Contactless liquid-level measurement through opaque container using millimeter-wave sensor

A novel contactless method for measuring liquid level through an opaque container is proposed. A millimeter-wave Doppler sensor is used to “see” through a target container and measure the liquid level on the basis of the absorption of millimeter waves in liquid. One of the challenges is to achieve accurate liquid-level measurement (within sub-millimeter error) despite the inherently large beam-spread angle of the millimeter wave due to diffraction. A piezoelectric vibrator enables accurate measurement by reflecting a limited portion of the spread beam and modulating it to distinguish it from the other portion of the beam. The feasibility of the proposed method for clearly detecting an air-liquid interface concealed in an opaque container was confirmed experimentally. Nonlinearity error of the measured liquid level was within ±0.5 mm, which is the first demonstration of accurate liquid-level measurement using a millimeter-wave sensor.