Environmental Engineering Reference
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passage of the acoustic waves through the pipe creates a small pressure change
on the inside of the pipe wall. (d) This small pressure change results in a dy-
namic strain of the pipe wall itself. (e) The mechanical dynamic strain signal is
converted to an electrical signal by each element of an array of passive sensors
wrapped partially or fully around the pipe exterior - no coupling gels or liquids
are used. These sensors are spaced at a set distance from each other along the
axial direction of the pipe. (f) The resulting electrical signal from each sensor
element is interpreted as a characteristic signature of the frequency and phase
components of the disturbance under the sensor. (g) An array processing algo-
rithm combines the phase and frequency information of the characteristic sig-
nature from the group of sensor array elements to calculate the velocity of the
characteristic signature as it propagates under the array of sensors. (h) The ve-
locity of the acoustic waves is used to calculate the gas hold-up percentage.
The advantage of this principle is that it is impervious to scale build-up and the
acoustic velocity measurement will not drift with time or temperature. Figure 6.8
shows this device.
The physical embodiment of the gas hol-dup meter consists of a 4 in PVC pipe,
open at both ends, with its axial array of sensors external to the PVC tube but en-
cased within a sealed watertight enclosure, designed to withstand the rigors of a
harsh minerals processing environment. The measurement accuracy is 5% of read-
ing over the gas hold-up range of 0.01% to 20%. The gas hold-up meter is placed
in the collection zone of the flotation device, thus allowing the free access of as-
cending bubbles and descending slurry through the device. The cited article reports
(a)
(b)
Figure 6.8
CIDRA's gas hold-up meter (a) External view and transmitter. (b) Cross-sectional view
illustrating air bubbles and naturally generated propagating acoustic waves
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