Designing Reed Level Transmitters: Selecting Resolution and Float Magnet Correctly

→ Product category: Level measurement technology

 

A reed level transmitter can measure liquid level reliably and continuously when the float, magnet, reed measuring chain and mechanical design are consistently matched to the medium and the measurement task.

Unlike a single float switch, it does not merely detect one limit level. A resistor measuring chain with multiple reed contacts is arranged along the guide tube. Depending on its position, the magnet in the float actuates the corresponding contacts and thereby generates an electrical signal that varies with the liquid level.

The achievable measurement quality depends, among other things, on:

• contact pitch of the reed chain,
• magnet system and geometry of the float,
• density of the medium,
• diameter and length of the guide tube,
• free mechanical movement of the float,
• required output signal,
• actually required measurement resolution.

A very small contact pitch alone does not automatically make a reed level transmitter the better measuring solution. Float selection, medium density, mechanics and signal processing must all be matched to the same measurement objective.

A suitable example from the ICS portfolio is the WIKA FLR-S, FLR-P and FLR-H reed level transmitters. They operate according to the float principle with magnetic transmission and a reed resistor measuring chain. Depending on the version, different contact pitches, materials, process connections and electrical output signals are available.

Further solutions can be found under continuous level measurement with float and under level measurement technology at ICS Schneider.

How does a reed level transmitter work?

A reed level transmitter operates with a float that moves along a guide tube according to the liquid surface level.

The float contains a permanent magnet system.

Inside the guide tube is an electrical measuring chain consisting of:

• reed contacts,
• resistors,
• PCB track or carrier board.

As the float moves up or down, its magnetic field acts contactlessly through the wall of the guide tube on the reed contacts.

This changes the effective resistance of the measuring chain according to the position of the float.

The liquid level is therefore represented electrically

In the conventional design, the resistor measuring chain corresponds to a:

3-wire potentiometer circuit

The resulting measuring voltage or resistance ratio is proportional to the float position and therefore to the liquid level.

Because the individual reed contacts are arranged at a defined pitch, the signal is not mathematically completely continuous but is instead very finely stepped.

In practice, this behaviour is therefore described as:

quasi-continuous

.

Design of the reed measuring chain

In simplified terms, the reed measuring chain consists of a series of resistors connected one after another.

Magnetically actuated reed contacts are located at defined positions.

Depending on its position, the float magnet switches a particular section of the resistor chain.

In simplified terms

At a low liquid level, the float is located at the lower end of the measuring range.

As the liquid level rises, the float moves upwards and changes the effective resistance step by step.

This change can subsequently be converted, for example, into a:

• resistance value,
• 4–20 mA signal,
• HART signal,
• digital fieldbus signal.

An important advantage of the principle

The electrical part of the measuring chain is located inside the guide tube and is mechanically isolated from the medium.

Transmission between the float and the measuring chain is:

magnetic and contactless

.

No electrical sliding contacts are therefore required between the moving float and the measuring chain.

What is the function of the float magnet?

The float performs two fundamental functions.

1. Mechanical detection of the liquid surface

Due to its buoyancy, the float follows the liquid surface.

2. Magnetic actuation of the reed chain

The integrated magnet system generates the magnetic field that actuates the reed contacts inside the guide tube.

The float is therefore not simply interchangeable mechanical accessory equipment.

Magnet position, magnetic strength, float geometry and guide tube must be mechanically matched to the reed measuring chain being used.

An incorrect float can cause problems

For example, if:

• the magnetic field does not act sufficiently on the reed chain,
• the magnet is positioned incorrectly in the axial direction,
• the internal diameter does not match the guide tube,
• the float immerses too deeply at the existing medium density,
• the float mechanically jams on the guide tube.

The float and measuring element should therefore be designed as a matched system.

What does contact spacing mean?

The reed contacts are not arranged continuously along the entire tube but at defined intervals.

This spacing is often referred to as:

contact pitch

or:

contact spacing

.

Typical pitches for reed measuring chains are, for example:

• 5 mm,
• 10 mm,
• 15 mm,
• 18 mm.

A smaller pitch means finer stepping

With a 5 mm pitch, significantly more switching positions are available along the measuring range than with an 18 mm pitch.

The signal can therefore reproduce the actual float movement more finely.

However

Contact spacing and specified measurement resolution are not automatically identical.

The magnetic switching behaviour and the evaluation of the resistor chain influence the actual specified resolution.

Distinguishing contact spacing from measurement resolution

When selecting a product, the question should therefore not only be:

How large is the pitch?

but rather:

What specified resolution results from it?

For current WIKA reed level transmitters, for example, the following resolutions are specified depending on the contact pitch:

Contact spacing	Resolution
5 mm	approx. 2.7 mm
10 mm	approx. 5.5 mm
15 mm	approx. 7.5 mm
18 mm	approx. 9 mm

The finest version with a 5 mm pitch therefore enables level resolution in the range of just a few millimetres.

Resolution is not the same as overall accuracy

Even a resolution of, for example:

2.7 mm

does not automatically mean that the actual liquid level can be determined under all process conditions with an overall measurement error of ±2.7 mm.

Additional influencing factors include:

• float buoyancy,
• density changes,
• temperature,
• wave motion,
• mechanical play,
• mounting position,
• signal conversion.

Example of different contact pitches

A tank has a usable measuring range of:

1,000 mm

Version A with 5 mm contact pitch

A very fine electrical stepping is available along the measuring range.

The specified resolution is, for example:

approx. 2.7 mm

Version B with 18 mm contact pitch

The electrical stepping is coarser.

The specified resolution is, for example:

approx. 9 mm

Which version is better?

That depends on the measurement task.

If the only requirement is to determine whether a large storage tank is approximately:

65% or 66%

full, the coarser pitch may be completely sufficient.

For a small dosing vessel, on the other hand, finer stepping may be required.

The technically smallest available pitch is therefore not automatically the most economical or metrologically sensible choice.

Why medium density is crucial for the float

A float does not move directly because of the liquid level, but because of buoyancy.

According to Archimedes’ principle, the buoyant force corresponds to the weight of the displaced medium.

In simplified form:

FA = ρ · g · V

where:

• ρ = density of the medium,
• g = gravitational acceleration,
• V = displaced volume.

If the density of the medium decreases

the float must immerse more deeply in order to displace sufficient liquid.

If the density increases

the same float rides higher in the liquid.

The float selection must therefore be suitable for the:

• minimum medium density,
• temperature,
• process pressure,
• material compatibility.

Depending on the version, WIKA FLR series devices can be used with minimum densities starting at approximately:

400 kg/m³

.

However, the specific float selection must always be based on the particular version and the actual process conditions.

Immersion depth and actual measuring point

The liquid level does not automatically correspond to the geometric centre of the float.

The immersion depth of the float depends on:

• its weight,
• its volume,
• the density of the medium.

The position of the magnet system relative to the liquid surface is decisive.

With changing density

the position of the float relative to the liquid surface may change.

This is particularly relevant in the case of:

• large temperature changes,
• different product batches,
• media with strongly varying concentration,
• interface measurements.

If a few millimetres are critical, not only the electrical resolution but also the hydrostatic buoyancy behaviour of the float must therefore be considered.

Selecting the correct guide tube diameter

The guide tube guides the float and at the same time contains the reed measuring chain.

Depending on the reed level transmitter model, diameters such as the following may be available:

• 8 mm,
• 12 mm,
• 14 mm,
• 18 mm.

The tube diameter influences several factors

• possible measuring length,
• mechanical rigidity,
• usable float size,
• process connection,
• installation through existing tank openings.

Long guide tubes require sufficient stability

A very long and thin tube can be subjected to increased stress from:

• flow,
• vibration,
• lateral loading,
• its own weight.

For this reason, the maximum possible measuring length increases with the guide tube diameter for certain product series.

Considering measuring length and dead zones

A common ordering mistake is to assume that the tank height is directly equal to the required measuring length.

A distinction must be made between:

• overall guide tube length,
• insertion length,
• active measuring range,
• upper dead zone,
• lower dead zone,
• mechanical float stop.

The float cannot travel all the way to every mechanical end of the tube

A float stop is normally located at the lower end.

The process connection and housing also limit the upper travel range.

The following question should therefore be clarified during the design process:

Which liquid height should actually correspond to 0 to 100%?

and not merely:

What is the total height of the tank?

Mechanical guidance of the float

The float must be able to move freely over the entire measuring range.

Potential problems include:

• bent guide tube,
• mechanical stress,
• deposits,
• foreign objects,
• insufficient clearance from internal tank components,
• strong lateral flow.

Particularly with long probes

it should be checked whether additional mechanical guidance or support is required or specified by the manufacturer.

However, additional guidance must not obstruct the float.

Turbulence, wave motion and foaming

A very high-resolution reed chain can only provide a stable value if the float itself moves sufficiently smoothly.

With a strongly moving liquid surface

the float may constantly move between several positions.

The output signal then follows this movement.

Typical causes include:

• incoming medium directly next to the float,
• agitators,
• pump operation,
• strong wave motion,
• tank vibrations.

Possible measures

• select a calmer installation location,
• increase the distance from the filling line,
• provide a mechanical stilling tube,
• damp the signal in the evaluation system.

Electronic damping cannot fully compensate for a mechanically unsuitable measuring point.

Deposits and viscous media

The float principle requires sufficiently free mechanical movement.

With:

• sticky media,
• crystallising liquids,
• slurries,
• heavy deposits

friction between the float and guide tube may increase.

The float may then follow the actual liquid surface only with a delay or may stop moving altogether.

Typical fault pattern

The liquid level falls, but the indication initially remains at the previous value.

The float only moves further after vibration or mechanical disturbance.

In such applications, it should be checked whether the float principle is suitable at all.

Output signal of the reed chain

The reed chain itself initially forms a variable resistance or potentiometer circuit.

This signal can be evaluated directly or converted into an industrial standard signal using a transmitter.

Depending on the model, possible versions include, for example

• 3-wire potentiometer signal,
• 4–20 mA,
• 4–20 mA with HART,
• PROFIBUS PA,
• FOUNDATION Fieldbus.

The most suitable version depends on:

• cable length,
• control system,
• diagnostic requirements,
• explosion protection,
• existing infrastructure.

4–20 mA output and head-mounted transmitter

A 4–20 mA signal is particularly practical for conventional industrial installations.

The entire active level range can, for example, be scaled as:

4 mA = 0%

and:

20 mA = 100%

Example

Active measuring range:

0 … 1,200 mm

With linear scaling, this corresponds to:

Level	Output
0 mm	4 mA
300 mm	8 mA
600 mm	12 mA
900 mm	16 mA
1,200 mm	20 mA

The reed chain itself nevertheless remains finely stepped.

The transmitter converts this into a correspondingly stepped 4–20 mA signal.

What resolution is actually useful?

The required resolution should be derived from the process requirement.

Example: storage tank

Tank height:

3,000 mm

A resolution of:

9 mm

corresponds to only:

0.3% of the tank height

This may already be more than sufficient for a normal inventory indication.

Example: small dosing vessel

Measuring height:

300 mm

A resolution of:

9 mm

corresponds to:

3% of the measuring height

In this case, a finer pitch may be useful.

Define the process requirement first

Before selecting the transmitter, the following question should therefore be asked:

What is the smallest actual level change that must be reliably detected?

rather than:

What is the finest available contact pitch?

Tank geometry and volume indication

The reed level transmitter initially measures a:

height

or float position.

A percentage level indication is directly proportional to the volume only if the tank has a constant cross-section over its entire height.

For a vertical cylindrical tank

with a constant cross-section:

50% height ≈ 50% volume

For a horizontal cylindrical tank

this is not generally the case.

Due to symmetry, half the liquid height does correspond to 50% volume, but, for example, 25% height does not correspond to 25% volume.

Conical or irregular tanks

also require additional:

• linearisation,
• tank characteristic curve,
• PLC calculation.

A high-resolution level measurement alone therefore does not guarantee an equally accurate volume measurement.

Mounting position and process connection

The float should be able to move as freely as possible in the vertical direction along the guide tube.

Depending on the version, vertical or limited inclined installation may be specified.

