Depth gauge

2/ A system according to claim 1, wherein both active portions come from a common piezoelectric pellet.

3/ A system according to claim 2, wherein the piezoelectric pellet is substantially disk-shaped, the active portion suitable for directing a sound wave towards the liquid surface being situated in the center of the disk and having a circular outline, the active portion suitable for directing a sound wave towards the reference obstacle extending around the central portion.

4/ A system according to claim 1, wherein the opening in the wall of the housing facing the measurement active portion is of substantially the same dimensions as said measurement portion, said dimensions of the opening nevertheless being slightly greater.

5/ A system according to claim 1, including temperature-measuring means connected in parallel with the piezoelectric measurement means, the temperature-measuring means having a characteristic which varies as a function of temperature, the resistance of said means being lower than the resistance of the piezoelectric measurement means over the entire range of vehicle utilization temperatures.

6/ A system according to claim 5, wherein the temperature-measuring means comprise a thermistor.

7/ A system according to claim 6, wherein the temperature-measuring means are connected across any one of the pairs of power supply wires to the measurement pellet.

8/ A system according to claim 5, wherein the temperature-measuring means comprise a diode.

9/ A system according to claim 1, further comprising calculator means enabling the depth of liquid in the tank to be determined on the basis of the time separating transmission and reception of sound waves by the two active portions.

10/ A system according to claim 9, wherein the calculator means are also suitable for determining the temperature of the liquid in the tank by measuring the resistance across the terminals of the thermistor.

11/ A system according to claim 1, including support means for fixing the housing in position at the bottom of the tank.

12/ A system according to claim 1, wherein the housing has means suitable for holding the piezoelectric means in position inside the housing.

13/ A system according to claim 1, wherein the housing includes means suitable for holding the electrical connections of the pellet in position inside the housing.

14/ A system according to claim 1, wherein the reference obstacles comprise wave reflectors oriented relative to a vertical direction of the device when placed in the tank at an angle equal to half the angle through which the sound wave path is deflected, said reflectors having their bottom surfaces facing towards the nearest side walls of the housing.

15/ A system according to claim 14, wherein the reflectors are oriented at 45°.

16/ A system according to claim 1, wherein the area of the reference obstacles is substantially equal to the area of the measurement opening.

17/ A system according to claim 1, wherein the inside of the housing is subdivided into two volumes:

• a reference first volume;
• and a measurement second volume;
• the two volumes being separated by means of a separator tube extending from the top wall of the housing substantially in register with the separation between the two active portions of the pellet, and terminating before making contact with the active pellet fixed to the bottom of the housing.