Level Switches

A level switch is a device used to detect the presence or absence of liquid at a specific point within a tank, vessel or pipe. It is typically used to trigger an alarm, start or stop a pump, or protect equipment from conditions such as overflow or dry running.

Unlike continuous level measurement devices, a level switch provides a simple on/off output based on whether the media is present at the sensor point. This makes them widely used in applications where reliable, repeatable switching is required rather than full level profiling.

A thermal level switch works using thermal dispersion technology. The sensor is heated, and when it is exposed to air or gas, it retains heat differently than when it is in contact with liquid.

When liquid comes into contact with the sensor, it removes heat more efficiently. The instrument detects this change and switches output accordingly.

This method provides reliable detection even in challenging conditions such as coatings, foam, varying densities or turbulent flow, where mechanical or float-based switches can struggle.

Thermal level switches offer several key advantages:

• No moving parts, reducing wear and maintenance
• Reliable operation in difficult media including slurries and foam
• Suitable for high temperatures and high pressures
• Stable performance even with changing process conditions
• Ability to detect both level and interface conditions

In our experience, they are often chosen where conventional level switches fail due to fouling, vibration or process variability.

Level switches are used across a wide range of industrial applications, including:

• Pump protection (detecting dry running conditions)
• High and low level alarms in tanks and vessels
• Overflow prevention
  Separator interface detection (oil/water, liquid/liquid)
• Sump and wastewater level monitoring
• Process vessel level control

They are a critical part of many safety and control systems, particularly where a simple and reliable switching signal is required.

Yes, level switches are widely used for pump protection, particularly to prevent dry running.

When liquid flow or level drops below a safe threshold, the switch can trigger an alarm or shut down the pump. This helps prevent overheating, seal damage and premature failure.

Thermal level switches are especially effective in this role because they respond quickly and reliably, even in low flow or difficult conditions.

Yes, many level switches are designed for use in hazardous areas and are available with ATEX and IECEx certifications.

For applications in oil & gas, chemical processing or other explosive environments, it is important to select equipment with the correct certification and safety rating. Many models are also available with SIL (Safety Integrity Level) ratings for use in safety instrumented systems.

Correct specification is essential, and we always recommend reviewing the application in detail to ensure compliance.

Yes, thermal level switches can be used to detect interfaces between two different liquids, such as oil and water.

Because different fluids transfer heat differently, the sensor can distinguish between them and provide a reliable switching point.

This makes them particularly useful in separators, storage tanks and process vessels where interface control is important.

Choosing the right level switch depends on several factors:

• Type of media (liquid, slurry, foam, interface)
• Temperature and pressure conditions
• Hazardous area classification
• Required response time and switching accuracy
• Installation constraints and process connections

For example:

Pump protection may require fast-response switching
Interface detection may need a more sensitive configuration
Harsh environments may require specialised materials such as Hastelloy

The best approach is to review the application in detail and match the technology to the real process conditions.

Allison Engineering has extensive experience supplying level switch solutions across oil & gas, chemical, wastewater and process industries.

We don’t just supply products, we help ensure the correct solution is selected for the application. That includes understanding process conditions, recommending the right technology and supporting installation and long-term performance.

With access to proven, industrial-grade instrumentation and hands-on engineering support, customers benefit from solutions that are designed to work reliably in real-world conditions.