Flow Measurement

Flow is nothing but the quantity of fluid passing through the section from which it is measured in a given period. Flow sensors or flowmeters are the devices which measure this quantity. Some flowmeters measure the amount of fluid passing through the section in the given period ( like 100 gallons per minute), while some other flowmeters are designed to measure just the quantity of fluid passing through the section (like 100 gallons).

Flow meter, basically, consists of a primary device, a transducer and a transmitter. The transducer senses the fluid passing through the primary device and the transmitter converts the signal received from the transducer to a standard transmission signal.

Flow can be calculated using the following basic formulas:

The Volume of the fluid Q, passing through a pipe of cross-section area A with a velocity v, is given by

 Q = A.v 

The mass flow of the fluid W, of density r, flowing with velocity Q, is given by
 W = r.Q 

Units for flow measurement are as follows:
  • For Solids:   Tons/hr, Kg/min
  • For Liquids: Liters/min, Metre3/min, Gallons/min
  • For Gases:   Metre3/hr

Reynolds Number

The Reynolds number is a dimensionless quantity used in fluid dynamics to characterize the fluid flow. It is also defined as the ratio of fluid's inertial forces to its drag forces. In flow measurement, this quantity is used to characterize the type of flow. For flow in pipes, the Reynolds number is defined as:

Re = rVD


where, Re = Reynolds number
           V = Mean velocity of fluid (m/s)
           D = Hydraulic Diameter of pipe (m)
           µ = Dynamic Viscosity of the fluid (kg/(m.s))
           r = Density of fluid (kg/m3)

If  R<  2000, flow is laminar.
    2000 Re  <  4000, flow is transitional.
    Re > 4000, flow is turbulent.

The image (a) shows a laminar flow in a pipe of diameter d, while image (b) show turbulent flow. Transitional flow is nothing but the flow when the turbulence has just started forming in the fluid flow. 

Bernoulli's Principle

The Bernoulli's principle states that for an ideal fluid, an increase in the flow velocity of fluid results in a decrease of that fluid's pressure. An ideal fluid is a fluid assumed to have practically no viscosity.This principle is widely used by various volumetric flowmeters. Its derivation is based on law of conservation of energy.

Types of flowmeters

Different types of flowmeters are available depending upon the principle used for measurement.

  1. Variable Head Type
    This type of flowmeters operate on the principle that a restriction in pipeline of flowing fluid produces a differential pressure across the restriction element which is directly proportional to the flow rate.
     Q √P

    Thus, there is a square root relationship between flow and pressure. This relationship can be removed using square root extractor. Then we can have QP. The flowmeters working on this principle are

    Orifice Plate
    Flow Nozzle
    Venturi Tube
    Pitot Tubes
    Target Flowmeters
  2. Positive Displacement Type flowmeters.
  3. Variable Area meter (Rotameter)
  4. Magnetic flowmeter
  5. Ultrasonic flowmeter
  6. Turbine type flowmeter
  7. Vortex flowmeter