The invention relates to a filling level measuring device (10) for measuring the filling level in a container (2) through the wall (9) thereof by means of ultrasound, having an ultrasonic measuring head (12), a controller (20), and a fastening device (24) by means of which the filling level measuring device (10) can be fastened to the container (2) such that the ultrasonic measuring head (12) is pressed against the wall (9) of the container (2). The invention further relates to a method of operating such a filling level measuring device (10), wherein a sampling rate is used which is situation-dependent. The invention finally relates to an assembly made up of such a filling level measuring device and at least one spacer (50) which can be attached to the lower edge of a container (2) to be provided with the filling level measuring device (10).

A liquefied gas monitoring system includes: a container containing liquefied gas, which is obtained by liquefaction of single-component gas; a liquid level sensor, which detects liquid level height in the container; and control circuitry, which receives, each time the liquefied gas is fed into the container, an input of an environmental value of the gas. Before the liquefied gas is fed into the container, the control circuitry determines an amount of the liquefied gas previously fed into and still remaining in the container based on the height detected by the liquid level sensor, and stores the determined amount together with the environmental value. After the liquefied gas is fed into the container, the control circuitry determines an amount of the liquefied gas fed at this time based on the height detected by the liquid level sensor, and stores the determined amount of the liquefied gas together with the environmental value.