Thermo Scientific HB-S2000, User Manual

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Sarasota 2000 Ultrasonic Multipath
Flowmeter
Section 1 INTRODUCTION Page 1-6 Thermo Fisher Scientific
INTRODUCTION
H
min
=
27
√
( L / ƒ )
1.2.6 Path configurations
The simplest arrangement is to have a number of paths “ in line” above each other. This
would be suitable for a channel of regular cross section shape, which is straight for a long
distance compared with its width (5 to 10 times).
Other configurations are often used in other circumstances. For example:
•
Crossed paths where there is uncertainty about the flow direction
•
Sloping paths where the depth is greater on one side compared with the other
•
Transducers inset from the banks
•
Multiple sets of paths for compound channel shapes
•
V configuration used to divide the width because of size or uneven profile
•
Multiple channels
•
Reflected paths where transducers are on one side only and reflectors “bounce” the
sound pulses back from the far side. This method saves cable but increases path lengths
and is very sensitive to misalignment.
The Sarasota 2000 flowmeter is capable of being configured for these and many other
situations. Please consult Thermo Fisher Scientific for examples and advice.
1.2.7 Transducer frequency
Transducers are manufactured with characteristic frequencies. These will be in the range 1
MHz to 100 kHz. For propagation reasons, the greater the path lengths the lower the
frequency and the larger the transducer. As a guide, path lengths below 10 metres would use
1MHz transducers, 10 to 80 metres 500 kHz, 80 to 150 metres 250 kHz. These figures are for
guidance and the selection may be influenced by other factors relating to the application. For
example, lower frequency transducers may be used to improve penetration in conditions of
high suspended solids provided there is sufficient depth and velocity.
Please consult Thermo Fisher Scientific for advice.
1.2.8 Minimum depth of water
In order to avoid reflections from the bed or surface causing distortion of the ultrasonic
signals, a minimum height of water is necessary above each path. This depends on the
transducer frequency and the path length.
Where:
H
min
is the minimum height of water above the path, in metres
L is the path length, in metres
ƒ is the transducer frequency, in hertz
A similar restriction applies to the channel bed, particularly if it is smooth and reflects rather
than absorbs an acoustic signal. The minimum depth of water is therefore usually 2 x
H
min