The following must be considered during process planning

• flange or thread size,
• tank opening,
• float diameter,
• mounting direction,
• sufficient installation space.

Do not forget the float diameter

A common mechanical planning error is to consider only the diameter of the guide tube.

However, the float can be considerably larger.

It must either:

• fit through the existing tank opening

or:

• be installable only after the guide tube has been inserted.

The specific mounting arrangement should therefore be clarified before ordering.

Separating continuous measurement and limit detection

Continuous level indication and independent overfill protection are functionally different tasks.

A reed level transmitter can, for example, continuously indicate:

0 … 100%

.

However, if a safety-related:

high-high level

is required, it should be checked whether this requires:

• separate limit detection,
• an independent measuring principle,
• a suitably approved safety device.

The continuous operating indication should not automatically be regarded as an independent protection function.

Practical example: 1,500 mm tank

A process vessel has a usable level span of:

1,500 mm

The medium is a low-viscosity process oil.

Requirements

• continuous level indication,
• 4–20 mA output,
• resolution better than 10 mm,
• vertical installation from above,
• medium density throughout the operating range sufficient for the selected float.

Selecting the contact pitch

A contact spacing of:

18 mm

can already provide a specified resolution of approximately:

9 mm

.

This would fundamentally meet the defined requirement.

Would 5 mm still make sense?

Not necessarily.

A 5 mm contact pitch would provide a finer resolution, but this only offers a practical advantage if:

• the process actually requires this resolution,
• the float is guided with sufficient stability,
• the liquid surface is sufficiently calm,
• the downstream display or control system makes meaningful use of the additional information.

Float selection

The lowest expected medium density at the maximum process temperature must also be considered.

A float is then selected that still provides reliable buoyancy at this density and mechanically matches the guide tube.

Result

Only the combination of:

measuring length + contact pitch + float + density + process connection + output signal

defines the actually suitable reed level transmitter.

Typical fault patterns

Observation	Possible cause	Recommended check
Indication changes only in large steps	coarse contact pitch	check pitch and specified resolution
Float immerses unusually deeply	medium density lower than specified	check density and float selection
Measured value remains stuck while the tank is being emptied	float is sticking or deposits are present	mechanically inspect guide tube and float
Measured value constantly jumps by several steps	moving liquid surface	check installation location and damping
0% point does not correspond to the tank bottom	mechanical dead zone not considered	check active measuring range
100% point is not reached	upper float travel is limited	check insertion length and upper stop
Float does not fit through the process opening	float diameter overlooked during planning	check installation route and process connection
Indication fluctuates despite a calm liquid level	vibration or magnet/contact problem	check mechanical guidance and measuring chain
4–20 mA signal does not correspond to float position	head transmitter scaling incorrect	check 0% and 100% points
Volume indication is incorrect despite correct level measurement	non-linear tank geometry	store tank linearisation curve
Measurement changes after a product change	different medium density changes float immersion depth	check density range and float

Recommended selection procedure

1. Define the measurement task: Determine whether level, volume indication or both are required.
2. Determine the active measuring range: Define the required 0% and 100% points.
3. Define the medium: Consider material compatibility.
4. Determine the density range: Consider minimum and maximum density throughout the process.
5. Determine the temperature: Check its influence on density and materials.
6. Determine the process pressure: Select float and guide tube accordingly.
7. Select the float: Match buoyancy, magnet, material and dimensions.
8. Select the guide tube diameter: Consider length, stability and float.
9. Determine the contact pitch: Derive it from the actually required resolution.
10. Check the resolution: Do not equate contact spacing with resolution.
11. Consider dead zones: Allow for mechanical float stops.
12. Check the process connection: Define thread, flange and tank opening.
13. Check the float diameter: Ensure installation through the process opening.
14. Evaluate the installation location: Avoid turbulence and lateral flow.
15. Select the output signal: Define potentiometer, 4–20 mA, HART or fieldbus.
16. Check volume linearisation: Provide an additional characteristic curve for non-linear tanks.
17. Evaluate the limit-level function: Provide a separate safety function if required.
18. Document the overall design: Clearly specify measuring length, pitch, float, density and output.

