Non-Contact Ultrasonic Level Measurement
N
on-contact ultrasonic level technology is a
proven method for accurate liquid level meas-
urement. Process control instruments utilizing
this technology sense and measure liquid level, volume,
or open channel flow without making physical contact
with the liquid itself. This offers distinct advantages
over many contact technologies in applications where
corrosive media, suspended solids, changing media
characteristics, or coating media are present.
Measuring with Ultrasound
Two components make
up a typical non-contact
Ultrasound 101:
ultrasonic measurement
The Basics
device: the transmitter, or
electronics; and the trans-
ducer, or sensor. A piezo-
electric crystal within the
transducer converts electrical
signals generated by the
transmitter into a series of
ultrasonic pulses. Under
ideal conditions, these pulses
are transmitted through free
air at the speed of sound
until they come into contact
with the liquid surface
where they are reflected
back to the transducer. The
transmitter’s electronics then
measures the pulse’s round-
trip time and digitally
processes the information to
indicate liquid level. A trans-
mitter programmed with a
vessel’s geometry can calcu-
late the liquid volume of a
vessel. A transmitter pro-
grammed to convert the
level reading into units of volume per time can meas-
ure the liquid flow rate in an open channel.
Assuring Signal Integrity
In the real world of process management, an ultra-
sonic signal launched into free air is vulnerable to
distortion from many sources. Unless safeguards are
taken by mechanical means or advanced
signal processing, non-contact measurement
can be greatly compromised.
The most obvious form of inter-
ference are the physical structures
inside of a vessel—pipes, ladders, struts,
agitator blades, or irregular tank geometry. All of these
can fall within the path of the ultrasonic beam to com-
promise the signal.
The greater the distance from the transducer, the
wider the beam spreads,
thereby increasing the like-
lihood that a “false target”
will enter the beam’s path.
At 2 feet from the trans-
ducer, a typical ultrasonic
beam may be less than 6
inches in diameter; but at
35 feet from the transducer
The
Transmitter
houses
the electronics for signal
the conical shaped beam
processing and control
will expand to a diameter
outputs. Depending on
of 7 feet.
the model, transmitters
Magnetrol’s 335, 355,
can be mounted either
344 and 345 transmitters
integrally or remotely.
have a False Target
The
Transducer
is
Rejection feature which
located inside the
filters out obstructions that
vessel. It transmits and
may lie within the path of
receives the ultrasonic
the beam. By entering the
signals.
locations of the obstruc-
tions into the transmitter’s
false target memory, the
user can program out the
erroneous targets.
As an additional
measure to assure instru-
ment integrity, Magnetrol
transmitters feature a self-
diagnostics capability which continuously checks all
relays, outputs and the overall function of the trans-
ducer and electronics.
Ambient temperature can also have a significant
effect on the accuracy of a non-contact ultrasonic
transmitter. As temperature decreases, air becomes
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MAGNETROL [ MAGNETROL INTERNATIONAL, INC. ]
