In the field of measurement and control, there are many applications where indirect measurement of pipe length plays an important role. This paper deals with the on-line measurement of the position of a sand surface in casing pipe related to the improvement of weak foundation. In the construction, how fast does the sand surface sink influences the form of the sand pillar. If the sinking velocity of the sand surface is measured on-line and with high accuracy, not only precise quality control but also smooth construction and consequently saving of much energy are achieved. The measurement can be reduced to that of the length of the pipe terminated with a metallic cover and pressurized sand surface.
For the length measurement, light and sound waves are recollected immediately as a medium, but it is difficult to use light wave because of much vapor produced by pressurization and decompression in the pipe. This paper presents, using sound wave, a method which enables the pipe length to be measured accurately and low-costly. The method is based on the pattern matching between the acoustic pressure in the pipe and a linear combination of the elementary and its integralfold frequencies of stationary waves.