The shape of the Dall tube is similar to the classical Venturi tube, however, it is shorter and it does not contain any rounded sections. It consists of two reversely placed cones separated by a throat with a narrow slot into the collection chamber. The Dall tube was developed in the last century and therefore its design and technical operation was proved to be reliable flow meter in the long term. Dall tube is made of two reverse cones separated by a throat of 0.03d to 0.1d. Downstream the “plus” corner tapping upstream the face reminding the plate there is an inlet divergent section of diameter Dm, that is installed in the narrowest throat of diameter d. There are “minus” tappings and then there is a divergent section. The angle of the convergent section is within 40° to 50°, the angle of the divergent section is 12°to 17°. At the narrowest point of the tube, there is a narrow tapping slot enabling fluid flow to the circular chamber between the inner profiles and the sleeve, and the differential pressure is transferred to the circular chamber, whose reading is transmitted to the measuring device.
The Dall tube has a significant advantage of a very low permanent pressure loss. The permanent pressure loss in the Dall tube is 30-50 % lower than the permanent pressure loss in the classical Venturi tube, that has the lowest pressure loss described by the standard. It’s another advantage is, that the stability of the discharge coefficient CE of the Dall tube is not influenced by ReD. The Dall tube is inserted into the pipeline including straight pipe lengths as a unit. Required straight pipe length upstream the Dall tube is 10D. Downstream the Dall tube the straight pipe length is not required. Short length is based on compact design of the measuring body of the Dall tube. The only disadvantage of the Dall tube is, that its parameters are not described in the standard.
Measurement uncertainty of the Dall tube is comparable to basic primary elements described in the standard – orifice plates, nozzle, Venturi tubes. Before the installation of the Dall tube, the measurement uncertainty is verified with fluid or gas in the test room.
The Dall tube is suitable for measurements of homogenous or clear liquids – fluids, gases, steams or other technical gases. The Dall tube is not suitable for liquids with content of solid admixtures, when the sediments can be deposited in the tapping chamber.