About Dall Flow Tube
The Dall flow tube consists of two conical reducers inserted into the fluid-carrying pipe. It has a very similar internal shape to the venturi tube, except that it lacks a throat. This construction is much easier to manufacture than a venturi tube which requires complex machining, and this gives the Dall flow tube an advantage in cost over the venturi tube, although the measurement accuracy obtained is not quite as good (approximately ±1.5%).
Another advantage of the Dall flow tube is its shorter length, which makes the engineering task of inserting it into the flow line easier. The Dall tube has one further operational advantage, in that the permanent pressure loss imposed on the system is only about 5% of the measured pressure difference and thus is only half that due to a venturi tube. In other respects, the two instruments are very similar, with their low maintenance requirement and long life.