Magnetic Level Gauges - Bypass Level Indicators

Magnetic level gauges are externally mounted level instrumentation used on tanks to provide visual indication of the liquid level in the tank. If on-site visual level indication and level control is required, magnetic level gauges are an ideal solution. They can also be fitted with switches to control certain functions within a tank or transmitters to provide a constant reading of liquid level via current loops, analog outputs, or digital signals like HART and Profibus.

Magnetic level gauges operate on the principal of hydrostatic pressure. A bypass tube is attached to the side of the tank via two flanges, one on the top and one on the bottom, in a vertical orientation. The liquid from the tank fills the bypass tube and the level in the tube is the same height as the liquid within the tank itself. When the liquid level in the tank increases or decreases, the level in the bypass tube also changes accordingly.

Within the bypass tube, a magnetic float moves on top of the liquid level and transfers the level in a non-contact manner to a display, scale, or transmitter. There are a variety of displays and options.

While it varies by manufacturer, KOBOLD’s standard NBK Magnetic Level Gauges are available in lengths up to 18 feet for single piece models. However, KOBOLD also has longer multi-piece models available up to approximately 36 feet.

KOBOLD created the NBK-04 Top-Mount Magnetic Level Gauges to provide the same reliable system for underground tanks. The principal of operation is similar in nature to the normal NBK models except that the float is in a dip pipe that is connected with the actuator inside the above-tank display via a rod connection. The NBK-04 also offers similar options as our regular NBK-series.

Chemical, caustic, acidic, and alkaline media are typically not compatible with metallic instrumentation. KOBOLD offers the NBK 16/17 Magnetic Level Gauges for Chemicals in polypropylene or PVDF which offers a wide range of compatibility with many different types of chemicals.

Magnetic level gauges do not employ floats in the traditional sense of float level switches or even the floats within rotameter flow meters. The magnetic element within the bypass tube is not visible through the indication sight glass. Instead, the magnetic passing by the rollers/balls create the visible indication by flipping the color. You do not read the float in the sight glass, but movement of the magnetic element within the bypass tube provides the colored reading.

Magnetic level gauges provide rugged and reliable level indication and do not require power to operate unless they are fitted with additional instrumentation along the tube for transmitting the liquid level. In applications where “seeing” the tank level is desired, and is not possible because the tank does not have any transparent areas for direct observation of liquid level, magnetic level gauges can provide that functionality. They are also simple and straightforward, so repair and maintenance is easier than other complex level instrumentation.

They are also ideal for applications with high pressures and high temperatures as they are of a more rugged build than traditional sight glass models that can be prone to breaking and leakages.

They are also useful for tanks that are not uniform in shape or that have protrusions or items within the tank that could cause false readings for other level technology.

Because the primary unique feature of magnetic level gauges is providing onsite level indication for tanks, applications looking to meet that simple need do not typically require high accuracy. For applications that require level measurement in conjunction with the bypass indicator, the accuracy is determined by the type of sensor/transmitter used.

For resistor chain models, the resolution is roughly 10mm. For many applications this is sufficient. For applications that require more accuracy, a magneto-strictive option is available that provides a finer resolution of 1 mm. Models built with a guided wave radar in a secondary bypass tube also offer about 1 mm. To learn more about resolution, visit our easy-to-understand article on accuracy, repeatability, and resolution.

Reading a magnetic level gauge is quite simple. The rotating roller/balls automatically display the level of the tank. The surface of the liquid is where the white transitions to colored.

Magnetic level gauges must always be installed in a vertical orientation for the hydrostatic pressure principal to operate correctly. The density of the liquid is also a consideration as it can affect the placement of the float in relation to the liquid surface. The units are typically built for a specific liquid gravity/media type and using them with liquids of a different density is likely to create erroneous readings.

Magnetic level gauges cannot be installed within close proximity to any magnetic fields or elements that may affect the magnetic components.

Installing a magnetic level gauge typically requires modifications to the tank, which can create particulate matter. Always ensure that the tank is fully cleaned before commissioning to remove any particulate matter. Alignment is also a consideration. If elements of the tank or instrumentation are stressed because of improper alignment or if screws are over tightened, it can compromise the integrity of the system and the tank.

Other application considerations for magnetic level gauges are that they cannot exceed the specified maximum pressure or temperature values as this can lead to damage and malfunction of the instrument. The tube materials must also be compatible with the media type so that the tubes are not compromised.

Bypass tubes are not ideal for liquids that have a high degree of soiling/coating, lots of suspended solids, crystallization, or ferrous particles. There are other level monitoring and measuring options that are better suited to these media characteristics.

Magnetic level gauge models are also sometimes limited in length, but the KOBOLD NBK is available in longer than average lengths due to our multi-piece assembly capabilities.

Magnetic level gauges are also kept in the best condition when the drain plug is opened occasionally to wash out any deposits, so they do not settle in the bypass tube and disrupt the system. If coating or crystallization does place, the tank must be emptied and the lower cover flange of the bypass tube must be removed. The float should be cleaned and reinstalled.

Because the system is simple and mechanical in nature, they are easy to install, understand, and operate. They also do not require much maintenance if they are used within the stated requirements.