Flow measurement

Flow measurement refers to the process of quantifying the flow of a fluid as it travels through a section of piping for monitoring (e.g. determining the flow rate) and control (e.g. restricting the flow rate) purposes (also see Metrology). 

Measuring instruments that leverage differential pressure (e.g. standard orifices, standard nozzles, standard Venturi nozzles and Venturi tubes or electrical effects (e.g. magnetic-inductive or ultrasonic flow measurement) can be used.

Recently, optical methods have also been introduced for measuring particle speed as well as Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV).

Magnetic-Inductive Flow Measurement (MIF)

• This is a system that is based on Faraday's Law and does not require throttling or moving components. See Fig. 1 Flow measurement

• The MIF method requires a minimum electrical conductivity of approximately 20 μS/cm for the measured fluid. Regular tap water has an electrical conductivity of 400 μS/cm, whereby 1 S (Siemens) quantifies the conductivity of a conductor with an electrical resistance of 1 (Ohm).
• The measurement is carried out by applying an alternating magnetic field to the cross-sectional test area of the liquid. This allows a voltage to be induced at the measuring electrodes (3) that is directly proportionate to the velocity (v) at a given geometry (D) and to the flow rate (Q), if the cross-section is known. 
• Since this method is virtually independent of the flow profile, significantly shorter upstream and downstream stabilisation distances than with a throttling device (e.g. standard orifice, standard nozzle) can be chosen while achieving the same level of system accuracy.

Ultrasonic flow measurement

• The ultrasonic flow measurement system can be fixed to the outside of a piping section and does not require direct access to the liquid.
• The transit time difference method is typically applied to measure the velocity of a fluid. See Fig. 2 Flow measurement

• The properties of the liquid can vary greatly. Bubbles, solids, or high viscosity can skew measurement results, however.
• The doppler method, with the same sensor arrangement, is the better choice if the fluid contains solids.

Pitot tube sensors

• Pitot-static tubes such as the Prandtl tube are the preferred means of measuring localised velocity in a flow. See Fig. 3 Flow measurement

• Static and total pressure sensors such as the pitot tube utilise the correlation between static and dynamic pressure in a steady-state flow as defined by Bernoulli's equation.
• The direction field of a flow is measured using direction-sensitive cylindrical, conical, or wedge-shaped flow sensors or can be determined via tufts, the most basic and graphic directional indicators, if accuracy requirements are not quite as high.