Abstract: The present invention relates generally to a fluid level detector 1 designed to be mounted in a vessel which is arranged to contain a liquid which may be hazardous such as a petrochemical. The liquid level detector is mounted within a port 2 in a wall 3 of a vessel. The electrical componentry of the liquid level detector 1 includes a light transmitter 6 for transmitting light towards an optical element 7, and a light receiver 8 for receiving light reflected back from the optical element 7. Electrical circuitry is mounted on a printer circuit board 9 for detecting signals from the light receiver and controlling the transmission of light via the light transmitter 6. the entire componentry is potted in a potting compound 11 to protect the componentry from chemical and physical damage and the physically support it. The arrangement of epoxy and componentry is substantially enveloped by a glass housing 12.