Operating on the principle of conductivity (the ability of a liquid to conduct electricity), Curtis Industries Liquid Level controls sense and control the level of almost any fluid or moist solid. These units are available with fixed or adjustable set points to control levels in fluids having an external circuit resistance up to 3,000,000 ohms.
In operation, two or more submerged liquid level probes pass a minute electrical current through the liquid to be controlled. This current, typically 500 microamperes, is introduced to a solid-state resistance-sending amplifier which provides sufficient gate signal to trigger a silicon-controlled rectifier (SCR). The firing threshold is accurate to within 2% of operating differential and is adjustable by means of a potentiometer. The output of the SCR is used to operate the plug-in control relay. When anticipated loads exceed that of the standard relay, an auxiliary contactor is utilized.
Control Models ELS, LCS/LHS, LCF and RSA are designed for forward mode operation. They interpret a decrease of external circuit resistance from set point as an “ON” signal. Model LCR is designed for reverse mode operation. It energizes when external circuit resistance increases above set point. Model DLC is intended primarily for pump and heater level/lockout applications. See the appropriate catalog sections for specific application information.
When selecting a liquid level control, several factors must be taken into consideration. Questions arise as to what mode of operation is required, what is the resistivity range of the fluid to be controlled, is a fixed or variable set point called for, are special functions required such as a pump or heater lockout, etc. Use the connection diagrams in the Curtis Industries Catalog to select the ideal control for your specific application.