Abstract
In small well pipes, we must use small fioats to measure the water-level with automatic water -level gauge. Small float causes small buoyancy force which is sometimes less than the sum of inner frictional force of the machine and outer frictional force between the float and the wall of the well pipe, then the gauge cannot record the water-level change. The inner frictional force is measured for some machines and the outer frictional force is estimated for some cases. And then, the relation between the size of the float and the sensitivity of the water-level gauge is discussed.
