In many process plants, the level of a vessel must not only be available in the control system or process control system, but also remain clearly visible directly on site. Especially for tanks, pressure vessels, process vessels, feed tanks, steam systems, chemical plants or media with demanding process conditions, a robust local indication is often an important part of plant safety and operability.
A bypass level indicator is a proven solution for this. It displays the level mechanically and clearly visible next to the vessel. The measuring principle is based on a laterally mounted bypass chamber that is connected to the vessel via process connections. The level in the bypass chamber corresponds to the level in the vessel. A float with a magnetic system moves with the liquid level and actuates an external magnetic indicator.
This article explains when a bypass level indicator makes sense, how the measuring principle works and which points must be considered during sizing. The focus is on the bypass chamber, communicating vessel principle, float, magnetic flap indicator, lateral mounting on the vessel, process connections, pressure, temperature, medium density, materials, limit switches, 4–20 mA transmission and typical applications in process plants.
Table of contents
- Basics: what is a bypass level indicator?
- How it works: bypass chamber, float and magnetic indicator
- Why a mechanical level indication can be useful
- Typical applications in vessels and process plants
- Sizing: consider density, pressure, temperature and medium
- Process connections, installation position and lateral mounting
- Materials, corrosion and difficult media
- Limit switches, level transmitters and 4–20 mA signal
- Bypass indicator, sight glass or electronic level sensor?
- Maintenance, visual inspection and typical sources of error
- Practical example: local indication and limit switch on a process vessel
- Which measuring instruments / products are suitable?
- Conclusion: robust level indication directly on the vessel
- FAQ: frequently asked questions about bypass level indicators
Basics: what is a bypass level indicator?
A bypass level indicator is a mechanical level indicator that is mounted laterally on a vessel. At its core, it consists of a bypass chamber, a float and an externally mounted magnetic indicator. The bypass chamber is connected to the vessel via at least two process connections. This creates a communicating vessel: the liquid level in the bypass chamber adjusts to the same level as in the vessel.
The float is located inside the bypass chamber and is matched to the density of the medium. As the level in the vessel rises or falls, the float also moves in the bypass chamber. A magnetic system inside the float transfers the movement contactlessly to the external indicator. The liquid does not come into contact with the external indicator.
The major advantage of this design is the clear separation between process medium and display element. The medium remains inside the bypass chamber, while the display is mounted outside in a clearly visible position. This allows the level to be read locally even on closed, pressurized or hot vessels without requiring a direct view into the vessel.
Bypass level indicators are often used wherever a robust, easy-to-read and process-oriented level indication is required. Depending on the version, electrical limit switches or a continuous level transmitter can also be mounted. This allows the mechanical local indication to be combined with signals for the control system, process control system or alarm system.
| Component | Function | Why important? |
|---|---|---|
| Bypass chamber | Laterally mounted measuring chamber connected to the vessel | The level in the chamber corresponds to the level in the vessel. |
| Float | Moves with the liquid level in the bypass chamber | Must match density, temperature and medium. |
| Magnetic system | Transfers the float position contactlessly to the outside | Enables indication without direct media contact of the display elements. |
| Magnetic indicator | Displays the level visibly on the outside of the bypass chamber | Allows clear local reading at the vessel. |
| Optional switches / transmitters | Output limit levels or continuous signals | Extend the mechanical indication with electrical feedback. |
How it works: bypass chamber, float and magnetic indicator
The function of a bypass level indicator is based on the principle of communicating vessels. When the vessel and bypass chamber are connected via suitable process connections, the same liquid level is established in both areas. The bypass therefore does not display an indirectly calculated value, but the actual level of the connected vessel via the liquid level in the separate chamber.
A float is located inside the bypass chamber. This float has an integrated magnetic system and is designed to float on the medium. The density of the medium is therefore a decisive sizing factor. If the float is not selected to match the medium density, the indication may become inaccurate or the float may not operate reliably.
The magnetic indicator is mounted on the outside of the bypass chamber. A red-white magnetic flap indicator or a comparable optical indicator is often used. When the float passes the display element, the flaps are switched by the magnetic field. This creates a clearly visible level line. The operator can directly see at the vessel how high the medium level is.
