ES2001243A6 - Fluid level sensor for vessel - Google Patents

Abstract

The assembly includes a sealed three pin connector with threaded mounting, which supports a two sided ceramic substrate about 0.04 inches thick and projects into the interior of the vessel. On one side of the substrate is a heater circuit, surface mounted in printed circuit form. On the other side of the substrate is a detection circuit (24) consisting of four thick film, negative temperature coefficient thermistors (30,32,34,36) in a bridge configuration. - The pairs of thermistors are positioned a selected distance from each other depending upon the liquid level range being measured. An electronic circuit (42) connected to the bridge functions as an amplifier/switch and is dependant upon which thermistors are dry (in air) and which are wet (i.e. submerged). The potentials (Va and Vb) in the detector bridge are measured and, liquid level detection is provided. (7pp Dwg.No.3/5) USAB- US4609913 A A fluid level sensor is provided which includes a probe having a temperature-responsive circuit positioned adjacent to a heater element or circuit. The temperature-responsive circuit may include a number of thermistors wired in a bridge configuration on one side of a substrate. The heater circuit is defined on the other side of the substrate. When the bridge and heater circuits are energised, the voltages on either side of the bridge are monitored. - If the fluid level is correct, only the lower portion of the bridge will be immersed. Since fluid (i.e. liquid) removes heat faster than air, the immersied thermistor(s) will be cooler and will thus affect the relative voltage outputs from the opposite sides of the bridge. Different voltage levels will also be provided if all the thermistors are immersed or in the air. These relative voltages can be utilised in a circuit that will signal an operator that the fluid level is incorrect.