Abstract: Apparatus (20) for measuring a volume of a fluid (38) in a container (24) having a length and a circumference. The apparatus includes one or more gauges (28A, 28B, 28C, 28D) that are configured for coupling to the container. Each gauge includes a hollow, at least partially transparent structure (30, 32, 34) containing a lumen, which is segregated from contact with the fluid in the container and which extends along at least a part of the length and around at least a part of the circumference when the gauge is coupled to the container. A measurement liquid, contained in the lumen, in an amount selected so that a level of the measurement liquid provides a tilt-invariant measurement point (36A, 36B, 36C, 36D) indicative of the volume of the fluid in the container.