Because the magnetic indicator is located outside the pressure-retaining area, it is separated from the medium. This is particularly advantageous for hot, pressurized, aggressive or opaque media. The indication remains readable without a sight glass being directly exposed to the process medium.
Why a mechanical level indication can be useful
Mechanical level indicators continue to be used even in modern plants because they offer a very practical advantage: the level is visible directly on site. Operators, maintenance staff and plant personnel can immediately assess the vessel condition without first having to open a PLC display, HMI or process control system.
This is particularly helpful during commissioning, maintenance, troubleshooting and inspection rounds. If an electronic level value in the process control system appears implausible, the mechanical indication can serve as independent local information. It does not replace complete process diagnostics, but provides direct orientation at the vessel.
Another advantage is the robust design. A bypass level indicator requires no electrical auxiliary power for the pure indication. The level indication is generated mechanically or magnetically by the movement of the float. This means that the local indication remains available even if an electrical signal, transmitter or control system is not being evaluated.
At the same time, a bypass indicator can be flexibly expanded. Magnetic switches can be used to monitor limit levels, for example minimum, maximum or dry-run protection. With a level transmitter, a continuous signal, such as 4–20 mA, can also be transmitted to the control system. In this way, the bypass level indicator combines mechanical indication with electrical process integration.
| Advantage | Practical benefit | Typical situation |
|---|---|---|
| Direct local indication | Level is visible at the vessel | Inspection round, commissioning, fault, maintenance. |
| Indication without auxiliary power | Mechanical indication works independently of the output signal | Comparison with process control system or during electrical fault. |
| Closed process space | Medium remains in vessel and bypass chamber | Pressure vessels, hot media, aggressive liquids. |
| Optional limit switches | Min. or max. levels can be reported electrically | Pump protection, overfill protection, alarming. |
| Optional transmitter | Continuous level can be transmitted to PLC or process control system | Automation, process monitoring, documentation. |
Typical applications in vessels and process plants
Bypass level indicators are used in many process areas. Typical applications include vessels, tanks, pressure vessels, feed tanks, steam generators, condensate vessels, separators, reactors, storage tanks and process equipment. Wherever liquids are stored in a closed vessel and the level should be visible on site, a bypass indicator can be a sensible solution.
In the chemical and petrochemical industries, the robust design and wetted materials are particularly important. Materials, seals, process connections and float must match the medium. For aggressive, hot or corrosive liquids, a bypass level indicator with suitable materials can provide reliable local indication.
In power plants, boiler houses, process water systems and utility areas, clear indication directly at the vessel is often an important operating advantage. During inspection rounds, the plant condition can be checked quickly. In addition, limit switches can be used for pump control, min. alarm or max. alarm.
Mechanical level indicators can also be useful in food, beverage, pharmaceutical or water treatment plants if materials, cleaning, hygiene requirements and process connections are properly designed. The specific application is always decisive: medium, temperature, pressure, density, cleaning, installation location and desired signal output must be considered together.
Sizing: consider density, pressure, temperature and medium
The sizing of a bypass level indicator begins with the process data. The density of the medium is particularly important. The float must be designed so that it floats reliably under real operating conditions. It is not only the density at room temperature that is relevant, but the density in the actual process condition. Temperature, pressure and composition can change the density.
Pressure and temperature also determine the design. The bypass chamber, process connections, seals, float and optional electrical components must be suitable for the operating conditions. Careful sizing is particularly important for high temperatures, cryogenic media, pressure vessels or changing operating conditions.
The medium itself also influences the selection. Aggressive liquids require suitable materials. Viscous media can dampen the float movement. Crystallizing or heavily contaminated media can lead to deposits. Media containing solids, foam formation or strong density changes must be assessed particularly carefully.
In practice, therefore, not only the desired measuring range should be specified. For reliable sizing, medium, density, operating pressure, operating temperature, vessel connections, installation situation, material requirements, desired indication, limit switches, electrical signal and possible cleaning or maintenance requirements are needed.
| Sizing variable | Why important? | Typical question |
|---|---|---|
| Density of the medium | Determines float design | Which density is present under operating conditions? |
| Pressure | Determines chamber, connections and mechanical design | Which operating and design pressure is required? |
| Temperature | Influences materials, seals and density | Which minimum and maximum process temperature occurs? |
| Medium | Determines materials, corrosion resistance and cleanability | Is the medium aggressive, viscous, contaminated or crystallizing? |
| Measuring range | Defines length and connection positions | Which level range should be displayed visibly? |
Process connections, installation position and lateral mounting
A bypass level indicator is usually mounted laterally on the vessel. At least two process connections are required for this: a lower and an upper connection. Via these connections, the liquid level in the bypass chamber can adjust according to the vessel level. The connection positions define the visible measuring range.
