Example - Measuring Air Concentration in Fluids

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The Proceptor can measure air concentrations in fluids, even when their permittivities are relatively low, as shown in experiments at General Motors Powertrain Division in February 2000 with flowing, heated automatic transmission fluid.

Optical methods are inapplicable for measuring bubble concentrations in oils because the materials are opaque. The Proceptor dielectric analyzer proved well suited for making these measurements. ATF was pulled from the bottom of a heated bath at a measured flow rate (10 liters / min) and routed through a pump, through a 45 mm ID Proceptor sensor and back to the bath.

Automatic transmission fluid is a relatively non-polar hydrocarbon oil. Proceptor measurements indicate that its permittivity does not vary significantly with frequency over the range 500 to 100,000 Hz. The permittivity of pure ATF was 2.09 at 12,011 Hz and 93 C. This permittivity value is in the typical range for hydrocarbons.

A compressed air supply and an air flow meter were used to inject air at known flow rates (alternately 0.47 and 0.94 liters / min)into the circulating fluid. The Wallis procedure for gas-liquid two-phase flows, assuming a terminal bubble velocity of 18 cm / sec, was applied to determine air volume fractions (alternately 1.7 and 3.5%) from the measured air and oil flow rates.

When air was added to the oil, the permittivity of the resulting mixture went down, consistent with the fact that the permittivity of air (1.0) is lower than that of oil.

The first figure shows permittivity at 12,011 Hz versus time as the air flow rate was switched between different levels.

The second figure shows that the numerical permittivity values obtained at different air concentrations agreed quantitatively well with the standard mixing rule. This enabled the Proceptor's in-line dielectric measurements to quantitatively determine air concentrations in oil. When that was done in this case, the air concentration was determined with a standard deviation of 0.4%.