Point level measurement: how to choose the right point level switch!

During point level measurement, point levels are detected using point level switches and/or level switches. This allows you to monitor maximum and minimum fill levels or individual filling levels in silos, storage tanks, and processing tanks, as well as pipelines. Overfilling, overflows, and dry running are thus prevented.

When selecting the suitable point level switch, you must adopt a systematic approach and consider a number of important criteria.

1. Which medium would you like to measure?

2. What are the process conditions?

3. What are your installation options?

4. Which ambient conditions are you required to adapt to?

5. Suitable measuring principles for the point level detection of liquids

5.1 Vibronic measuring principle for liquid measurement

5.2 Mechanical measuring principle

5.3 Conductive measuring principle

5.4 Capacitive measuring principle for the detection of point levels in liquids

5.5 Optical measuring principle

6. Suitable measuring procedures for the point level measurement of bulk solids

6.1 Vibronic measuring principle for the detection of point levels in bulk solids

6.2 Electromechanical measuring principle

6.3 Capacitive measuring principle for bulk solid measurement

7. Overview of the selection of the suitable measuring principle for point level measurement

8. Select the right point level switch at Automation24 now

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1. Which medium would you like to measure?

In order to select the suitable measuring principle for the point level measurement, you should first consider which medium you wish to detect. Is it liquids or bulk solids? It is important to note that not all available principles are suitable for both media, as they often differ considerably in terms of their properties.

2. What are the process conditions?

In addition to the medium present, the process conditions within your application represent another criterion that must not be underestimated.

If there are high pressures and temperatures within your application, then, for the point level measurement of liquids in particular, it is vital that you choose a solution that is suitable for these challenging conditions.

If a particularly high hygiene standard is required – for example in case of the use of paste-like or adhesive media, which could repeatedly make its way into the production circuit – then point level switches with a hygienic design must be used.

If chemical or aggressive agents come into contact with the point level switches, you should select switch models made of durable and corrosion-resistant materials.

3. What are your installation options?

Initially, your installation options will be dependent on which point level you want to measure - whether it is a maximum or minimum fill level, or perhaps a specific fill level. This selection influences whether you can flexibly install a point level switch, or whether they must be installed at specific positions - either at the top, to the side, or on the base of the container.

When determining the installation position, you should also consider whether certain objects such as agitators or other obstacles could prevent the point level measurement from being performed properly.

It is therefore important when making your decision that you consider and check whether your chosen switch offer flexible installation capabilities.

4. Which ambient conditions are you required to adapt to?

Is your container in an unprotected outdoor area or is it protected against influences such as humidity, vibrations, dust, or extreme temperatures?

If there is no protection against such influences, it is recommended that you select a switch made from robust materials for your point level measurement, with a suitably high protection rating. Otherwise, it will be necessary to protect the installation with a suitable tarpaulin or guard.

If electromagnetic interference (EMI) occurs at the respective place of use, you should shield your switch from this disturbance. You can do so, for example, by using switch earthing or a shielded cable.

In explosive environments, you must select point limit switches that hold ATEX or IECEx certification.

When utilizing substances that have the potential to contaminate water, it is crucial to ensure that the point level switches are certified in compliance with the relevant water management regulations. This way, effective overfill protection and leak detection are guaranteed, with a view to protecting the water against contamination by pollutants as effectively as possible.

5. Suitable measuring principles for the point level detection of liquids

Liquids can differ significantly in terms of their properties. For instance, they can be adhesive, paste-like, thin, viscous, chemically aggressive, clear, cloudy, frothy, conductive, or non-conductive. In order to detect the point level, different measuring principles are therefore also required. These are in turn dependent on the application requirements and the respective field of use, as well as the ambient conditions. Below, we will focus on five of the possible measuring procedures for level detection.

Fig. 1: Not all liquids are created equal

5.1 Vibronic measuring principle for liquid measurement

Vibrating level switches, which are based on the vibronic measuring principle, are suitable for uses involving contact with non-adhesive liquids that do not form deposits. They can be used irrespective of liquid properties such as conductivity, density, pressure, and temperature. What's more, they are affected neither by the formation of foam and bubbles, nor turbulence.

As the name suggests, the vibronic measuring principle, also known as the tuning fork principle, is based on the basic functionality of the tuning fork. These resonate at certain frequencies and, in so doing, generate characteristic vibrations. In the case of vibrating level switches, the resonant frequency serves as a reference for the point level measurement. If the tuning fork is covered with liquid, a frequency change occurs. This change enables conclusions to be drawn regarding the point level. This explains why the ends of the vibrating level switches resemble tuning forks.

