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Velocity-Only Sensor
Section 4
4.11.4.3 Setting the Invisible Range
The 980 Flow Meter is equipped with an invisible range feature to prevent
false echoes from tops of channel walls, ladder rungs, shelves, etc. A user-
selected range is defined that is invisible to the flow meter. Extend the
invisible range to where it meets or overlaps the highest expected level in the
channel. Have a gap of at least 2 in. (5 cm) between the invisible range and
the highest expected level. Only objects beyond the invisible range can be
detected.
Note: When programming the invisible range, 18.21 cm (7.17 in.) must be added to the
desired range to compensate for the internal deadband distance between the sensor,
the reflector, and the bottom of the sensor housing. See Manual Figure Side View of
In-Pipe on page 4 of this
change notice.
1. From the Main Menu, select OPTIONS>ADVANCED OPTIONS>
CALIBRATION> ULTRASONIC SENSOR.
2. Select the Invisible Range option using the up and down arrow soft keys.
Press
SELECT to continue.
3. Enter the Distance to End of the Invisible Range using the numeric
keypad.
4. Select either inches or centimeters using the
CHANGE UNITS soft key.
Press the
ACCEPT soft key when finished.
4.11.5 Protecting the In-Pipe Ultrasonic Sensor
Keep the sensor and the reflector free of grease and dirt. Since the logger
“listens” for the relatively faint sound of the returning echo, a heavy coated
sensor will not be able to detect the echo well and may not provide accurate
level measurement.
4.11.6 In-Pipe Ultrasonic Sensor Troubleshooting
During surcharged conditions, the ultrasonic level will give random depth
readings that will need to be edited in the application software.
Symptoms of this hydraulic condition occur when the level appears erratic
within 5.7 cm (2.25 in.) of the pipe diameter. The readings will first appear to
flat-line. Then the erratic readings (that in some cases exceed the pipe
diameter) will occur.
If the hydraulics are frequently in surcharge conditions, use a submerged level
sensor instead of an in-pipe ultrasonic sensor.
4.12 Velocity-Only Sensor
The Velocity-Only Sensor is an extremely low-profile velocity sensor. It does
not measure level. The streamlined shape of the wafer probe allows velocity
measurement in very low-flow conditions. When used in conjunction with a
level sensor (such as the Zero-Deadband Ultrasonic Level Sensor), the meter
can calculate flow. Refer to Figure 39.