Diagnostic parameter Description Typical values
Lower sound velocity
limit
The minimum allowed fluid sound velocity for reliable flow measure‐
ment. Operation below this limit may impact measurement accuracy or
prevent proper signal detection. A configuration alarm will also be gen‐
erated.
Upper sound velocity
limit
The maximum allowed fluid sound velocity for reliable flow measure‐
ment. Operation above this limit may impact measurement accuracy or
prevent proper signal detection. A configuration alarm will also be gen‐
erated.
9.6 Improve the application (A4)
If the application still gives unstable or incorrect measurements, a number of measures can be
taken to improve the installation. In the following it is described how to find the causes of a high
measurement error or unstable flow measurement and other measures that will improve the
installation.
Checking the zero point offset
In order to observe if the zero point (or zero flow offset) exceeds the reasonable limit for the
application, it will be necessary to first set the Low flow cut-off to 0.0 m
3
/s. Any zero point offset
will then be visible on the flow display.
If necessary, perform the Zero point adjustment again.
Incorrect installation of the sensors
● Are the sensors properly aligned and spaced along the pipe axis?
● Sensors should not be installed on the very top or bottom of a horizontal pipe, where gas or
solids can interfere with the acoustic signal.
● Are the sensors properly coupled (with supplied grease couplant) to the pipe wall?
● Is the pipe surface free of any loose or bubbling paint? If necessary, sand or condition the
pipe surface to ensure reliable sensor coupling.
Air in the liquid
● When air is present in the liquid, the flow measurement becomes unstable, which leads to
a poor measurement accuracy.
Checking for air:
● Look for unstable path gain and SNR diagnostics.
● Observe the path receive signals and look for rapidly varying waveform characteristics that
would result from scattering of the acoustic beam by air bubbles.
Diagnostics and troubleshooting
9.6 Improve the application (A4)
SITRANS FST030 (Modbus)
Operating Instructions, 08/2018, A5E37367739-AD 99
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