Troubleshooting The Downlook Ultrasonic Sensor Installation - Hach 911 User Manual

Sensor Installation

6.1.2 Troubleshooting the Downlook Ultrasonic Sensor Installation

Problem/Error Message
Ultrasonic Failure
(No signal from sensor)
Echo Loss
(Flow meter does not get a
return signal from the
sensor)
Convection Currents
Obstructions
Transducer Ringing
(False return echoes mask
"real" echoes)
RS485 Time Out
(Flow meter did not receive
data within the specified
time)
48
Solutions
Check the connection between the flow meter and the sensor.
Check the sensor cable for cuts or nicks.
Recalibrate the sensor. Look for unusual temperatures or the inability for a new calibrated
level to be read.
Try a different sensor on the same flow meter to rule out instrument problems, and try a
different flow meter with the same sensor to rule out sensor problems.
Check for excessive foam on the water surface. Foam may cause sound waves to be
absorbed rather than reflected.
Check the sensor height. See
Check the sensor cable for cuts or nicks.
Check the connection between the flow meter and the sensor.
Make sure that the sensor is level. The sensor must be level for proper return of the signal.
Try shielding the transducer from convection currents.
Repeat the temperature calibration procedure. If the unit reads extreme high or low
temperatures, the sensor may have a bad temperature.
Try a different sensor on the same flow meter to rule out instrument problems, and try a
different flow meter with the same sensor to rule out sensor problems.
Check to see if convection components between the sensor target are present. If these
components are present, install a shield around the sound beam to eliminate temperature
variation due to convection currents. The system is designed with averaging routines to help
mitigate this problem.
Check for excessive foam and oil. While the 940 Flow Meter is designed with an AGC
(Automatic Gain Control) scheme to reduce these factors, it is recommended that sites
without these anomalies be selected.
Use the invisible range to select targets beyond peripheral obstructions. Care must be taken
to set the range higher than the expected level.
Try moving the transducer farther from the liquid.
Check under the transducer for obstructions. This includes the front and sides
of the transducer.
Check the face of the transducer for a coating. This is rare, but cleaning the transducer face
may correct the problem. If this is a constant problem due to site conditions, coat the face of
the sensor with a thin film of silicone grease to keep debris from clinging.
Make sure that the proper rubber isolation washers are used on the mounting bracket. The
transducer can resonate against steel mounting rails. See
If the CPU has trouble communicating with the ultrasonic board, RS485 time outs may occur.
Wait a few minutes and see if the trouble has stopped. If it continues, a problem with the
ultrasonic, velocity, or CPU board may exist.
If the logging interval is 1 or 2 minutes, or if conditions are poor, or if the problem continues
indefinitely, try increasing the logging interval to capture more signals.
When using a combination of area velocity and ultrasonic sensors, this condition may occur
more frequently if site conditions are less than ideal. This is not necessarily an indication of a
defective sensor. This condition may occur occasionally.
Check to see if pins are shorted. Do not short pins on receptacle to avoid damage.
Figure 22 on page 44.
section 6.1.1.1 on page 45.