ultrasonic sensors are mounted at the top of a tank or in
a position above the liquid being measured. The sensor
continuously transmits pulses of high frequency sound
(typically 42 kHz) which travel away from the sensor, hit
the surface of the liquid and return to the sensor.
instrument electronics measure the time it takes from
transmitted sound to return of the echo. With reference
to the speed of sound in air, the exact distance of the
liquid surface from the sensor can be calculated with
high accuracy (±0.25% of maximum range).
Since the speed of sound is affected by air
temperature, Greyline ultrasonic level sensors include a
built-in temperature sensor. Level/distance measurements
are automatically temperature compensated throughout the
operating temperature range of the sensor.
The sensor should be positioned so that
it has a clear "view" of the liquid surface
and away from ladders, pipes or other obstructions.
Greyline recommends 1 ft. from the sidewall for every 10
ft. depth (300 mm for every 3 m depth). False echoes from
agitators (sweeping under the sensor), turbulence and
waves can be filtered and disregarded by the instrument.
Greyline models range from simple
4-20mA level indicating transmitters, to sophisticated
monitoring, controlling and data logging models. Return to
Greyline Products for Level
model details and specifications. For information on a wide range of
level instrument technologies including ultrasonics, read Wastewater Level
flow meters measure flow from outside a pipe with a
clamp-on sensor. Greyline Doppler meters continuously
transmit high frequency sound (640 kHz) that travels
through the pipe wall and into the flowing liquid. Sound
is reflected back to the sensor from solids or bubbles in
the fluid. If the fluid is in motion, the echoes return
at an altered frequency proportionate to flow velocity.
Doppler flow meters continuously measure this frequency
shift to calculate flow.
Measure Flow of dirty or aerated
Liquids from Outside a Pipe
The Doppler effect was first documented in 1842
by Christian Doppler, an Austrian physicist. We hear everyday examples of Doppler: the sound
of a train whistle changing pitch as it passes by, or the
exhaust noise from a race car as it speeds past our
The Doppler technique
only works on liquids which contain solids or gas bubbles
to reflect its signal. These are "difficult"
liquids that may damage regular flow meters: slurries,
sludge, wastewater, abrasives, viscous and corrosive
chemicals. Because the sensor mounts on the outside of
the pipe, there is no pressure drop and no obstruction to
For best performance Doppler sensors
should be mounted away from turbulence creating devices
like pipe elbows and tees, and away from velocity
increasing devices like controlling valves and pumps.
Typical accuracy is ±2% of full scale.
Doppler instruments include a clamp-on
ultrasonic sensor, connecting cable and an electronics enclosure
which can be mounted at a convenient location nearby
(within 500 ft / 152 m). Sensors can be rated
intrinsically safe for mounting in hazardous-rated
Return to Greyline
Doppler model details and specifications.
Transit Time flowmeters measure the time it
takes for an ultrasonic signal transmitted from one transducer to
cross a pipe and be received by a second transducer. Upstream and
downstream time measurements are compared. With no flow, the transit
time would be equal in both directions. With flow, sound will travel
faster in the direction of flow and slower against the flow.
Very accurate timing circuits are
required but 1% accuracy is quite typical when the transducers can
be mounted on a pipe section with evenly distributed flow.
Because the ultrasonic signal must cross the pipe to a receiving
transducer, the fluid must not contain a significant concentration
of bubbles or solids (less than 2%). Otherwise the high frequency
sound will be attenuated and too weak to traverse the pipe.
Applications include potable water, cooling water, water/glycol
solutions, hydraulic oil, fuel oils and chemicals.
Time transducers typically operate in the 1000-2000 MHz frequencies.
Higher frequency designs are normally used in smaller pipes and
lower frequencies for large pipes up to several meters in diameter.
Return to Greyline
Transit Time model details and specifications.
The common method of measuring flow
through an open channel is to measure the height or HEAD of the liquid
as it passes over an obstruction (a flume or weir) in the channel. Using
ultrasonic level technology, Greyline open channel flow meters include a
non-contacting sensor mounted above the flume or weir. By measuring the
time from transmission of an ultrasonic pulse to receipt of an echo, the
water level or "Head" is accurately measured.
weirs are specially designed channel shapes that
characterize the flow of water. Common types are
Rectangular Weirs, V-Notch Weirs, Parshall flumes and
Palmer Bowlus flumes. The choice of flume or weir type
depends on the application: flow rate, channel shape and
solids content of the water. Contact Greyline Instruments(firstname.lastname@example.org) for
advice on selection of a suitable flume or weir for your
channel flow meters can be calibrated to any flume or
weir by menu selection. The open channel flow meter
electronics use an internal formula to calculate flow
rate (Flow = K Hn, where 'K' and 'n' are
constants and 'H' is Head as measured by the instrument).
Calibration to uncommon or custom flumes can be done by
direct entry of 'K' and 'n' constants. Greyline also
offers a PC software program "Find K&n" to
develop calibration constants from a flume or weir flow
Greyline open channel flow meters
include a non-contacting ultrasonic sensor, connecting
cable and an electronics enclosure which can be mounted
at a convenient location nearby (within 500 ft / 152 m).
Sensors can be rated intrinsically safe for mounting in
hazardous-rated locations. The instruments display,
totalize, transmit and control, and some models include
data logging/flow reporting systems.
Return to Greyline
Channel Flow model details and specifications.
Flow Meter continuously measures both Level and Velocity to calculate
flow volume in an open channel or pipe.
The ultrasonic sensor is installed at the bottom of a pipe or channel. To measure water level the sensor
transmits ultrasonic pulses that travel through the water and reflect
off the liquid surface. The instrument precisely measures the time it
takes for echoes to return to the sensor. Based on the speed of sound
in water, the level is measured with accuracy of ±0.25%.
Flow velocity is measured with an
ultrasonic Doppler signal continuously injected into the water. This
high frequency sound (640 KHz) is reflected back to the sensor from
particles or bubbles suspended in the liquid. If the fluid is in motion, the echoes return
at an altered frequency proportionate to flow velocity. With this
technique the instrument measures flow velocity with accuracy of ±2%.
Greyline Area-Velocity Flow Meters
work in partially full and surcharged pipes, rectangular, trapezoid
and egg-shaped channels.
Optional - Separate
Level and Velocity Sensors
A separate down-looking ultrasonic
sensor can be used for highly aerated or turbulent flow applications.
It measures level by transmitting ultrasonic pulses through the air to
the liquid surface with accuracy of ±0.25%. Along with the level
sensor, a submerged Doppler velocity sensor is used to measure the
Return to Greyline
Area-Velocity Flow Meter model details and specifications.
complete product literature and applications information
on Ultrasonic Level, Open Channel Flow and Doppler Flow
please contact Greyline Instruments. (To receive complete
information please provide your
name, mailing address, phone and fax numbers):
Sixsmith Drive Long Sault, Ont. K0C 1P0 Tel: 613-938-8956