Sonic level measuring method

A sonic level measuring method including steps of disposing a plurality of N sonic receivers at a constant interval l toward a water surface along the longitudinal portion of a waveguide tube, constituting a position of first sonic receiver as an original point of the water level measurement and emitting sonic pulses from the upper portion of a waveguide tube to measure a distance Lx from the original point to the water surface, comprising steps of:

• measuring a transit time t1 that it takes for advancing sonic pulses to be transited between two sonic receivers nearest to the water surface;
• measuring a transit time t2 from the receiving moment of the advancing pulse until the sonic pulse is reflected on the water surface and again received by the sonic receiver nearest to the water surface;
• computing an interval ΔL between the sonic receiver nearest the water surface to the water surface by the following expression;
• Δ ⁢ ⁢ L = t 1 2 ⁢ t 2 × l
• computing a distance Ll from the original point till the sonic receiver disposed nearest to the water surface by the following expression;
• and
• Li=(Nl−1)l
• adding the interval ΔL to the distance Ll thereby to computing Lx.
• wherein, Ni is the number of sonic receivers including the sonic receiver at the original point the sonic receiver nearest to the water surface, l is an interval between the sonic receivers.