Other meters:

    Wide variety of flow meters other than the traditional full-Bore meters and Area meters are available.
 

•Non-intrusive meters:

Magnetic meters
Ultrasonic meters

• Thermal meters

Non-intrusive meters:
 

•No obstruction is placed in the fluid stream

•No reduction of flow channel

•No creation of pressure drop in the fluid

•Rate of fluid flow is measured from outside the tube

Magnetic meters:
 

•Flow tube is lined with a non-conducting material

•Metal electrodes are mounted flush with the inner wall

•Electromagnetic coils surrounding the tube generate a uniform magnetic field

•The magnetic field generated is directly and linearly proportional to fluid velocity as per Faraday’s law.

•Except velocity fluctuations, changes in viscosity and density do not affect the meter reading

•Commercial magnetic meters can measure the velocity of almost all fluids except hydrocarbons (too low electrical conductivity)

Two types of ultrasonic meters:

Transit time meters:

• In this type of meter, a high frequency pressure wave is beamed at an angle across the pipe.

• The velocity of the wave is found from its time of transit.

• From the change in transit time from that of a quiescent fluid the fluid velocity can be determined.

•Are only applicable to clean fluids

Doppler-shift meters:

•They depend on the reflections of the pressure wave from suspended particles in the fluid moving with velocity of fluid.

•The pressure wave is projected at an angle to the direction of the flow

•The difference between the frequency of the projected wave and that of the reflected wave is proportional to the velocity of fluid stream.

•Mostly used in flow measurement of corrosive fluids.

•Measure the flow rate directly, by measuring the temperature of the fluid.

•Temperature is measured when the fluid passes over a heating element or

•The rate of heat transfer to the stream from a heated surface

•These measurements coupled with the specific heat and thermal conductivity of the fluid can be related to mass flow rate.

•Mostly used to measure gas flow in small lines

'Unit Operations in Chemical Engineering' - Mc Cabe, Smith, and Harriott