Capillary depth gauge (1) including at least one capillary tube (2) of triangular section arranged against a background (3) opposite an observation surface (5) and visible by transparency from the observation surface when the tubular section is substantially filled with water, at least one reflection surface (4), visible by reflection from said observation surface when the tubular section is substantially filled with air, said tube forming an isosceles triangle whose equal angles (α) have an angular value of between 48 and 60 degrees.