Non-invasive measurement of lubricating oil viscosity using an ultrasonic continuously repeated chirp shear wave.

•Use of a continuously repeated chirp to form a pseudo-standing wave for viscosity measurement.•In-situ real time Newtonain viscosity measurement using non-invasive ultrasound.•Shear standing wave used to increase sensitivity of viscosity measurement.•Acoustic matching layer significantly improves ultrasonic viscosity measurement.