Vibrating level switches are also available in coated versions and are therefore also suitable for applications within the chemicals industry.

Depending on the application, the switches are generally mounted onto the external walls of containers, inserted directly into suitable container openings, installed using a flange, or attached to pipelines.

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Fig. 2: The measuring tip of the vibrating level switch resembles a tuning fork

Fig. 3: A float switch requires no power supply in order to operate

5.2 Mechanical measuring principle

Float switches that operate according to the mechanical measuring principle are suitable for use in conductive and non-conductive liquids. They are also intended for environments in which the use of electronic sensors is not suitable, where, for example, pumps or valves are controlled directly.

These affordable wetted switches are operated without a power supply and are capable of detecting the fill level of a liquid based on the buoyancy of a float, or by means of mechanical movements. If the fill level reaches a certain level, the switch is activated and a signal is issued so that actuators can be directly switched on or off.

The float switches are made using simple materials, which is why the manufacturer's recommended pressure and temperature limits must not be exceeded. It is also important to ensure that the switching capacity of the float switch is not placed under excessive stress, so as to avoid damaging the switching contact.

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5.3 Conductive measuring principle

If you work with conductive liquids such as water, acids, or bases, with wetted point level switches that operate according to the conductive measuring principle you can call upon an affordable solution for point level measurement.

Here, the electrodes or sensors immersed in the liquid generate an alternating current. If the electrical resistance between the electrodes changes, conclusions can be drawn regarding the point level on the basis of this change. In the event that the electrodes come into contact with the conductive liquid, the electrical resistance reduces. The resulting change in the alternating current triggers a switching signal.

In order to guarantee the proper function of the conductive point level switch, the respective temperature and pressure limits must be considered when selecting the suitable model.

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Fig. 4: Composition of a conductive point level switch

Fig. 5: Installation of a capacitive point level switch for point level detection in liquids.

5.4 Capacitive measuring principle for the detection of point levels in liquids

In order to detect conductive and non-conductive liquids, affordable capacitive point level sensors represent a suitable solution.

These work according to the capacitive principle, whereby their electrode - each consisting of a covered and an uncovered probe - form a capacitor with the respective container. A change in the fill level of the container changes the capacity of the capacitor. This change in capacity in turn enables conclusions to be drawn regarding the point level of the liquid.

To avoid damage to the capacitive point level switches in use, these should be selected in accordance with the manufacturer's specifications with regard to the pressure and temperature limits.

Capacitive level switches can also be purchased in special versions with an additional electrode, and are thus suitable for use in adhesive media.

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5.5 Optical measuring principle

For versatile applications whereby you want to detect the point levels of conductive and non-conductive either contactlessly or by means of a wetted solution, point level switches that operate according to the optimal measuring principle are a suitable option.

The other properties of the liquid, such as the temperature, pressure, density, conductivity, and dielectric constant are irrelevant, as the optical measuring principle is based on the emission of a light bean and the detection of the reflected or transmitted light signal. Here, the change in light intensity or the reflected light is used as the measurement indicator for the fill level.

Wetted optoelectronic switches are inserted directly into the liquids. This can be by means of integration into the containers themselves, or by mounting special installation components or flanges.

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Fig. 6: Wetted optoelectronic switches

Fig. 7: Contactless laser sensors are suitable for the point level measurement of aggressive media

In contrast to this, optical point level sensors work with light beams.

They do not come into contact with the media and are therefore also suitable for the detection of more aggressive liquids such as acetone or acids, and for hygienic applications.

Furthermore, they are also suitable for the point level detection of liquids in narrow tanks or in specially shaped containers. In such cases, they can be mounted at any point on the container lid or on the tank wall.

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6. Suitable measuring procedures for the point level measurement of bulk solids

Like liquids, bulk solids can have a variety of different properties. For example, they can be coarse-grained, fine-grained, powderous/dusty, adhesive, damp, dry, abrasive, pourable or non-pourable, or corrosive.

Here too, in order to perform a point level measurement, a differentiation must be made between suitable measuring principles, based on the properties of the bulk solid.

Three commonly used measuring principles are discussed below.

Fig. 8: Bulk solids can differ greatly from one another

Fig. 9: The vibrating level switch measuring tip can also be rod-shaped when used to measure the point level of bulk solids

6.1 Vibronic measuring principle for the detection of point levels in bulk solids

The vibronic measuring principle for bulk solids is comparable with the measuring principle for liquids. However, for the wetted point level detection of bulk solids, it is not the change in the frequency of vibrations that is used, but rather the change in the amplitude of the vibrations. The changes are converted into a switching signal.

