The dall flow tube, illustrated in figure, has a higher ratio of pressure developed to pressure lost than the venturi flow tube. It is more compact and is commonly used in large flow applications.
The tube consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube. This section, called the inlet shoulder, is followed by the converging inlet cone and a diverging exit cone. The two cones are separated by a slot or gap between the two cones.
The low pressure is measured at the slotted throat (area between the two cones). The high pressure is measured at the upstream edge of the inlet shoulder.
The Dall flow tube is available in medium to very large sizes. In the large sizes, the cost is normally less than that of a Venturi flow tube. This type of flow tube has a pressure loss of about 5 percent.
Advantages
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Low head loss
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Short lying length
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It is available in numerous materials of construction.
Disadvantages
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Pressure difference is sensitive to up-stream disturbances.
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More straight pipe required in the approach pipe length.
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It is not considered for measuring flow of hot feed water.