2-4 Installation and Operation 5707070-A
Advanced Energy
®
Assessing Particles or Air Bubbles
The ultrasonic beam from the transducer is transmitted through the pipe wall into the
liquid. The particles, air bubbles suspended in the liquid, have to be large enough to
act as reflectors to return the signal back to the transducer so that a measurement may
be taken. It is important to understand that the ultrasonic beam from the Doppler flow
meter does not penetrate air very well. A few bubbles will not affect flow meter
performance, but too many bubbles may cause the meter to malfunction. See
“Suspended Particle Test For Doppler Signals” on page 3-10 for information on how
to assess the particulate content of your flow.
Note: At a frequency of roughly 650 kHz, the minimum particle which can be sensed
is 40 microns @ 35 parts per million (ppm) and at least 0.2% density difference
from the liquid.
FLUCTUATIONS IN FLOW CONTENT
Wide fluctuations in flow suspended solids content can only be handled by an
instrument capable of monitoring such a wide fluctuation in liquid characteristics.
Extreme cases may require the use of both a transit time flow meter (such as the Sono-
Trak™ model) as well as a Doppler flow meter. For example, river water may require
a meter which is capable of measuring both clean and dirty liquids. Depending on
conditions, the river may vary considerably in suspended solid content.
PROCESS CHANGES
Changes in the process may cause the meter to cease proper operation. For example, a
Doppler meter had been operating satisfactorily for a long time in a certain
installation, and then suddenly it stopped functioning. The problem was eventually
traced to a particle filter that had recently been placed before the transducers,
effectively eliminating the reflectors from the stream.
Process changes in the other direction are also possible. The meter responds to
bubbles up to a point, but if the liquid starts to foam there may be too many air
bubbles to allow the ultrasonic beam to adequately penetrate the flow stream. This
may cause the Doppler flow meter to produce erroneous flow readings. Similarly, a
sludge may become too dense to allow proper penetration.
Attention:
The flow to be measured must always have a sufficient number of sonic
reflectors of ample size in order to provide reflection of the transmitted
sound. If the reflectors do not have sufficient density difference they
will neither rise nor fall in the solution but remain suspended. The
specific gravity will also affect the ability of a particle to reflect sound.
A good example of a reflector is a sand particle or an air bubble. A poor
example is algae or onion skins.
