½”
(12 mm)
Chain Mounting Cleat
Figure 30: Application of acoustic couplant for 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 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 reflects off 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 are reflected
at a frequency shifted (Doppler frequency) from the transmitted frequency . The shift in frequency is 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 .
Installation Method
Mount Doppler transducers on the pipe 180 degrees 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
. Orientation on vertical pipes does not matter .
OTEE:
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 in . (900 mm) straps, which are suitable for pipes up to 39 in . (1000 mm) diameter . Select the
proper number of transducer straps to allow a complete strap to go around the circumference of the pipe . See
.
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 in . (3 mm) thick by 1/2 in . (12 mm) wide, to the bottom
flat face of the two transducers .
4 . Place each transducer under the strap with the flat face—amber plastic window—positioned towards the pipe . The notch
on the back of the transducer provides a mounting surface for the strap . The transducer cables must be facing in the same
direction and in the downstream direction for proper operation .
Transducer Installation
Page 28
November 2016
HYB-UM-00090-EN-04
