em-tec SonoTT, Operating Instructions Manual

Page: 6 / 52

EN
2 Description of the Measurement Principle
The SonoTT
TM
Clamp-On Transducer is based on an acoustic measurement principle and utilizes the transit
time method to determine the flow. A typical transit time flow measurement system incorporates two
piezo ceramics that act as both ultrasonic transmitter and receiver. The measurement system, consisting
of the SonoTT
TM
Ultrasonic FlowComputer and the SonoTT
TM
Clamp-On Transducer, operates by alternately
transmitting and receiving an ultrasonic pulse between the piezo ceramics and measuring the transit time
difference that it takes for the pulse to travel between them.
This method determines the transit time difference of ultrasonic signals through a measuring section
and is used to estimate volumetric flow rates. There are at least two piezo ceramics required in order to
send and receive ultrasonic pulses both with and against the flow direction. For a higher accuracy, the
SonoTT
TM
Clamp-On Transducers incorporate two pairs of piezo ceramics, i.e. four piezo ceramics in total,
which are arranged in an X-configuration around the tube containing the measured flow.
When sending ultrasonic signals through the measuring section, the transit time difference depends on the
flow direction of the medium:
• The ultrasonic sound signals that are sent along the flow direction and volume flow of the medium, i.e.
downstream, need less time to travel the measuring section
than
• the ultrasonic sound signals that are sent against the flow direction, i.e. upstream.
Consequently, the transit time difference is measured for every pulse. The difference between upstream and
downstream measurements is proportional to the volumetric flow rate of the liquid in the tube. Therefore,
by accurately measuring the difference between upstream and downstream transit time T
up
and T
down
, it is
possible to estimate the flow velocity. Subsequently, the volumetric flow rate is computed from the inner cross
section area of the tube and the instrument coefficient.
Figure 2: Sensor Structure
SonoTT
TM
Clamp-On Transducer
Page 6 D162-700 SonoTT
TM
Clamp-On Transducer - Operating Instructions V3.0 Copyright em-tec GmbH