Protecting The In-Pipe Ultrasonic Sensor - Hach 911 User Manual

6.2.3.2 Calibrating the Liquid Depth
6.2.3.3 Calibrating the Sensor Height
6.2.3.4 Invisible Range/Deadband

6.2.4 Protecting the In-Pipe Ultrasonic Sensor

Liquid depth calibration requires knowing the level or the depth of the liquid in the channel
that is contributing to the flow. Liquid depth calibration is the recommended calibration
method for the in-pipe sensor. Take a physical measurement of the liquid depth and enter
the value into the application software. For a dry channel enter 0 depth.
Sensor height calibration is generally used when access to the primary device is difficult
(such as confined space entry in a manhole) or when there is no liquid flow during
installation of the flow meter. This calibration method requires knowing the distance
between the zero flow point and the bottom of the sensor. In a round pipe the zero flow
point is typically the invert or bottom of the pipe. Further, compensation is required for the
invisible range (internal deadband) in the sensor housing. Measurement uncertainty
increases to 1.28 cm (0.042 ft) for a ±30 cm (±1 ft) change in level from the calibration
point. Use this method
ONLY
Measure the distance from the bottom of the sensor to the zero flow point. Add 18.3 cm
(7.2 in.) to the measured distance to obtain the total zero flow distance. Enter the total zero
flow distance value into the application software.
Note: When programming the invisible range, 18.3 cm (7.2 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.
The 940 flow meter is equipped with an invisible range feature to prevent false echoes
from top of channel walls, ladder rungs, shelves, etc. A user-selected range is defined that
is invisible to the flow meter. Do not extend the invisible range to where it meets or
overlaps the highest expected level in the channel.
Measure the distance between the bottom of the sensor housing and the object that is to
be excluded from the level measurement. Add 18.3 cm (7.2 in.) to the measured distance
to obtain the total invisible range (deadband) distance. Enter the total invisible range value
into the application software.
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 heavily coated sensor will not be able to
detect the echo well and may not provide accurate level measurement.
if the Liquid Depth is not an option.
Sensor Installation
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