Suitable reed level transmitters from ICS Schneider

WIKA FLR-S, FLR-P and FLR-H for the process industry

The WIKA FLR-S, FLR-P and FLR-H are suitable for continuous level measurement in a wide variety of liquid media.

The measuring principle is based on:

• float with magnet system,
• reed resistor measuring chain inside the guide tube,
• 3-wire potentiometer circuit,
• quasi-continuous level measurement.

Depending on the model, the product family offers:

• different contact pitches,
• different floats,
• different process connections,
• different materials,
• optional explosion-protected versions.

Depending on the version, the operating limits extend up to:

• Temperature: -80 … +200 °C,
• Pressure: vacuum up to 80 bar,
• Minimum density: from approximately 400 kg/m³.

Optional configurable head-mounted transmitters are available for:

• 4–20 mA,
• HART,
• PROFIBUS PA,
• FOUNDATION Fieldbus.

WIKA FLR-SC with 4–20 mA and Bluetooth

The WIKA FLR-SC combines the reed measuring chain with a:

4 … 20 mA

output and an additional Bluetooth interface.

Via the Bluetooth connection, the measuring range and the 0% and 100% points of the head-mounted transmitter can be configured, among other parameters.

WIKA FLR-F for food and pharmaceutical applications

For more demanding hygienic applications, the WIKA FLR-F is available.

The series is designed for applications including:

• process water,
• drinking water,
• beverage industry,
• food industry,
• pharmaceutical industry.

Alternative for particularly high resolution requirements

If resolution in the range of a few millimetres is not sufficient, it should be checked whether another measuring principle would be more suitable.

Magnetostrictive level transmitters such as the WIKA FLM series provide, depending on the model, resolution significantly below:

1 mm

and are therefore suitable for higher-resolution float-based measurements.

Further solutions can be found under level measurement technology at ICS Schneider.

Conclusion

A reed level transmitter is a robust and versatile solution for continuous level measurement when the mechanical and electrical design is consistently matched to the application.

The float is part of the measuring system

Its buoyancy and magnet system directly determine how reliably the reed chain is actuated.

Medium density influences the float position

At a lower density, the float immerses more deeply. The float selection must therefore take the entire expected density range into account.

Contact spacing and resolution are not the same thing

A 5 mm contact pitch can, for example, provide a specified resolution of approximately 2.7 mm, while an 18 mm contact spacing can provide approximately 9 mm.

The finest pitch is not always necessary

The selected resolution should match the actual process requirement, tank size and stability of the liquid surface.

Free mechanical movement is crucial

Deposits, turbulence, lateral forces or an unsuitable float can influence the measurement more strongly than the electrical resolution of the reed chain.

Tank height and measuring range must be considered separately

Dead zones, float stops and insertion length determine which part of the tank can actually be used as the active measuring range.

For practical applications

Define measuring range → determine medium and density range → check process pressure and temperature → select suitable float → define guide tube and mechanical installation conditions → determine required measurement resolution → select appropriate contact pitch → consider dead zones → define output signal → consider tank geometry and, where necessary, volume linearisation → evaluate independent limit-level functions separately.

FAQ: Selecting Reed Level Transmitters Correctly

What is a reed level transmitter?

A reed level transmitter is a continuous level measuring instrument in which a magnetic float moves along a guide tube and actuates a reed resistor measuring chain arranged inside the tube.

How does the reed chain work?

Reed contacts and resistors are located inside the guide tube. Depending on the float position, its magnetic field actuates the corresponding contacts and thereby changes the electrical resistance of the measuring chain.

Is a reed level transmitter a continuous measurement?

Due to the contact pitch, the measurement is finely stepped and is therefore described as quasi-continuous.

What does contact pitch mean?

The contact pitch describes the spacing between the individual reed contacts or switching positions along the measuring chain.

Which contact pitches are available?

Depending on the version, corresponding WIKA reed measuring chains are available, for example, with 5, 10, 15 and 18 mm pitches.

Does a 5 mm pitch automatically mean 5 mm accuracy?

No. Contact spacing and specified measurement resolution are different parameters. For corresponding WIKA versions, a 5 mm contact spacing can, for example, provide a resolution of approximately 2.7 mm.

