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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.

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Two components make

up a typical non-contact

ultrasonic measurement

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.

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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 conical shaped beam

will expand to a diameter

of 7 feet.

Magnetrol’s 335, 355,

344 and 345 transmitters

have a False Target

Rejection feature which

filters out obstructions that

may lie within the path of

the beam. By entering the

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

Non-Contact Ultrasonic Level Measurement

The Transmitter houses

the electronics for signal

processing and control

outputs. Depending on

the model, transmitters

can be mounted either

integrally or remotely.

The Transducer is

located inside the

vessel. It transmits and

receives the ultrasonic

signals.

Ultrasound 101:

The Basics