A fluid level sensing device for determining the level of the interface between a first fluid and a second fluid in a container, the device comprising: an optical waveguide for disposal in the container for at least partial immersion in the first fluid; a light source for providing input light to the optical waveguide; and a light detector for sensing output light from the optical waveguide, wherein the optical waveguide comprises a plurality of curved portions (22a-d), successive ones of the curved portions being curved in alternating directions, and wherein the optical waveguide is configured such that a curved portion immersed in the first fluid has a greater light loss profile than a curved portion not immersed in the first fluid, and the output light from the optical waveguide provides a measure of the level of the interface based on light loss of the input light through the optical waveguide, wherein a sensing portion of the optical waveguide is configured to extend in a first direction, the first direction being substantially perpendicular to the interface between the first and second fluids (14,16), and successive curved portions (22a-d) are spaced in the first direction and overlap such that at least one curved portion is present at any point in the first direction in the sensing portion of the optical waveguide, and wherein a first plurality of the curved portions comprises a plurality of curved portions having curved portions of a first size and one curved portion having a curved portion of a second size smaller than the first size, the fluid level sensing device being configured to detect an expected change in detected output light at the smaller curved portion and to correlate the expected change in detected output light to a known level of the smaller curved portion, optionally further to calibrate a determined level of the interface based on the known level of the smaller curved portion. an optical waveguide for disposal in the container for at least partial immersion in the first fluid; a light source for providing input light to the optical waveguide; and a light detector for sensing output light from the optical waveguide, wherein the optical waveguide comprises a plurality of curved portions (22a-d), successive ones of the curved portions being curved in alternating directions, and wherein the optical waveguide is configured such that a curved portion immersed in the first fluid has a greater light loss profile than a curved portion not immersed in the first fluid, and the output light from the optical waveguide provides a measure of the level of the interface based on light loss of the input light through the optical waveguide, wherein a sensing portion of the optical waveguide is configured to extend in a first direction, the first direction being substantially perpendicular to the interface between the first and second fluids (14,16), and successive curved portions (22a-d) are spaced in the first direction and overlap such that at least one curved portion is present at any point in the first direction in the sensing portion of the optical waveguide, and wherein a first plurality of the curved portions comprises a plurality of curved portions having curved portions of a first size and one curved portion having a curved portion of a second size smaller than the first size, the fluid level sensing device being configured to detect an expected change in detected output light at the smaller curved portion and to correlate the expected change in detected output light to a known level of the smaller curved portion, optionally further to calibrate a determined level of the interface based on the known level of the smaller curved portion.