35
51-638 Echotel Model 338 Two-Wire Ultrasonic Transmitter
2.0
Reference Information
2.1
Description
The Model 338 ultrasonic transmitter performs level,
volume, and open channel flow measurement of liquids.
Measurement based on non-contact ultrasonic technology
is especially suited for applications where no physical
contact can be made to the surface of the liquid material
to be measured. This is especially important in applica-
tions containing corrosive materials, suspended solids or
coating media.
2.2
Theory of Operation
Non-contact ultrasonic measurement is made by emitting
an ultrasonic pulse from the transducer and measuring the
time required for the echo to reflect from the liquid sur-
face and return to the transducer. The powerful electronics
measure the time of the round trip pulse and, by knowing
the speed of sound, calculates the distance. Since speed of
sound is temperature dependent, the transducer also meas-
ures the temperature in the vessel to provide compensation
for changing temperature.
By inputting the type and geometry of the vessel, the intel-
ligent electronics can calculate the liquid volume in the
vessel. In a similar operation, the Model 338 can perform
open channel flow measurement by converting the level
reading into units of volume per time. Common tank
shapes, flumes, and weirs are stored in the 338 software.
A 32-point linearization table is also available for unusual
tanks or primary flow elements.
2.3
Performance
Model 338 Transmitters are high performance units that
feature a powerful 80 kHz transducer, highly collimated
beam angles, and advanced digital signal processing. These
features allow the 338 to accurately track the liquid level
in difficult applications involving agitation and light foam.
The extremely narrow 5° beam angle, shown at left, allows
the 338 to be used in applications where other units with
wider beam angles fail due to false reflections.
Beam Radius vs. Distance
r=2.6 inches
5 feet
r=5.2 inches
10 feet
r=7.9 inches
15 feet
r=10.5 inches
20 feet
r
