Siemens SITRANS FUS1020, Manual, Page: 148 / 251

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1020NFM-1D Section 6
6.4.10 ZERO FLOW ADJUST MENU
Unlike turbine meters ultrasonic transit-time meters provide active flow measurement right down to
zero flow, however, the measurement of the transit-time delta is dependent on the similarity or “match”
of the electronics, cables and ultrasonic transducers. Consequently some flow offset (or zero offset)
may be present in any installation. To eliminate this residual zero offset Controlotron has developed
several different methods to insure proper zero flow compensation. The following paragraphs describe
each method and when they should be used.
AutoZero
When the 1011 transducers are mounted in the Reflect Mode configuration (see paragraph 3.1.5) the
AutoZero routine is automatically invoked at the end of the Initial Makeup. Flow does not have to be
stopped to perform AutoZero since only the pipe wall signal is used in determining the zero offset and
not the liquid component. The AutoZero routine performs a one-time analysis of the pipe wall compo-
nent of the ultrasound signal to quantify any residual mismatch in the hardware. Once the AutoZero
routine is complete, the system memorizes this measured zero offset and subtracts this value from
the flow reading.
Actual Zero
The “Actual Zero” function simply averages the indicated “zero flow” readings (over a user defined time
period) then stores this average value in memory. Under normal operation the indicated flow reading is
zero compensated by simply subtracting this memorized value from the uncompensated flow reading.
Actual Zero is the most positive method for zeroing the system, however,
flow must be stopped with the
line blocked (if possible) before invoking this function. If stopping flow is not possible then an alternate
zeroing method should be selected.
ReversaMatic
This routine involves swapping the UP and Down transducers on the pipe (while keeping the cables
attached) such that the difference in the transit-time change represents the zero offset. The fixed zero
offset value is stored in memory in the same manner as described in Actual Zero. This routine would
generally be used whenever flow cannot be stopped and the transducers cannot be mounted in the
Reflect Mode configuration. Flow must be stable during the entire process.
ZeroMatic
When ZeroMatic is invoked the flowmeter first performs the same analysis as described above in the
AutoZero routine. However, after this analysis is complete the flowmeter continues to interrogate the
pipe wall signal and update the zero offset value under normal operation, such that the flowmeter
dynamically compensates for changing conditions which would normally result in zero drift. ZeroMatic
will only operate with the transducers mounted in the Reflect Mode configuration and is recommended
for applications which experience large temperature extremes.
NOTE: Invoking ZeroMatic will clear any existing “fixed” or memorized zero offset. If any zero
offset remains after flow is stopped, an Actual Zero can be performed without interrupt-
ing ZeroMatic operation. To disable ZeroMatic, invoke it again, but then press <Left
Arrow> to abort the installation.
NOTE: The ZeroClr command only resets the memorized zero offset registers not those set
when the AutoZero routine is invoked.