An assembly is disclosed herein that is configured to detect a liquid level in a liquid-containing vessel, the assembly comprising a first electrical circuit board disposed in a case and including an LED emitting component, an LED detecting component, and a switch-activating component, a sight window assembly configured to be mounted in an opening in a wall of the liquid-containing vessel at a location remote from the case, the sight window assembly including a housing with a first end section supporting a glass component with an apex configured to be disposed proximate the inner side of the wall of the vessel, and a second end section supporting a cable support module, a first fiber optic cable connecting the LED emitting component to the glass component and a second fiber optic cable connecting the LED detecting component to the glass component, the second fiber optic cable transmitting light reflected through the glass component to the LED detecting component when gas is present in the vessel at a designated level adjacent to the glass component, and not transmitting light when liquid is present in the vessel at the designated level adjacent to the glass component. An electrical switch is configured to be in a first position when reflected light is detected by the LED detecting component, and in a second position when reflected light is not detected by the LED detecting component.

A trap bowl is provided to accumulate liquid droplets from a filter, as a liquid content. The trap bowl includes a transparent vertical prism. The transparent vertical prism includes a face that forms a vertical transparent surface facing against a content of the section. The face can provide a first angle of total reflection when content of the section is a type of gas, and a second angle of total reflection when the content of the section is the liquid content. A light source may emit a light beam incident on the face at an angle of incidence. The angle of incidence results in reflection of the light beam, striking the light receiver, when the face has the first angle of total reflection, and results in refraction of the light beam, missing the light receiver, when the face has the second angle of total reflection.

To improve liquid level gauging inside liquid tanks, especially cryogenic liquid tanks, an apparatus and method for determining the liquid level are provided. A light source emits outgoing light rays that are back reflected or scattered by the tank wall of the liquid tank. The back reflected or scattered incoming light rays are received by a light conducting device, which conducts the incoming light rays towards a sensor device. The sensor device has optical sensors coupled to the light conducting device such that there is a one-two-one relationship between each optical sensor and a specific tank wall portion. Due to the change in the amount of light that is received by the optical sensor after the outgoing light rays were refracted by the liquid surface, the liquid level determining device is capable of determining the liquid level.

A trap bowl is provided to accumulate liquid droplets from a filter, as a liquid content. The trap bowl includes a transparent vertical prism. The transparent vertical prism includes a face that forms a vertical transparent surface facing against a content of the section. The face can provide a first angle of total reflection when content of the section is a type of gas, and a second angle of total reflection when the content of the section is the liquid content. A light source may emit a light beam incident on the face at an angle of incidence. The angle of incidence results in reflection of the light beam, striking the light receiver, when the face has the first angle of total reflection, and results in refraction of the light beam, missing the light receiver, when the face has the second angle of total reflection.