reflected by the product surface. By measuring the wave travel time between emission and reception of the reflected wave, the distance between the sensor and the product surface can be calculated.

This technology allows for non-contact measurement and has the same advantages as radar sensors: this type of sensor is easy to install and uninstall, it does not require the tank or vessel to be emptied.

An additional advantage is that the ultrasonic vibration of the sensor ensures permanent self-cleaning, which prevents the sensor from clogging due to dust deposits, for example.

These sensors also offer a wide measuring range, making them suitable for use in many applications.

Ultrasonic sensors are used to measure the level of liquids and solids, but are not suitable for foamy or dusty products, which dampen the waves. These sensors are also sensitive to temperature variations and are unsuitable for high temperatures and high pressures.

UWT capacitive level sensor

The principle of the capacitive level sensor is that a sensor and vessel form an electrical capacitor whose electrical capacitance value depends directly on the height of the product in the tank.

Capacitive sensors are particularly heavy duty and can withstand very high temperatures and pressures. They are easy to install and remain reliable, even with viscous or highly adhesive products. They can be used with liquids, granular products or powders. Unlike ultrasonic and radar sensors, capacitive sensors are not sensitive to electrically conductive products.

These sensors are generally used in the chemical, food and plastics industries.

Radiometric level sensors use a radioactive source that emits gamma rays that are attenuated as they pass through matter. This technology requires a source and a sensor set up so that the gamma rays pass through the tank. The gamma rays will be more or less attenuated depending on the height of the product in the tank.

The main advantage of this type of sensor is that it can be used for all applications. It allows for non-contact and non-intrusive measurement as it is installed outside the tank (gamma rays pass through the walls), which means you don’t have to stop production. This type of sensor is unaffected by product characteristics and can therefore be used with aggressive media and under extreme conditions (temperature and pressure).

This technique is reliable and despite the use of gamma rays, it is very safe for the user.

While it could be used everywhere, this technology is very expensive and is only deployed in cases when no other type of level sensor can be used.