Xylem FM500 ULTRASONIC FLOW METERS - Manual (Page 5)

1.0 Product Overview
The Hybrid Ultrasonic Flow Meter can measure fluid flow in virtually any fluid in
which sound waves can travel. The meter is considered “hybrid” because it can measure
fluid flow using either the Doppler or Transit Time methods. The system includes a set of
ultrasonic sound Transducers and a Signal Processing Unit (SPU). The transducers are
clamped to the outside of the pipe wall and include no moving parts. The SPU controls
and processes the transducer signals into useful flow measurement data. This method of
flow measurement is safe, non-intrusive and very easy to service.
The Transit Time measuring method requires relatively “clean” fluid to enable the sound
waves to complete their circuit. The meter may be operated in the Transit-Time mode
when the fluid contains
particles. To allow for changes
in the fluid’s particle count, the Sonic-Pro monitors the signal gain and employs an
Automatic Gain Control (AGC) algorithm that automatically adjusts the gain to maintain
the optimum power level.
The speed at which sound travels in the fluid must be known. The factory will configure
the meter for a known fluid during the initial configuration. The model 3 includes a 5-
button user interface that can be used to configure the meter. Many common fluids are
listed in the software and can be selected directly from the menu. Provided the speed of
sound in the fluid is known, custom “unknown” fluids can be input manually by the user.
A list of various fluids and their sound speeds are provided in the index of this user
manual.
Signal Processing
Unit)
0% to 10% (0 to 100,000 ppm) of
When operated in the Transit Time measurement method, a short ultrasonic signal burst
passes first in one direction and then in the other between two transducers separated
along the length of the pipe. When traveling in the same direction as fluid flow, the burst
is carried along by the fluid and arrives earlier as a result. When traveling against fluid
flow, the burst is held back by the fluid and arrives later. The SPU (
measures this difference in time-of-flight in the two directions. From this, the
actual time-of-flights, the distance traveled in the fluid and the angle of the ultrasonic
signal path, it calculates the fluid velocity.
As well as the V mounting method shown, the transducers could be on opposite sides of
the pipe with no reflections in the path (Z mount).
1.2
Transit Time
Operation
1.1
Operating
Principles
Page 5
TRANSIT TIME MEASUREMENT PRINCIPLE
Ultrasonic Flowmeter