Can Pressure Switches Be Used to Monitor and Control Tank Level Pressure?

Yes, pressure switches can be used to monitor and control tank level by sensing the pressure created by the liquid in the tank. They provide accurate, repeatable and versatile level control for applications including water treatment systems, bubblers, scrubbers, and pressurized tanks.

Industrial storage tanks are used for applications ranging from rainwater harvesting and municipal drinking water to irrigation and emergency fire suppression. Maintaining the correct liquid level in these tanks is important for efficient and reliable operation.

In this article, you will learn how pressure switches measure tank level and how different pressure switch configurations can be used to monitor and control liquid levels.

How do pressure switches monitor tank level?

Pressure switches monitor tank level by sensing the hydrostatic pressure created by the weight of the liquid column above the pressure connection. This pressure is proportional to the liquid level and is independent of tank diameter or piping configuration.

Switches can be installed in existing tank drains or access fittings and are typically mounted outside the tank, where they can be isolated with manifold valves for easier maintenance and replacement. Depending on the application, one or more switches can be used to monitor multiple tank levels from a single pressure connection.

The pressure in a container is the same at all points at the same level. For most applications, the pressure switch can therefore be connected to the tank anywhere below the level where switching is required. The switch is then adjusted to activate at specified pressure levels corresponding to desired liquid levels.

Compared with level switches that may require separate connections at each designated control level, pressure switches can monitor and control multiple levels from a single pressure connection. Setpoints can also be adjusted to accommodate changing operating conditions.

How does a bubbler system measure tank level?

A bubbler system allows a pressure switch to monitor liquid level when only the top of the tank is accessible or when side and bottom connections are difficult.

Air flows through a tube extending to the bottom of the tank and bubbles through the process fluid. A regulator controls the pressure to keep airflow to a minimum, while the pressure switch monitors the resulting backpressure.

This backpressure is proportional to the height of the fluid above the bottom of the tube, allowing the switch to determine the liquid level without requiring a pressure connection at the bottom of the tank. Plants and mills have successfully used this method for many years.

Depending on tank access, operating requirements and the number of control points needed, several pressure switch configurations can be used.

How can one pressure switch with two setpoints control tank level?

One pressure switch with two independent setpoints can control tank level and provide an additional alarm function in a bubbler system.

For slurry applications where side or bottom tank connections are difficult, a bubbler system can be an effective solution. A purge meter adjusts airflow to keep bubbling to a minimum, while the pressure switch monitors backpressure corresponding to the liquid level.

In this configuration:

• Backpressure equals the head or liquid level.
• Water flows continuously during normal operation.
• Setpoint one activates when level pressure reaches L1 and opens the solenoid valve to prevent spillage. The solenoid valve closes when pressure drops to the switch reset value.
• Setpoint two activates an alarm if the level pressure drops to L2.

Figure 1. Tank level control using one pressure switch with two setpoints and a bubbler system.

How can two pressure switches monitor and control tank level?

Two pressure switches can provide separate control points for managing high and low liquid levels in applications such as wet air scrubbers.

In sewage treatment plants, maintaining the correct liquid level in wet air scrubbers helps minimize chemical use and maintain consistent treatment. Two pressure switches with single setpoints can be used for this application. In some cases, one pressure switch with two independent setpoints may be used instead.

In this configuration:

• The suction pump operates continuously, with the make-up water solenoid typically open during normal operation to remove treated water at the same rate untreated water enters.
• Water passing through the baffles is aerated to enhance the treatment process.
• The first pressure switch (PS1) stops the suction pump if water pressure drops to level L1, helping prevent pump cavitation.
• The second pressure switch (PS2) stops the make-up water solenoid when the water level reaches L2, preventing high water levels from interfering with aeration at the baffles.

Figure 2. Pressure switch liquid level control in a sewage treatment plant wet air scrubber.

How can three pressure switches monitor and control tank level?

Three pressure switches can provide multiple control and alarm points for managing liquid levels in a water treatment system.

In this configuration, the first pressure switch (PS1) has an adjustable deadband to control the level between L1 and L2. The second and third pressure switches (PS2 and PS3) use single setpoints and fixed deadbands to control alarm levels L3 and L4.

The system operates as follows:

• PS1 activates when pressure drops to L1 and resets when pressure rises to L2.
• The solenoid valve opens at L1 to allow dirty water into the system and closes at L2.
• PS2 triggers an alarm and stops the suction pump if pressure drops to L3, helping prevent cavitation.
• PS3 activates if pressure reaches L4 to help prevent spillage.
• A separate control system manages treatment chemical addition, reaction timing and suction pump activation.

Figure 3. Three-pressure-switch level control in a water treatment system.

How do differential pressure switches control level in pressurized tanks?

Differential pressure switches can monitor liquid level in pressurized tanks by measuring the difference between the pressure at the bottom of the tank and the pressure above the liquid.

In a pressurized tank, the pressure above the liquid affects both the tank and the measurement. Using differential pressure allows this pressure to be accounted for so the resulting measurement corresponds to the hydrostatic pressure created by the liquid level.

For this application:

• The air pressure above the liquid acts on the low differential pressure port of the switch.
• The resulting differential pressure corresponds to the hydrostatic pressure created by the tank's liquid level.
• A differential pressure switch with a single setpoint and adjustable deadband is used.
• When the water level reaches L2, the switch activates and opens the output solenoid.
• The switch resets when the level drops back to L1.

Figure 4. Differential pressure switch level control in a pressurized tank.

What should you consider when selecting a pressure switch for tank level control?

The right pressure switch configuration depends on the tank design, process conditions, required control levels and how the switch will interact with the rest of the system.

Important factors can include the number of required setpoints, deadband requirements, tank pressure, available tank connections, process media and accessibility for maintenance.

Ready to learn more?

Pressure switches provide a flexible way to monitor and control liquid levels in water and wastewater applications, including storage tanks, bubblers, scrubbers and pressurized tank systems.

Check out the related resources below for more information, or contact us to speak to a product expert who can answer your questions. In the meantime,

Download our guide to learn more about switches and other pressure instrumentation used in water and wastewater applications, including instruments certified to NSF/ANSI/CAN 61 requirements.