Ultrasonic distance measuring device

An ultrasonic distance measuring device configured to measure a length of an ultrasonic wave propagation path between a liquid level of a detected fluid reserved in a tank and an ultrasonic element, the ultrasonic distance measuring device comprising:

• a transmission pipe including a reference pipe and a detection pipe, wherein the reference pipe has a predetermined length and extends in a horizontal direction orthogonal to a height direction along which the ultrasonic element and the liquid level are placed, wherein the detection pipe extends in the height direction toward the liquid level from a distal end of the reference pipe, such that a cavity of the reference pipe and a cavity of the detection pipe communicate with each other;
• the ultrasonic element configured to generate an ultrasonic wave in the cavity of the reference pipe toward the distal end of the reference pipe and to convert an input ultrasonic wave into an electric signal;
• a transmission circuit configured to output a drive signal that causes the ultrasonic element to generate an ultrasonic wave;
• a comparator circuit configured to output a detection signal when the electric signal converted by the ultrasonic element is greater than a threshold value;
• a storage unit configured to store
• the predetermined length of the reference pipe and
• a longest propagation path length and a shortest propagation path length of the propagation path;
• a first arithmetic circuit configured to compute a propagation velocity of an ultrasonic wave propagating through the propagation path based on a first time difference between an output timing, at which the ultrasonic element outputs an ultrasonic wave, and a reference timing, at which the comparator circuit outputs the detection signal based on an ultrasonic wave reflecting off the distal end of the reference pipe, and the predetermined length of the reference pipe stored in the storage unit;
• a period circuit configured to set a propagation path detection period to detect a liquid level timing, at which the comparator circuit outputs the detection signal based on an ultrasonic wave reflecting off the liquid level, based on the longest propagation path length and the shortest propagation path length stored in the storage unit and the propagation velocity of an ultrasonic wave computed by the first arithmetic circuit;
• and
• a second arithmetic circuit configured to compute the length of the propagation path based on a second time difference, which is between the liquid level timing and the output timing during the propagation path detection period, and the propagation velocity of an ultrasonic wave computed by the first arithmetic circuit.