Many different types of float switches have been developed for opening and closing an electrical circuit in response to the level of a liquid within a reservoir. Generically, float switches includes a floating buoy and a means, responsive to the vertical position of the buoy, for alternately closing an electrical circuit when the float achieves a predetermined maximum height and opening the electrical circuit when the float achieves a predefined minimum height (normally open) or visa versa (normally closed).
One type of available float switch is known as a mercury-actuated switch. Referring to FIGS. 1a and 1b, a typical, normally open, mercury-actuated float switch includes an electrode 10a", 10b" which extend into a sealed tube 11" containing mercury 12". The mercury 12" resides at a second end of the tube 11" and electrically closes the electrode 10a", 10b" after the float switch has attained a predefined upward vertical angle caused by a high liquid level within the reservoir (FIG. 1a). Conversely, the mercury 12" resides at a first end of the tube 11" and leaves the electrode 10a", 10b" electrically open after the float switch has attained a predefined downward vertical angle caused by a low liquid level within the reservoir (FIG. 1b).
Mercury-actuated float switches provide superior switching performance. However, because of environmental concerns relating to the use of mercury, alternatives to the mercury-actuated switch are being explored.
One type of mercury-free float switch is what is known as a sphere-actuated float switch. Generally, sphere-actuated float switches utilize movement of a sphere within a raceway caused by changes in the attitude of the raceway to effectuate opening and closing of an electrical circuit.
Examples of sphere-actuated float switches are provided in U.S. Pat. Nos. 4,644,117 (Grimes et al.) and 4,629,841 (Riback et al.). Referring to FIGS. 2a and 2b, the sphere-actuated float switch of Grimes et al. includes (i) a pair of positionable electrical contacts 63" attached to a shuttle 55" which is slidably retained within a raceway 33" defined by housing 25" and (ii) a corresponding pair of stationary electrical contacts 53" which are connected to the housing 25" and extend into the raceway 33". The shuttle 55" resides at a second end of the raceway 33" and provides contact between the electrical connections 53" and 63" after the float switch has attained a predefined upward vertical angle caused by a high liquid level within the reservoir (FIG. 2a). Conversely, the shuttle 55" resides at a first end of the raceway 33" and prevents contact between the electrical connections 53" and 63" after the float switch has attained a predefined downward vertical angle caused by a low liquid level within the reservoir (FIG. 2b). Movement of the shuttle 55" within the raceway 33" is effected by a sphere 75" which rolls within the shuttle 55" based upon the attitude of the shuttle 55". The float switch of Grimes et al. addresses the environmental concerns associated with the utilization of mercury-actuated float switches. However, the switch lacks the reliability associated with mercury-actuated float switches. Various factors contribute to this lack of reliability including specifically, but not exclusively, excessive friction between the shuttle 55" and the housing 25" resulting in failure of the sphere 75" to reposition the shuttle 55", wedging of the shuttle 55" within the raceway 33" again resulting in failure of the sphere 75" to reposition the shuttle 55", and generation of deposits upon the electrical contacts 53" and/or 63" resulting in poor electrical flow between the electrical contacts 53" and 63".
Referring to FIGS. 3a, 3b and 3c, the sphere-actuated float switch of Riback et al. includes (i) a longitudinally extended raceway 51" having a first longitudinal end 50" and a second longitudinal end 52" within which a sphere 54" is free to roll based upon the attitude of the raceway 51". A pivotally mounted cage 56" is positioned proximate the raceway 51" with legs 64" and 65" of the cage 56" alternately extending into the raceway 51". An actuator extension 60" extends from the cage 56" in a direction opposite the legs 64" and 65" for actuating a microswitch 62" based upon the pivoted position of the cage 56". The cage 56" resides in a first pivoted position with the sphere 54" at the second end 52" of the raceway 51", the actuator extension 60" detached from the microswitch 62", and the microswitch 62" in an electrically open mode after the raceway 51" has attained a predefined downward vertical angle caused by a low liquid level within the reservoir (FIG. 3c). Conversely, the cage 56" resides in a second pivoted position with the sphere 54" at the first end 50" of the raceway 51" and the actuator extension 60" in contact with the microswitch 62" so as to place the microswitch 62" in an electrically closed mode after the raceway 51" has attained a predefined upward vertical angle caused by a high liquid level within the reservoir (FIG. 3a). Pivoting of the cage 56" is effected by movement of sphere 54" between the first 50" and second 52" ends of the raceway 51". A locking mechanism (not shown) is employed to lock the cage 56" into the first and second pivoted positions in an effort to maintain synchronization between pivoting of the cage 56" and movement of the sphere 54". The locking mechanism releases the cage 56" to pivot between the first and second positions only when the sphere 54" depresses a pressure plate 86" which extends into the raceway 51".
The float switch of Riback et al., as with the float switch of Grimes et al., addresses the environmental concerns associated with mercury-actuated switches but lacks the reliability associated with mercury-actuated float switches. Various factors contribute to the lack of reliability including specifically, but not exclusively, insufficient momentum to effect a clean repositioning of the cage 56" and loss of synchronization between movement of the cage 56" and the sphere 54". In addition, the float switch of Riback et al. utilizes a microswitch to effectuate opening and closing of the electrical circuit based upon movement of the cage 56" rather than direct electrical contact.
Accordingly, a substantial need exists for a reliable, mercury-free float switch which can directly provide effective opening and closing of an electrical circuit under heavy electrical load conditions.