Screen Washer Pump

Unlike electronic components which are generally cheap and readily available, fluid handling devices for low budget research and student projects are harder to find.  One possibility for non-hazardous fluids is an automotive screen wash pump.  These can be obtained from vehicle dismantlers for less than £5.  The photo shows  a typical example.

Typically these units are in the form of a 12 volt motor directly coupled to a small centrifugal pump.

This particular pump's performance can be summarized as:

Voltage	12V
Current under load	Approx. 2A
Flow rate	Approx. 2.5 litres/minute
Head	greater than 5m

At a guess these units are designed for intermittent use rather than continuous service so are not suitable for 24/7 mission critical tasks, but might be a means of implementing a rain water toilet flushing scheme.

Flowrate

A portion of the pump's performance is show below.

In this test, the head is the physical distance the water was lifted, rather than pressure at the discharge.  It is possible that the loss of pressure due to friction in the piping was around 0.5m.

Pulse Width Modulation

In their intended application, screen washer pumps are binary devices (i.e. they are doing nothing or squirting water), but if controlled using Pulse Width Modulation, the flow rate becomes continuously variable.

The performance of the pump varied with the pulse frequency as shown in the graph below:

At the higher frequencies, the pulses are two short to overcome the mechanical and fluid inertia the pump.  At 15Hz, the pump is controllable down to a 20% pulse width, below that the flow rate becomes unstable.

The relationship between pulse width and head is shown below:

Page Updated: 11-Feb-2009

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