It has heretofore been proposed to employ so-called time domain reflectometry techniques to detect level of material in a storage vessel. In general, this technique involves placement of a conductive transmission line or probe in the vessel at an orientation to be contacted by material in the vessel. A microwave pulse of short duration is launched onto the transmission line probe, typically in a downward direction through air toward the material surface. When this pulse encounters an electrical impedance discontinuity, such as the change in dielectric constant at the interface between the air and material, a portion of the pulse energy is reflected back along the transmission line probe to detection electronics. Time-delayed gating techniques are employed in a process referred to as equivalent time sampling to locate the position of the reflecting discontinuity along the transmission line probe, and thereby to determine level of the material surface with respect to the probe.
Although the material level sensing technique so described has overcome problems and difficulties theretofore extant in the art, further improvements remain desirable. For example, although this conventional level detection technique is well suited for determining level of gasoline in a below-ground storage vessel at a service station or the like, a problem remains in association with detecting level of water accumulated at the bottom of the storage vessel. Conventional time delay reflectometry techniques will detect the interface between the gasoline and the water. However, because the dielectric constant of the gasoline is different from that of air, and the speed of pulse travel through the gasoline is thus slower than that in air, level of the gasoline/water interface will not be reliably measured unless compensation is made for these differences in dielectric constant and velocity. Furthermore, different grades of gasoline have different dielectric constants, and therefore different velocities of pulse travel through the gasoline layer to the gasoline/water interface.
It is a general object of the present invention to provide a system and method for measuring the position or level of the interface between immiscible materials, such as the gasoline/water interface in a below-ground gasoline storage vessel, which do not require input of gasoline dielectric constant. A more specific object of the present invention is to provide a system and method of the described character for automatically compensating the fluid interface measurement for fluid dielectric constant. A further object of the present invention is to provide a system and method of the described character that are economical to implement, and provide reliable operation over an extended operating lifetime.