Operation & Installation Manual V12-V6 2010-05-31.doc
31/05/2010
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transducer (transmit pulse) and digitizing and storing the signal on the receiving transducer
(receiving pulse).
Before starting the digitizing process a “receiving” window must open. The time to open and
to close this window (relative to the firing time of the transmit pulse) are dependant of the
size of the meter and are set as a parameters in the configuration file. This are factory
settings that should not be changed.
An accurate and high resolution timer is used to control the digitizing process.
The digitized and stored signal is evaluated to detect the arrival time of the acoustic signal.
As the signal is evaluated, it is checked for specific criteria to make sure it is the wanted and
genuine signal and that it can be used for a reliable transit time measurement. If the signal
does not meet the criteria it is rejected.
Based on the strength of the received signal, the setting of an variable gain amplifier is
updated in order to match the level of the signal to the operating envelope of the digitizing
circuit. The setting of the variable gain amplifier is available as a diagnostic value to indicate
the strength of the received signal. To capture the level of background noise the received
signal strength is also measured at a time when no signal is expected. The background noise
is used to evaluate the signal to noise ratio and is also available as a diagnostic value.
The repetition rate (called sample rate, expressed as times per second) for performing a
single transit time measurement can be set as a parameter in the configuration file. This is
not a critical value and has no impact regarding the measured flow values.
The sample rate can be set to a value that is too high to be physically achievable, meaning
the transit time does not allow such a repetition rate. In that case the meter will automatically
adjust the sample rate to the highest achievable value. This value is also reported as a
diagnostic value: The actual sample rate.
As a cycle the meter performs transit time measurements for all acoustic paths, each path in
both directions. After completion of a cycle all results are forwarded to the flow calculation
module. This includes the measured transit times, signal strength data and signal to noise
ratio data. In case a measurement had failed this will be indicated: the results are marked as
rejected values.
2.4.4. The flow calculation module.
The flow calculation module processes the data received from the basic transit time
measuring process. After each cycle a number of variables is updated, such as
•
Volume flow rate
•
Average gas velocity
•
Average speed of sound
•
Gas velocity for each measuring path
•
Speed of sound for each measuring path
•
Reynolds number
•
Reynolds correction factor
•
Reliability
•
Total (forward, accountable)
•
Total (forward, not accountable)
•
Total (reverse, accountable)
•
Total (reverse, not accountable)
•
Statistics
•
Averages
The flow calculation module will also report diagnostic information as:
