BioProTT
TM
Flow Measurement System Version 1.00 page 13 of 75
4 Description of Measuring Principle
The physical measurement principles applied in the BioProTT
TM
flow measurement system
are described in this chapter.
The BioProTT
TM
flow measurement system uses the ultrasonic transit-time method (TT =
transit-time) which delivers precise flow values in tube systems and pipes.
The ultrasonic converters (piezoceramics) in the flow sensors transmit high-frequency
acoustic signals in and against the flow direction. The time difference between these
signals is proportional to the volumetric flow.
The basic principle is explained below.
Transmitter/
Receiver
Receiver/
Transmitter
A
B
Tube
Flow
a
Figure 1: Principle of ultrasonic flow measurement (simplified using only two ceramics)
One piezoceramic (A) is excited by a set of high-frequency vibrations and transmits
ultrasonic waves to a second opposing piezoceramic (B) that acts as a receiver. The
piezoceramics are arranged at a specific angle α in relation to the flowing medium. The
transit time is influenced by the medium. The flow velocity of the medium can be
calculated using the measured transit time differences. The flow in litres per minute is
established using the known cross-section of the tube being scanned by ultrasonic
transmission.
Four ultrasound converters in a crosswise arrangement transmit high-frequency sound
signals alternately in and against the direction of the flow. The transit time for each
impulse is measured; the transit-time difference between the upstream and downstream
movement of the impulses is proportional to the volumetric flow.
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