An optical sensor includes: a sensor portion, having a transmitter and at least one receiver, configured to couple to a wettable component having a fluid flow channel, wherein the transmitter is disposed to emit a light that travels from the sensor portion to the wettable component where a majority of the light is directed towards the fluid flow channel at an angle between a first critical angle and a second critical angle; wherein with liquid filling the fluid flow channel, the majority of the light refracts at a liquid interface and travels through the entirety of the fluid flow channel; wherein with gas in the fluid flow channel, the majority of the light reflects at a gas interface and does not travel through the entirety of the fluid flow channel; wherein the amount of the light refracted and/or reflected and received by the at least one receiver, is used to determine if there is liquid or gas in the fluid flow channel.