A Dall tube flow meter, also known as a Dall tube, is a type of differential pressure flow meter used to measure the flow rate of fluid in a pipe. It consists of a specially designed tube inserted into the flow stream, typically at a right angle to the pipe axis. The Dall tube creates a constriction in the flow path, causing a pressure difference between the upstream and downstream sections of the tube. Here's how a Dall tube flow meter works and some of its characteristics:

1. Principle of Operation:

   • The Dall tube operates based on the principle of Bernoulli's equation, which relates the velocity of fluid flow to the pressure exerted by the fluid. As the fluid flows through the constriction in the Dall tube, its velocity increases, causing a pressure drop.
   • According to Bernoulli's equation, the pressure at the constriction point is lower than the pressure upstream of the tube. The pressure difference between the upstream (P₁) and downstream (P₂) sides of the Dall tube is proportional to the square of the flow rate (Q) through the tube.

2. Measurement of Pressure Difference:

   • The pressure difference across the Dall tube is measured using pressure sensors or transmitters installed at the upstream and downstream sections of the tube. These sensors detect the static pressure (P₁ and P₂) and transmit the pressure readings to a differential pressure transmitter.

3. Calculation of Flow Rate:

   • The differential pressure transmitter calculates the flow rate (Q) based on the pressure readings from the upstream and downstream sensors and the known characteristics of the Dall tube, such as its size, shape, and discharge coefficient.
   • The flow rate is determined using a calibration curve or equation that relates the pressure drop across the Dall tube to the flow rate through the tube.

4. Characteristics of Dall Tube Flow Meter:

   • Dall tube flow meters offer several advantages, including simplicity, reliability, wide rangeability, and suitability for a wide range of fluid types and flow conditions.
   • They have a low-pressure loss compared to other differential pressure flow meters, such as orifice plates, venturi tubes, and flow nozzles.
   • Dall tubes are available in various sizes and configurations to accommodate different flow rates, pipe sizes, and installation requirements.
   • They are commonly used in industries such as oil and gas, chemical processing, water treatment, HVAC, and power generation for flow measurement and control applications.

Overall, Dall tube flow meters provide a cost-effective and accurate solution for measuring the flow rate of fluid in industrial processes. Their simple design, reliability, and wide rangeability make them a popular choice for a variety of flow measurement applications. However, proper installation, calibration, and maintenance are essential to ensure accurate and reliable performance.