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A sensor for determining level of liquid in a reservoir, a reservoir containing such sensor, and a method for determining level using such sensor. The sensor comprises a liquid-proof housing, at least one actuator operatively connected to the inner side of the housing and configured to generate vibrations, and at least one vibration sensor operatively connected town inner side of the housing and configured to measure vibrations. The actuator comprises an electromagnet directly or indirectly fixedly connected to the inner side of the housing, and a permanent magnet movably suspended in the housing, wherein the electromagnet and the permanent magnet are configured such that activation of the actuator creates an oscillating mutual displacement in parallel with a longitudinal center axis of the electromagnet.