Banner U-GAGE ULTRA-BEAM Series - Manual (Page 3)

TABLE: Relative Effectiveness of Various Materials as
Reflectors of Ultrasound (rough order, best to worst)
c) Object orientation, shape, and surface characteristics: Ultra-
sonic waves follow the same laws of reflection as do light waves. The
angle of incidence equals the angle of reflection. This means that
ultrasonic waves are reflected from a smooth, flat surface at the same
angle (to the surface) as the angle at which they arrive. A perfectly flat
object that is exactly perpendicular to the direction of travel (the "axis")
of the ultrasonic waves will reflect the waves back along the same path
(figure C). Objects thus oriented produce strong reflections when
sensed.
Smooth, flat steel plate (best)
Smooth, flat plywood sheet
Undisturbed liquid surface
Aggregate (coal, ore, etc.)
Smooth, flat corrugated cardboard Foam insulation panel
Fine particulates (flour, grain, etc)
Liquid with heavy surface foam
Wool, cotton, felt
Fiberglass insulation (worst)
General rules:
1) The higher the density of the object, the stronger the reflection.
2) The smoother the surface of the object, the stronger the reflection.
d) Location of the object within the sensor's response pattern: the
ultrasonic signal radiated from the ULTRA-BEAM is strongest along
the axis of the response pattern (the "sensing axis"), and drops off with
increasing angle away from the axis. Objects can be most reliably
sensed when they are as close as possible to the sensing axis.
e) Location of sidewalls with respect to the beam pattern. Sidewalls
located close to the sensing axis may sometimes cause unwanted signals
to be reflected back to the sensor. Unwanted reflections may also occur
from deposits of material adhering to the sidewalls of silos, tanks, etc.
If possible, align the sensor so that its beam pattern will not encounter
sidewalls, and try to keep sidewalls free of buildup.
3) Extreme environmental conditions may affect ultrasonic sens-
ing. Factors which may need to be considered include: temperature,
high winds, high levels of sounds of certain types, humidity, atmos-
pheric pressure, and dirt or moisture on the transducer.
a) The speed of sound increases and decreases slightly with in-
creases and decreases in ambient temperature. A large temperature
increase will move the window slightly towards the sensor. A large
temperature decrease will move the window slightly away from the
sensor.
The amount of shift is 3.5% for every 20
°
C of temperature change. For
this reason, it is a good idea to set the sensing window limits when
the ambient temperature is midway in the expected environmental
operating temperature range of the sensor. Also, whenever it is
consistent with the application, adjust the sensing window so that
the object(s) to be sensed will pass as much as possible through the
midpoint of the window.
Fluctuations in the speed of sound can result when hot objects are
sensed. A small fan directed along the sensing axis can help to thermal-
ly stabilize the sensing path and make accurate readings possible.
b) In outdoor applications, crosswinds can blow an ultrasonic beam
off target. The effect becomes more noticeable as the wind velocity and
the distance to the object being sensed increase. Try to avoid sensing
in areas of high crosswinds. When it is necessary to use ultrasonics in
windy areas, keep the sensing range as short as possible, and shield the
area from the wind as effectively as possible. Winds blowing steadily
along the sensing axis, toward or away from the sensor, have less effect.
Gusty winds along the sensing axis may affect output stability.
c) Care should be taken to shield ultrasonic sensors from sustained,
loud sounds such as factory whistles and similar sources. Sound
sources produce harmonics (sounds at frequencies above the funda-
mental frequency of the source). Harmonics may fall in the ultrasonic
range and "confuse" ultrasonic sensors. High pressure air blasts are
especially good producers of harmonics in the ultrasonic range. Since
sound waves travel in a straight line from the harmonic source to the
sensor, the solution is simple: a wall or baffle placed between the
sensor and the harmonic source is nearly always all that is required. This
tactic can also help prevent interference between adjacent ultrasonic
sensors.
d) Humidity influences ultrasonic sensing by a maximum of 2% with
extreme changes of humidity. The speed of sound increases with
increasing humidity. Heavy atmospheric fog can increase sound
absorption and reduce sensing range.
e) Atmospheric pressure: a 5% increase in atmospheric pressure
increases the speed of sound by 0.6%. A 5% decrease in pressure slows
the speed of sound by 0.6%.
f) Condensation or other contamination on the transducer face can
seriously impede sensor performance, and should be avoided. In
order to function, the transducer must be able to vibrate freely and at a
high rate. Condensation or particulates on the transducer dampens its
movement. While the transducer is not harmed by mists or non-
condensing humidity, it should be clean and dry to operate most
effectively.
Most contamination can be prevented by mounting the sensor in the
driest, cleanest location possible that still allows reliable sensing
performance in a given application. Never mount the sensor "face
up" in areas where contamination might be a problem.
As the object's reflecting surface is tilted away from the axis of the
waves, however, less and less of the ultrasonic signal is reflected back
to the sensor. Eventually the point is reached beyond which the object
can no longer be sensed. When attempting to sense an object with a flat,
smooth, highly reflective surface, the angle of the reflecting surface to
the sensing axis should never be more than 3
°
off of perpendicular.
Irregularly shaped objects and aggregate matter (coal, ore, sand, flour,
etc.) have many reflecting faces of many different angles. Although this
scatters much of the ultrasonic energy away from the sensor, enough
sound energy may be reflected back to the sensor for reliable sensing.
In fact, due to the large number of reflecting surfaces, the "perpendicu-
larity requirement" for smooth objects is not nearly as critical for these
materials. Sensor angles of up to several degrees away from perpen-
dicular often produce adequate reflections (figure D). Some materials
may actually produce just as good reflections when sensed "at an angle"
as when sensed "straight on". This allows a degree of freedom in
choosing a sensor mounting location for some applications. Some trial-
and-error experimentation may be required.