Depending on the vessel and application, the process connections can be designed as flange, threaded or welded connections. It is important that connection type, nominal size, pressure rating, sealing face and material match the plant. For existing vessels, it must also be checked which connections are already available and whether they are suitably positioned for a bypass indicator.
The installation position must ensure that the bypass chamber communicates correctly with the vessel. Shut-off valves, venting, draining and maintenance options should be considered during planning. Additional fittings are useful, especially for media that form deposits or must be drained during maintenance.
Readability also plays a role. The display section should be clearly visible to operating personnel. If the bypass indicator is mounted behind pipes, insulation or plant components, it loses part of its practical benefit. Mechanical mounting, visibility and maintenance access should therefore be planned together.
Materials, corrosion and difficult media
The wetted materials of a bypass level indicator must match the application. Bypass chamber, float, process connections, seals and, if applicable, other components come into contact with the medium or its vapor phase. For corrosive media, careful material selection is crucial.
Stainless steels are used in many applications. For particularly aggressive media, however, other materials or special versions may be required. The selection depends on medium, concentration, temperature, pressure, impurities and cleaning chemicals. A general statement on media compatibility is not meaningful without precise process data.
Viscous, sticky, crystallizing or contaminated media can additionally challenge mechanical level indicators. If deposits form in the bypass chamber or the float no longer remains freely movable, the indication can become delayed, blocked or inaccurate. In such cases, chamber geometry, connections, cleaning, draining and maintenance access must be considered particularly carefully.
For hot or cryogenic media, insulation, frost protection, heat tracing or special display elements may be relevant. Insulation must not make the display unreadable and should not impair the function of the magnetic system. In hazardous areas, the components and electrical options used must also match the zone and application.
Limit switches, level transmitters and 4–20 mA signal
A bypass level indicator can do more than provide local indication. By mounting magnetic switches, limit levels can be detected. Typical applications are min. alarm, max. alarm, pump enable, dry-run protection, overfill alarm or switching points for simple control logic. The switches are mounted externally on the bypass chamber and actuated by the magnetic system of the float.
If a continuous electrical signal is required, a level transmitter can also be used. This allows the level not only to be displayed locally, but also to be transmitted to a PLC, process control system, display unit or data logger. An analog 4–20 mA signal is particularly common for this purpose.
For 4–20 mA transmission, the scaling must be clearly defined. It must be clear which level corresponds to 4 mA and which level corresponds to 20 mA. Fault behavior, wire break, load, supply and signal assignment in the process control system should also be documented. A mechanically correctly operating bypass indicator can still be displayed incorrectly in the control system if the electrical scaling does not match.
The UPS4E loop calibrator is suitable for testing 4–20 mA signals. It can be used to measure and simulate mA signals, test current loops and detect scaling errors between level transmitter, PLC, display or process control system. This is particularly helpful during commissioning, device replacement, parameterization or troubleshooting.
| Electrical option | Typical benefit | What to pay attention to? |
|---|---|---|
| Magnetic switch | Limit level indication for min. or max. | Document switching point, contact type, switching capacity and position. |
| Level transmitter | Continuous electrical level signal | Define measuring range, output signal and integration into the control system. |
| 4–20 mA | Robust analog transmission to PLC or process control system | Check scaling, load, supply and fault current. |
| Alarm contact | Overfill protection, dry-run protection or fault message | Clearly define logic and plant response. |
| Ex version | Use in hazardous areas | Observe approval, zone, wiring and intrinsically safe circuits. |
Bypass indicator, sight glass or electronic level sensor?
A bypass level indicator is not automatically the best solution for every application. It is particularly useful when a robust local indication directly on the vessel is desired and the medium is suitable for a float. Compared with a simple sight glass, the bypass indicator often offers advantages in terms of pressure, temperature, readability and optional electrical expandability.