In spite of this difference, both processes work in essentially the same way and enable both reliable and precise point level measurement. Whereas rod-shaped vibrating level switches are suitable for coarse-grained bulk solids, the tuning fork-shaped variants are suitable for fine-grained or powderous solids with a low density.

Physical properties of the medium such as conductivity, dielectric constant, density changes, pressure or temperature, turbulence, foam formation or bubbling liquids have no negative influence on the point level measurement.

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6.2 Electromechanical measuring principle

If you wish to detect the point levels of bulk solids with various granularities and densities, then rotary paddle switches or rotary paddle limit switches that operate according to the electromechanical measuring principle will be a suitable option for you. The electrical properties of the media are irrelevant with this measurement variant.

With the electromechanical measuring principle, a certain fill level causes a simple mechanical movement in the switch, which generates an electrical signal that is used to detect the point level.

Due to the mechanical stress on the moving parts such as the rotary paddle, this measuring procedure requires the regular maintenance of the switch so as to avoid wear or malfunction.

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Fig. 10: Simple and proven: rotary paddle switches for point level measurement

Fig. 11: Bottom-mounted capacitive point level switch for the point level measurement of bulk solids

6.3 Capacitive measuring principle for bulk solid measurement

The basic principle of capacitive measurement for the point level measurement of liquids can also be applied to the detection of point solids.

Capacitive level switches can be used for all bulk solids, provided the dielectric constant is also considered when detecting the point level on the basis of the capacity change.

The difference lies in the fact that the dielectric constant of the respective bulk solid must also be considered, as this influences the capacity change that is used to detect the point level. The dielectric constant is in turn determined, for example, by the composition, temperature, and humidity of the bulk solid.

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7. Overview of the selection of the suitable measuring principle for point level measurement

Selection criterion	Vibronic measuring principle	Mechanical measuring principle	Conductive measuring principle	Capacitive measuring principle	Optical measuring principle	Electromechanical measuring principle
Medium	liquids & bulk solids	liquids	liquids	liquids & bulk solids	liquids	bulk solids
Required liquid properties	non-adhesive, non-deposit-forming	conductive & non-conductive, non-adhesive, non-deposit-forming	conductive, adhesive *	conductive & non-conductive, adhesive *	conductive & non-conductive	–
Required bulk solid properties	fine & coarse grained as well as powdered with low density ***	–	–	fine grained & powdered	–	var. granularities & densities
Resistance to aggressive media	*	*	*	*	*
Suitable for hygienic applications	**		**	**	**
Wetted measuring process					***
Non-wetted measuring process				***	***
Power supply
Installation cost	moderate	simple	moderate	moderate	simple	moderate
Costs	–		–		–
Particular attention must also be paid to:	temperature & pressure limits	temperature & pressure limits, use of chemically resistant cables	temperature & pressure limits, use of earth probe required if there is no earthing	temperature & pressure limits, dielectric constant	temperature & pressure limits	regular maintenance recommended
Compatible product families at Automation24	Endress+Hauser: Liquiphant FTL31/33, FTL41, FTL51B, FTL50H; Soliphant FTM20/21, FTM50/51 VEGA: VEGASWING 51, 53; VEGAWAVE S61; VEGAVIB S61	WIKA: RLS-1000, RLS-7000, RLS-8000 Honsberg: SB, NW, NM, RW Reltech: SKW, SSW Georg Fischer: 2282 Endress+Hauser: FTS20	Endress+Hauser: Liquipoint FTW31, 32, 33	ifm: LMT, LMC, LI, LK, KQ Endress+Hauser: Liquipoint FTW33, Nivector FTI26, Minicap FTC260/262	WIKA: OLS-C01, OLC-C05 ifm: O1D, OID, OGD	Endress+Hauser: Soliswitch FTE20
* depending on the wetted material of the product depending on the wetted material of the product and the max. temperature * depending on the product model

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8. Select the right point level switch at Automation24 now

You are now familiar with the measuring principles suitable for point level measurement and know what to bear in mind when selecting a suitable switch.

You can now select the point level switch suitable for your application from the Automation24 product range:

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If there are other levels you wish to detect in addition to point levels, our solutions for continuous level measurement are just what you need.

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Would you like to read up on more general information on the different level measurement techniques?

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