Fail safe circuit for a salinity monitor

Claim 1

In a salinity monitor of the type wherein a salinity cell is employed to measure salinity of a liquid and is connected to a salinity indicating panel by means of first, second and third electrical leads, a fail safe circuit comprising: first means within the salinity cell for generating a current signal, the absence or presence of which represents whether the first and third electrical leads are connected to the salinity indicating panel and a salinity level signal representative of the salinity of the liquid; second means within the salinity indicating panel for providing an a.c. signal representing a standard signal for comparison to signals provided by the salinity cell; third means responsive to the a.c. signal and the absence or presence of the current signal for providing an a.c. differential voltage signal having a level which represents whether the first and second electrical leads are connected to the salinity indicating panel; reference means for providing a reference voltage representing a voltage level below which indicates a disconnected electrical lead and above which indicates a connected electrical leads; voltage comparator means responsive to the reference voltage and the a.c. differential voltage signal for providing a control signal representing a disconnected electrical lead by a high voltage connected electrical lead by a low voltage; electronic switch means responsive to the control signal for causing said salinity level signal to indicate a high salinity level thereby enabling an alarm; and circuit means responsive to a disconnected condition of the second electrical lead, said circuit means causing said salinity level signal to indicate a high salinity level thereby enabling an alarm.