A sight glass can appear very direct because the medium is visible. However, it is more dependent on contamination, discoloration, medium appearance and mechanical design. With opaque, fouling or hazardous media, a magnetic bypass indicator is often easier to read because the indication is located outside and does not rely on viewing the medium itself.
Electronic level sensors such as radar, guided radar, hydrostatic sensors or capacitive sensors have different strengths. They can operate without moving floats, are better suited to certain vessel geometries or directly provide an electrical signal. Depending on the version, however, they lack the simple mechanical local indication directly on the vessel.
In many plants, a combination is useful: a bypass level indicator shows the level locally, while a transmitter or limit switch provides electrical signals. This gives operating personnel, control system and maintenance staff the information they need.
Maintenance, visual inspection and typical sources of error
A bypass level indicator is generally robust, but should be checked regularly. Visual inspection includes the display, flap strip or indicator rail, bypass chamber, process connections, seals, valves, venting, draining and optional electrical components. The indication should be clearly readable and respond plausibly to level changes.
Typical sources of error are deposits in the bypass chamber, blocked float, dirty or damaged display elements, incorrect float design, changed medium density, closed or partially closed shut-off valves, leaking process connections or incorrectly positioned limit switches.
If the indication does not match the actual vessel level, it should first be checked whether the bypass chamber is correctly connected to the vessel. Are both shut-off valves open? Are there deposits or blockages? Can the float move freely? Has the medium been changed? Has the density changed? Has the plant been modified?
For electrical options, the signal path should also be checked. Does the limit switch switch at the correct point? Is the transmitter correctly scaled? Does the 4–20 mA signal match the local indication? Are limit values evaluated correctly in the process control system? Mechanical and electrical checks should be considered together.
Practical example: local indication and limit switch on a process vessel
In a process plant, a feed vessel is operated with a liquid. Operating personnel want to be able to see the level directly during inspection rounds. At the same time, a low level should protect a pump and a high level should trigger a warning message. A purely electronic measurement in the process control system would not replace the local reading.
A bypass level indicator is specified for the vessel. The bypass chamber is mounted laterally on the vessel via two flange connections. The float is designed to match the medium density. The magnetic indicator displays the level directly on site. In addition, two magnetic switches are mounted: a lower switching point for minimum level and an upper switching point for maximum level.
During commissioning, it is checked whether the bypass chamber fully communicates with the vessel, whether the float moves freely and whether the magnetic indicator switches plausibly. The limit switches are then tested with the control system. The switching points are documented so that later maintenance and troubleshooting remain traceable.
The example shows the typical advantage of a bypass level indicator: it combines clear mechanical indication with simple electrical feedback. Operating personnel can see the level at the vessel, while the control system can simultaneously evaluate min. and max. signals.
Which measuring instruments / products are suitable?
For robust local level indication on vessels, the WIKA Type BNA bypass level indicator with magnetic indicator is a suitable solution. It is mounted laterally on the vessel and displays the level via a bypass chamber, a float with magnetic system and an external magnetic indicator. This makes it particularly suitable for process plants where the level should remain visible directly on the vessel.
The category level indicators is the right starting point when different mechanical and magnetic level indicators for vessels and apparatus are to be considered. Bypass indicators, sight glass indicators and other robust local indicators can be classified there.
If limit values or continuous electrical signals are required in addition to the local indication, the version with magnetic switches or level transmitter should be checked. This means the bypass level indicator can not only display the level, but also provide min./max. messages or a continuous signal to the PLC or process control system.
The UPS4E loop calibrator is suitable for testing and commissioning 4–20 mA signals. It can be used to check whether level transmitter, display, control system and process control system use the same scaling and whether the current loop is working correctly.
| Product / area | Typical use | Particularly relevant for |
|---|---|---|
| WIKA Type BNA bypass level indicator | Mechanical level indication with magnetic indicator directly on the vessel | Process vessels, pressure vessels, chemicals, energy plants, water treatment and local reading |
| Level indicators | Selection of mechanical and magnetic level indicators | Vessels, apparatus, tanks, sight glass indicators and bypass systems |
| Magnetic switches / limit switches | Electrical indication of min. or max. levels | Pump protection, overfill alarm, alarming and simple control functions |
| Level transmitter | Continuous electrical transmission of the level | PLC connection, process control system, process monitoring and data recording |
| UPS4E loop calibrator | Testing and simulation of 4–20 mA signals | Commissioning, scaling check, troubleshooting and signal comparison |
Conclusion: robust level indication directly on the vessel
A bypass level indicator is useful when the level of a vessel should be displayed robustly, directly and clearly visible on site. The measuring principle with bypass chamber, float and magnetic indicator is particularly helpful in process plants where operating personnel need an independent local indication.
