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Gas holdup is the volume fraction of gas in an aerated liquid. It is important in processes where gas dispersion is important, e.g., froth flotation or stripping volatile components out of a liquid. It is also important when characterizing or scaling processes with bubbly flow in tanks, mixers, or pipes.

There are various methods for measuring gas holdup. Understanding the differences between them is important to select and interpret measurements. The simplest method is looking at the difference between aerated and unaerated liquid levels in a vessel. Gas holdup can be calculated from the rise height after aeration. This is typically a simple and effective method. However, if the fill line is obscured by the vessel, fluid motion, froth etc. then this method is not practical.

Other methods for measuring gas holdup include differential pressure, conductivity, and capacitance measurements. In differential pressure measurements, the gas holdup is determined from the pressure difference at different heights in the vessel. Differential pressure measurements can be highly sensitive to fluid motion, which can make them challenging in agitated vessels. Conductivity and capacitance measurements use the differences in the electrical properties of the gas and liquid phases to determine gas holdup. These require well-characterized fluids and a vessel with appropriate electrodes.

Nuclear densitometry is often the most robust method for gas holdup measurement, however, there are several considerations. A gamma source is placed on one side of a vessel with a detector on the other. The detector current corresponds amount of incident gamma rays, which depends on the density of the intervening fluid. Calibration with liquids of known density is required to translate the measured currents into density values. When the fluid is aerated, the change in density and thus gas holdup can be inferred from the change in detector current. The gas holdup is only measured along the beam path, however, so for vessels that are not well-mixed, the measurement location must be considered.