초록 ▼

A pair of PTC elements, preferably of the barium titanate variety, are mounted in a container to sense liquid level, for example oil level in an engine crankcase. The elements have the same resistance, mass and switching point but different thermal loss coefficients which have one value in air and a

A pair of PTC elements, preferably of the barium titanate variety, are mounted in a container to sense liquid level, for example oil level in an engine crankcase. The elements have the same resistance, mass and switching point but different thermal loss coefficients which have one value in air and another in liquid. In one embodiment, the elements are both positioned at the set liquid level and are electrically energized at the same moment. When the elements are in liquid, one of them will heat to the switch point before the other and change to a much higher resistance. When the elements are in air, the other element reaches the switch point first. When the elements are connected in series across a power source, the switching causes the circuit to latch in that state even though the liquid level condition changes. The junction of the elements provides a convenient signal output indicating liquid level. In a second embodiment, one PTC element is at the set liquid level and the other is in air above that level. Again, the liquid level determines which PTC element wins the race when electrically energized.

대표청구항 ▼

A liquid level detector for monitoring the presence of liquid in a container at a set level when the detector is energized comprising, first and second PTC resistors each of which are self-heating upon application of a current and having a switch point effecting a substantial resistance increase at

A liquid level detector for monitoring the presence of liquid in a container at a set level when the detector is energized comprising, first and second PTC resistors each of which are self-heating upon application of a current and having a switch point effecting a substantial resistance increase at a unique temperature above the maximum temperature of the liquid, at least the first of the PTC resistors being positioned at the said set level and the second being positioned at the set level or in the air above the liquid, the first PTC resistor having a heat loss rate in air that is less than that of the second, and a heat loss rate in liquid that is greater than that of the second, so that when heating current is applied to the resistors, one will reach the switch point before the other depending on the liquid level, and a circuit including the PTC resistors in series and a voltage source for applying a current to the resistors and providing a voltage signal indicative of liquid level whereby the voltage signal assumes a high or low value depending on which of the resistors first switches to a high resistance.