What resolution results from a 10 mm contact pitch?

Depending on the version, the specified resolution is approximately 5.5 mm.

What resolution results from a 15 mm contact pitch?

Depending on the version, the specified resolution is approximately 7.5 mm.

What resolution results from an 18 mm contact pitch?

For corresponding versions, the specified resolution is approximately 9 mm.

Is the smallest contact pitch always the best choice?

No. The pitch should be derived from the actually required process resolution. A higher resolution offers no practical advantage if, for example, wave motion or process fluctuations are significantly larger.

What is the function of the magnet in the float?

It actuates the reed contacts of the measuring chain contactlessly through the wall of the guide tube.

Can I use any float?

No. Magnet system, float dimensions, density range and guide tube must all match one another.

Why is the density of the medium important?

The density determines the buoyant force and therefore how deeply the float immerses in the liquid.

What happens if the medium density is too low?

The float immerses more deeply. If the density is too low for the selected version, the float may no longer reliably perform its intended measuring function.

What minimum density do WIKA FLR level transmitters support?

For various FLR versions, minimum densities starting at approximately 400 kg/m³ are specified. However, the specific float selection depends on the particular device configuration and process conditions.

Can a change in density influence the measured value?

Yes. A change in density alters the float immersion depth and therefore the position of the magnet system relative to the actual liquid surface.

Which guide tube diameters are available?

Depending on the FLR version, diameters such as 8, 12, 14 or 18 mm are available.

Why is a larger guide tube required for long measuring ranges?

As the length increases, the mechanical stability requirements for the guide tube increase. The maximum permissible length therefore depends, among other things, on the tube diameter.

Is the insertion length the same as the measuring range?

Not necessarily. Mechanical float stops and design-related dead zones can reduce the active measuring range.

Why can the float not travel all the way to the tank bottom?

A mechanical float stop is normally located at the lower end of the tube. In addition, the float itself has a certain height.

Can turbulence influence the measurement?

Yes. A strongly moving liquid surface causes corresponding float movement and can therefore result in a fluctuating measured value.

What can be done if the liquid surface is unstable?

The installation location can be optimised, the distance from filling lines can be increased, a stilling tube can be used or the output signal can be damped accordingly.

Are reed level transmitters suitable for sticky media?

That depends on the application. Heavy deposits or sticky media can restrict the free movement of the float and therefore impair the measurement.

What output signal does a reed measuring chain provide?

The basic version operates as a resistor measuring chain or 3-wire potentiometer circuit. Suitable transmitters can convert this into industrial standard output signals.

Is 4–20 mA available?

Yes. Depending on the version, WIKA FLR series devices can be equipped with a head-mounted transmitter for 4–20 mA.

Is HART available?

Yes. Configurable head-mounted transmitters with 4–20 mA and HART are available for corresponding FLR versions.

Can a reed level transmitter directly measure tank volume?

It initially measures the liquid level or float position. With non-linear tank geometries, conversion into volume must be carried out using a corresponding tank characteristic curve.

Why does the percentage indication in a horizontal tank not automatically correspond to the volume?

Because the cross-sectional area changes with the fill height. The relationship between liquid height and volume is therefore non-linear.

Can a reed level transmitter also be used as overfill protection?

Continuous operating indication and a safety-related limit-level function must be evaluated separately. Depending on the application, an independent and appropriately approved limit detection system may be required.

Which WIKA reed level transmitter is suitable for process applications?

The WIKA FLR-S, FLR-P and FLR-H are specifically designed for continuous level measurement in numerous industrial liquid applications.

Is there a reed level transmitter with Bluetooth?

Yes. The WIKA FLR-SC combines a 4–20 mA output signal with a Bluetooth interface for configuration and level monitoring.

What alternative is available for significantly higher resolution?

For particularly high resolution requirements, magnetostrictive float level transmitters may be a suitable alternative. Depending on the model, corresponding WIKA FLM versions can achieve resolutions of less than 0.1 mm.

Where can I find further level measurement technology?

Further devices can be found under level measurement technology at ICS Schneider.