A filter trap apparatus for a therapeutic gas delivery system, comprising: a trap bowl configured to accumulate liquid droplets from a filter, as a liquid content, comprising a transparent circumferential prism, having a face that forms a circumferential interior surface of the trap bowl, the face having a first angle of total reflection when the gas is against the circumferential interior surface, and a second angle of total reflection when the liquid content is against the circumferential interior surface; a light source, configured to emit a light beam incident on the face at an angle of incidence; and a light receiver, wherein 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, wherein the face is an upper face, wherein the circumferential interior surface of the trap bowl is an upper circumferential interior surface, wherein the transparent circumferential prism further comprises a lower face, wherein the lower face forms a lower circumferential interior surface of the trap bowl, and wherein the upper face and the lower face form an included angle that opens outwardly, circumferentially around the trap bowl, and wherein the upper face and the lower face intersect at a vertex, the vertex being circumferential around the trap bowl, and wherein the included angle is arranged symmetrically about a reference bisector line that extends outward from the vertex, and wherein the light source is configured to emit the light beam as a collimated light beam, and to emit the collimated light beam in a direction approximately parallel to the reference bisector line, and wherein irrespective of rotational orientation of the trap bowl, 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. a trap bowl configured to accumulate liquid droplets from a filter, as a liquid content, comprising a transparent circumferential prism, having a face that forms a circumferential interior surface of the trap bowl, the face having a first angle of total reflection when the gas is against the circumferential interior surface, and a second angle of total reflection when the liquid content is against the circumferential interior surface; a light source, configured to emit a light beam incident on the face at an angle of incidence; and a light receiver, wherein 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, wherein the face is an upper face, wherein the circumferential interior surface of the trap bowl is an upper circumferential interior surface, wherein the transparent circumferential prism further comprises a lower face, wherein the lower face forms a lower circumferential interior surface of the trap bowl, and wherein the upper face and the lower face form an included angle that opens outwardly, circumferentially around the trap bowl, and wherein the upper face and the lower face intersect at a vertex, the vertex being circumferential around the trap bowl, and wherein the included angle is arranged symmetrically about a reference bisector line that extends outward from the vertex, and wherein the light source is configured to emit the light beam as a collimated light beam, and to emit the collimated light beam in a direction approximately parallel to the reference bisector line, and wherein irrespective of rotational orientation of the trap bowl, 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.