Liquid level sensor

Claim 1

In combination with a gas cylinder capable of storing a liquefiable gas, the gas cylinder having a threaded recess disposed at the top of the cylinder, a sensor for sensing upper and lower levels in the gas cylinder comprising: (a) a sensor assembly including a first level sensor and a second level sensor both of identical design but of different lengths, the lengths of said first and second level sensors being selected to represented an upper level and a lower level, each level sensor comprising: (i) an input light pipe with an axis having an input surface at one end and terminating in an essentially 45° surface with respect to its axis at the other end; (ii) an output light pipe with an axis adjacent said input light pipe having an essentially 45° surface with respect to its axis opposite that of said input light pipe and having an output surface at its other end, an essentially non-refractive light path formed between said 45° surfaces, whereby, when said 45° surfaces of said light pipes are in a material of relatively low index, light at said input surface will be transmitted through said input light pipe, will be reflected at said 45° surfaces from said input light pipe to said output light pipe and will be transmitted to said output surface in said output light pipe, due to total internal reflection, but, when said 45° surfaces are in a material of higher index, light will be refracted at said 45° surfaces of said input light pipe and will not reach said output surface; said first and second level sensors in abutting relationship; and (iii) a baffle disposed between said first and second level sensors; (b) a seal disposed in said cylindrical recess; (c) a transparent cylindrical cap member having an internal bore adapted to accept said sensor assembly with a press fit, having a flange at its bottom sized to fit in said cyindrical recess atop said seal; and (d) a metal cover including an internal bore sized to tightly surround said transparent cap member and having external threads for screwing into said threaded recess, said metal cover having an input opening in the side thereof overlying the input surfaces of said first and second sensors and first and second output openings in the top thereof overlying the output surfaces of said first and second sensors.