Correct sizing is decisive. Medium, density, pressure, temperature, materials, process connections, measuring range, installation position and electrical options must match the application. Especially for aggressive, hot, viscous, crystallizing or contaminated media, the version should be checked carefully.
The most important recommendation is: do not consider bypass level indicators only as simple displays. With limit switches and level transmitters, the mechanical local indication can be connected to the control system, alarm system and process control system. This creates a robust and practical level solution for vessels and process plants.
FAQ: frequently asked questions about bypass level indicators
What is a bypass level indicator?
A bypass level indicator is a mechanical level indication mounted laterally on a vessel. The level is detected in a bypass chamber via a float and made visible externally via a magnetic indicator.
How does a bypass level indicator work?
The bypass chamber is connected to the vessel via process connections and acts like a communicating vessel. The level in the chamber corresponds to the level in the vessel. A float with a magnetic system transfers the position contactlessly to the external indicator.
Why is a magnetic indicator used?
The magnetic indicator allows a clearly visible display of the level without the display elements coming into direct contact with the medium. This is advantageous for hot, aggressive, pressurized or opaque media.
Does a bypass level indicator require electrical auxiliary power?
No electrical auxiliary power is required for the pure local indication. The level is displayed mechanically or magnetically. Electrical power is only required when limit switches or a transmitter are used.
When is a bypass indicator useful?
It is useful when the level should be clearly visible directly on the vessel, when a robust indication is required or when limit levels or a continuous signal should additionally be transmitted to the control system.
What role does the density of the medium play?
Density is decisive for float design. The float must float reliably under real operating conditions. If the density changes significantly, this can influence the indication.
Can a bypass level indicator be used on pressure vessels?
Yes, depending on the version, a bypass indicator can be designed for pressurized vessels. Pressure rating, process connections, materials and seals must match the plant.
Which process connections are possible?
Depending on the version and plant, flange, threaded or welded connections are possible. It is important that connection type, pressure rating, material and position match the vessel.
Can a bypass indicator report limit levels?
Yes. Externally mounted magnetic switches can electrically report min. or max. levels. This is useful, for example, for pump protection, overfill indication or alarming.
Can a bypass indicator provide a 4–20 mA signal?
Yes, with a suitable level transmitter, the level can be transmitted continuously as an electrical signal. The scaling must match the measuring length and the control system.
How do you test the 4–20 mA signal of a level transmitter?
A loop calibrator can be used to measure or simulate the mA signal. This makes it possible to check whether transmitter, display, PLC and process control system use the same measuring range.
What is the difference between a bypass indicator and a sight glass?
With a sight glass, the medium is viewed directly. With a magnetic bypass indicator, the level is displayed via a float and an external magnetic indicator. This often keeps the indication easier to read and separated from the medium.
Which media are critical?
Highly viscous, sticky, crystallizing, contaminated, solids-containing or strongly corrosive media can be critical. Chamber geometry, materials, float, cleaning and maintenance must be checked particularly carefully in these cases.
What happens if the float is blocked?
If the float is blocked, the indication no longer reliably follows the actual level. Causes can include deposits, contamination, incorrect sizing, mechanical damage or unsuitable media conditions.
How is a bypass indicator maintained?
Maintenance includes visual inspection, checking the indicator, checking the process connections, draining or cleaning if necessary and functional testing of limit switches or transmitters. The exact intervals depend on medium and plant.
Can a bypass level indicator be insulated?
For hot or cold media, insulation can be useful. However, it must be ensured that indication, magnetic function, readability and maintenance access are maintained.
When is an electronic level sensor better?
An electronic sensor may be more suitable if no moving parts are desired, if the vessel geometry is unfavorable or if only an electrical signal is required. A combination of bypass indicator and transmitter is also often useful.