A device is described which measures, on line, the bubble size distribution and volume fraction of gas in a liquid foam. There are four elements in the device: a flow channel, meniscus detection, velocity detection, and microcomputer data acquisition. The foam flows into a gently converging capillary to reorder the foam bubbles into alternating slugs of gas and liquid. The passage of the meniscus between a gas and liquid slug is detected by a matched light emitting diode/phototransistor pair. When gas is in the detection path, the detector signal is "low," and when liquid is in the path, the signal is "high." The volume fraction of gas is determined from the ratio of time the detector signal is low to the total elapsed time. A second detector pair is located a fixed distance further down the capillary. From the time signals at the first detector and those at the second, the velocity of the foam in the capillary can be calculated. From the velocity, time that the detector signal is "low," and known capillary geometry, the volume of a gas bubble can be determined. An IBM PC is used for rapid data acquisition and calculation. The device has been tested under a range of foam conditions to validate its performance.
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