4210 Flow Meter
Section 3 Installation
3-16
lating the velocity of sound in the air is multiplied by the dis-
tance from the level sensor to the surface of the flow stream.
Minimizing the distance will minimize the error.
Calibrate at Expected Temperature – The user should cali-
brate the level reading under temperature conditions as near as
possible to those expected during operation. For small changes of
level, the error due to temperature is determined by the product
of the distance (from the transducer to the flow stream surface)
and the temperature change. Calibrating the flow meter at the
same temperature as the expected operating temperature will
minimize this error.
Avoid Water Condensate – The ultrasonic level sensor will not
operate properly if the bottom surface collects water droplets.
This may occur if water condenses on the transducer surface as a
result of high ambient humidity. Some users have found that
mounting the ultrasonic level sensor horizontally and aiming it
at a 45° angled reflector will keep water from collecting on the
level sensor's radiating surface.
Avoid Foam, Oil, and Turbulence – If the flow stream surface
is absorbent (such as with foam) or very irregular (such as highly
turbulent water), the ultrasonic echo may not be correctly
reflected back to the ultrasonic level sensor. This can result in a
false measurement or no measurement at all. If the foam is
reflective, the system will detect the top of the foam rather than
the liquid surface. Also, if grease or oil is floating on the flow
stream surface, it will be detected rather than the liquid surface.
(See Figure 3-6.)
Small Pipes and Channels – Small circular pipes, narrow
channels, and small flumes may also cause problems with ultra-
sonic distance measurement. Since the ultrasonic pulse expands
outward at a beam angle of approximately 10° as it travels away
from the ultrasonic level sensor, it may strike the sides of a
channel or the sloping sides of a circular pipe with low flow. (See
Figure 3-7). This can result in false echoes and incorrect level
readings. The term “small channels” generally refers to “U”
shaped channels and pipe inverts 10" in diameter and less. The
term “small flumes” generally refers to 1" and 2" Parshall flumes.
It should be noted that the level measuring point for many types
of flumes (Palmer-Bowlus, Leopold-Lagco, etc.) is not in the
flume, but upstream in the invert of the pipe; for these types of
flumes the section of interest is in the pipe invert, not in the
flume itself. Thus, care should be exercised in the use of 10" or
smaller Palmer-Bowlus and Leopold-Lagco flumes.
Determining Suitability – The channel to be measured can be
“pre-qualified” by a simple equation that will determine whether
or not the channel is wide enough to allow correct positioning of
the ultrasonic sensor. Since the beam angle is 8°, the equation is:
...Where Distance is the distance from the bottom of the ultra-
sonic level sensor to the minimum expected level.
MinimumWidth 0.14 Dis cetan×=
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