Patent ID: 7243540

Claim:
Apparatus responsive to presence of a fluid at a predetermined level within a fluid-receiving space in which the fluid can rise to the predetermined level and for providing a control function if a level of the fluid shifts from the predetermined level, comprising: signal generating circuitry capable of introducing a level-determining signal into the fluid-containing space such that the level-determining signal is present for sensing within fluid at the predetermined level if, and only if, the fluid level is at least as high as the predetermined level, probe and probe-responsive circuitry having a sensitivity capable of sensing the presence of the level-determining signal within the fluid at the predetermined level, wherein the probe is configured for being inserted into the fluid-receiving space at the predetermined level, and control circuitry capable of providing a control function in response to whether the level-determining signal is so sensed, in order to indicate whether the fluid has a level at least as high as the predetermined level, whereby the control function may be used for alarm or cut-off purposes if the fluid level shifts relative to the predetermined level, said level-determining signal being transmitted through the fluid in the fluid-receiving space according to the value of electrical conductivity of the fluid, and the system including a provision for selectively adjusting the sensitivity of the probe-responsive circuitry according to a predetermined threshold of said value, wherein said level-determining signal is bipolar, the control circuitry providing multiple signal paths for responding to respective different polarities of the bipolar signal sensed by the probe, whereby the control function is provided with fail-safe operation.