Bubbler tubes comprise a superior way to measure the level of liquids because no mechanical apparatus needs to be immersed in the fluid. Reliability is enhanced and maintenance reduced, especially with corrosive, sticky, viscous, or otherwise difficult fluids. However, accuracy may suffer if the pressure is not correctly determined. A number of factors can generate errors in the pressure measurement and the prior art includes a number of proposed designs to insure better determination of the fluid pressure.
For example, U.S. Pat. No. 5,115,679, to Uhlarik, teaches making the bubbler tube horizontal at its outlet tip so that the gas/liquid interface remains at the same vertical height when it moves somewhat within the bubbler tube as the pressure stabilizes. This insures that one is measuring the pressure at a known height location rather than the pressure at a somewhat higher location where the gas/liquid interface has been pushed by the fluid pressure. But even if one has an accurate pressure reading at a known location in the tank, the height of the liquid above that location cannot be calculated unless the weight of the liquid per unit volume, compared to water, that is, the specific gravity, is also exactly known. This is not always the case. Even common fluids like water vary in specific gravity depending on mineral content and temperature. Fluid mixtures vary their specific gravity over time as more volatile constituents evaporate off.
To solve this problem, U.S. Pat. No. 2,613,535, to Born, teaches the use of two bubbler tubes, vertically separated by a fixed distance, to establish the pressure differential over this fixed distance and calculate therefrom the actual specific gravity or the fluid. Thereafter, the level of the fluid can be calculated from the pressure sensed by the lower of the two bubbler tubes. Similar solutions are proposed in U.S. Pat. No. 4,669,309, to Cornelius, U.S. Pat. No. 4,006,635, to Khoi, and U.S. Pat. No. 4,630,478, to Johnson. All of these prior art approaches fail to overcome certain inherent problems with fluids, however.
No measurement of the density (specific gravity) of a fluid, no matter where taken, or how taken, can be extrapolated to accurately characterize all of the fluid in the tank due to the problem of stratification. Heavier components tend to sink toward the bottom making the liquid denser at lower heights. Temperature variations induce density variations. For example, during the course of the day, or as clouds pass by, the sun may warm one part of the tank and the fluid proximate thereto. The warmed fluid expands and becomes less dense. Convection currents may then begin with lighter fluid rising and heavier fluid sinking. The currents may interact with the component induced stratification in unpredictable ways. The end result is a continuously changing dynamic, non-linear, mathematically chaotic specific gravity distribution. Thus, accuracy is inherently limited. An average approximation of the specific gravity can be measured and used to calculate an approximation of the level and volume of the fluid, but high accuracy is unobtainable using these prior art methods. This problem grows progressively worse as tanks become taller. However, the present invention overcomes the problem.