Ultrasonic liquid flow metering is a convenient method of determining liquid flow within rigid pipes. Instruments are portable, easily configured, self-powered, log data, and are non-intrusive, so no pipefitting or modification required, no need to halt production, no risk of contamination. They cater for a wide range of fluids and conditions by utilising one of two methodologies.
METHOD 1: DOPPLER EFFECT
Applications:
Raw sewage, Primary sludge, Slurries,
Carbonated Beverages, Paper pulp, Waste
water, Grey water, Aerated fluids.
Principle:
In contrast to Transit Time
flowmeters, ‘Dopplers’ are
reliant on ultrasonic pulses bouncing off
targets so do require a degree of aeration
or suspended (not dissolved) solids. Each
transducer sends pulses which are then
reflected back at a rate dependant on the
speed of the moving target (bubble / solid
etc). The ‘Doppler Effect’ is widely known for
the change in pitch when an object such as a
vehicle (plane etc) approaches and passes an
observer. The difference between the reflected
frequencies and transmitted frequencies is
directly proportional to the speed of the sonic
reflectors, and thus the instrument calculates
the rate of liquid flow.
METHOD 2: TRANSIT TIME – TIME OF FLIGHT
Applications:
Potable water, Wastewater, Cooling and
heating water, Ultrapure water and liquids,
Water/glycol solutions, Crude oil, Refined
hydrocarbons, Diesel and fuel oils,
Chemicals, Lubricating oils, Beverages.
Establishing flows of water, waste, coolant,
lubricant, fuel, beverages.
Determining consumption of water, waste,
coolant, lubricant …
Energy auditing of heat exchangers
Principle:
Two transducers are coupled
to the surface of rigid pipe. Each acts as a
transmitter and receiver. Ultrasonic signals
are sent [and received] by both transducers
alternately. Sound energy in a moving liquid
is carried faster when it travels in the direction
of fluid flow (downstream) than it does when it
travels against fluid flow (upstream), therefore
a differential in time of flight will result, and
the instrument calculates from this the rate of
liquid flow.
It is important that the fluid is homogenous
and carries minimal solids and bubbles as
these tend to scatter the sound pulses and
reduce the signal strength.
Common transducer positions allow for
one (Z) traverse where transducers are
diametrically opposed but longitudinally
spaced, two (V) traverses where the
transducers are mounted on the same side
of the pipe, or multiple even (W) traverses.
Pipe diameter largely dictates which method
is used