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Lab Station: Buffer tank

University of South-Eastern Norway (campus Porsgrunn) has 10 laboratory buffer tanks, see Figure 1. The tanks are used in control courses in both bachelor and master programmes in technology.

Figure 1: Buffer tank


Averaging level control is an important part of several process systems. Some important practical examples are:

·               The equalization or buffer magazine at the inlet of a wasteater treatment plant.

·               Oil/water separators in the oil industry.

·               Water magazines in hydropower systems.

Both the equalization magazine and the separator can here be regarded as liquid buffer tanks. In both examples, the level should be compliant to flow variations so that variations in the inflow are damped through the tank, making the outflow considerably smoother than the inflow. Smoother outflow is advantageous for the subsequent processes, e.g. for the biological treatment processes and the oil production. The level controller must be tuned for compliant (or soft, or sluggish) level control so that the volume of the tank can absorb the inlet variations.

Technical description

Figure 2 shows a piping and instrumentation diagram (P&I D) of the tank including a level control system.

Figure 2: Piping and instrumentation diagram (P&I D) of the buffer tank a level control system.

The measurement signals (analog inputs) from the sensors LT1, FT1 and FT2 are voltage signals in the range 0 5 V.

The control signals (analog outputs) to the actuators pump P1and pump P2 are voltage signals in the range 0 5 V.

A built-in NI USB-6001 IO device handles the IO. This IO device can be connected to a PC with a USB cable.

As an alternative to using the NI USB-6001 IO device, there are terminals on the rig for direct analog IO.

Technical information

To appear.

Video presenting the buffer tank

To appear.

Level controller

The control signal generated by the level controller is a demanded outflow:

u = F_out

The level controller may be a PI controller or some other controller.

Mathematical model

Mass balance of the water in the tank leads to the following “volume balance” assuming water density is the same all over:

h’ = (F_in - F_out)/A


·       h [cm] is the water level in the tank.

·       F_in [cm^3/s] is water inflow.

·       F_out [cm^3/s] is water outflow.

·       A [cm^2] is cross-sectional area of tank.

Nominal values and ranges:

·       h_nom = 15. Range: 0 - 30.

·       F_in_nom = 50. Range: 0 - 100.

·       F_out_nom = 50. Range: 0 - 100.

·       A = 56.7.


The project has been initiated by Olav Vangen and Finn Aakre Haugen. The construction as been accomplished by Olav Vangen, Cecilie Gløsmyr, Fredrik Hansen, Marius Bergflødt, and students at Skogmo high school.

Updated 22 October 2023 by Finn Aakre Haugen. E-mail: