Principle Of Operation - Thermo Scientific Sarasota 2000 User Manual

Sarasota 2000 Ultrasonic Multipath
Flowmeter
1.2

Principle of operation

1.2.1 The Standard
The Sarasota USMP is a velocity x area open channel flowmeter which uses the ultrasonic
"time of flight" method. This is also known as the "transit time" method. It complies with the
International Standard ISO 6416. The UK version BS 3680 part 3E is identical. The transit
time method belongs to the general category of velocity x area methods. A full description of
the method and its applications is to be found in the Standard. A brief summary is given
below.
1.2.2 Velocity x area method
Velocity x area methods require a means of determining the water velocity and the cross
section area. The product of the two produces the flow rate in a manner which is not
dependent on factors influencing the level, for example downstream constrictions, tidally
influenced water level etc.
Assuming the shape of the channel cross section is stable, determination of the area
becomes a matter of measuring water level. This may be done by a variety of methods.
1.2.3 Water level
Water level is required in order to determine the cross section area in an open channel.
Though a single level measurement may be used, it is common to use more than one and to
average them. This has the advantage of a more representative level, particularly if the
measurements are made at different positions, for example on either side of the channel.
Another advantage is that flow may still be computed even if a level sensor fails.
Level may be determined by using one or more ultrasonic transducers in the water facing
upwards. The time taken for a pulse of sound to return to the transducer after being reflected
from the surface is converted into level using the velocity of sound in water as measured on
the water velocity paths (see Section 1.2.4). There is a minimum depth of water required
above the transducer for it to carry out a measurement. This is given in Appendix 3:
Specification.
Water level may also be provided by external auxiliary depth gauges, for example pressure
transmitters, downward facing ultrasonic devices and float systems with shaft encoders.
1.2.4 Water velocity
In the transit time method, water velocity is determined at a number of heights within the body
of water by measurement of time taken for pulses of ultrasound to travel across the channel
at an angle to the flow direction.
Transducers are mounted in the water at or near the sides of the channel with each pair
usually at the same height and aligned so as each one can transmit a "beam" of ultrasound
towards its partner. The ultrasonic "path" between the transducer pairs must be at an angle
o
(usually about 45 ) to the flow direction.
Each transducer acts as a transmitter and receiver and is connected to a processing unit,
which measures the transit time and the time difference.
Section 1 INTRODUCTION Page 1-2
INTRODUCTION
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