COMPLIANCE INFO_1993-1998 - Laserfiche WebLink

Home Browse Search

A number of factors affect the quality of volumetric tank testing. Among these are product thermal currents, water table effects, tank deformation, and stiction. Thermal currents are tracked by the temperature sensors located in the probe to provide the statistical model with the required information to filter out the unwanted effects of expansion or contraction during the test period. Ground water and tank deformation are also easily handled by the test algorithm. Stiction, however, is an event which occurs randomly and is more difficult to identify. Stiction is the "sticking" of the float to the shaft and failing to move with the liquid. The motive forces on the float are the result of buoyancy and gravity. When the liquid level changes the balance between the two forces is upset, and a small net force is created moving the float up or down. If the float's static frictional force against the shaft is greater than the net motive force, the float will not move in response to the level change. In fact, since the float is stuck on one side, the float will actually pivot around the point of contact. This pivot action will be recorded as a change in level. However, the level change reported will be much smaller than the true level change. This pivoting action continues until friction is insufficient to resist the opposing force and a correction jump occurs. Although the effects of stiction are of no significance to inventory management, it is of great significance to tank testing. Stiction is the single greatest limitation on leak detection performance with the magneto-strictive level sensor. The technology is capable of measuring the magnet position to 0.0001 inch but is limited to 0.001 inch primarily because of stiction. Sophisticated programs must be used when tank testing to accommodate this event. Control Engineers, U.S.A. Page 1�,'.