Panametrics Sentinel, User Manual, Page: 14 / 166

Chapter 1. Installation
4 Sentinel™ User’s Manual
1.5.1 Transit-Time Method
The transit time technique uses a pair of transducers, with each transducer alternately sending and receiving coded
ultrasonic signals through the fluid. Figure 3 below shows the paths used in the Sentinel. When the fluid is flowing,
signal transit time in the downstream direction is shorter than in the upstream direction; the difference between
these transit times is proportional to the flow velocity. The Sentinel measures this very small time difference and,
using various digital signal processing techniques combined with programmed pipe parameters, determines the
flow rate and direction.
Figure 3: Path Configuration
1.5.2 Transducers
When in a transmit cycle, transducers convert electrical energy into ultrasonic pulses and then convert the ultrasonic
pulses back to electrical energy when in a receive cycle. In other words, they act like loudspeakers when transmitting
the signal and like microphones when receiving it. They perform the actual data transmission and collection, thus
interrogating the flow.
The transducers in the Sentinel Measurement System were specifically designed to work with the available Insertion
Mechanism. In the event that a transducer becomes damaged or non-functional, it can be replaced without shutting
down the pipeline. The insertion mechanism is an option available with all offered versions of the Sentinel. To keep
the fluid from escaping while the transducer is being replaced, it is recommended that a shutoff valve be part of the
original transducer holder assembly.
Downstream
Transducer
Upstream
Transducer
Upstream
Transducer
Downstream
Transducer
Fluid
Flow
Signal Path
Signal Path