A Perspex tank ure 1), which has parallel sides for the top part of its height and a 'V' shaped section for the bottom part and a constant width throughout, is used as a receptacle for water.

A centrifugal pump feeds the tank from a rectangular sump tank and discharges to the sump tank through a manually adjustable ball valve. A D.C., p.m. motor drives the pump and its characteristics are not linear (though they will be assumed to be so). The motor is driven by a Pulse Width Modulated (PWM) power amplifier, which supplies power to the motor proportional to a voltage signal from the controller.

The head (h) in the tank is measured by a pressure transducer that produces a voltage signal proportional to head.

The signal representing h is supplied to a PC through the ADC of a National Instruments (NI) card. Similarly the PC can send a control signal from the DAC of the same card. It is therefore possible to write programs to control the tank level, and the software used is the graphical programming language supplied by NI, called LabVIEW. This graphically based programming tool has drivers supplied for the NI card and so makes the programming of control algorithms reasonable straightforward.