If the measured phase angle is ?? and the wave length of the ultrasound signal is ??, and the distance between the transmitting transducer (numeral 52) and the receiver (numeral 51) is d, then: d=???????/360 [0103] As can be seen from FIG. 6, A(d)=A0 sin(??), where A(d) is the measured signal and A0 is a known value determined from a previous calibration measurement.