A clamp on apparatus 10,110 is provided that measures the speed of sound
or acoustic disturbances propagating in a fluid or mixture having
entrained gas/air to determine the gas volume fraction of the flow 12
propagating through a pipe 14. The apparatus includes an array of
pressure sensors clamped onto the exterior of the pipe and disposed
axially along the length of the pipe. The apparatus measures the speed of
sound propagating through the fluid to determine the gas volume fraction
of the mixture using adaptive array processing techniques to define an
acoustic ridge in the k-.omega. plane. The slope of the acoustic ridge 61
defines the speed of sound propagating through the fluid in the pipe.