Dynasonics DXN Portable Ultrasonic Measurement System - Manual (Page 29)

NOTE: N
The chains do not need to be taught at this point. Any slack in the chains will be removed when the thumb screws
are adjusted.
½”
(12 mm)
Chain Mounting Cleat
Figure 35: Application of Acoustic Couplant – DTTSU Transducers
4. Finger tighten the thumb screws so that the acoustic coupling grease begins to flow out from the under the
transducer Do not over tighten.
5. If signal quality is less than 10%, remount the transducer at another location on the piping system.
Doppler Transducer Installation
For Doppler installation the only pipe information needed is the Pipe ID (Inside Diameter).
Sonic Reflectors for Doppler Mode
Flow meters based on Doppler shift principals operate by transmitting an ultrasonic sound from its transmitting transducer
through the pipe wall into the flowing liquid. The sound will be reflected by useful sonic reflectors suspended within the
liquid and recorded by the receiving transducer. If the sonic reflectors are moving within the sound transmission path, sound
waves will be reflected at a frequency shifted (Doppler frequency) from the transmitted frequency. The shift in frequency will
be directly related to the speed of the moving particle or bubble. This shift in frequency is interpreted by the instrument and
converted to various user defined measuring units.
The four criteria for a good Doppler reflectors are :
1. The scattering material must have a sonic impedance (sound speed difference) different from the fluid. The minimum
difference must be at least 10%.
2. There must be some particles large enough to cause longitudinal reflection - particles larger than 35 micron
(435 mesh).
3. For a given pipe size, the longitudinal reflection must have sufficient energy to overcome the Rayleigh (energy
wasting) scattering caused by smaller particles.
4. The reflecting material must travel at the same velocity as the fluid for good accuracy.
Doppler Installation
Mount Doppler transducers on the pipe 180° apart and facing each other, with the cables on the downstream side of the
transducers. If the pipe is horizontal, the preferred mounting orientation is 3 and 9 o’clock, with 12 o’clock being the top of the
pipe. See Figure 36 . Orientation on vertical pipes does not matter.
NOTE: N Doppler transducers may be mounted on the same pipe as transit time transducers without encountering acoustic
cross-talk.
1. Large pipe installations use stainless steel straps to secure the transducers to the outside of the pipe. The DXN system
is shipped with four 36" (900 mm) straps, which are suitable for pipes up to 39" (1000 mm) diameter. Select the
proper number of transducer straps to allow a complete strap to go around the circumference of the pipe.
2. Wrap the strap around the pipe in the area where the transducers are to be mounted. Leave the strap loose enough
to allow the transducers to be placed underneath. If multiple straps are being used, it can be beneficial to wrap
electrical tape around all but one strap connection to secure the strap worm screws in place.
3. Spread an even layer of coupling compound, approximately 1/8" (3 mm) thick by 1/2" (12 mm) wide, to the bottom
flat face of the two transducers.
Installation & Operation Manual
Page 29 February 2013