Thermo Scientific HB-S200, User Manual

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Sarasota 200 Ultrasonic Multipath
Flowmeter
Section 2, SYSTEM COMPONENTS, Page 2-10 Thermo Fisher Scientific
SYSTEM COMPONENTS
2.4 Ultrasonic
transducers
Each velocity path requires two transducers and each ultrasonic depth requires one.
The transducers are available in a number of frequencies. See Section 1.2.7 for frequency
selection. The frequency of the ultrasonic depth transducer, if used, is not normally critical.
1 MHz is usually used if the velocity transducer frequency is 1 MHz or 500 kHz otherwise.
The diameter of the transducer is different for different frequencies. This is to maintain the
angular spread of the beam, which is a function of frequency and diameter.
The transducers for the Sarasota 200 are “smart”, with local circuits for the drive voltage (HT)
generation and the receiver amplifier built in to minimise losses and optimise signal-to-noise
ratio. In the case of the 1 MHz transducers the size of the transducer is limited and the local
circuit is separated from the transducer in a potted in-line housing called a “Tbox”,
Transducers are wired to the flowmeter using the multi-drop method, via “star” junction boxes
in each transducer array. Up to 4 transducers may be joined together in this way.
The maximum capacity with multi-drop is 4 paths and 4 ultrasonic depths. (Auxiliary depths
may also be used via 4-20 mA inputs).
The transducers connected to each star box must be pre-programmed with different
addresses from 1 to 4 to enable each one to be operated separately by the flowmeter.
Specialist equipment is necessary to programme the transducers and it is usual for them to
be supplied with specified addresses.
2.4.3 1 MHz transducers
For use with path lengths up to 10 metres. In cases where serious attenuation is anticipated
(for example, sewage) it is recommended that the use be restricted to 5 metres and a lower
frequency be used above that. Of course the lower frequency requires a greater depth of
water in which to operate and this must be taken into account (see Section 1).
An in-line Tbox is fitted in line with each transducer.
The co-axial cable from the T box must be connected to a star box or extended to run directly
to the flowmeter. Coaxial cables to the flowmeter enclosure are usually made up on site.
Cables may be supplied to length with made up ends if specified with the order. However this
often makes the installation more difficult where there is the need to pull cables through ducts.
2.4.3 1 MHz intrinsically safe (IS) transducers
1 MHz transducers are available for use in hazardous areas which have atmospheres with a
risk of explosion. The IS specification is given in Appendix 2, specification. The transducer is
internally different from the standard one and a special barrier is wired between it and its
Tbox. The transducer and connecting cable can be used in the hazardous area but the barrier
and all other electronic parts must be located in safe areas.