Wednesday, September 7, 2011

Mechanically Actuated Water Level Detection Float Ball Switch with Mercury Switch

The use of float balls as a component part of a liquid level detection device to mechanically actuate a switching mechanism such as a mercury switch or a micro switch is an effective and economical method of controlling liquid level in liquid tanks or any liquid reservoir. Float level switch is not only simple and convenient in terms of its concept and adaptability in various applications due to its uncomplicated and easy to understand principle but is also cheaper than its more advanced opposite counterpart most commonly known as the floatless liquid level controllers.

In general, liquid floats use the theory of buoyancy, which is brought about by the natural consequence of the upward force exerted by the liquid on objects that are lighter in density than the liquid, hence causing the object to stay afloat. In actual applications such as detecting water level as shown on the drawing below, the plastic ball floats due to the force exerted against its associated weight by means of the upward push of the rising water acting on the buoyancy of the plastic ball.

mechanically actuated water level detection float switch with mercury switch
Water Level Detection Float Switch equipped with Mercury Switch
This typical float switch works by switching the direction of its lever's tilt position influenced by the effect of the exchanging prevalent force between the two balanced counter weights suspended on both ends of the see-saw lever. The see-saw lever is fixed on its center axis by a fulcrum that supports the pivoting action of the lever as it changes its tilt direction influenced by the magnitude of whichever of the two counter weights exerts the most dominant pull-down force on the ends of the lever.

To provide protection for outdoor use, the internal switching mechanism that holds the mercury switch is housed inside a protective weatherproof plastic enclosure. The mercury switch is a sealed glass vial containing liquid mercury and two electrical contact points which provides a close contact only when the contacts are immersed in the liquid mercury. The mercury switch is mechanically attached physically parallel to the center of the see-saw lever which provides the mercury switch with equal tilt axis position common to the lever, that is, the tilting direction of the lever causes the mercury switch to move in tandem according to the same inclined angle of the lever.

The electrical contact points inside the glass vial of the mercury switch is a normally open contact which is provided with external wires intended for external wiring connection to a pump control circuit, or alternatively, it can also be used to signal an alarm. But the most common application is for running or stopping a water pump. When used for starting a water pump, the normally open contact of the mercury switch is directly connected in parallel across the start push button switch of the water pump control circuit, which then acts as a closing contact to run the pump in place of the start push button switch.

Conversely, when used for stopping a water pump, it is necessary to reverse the condition of the normally open contact in the mercury switch, this can be achieved by incorporating an additional relay and using the normally close contact of the relay to connect in parallel across the start push button switch of the water pump control circuit. The mercury switch open contact will be used to activate or deactivate the relay coil. When the mercury switch contact is open, the relay coil is not energized which retains the relay's normally close contact at close state and maintains the water pump at run mode. When the mercury switch changes position wherein the glass vial is tilted to the direction that causes its internal contacts to be immersed in the liquid mercury, the mercury switch will then provide a shorted contact output as its contacts changes state from open to close which energizes the relay coil to open the relay's normally close contact, placing the water pump in stop mode.

The position of the stopper for the plastic ball is adjustable, it is adjusted depending on the water level where the float ball must be kept stopped from sliding up the string which is inserted through a hole in the center of the plastic ball. The stopper serves as a knot that holds the float ball at a certain preferred water level. When the ball reaches the stopper it will cause the force of the rising water to elevate the floating plastic ball to pull up counter weight A below it, thereby relieving the pull down force of counter weight A on the lever as it becomes lighter while it is being carried upward by the floating ball. The force of the rising water on the plastic ball would also provide an upward push action to one end of the lever which correspondingly causes counter weight B on the other side to pull down the other end of the lever. Once in this condition where counter weight B surpasses the downward force of counter weight A, the mercury switch will be tilted to the direction where the electrical contacts inside the glass vial are away from the liquid mercury which results in an open electrical contact (as shown in the drawing).

When water recedes and the water level in the tank or reservoir falls, the plastic ball would also be lowered down to follow the falling water level until the plastic ball is no longer afloat, this would eventually cause the weight of counter weight A to pull down its end of the lever to surpass counter weight B on the other end of the lever. Once the counter weight A prevails over the counter weight B, the mercury switch would then be tilted to the direction wherein its internal contact points will be immersed in the liquid mercury, hence providing a close contact connected to the external water pump control circuit.

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