eFunda: Introduction to Target Flowmeters
engineering fundamentals Target Flowmeter
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Overview

Target flowmeters, also known as drag force flowmeters, insert a target (drag element), usually a flat disc or a sphere with an extension rod, into the flow field. They then measure the drag force on the inserted target and convert it to the flow velocity.

One major advantage of the target flowmeter over other flowmeters is, with a sphere drag element, a proper strain gage layout, and well thought-out mathematical formulas, a target flowmeter is capable of measuring sporadic and multi-directional flows.

Further Information
The key to the success of a target flowmeter is the measurement of the drag force.

The drag force Fd is given by the drag equation of incompressible flow:

where V is flow velocity, rho is the density of the fluid, A is the projected area of the target, and Cd is the drag coefficient to be determined experimentally based on the flow conditions and the geometry of the drag element. For flat plate and sphere, the drag coefficients typically are:

Please note that the 1.28 drag coefficient is for flat plates that are perpendicular to the flow direction and the drag coefficient for spheres is related to the Reynolds Number.

For a given design, A and Cd are constant. If the density of the liquid rho is also constant, then the drag force Fd is solely a function of V2.

With strains at certain points measured by strain gages, the drag force can be calculated by a beam-bending formula (cantilever beam with an end load). The flow velocity can consequently be obtained.

Suppose the strain gage is attached at the front and/or back surface(s) of the extension rod () at location . The stain measured will be

where is the stress at the cross section along the x direction, M is the bending moment, y is the coordinate perpendicular to the rod's longitudinal (x) direction, E is Young's (elastic) modulus of the rod material, and I is the rod's cross section's area moment of inertia. (Please refer to the beam theory for further details. The area moment of inertia can be found in Area of the Mathematics section.)

The bending moment M of a cantilever beam with length L under a concentrated end load P is

where x is measured from the fixed end. In this case, L is the length of the extension rod, and P is the drag force Fd.

The strain becomes

and the drag force can be expressed in terms of strain:

Plug the above formula into the drag equation we will have the following expression:

This formula for V is valid under the following assumptions:

  1. The cross section of the extension rod is much smaller than the diameter of the drag element.
  2. The drag element is much smaller than the inside diameter of the pipe.

Otherwise, further calibration is needed to determine the correction factors.

Common Specifications

Common specifications for commercially available target flowmeters are listed below:

  Fluid Phase:

Score Phase Condition
 Cryogenic  
 Gas  Clean 
  Dirty 
 Liquid  Clean 
  Dirty 
  Viscous 
 Steam  Saturated 
 Liquid  Corrosive  
: Recommended
: Limited applicability

  Line Size: Inline models: 15 ~ 150 mm (0.5 ~ 6 inch)
Insertion models: 100 ~ 1500 mm (4 ~ 60 inch)
  Turndown Ratio: 15 : 1

Pros and Cons

  • Pros:
  - Low initial set up cost
  - Can be used in abrasive, contaminated, or corrosive fluid flow
  - Can be made to measure flow velocity that is sporadic or multidirectional with sphere drag element designs
  • Cons:
  - Pressure drop is inevitable due to the rod and the drag element
  - Less popular than it was before

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