Bronkhorst Mini CORI-FLOW - Manual (Page 25)

BRONKHORST CORI-TECH B.V.
3.4 Zeroing
3.4.1 What is zero-stability ?
Due to mechanical construction of the sensor tubes each Coriolis sensor will have a very small offset signal,
even when the mass flow is zero. This is called the zero-stability error and is specified for accuracy separately
for all Coriolis instruments. Main reason for this is the fact that this error can be neutralized after performing a
zero-action.
Immediately after zeroing, zero-stability error is 0%. However it is allowed to move between a certain band
depending on the environment and fluid conditions.
In ideal situations where actual process conditions do not change, this error will remain the same.
Each time process conditions have been changed significantly a mini CORI-FLOW needs to be zeroed in order
to get rid of the offset error due to zero-stability. At least the very first time an instrument is used a zero
procedure will be required.
The zero-stability error might change when one or more of the following items change significantly:
Mainly:
• Temperature (of fluid and environment)
• Mounting position of instrument
Less important:
• Dynamic viscosity of fluid
• Vibrations working on instrument via environment
• Pulsation of supply pressure working on instrument
• Pressure
• Density of fluid
3.4.2 Procedure for zeroing
(See also Short form instruction manual mini CORI-FLOW, document nr. 9.17.052)
• Warm-up instrument for at least 30 minutes.
• Purge as long as needed with fluid to make sure there is no gas in the liquid or no condensate in the gas
• Fill instrument with fluid under process conditions.
• Close all valves at output to establish zero flow. Closing valves at input will not be critical (under
circumstances even better to let them open) but extra shut-off valves at output should be closed, as well as
control valve of instrument e.g. by means of setpoint = 0% or control mode = “close valve” .
• To start the zero procedure: Press push-button switch (#) on the side of the instrument and hold it (LED’s
will respond) until only the green LED is burning and then release the button.
This command is also possible via any available fieldbus- or RS232 interface.
• The zeroing procedure will start at that moment and can be monitored by the LED’s.
The green LED will blink until the procedure is ready. This will take about 30 seconds.
• When it is ready it will burn green continuously again.
3.4.3 Zero-procedure information
If the instrument has problems finding a proper and stable zero, it will repeat the auto-zero step a few times
(max. 4 times). Each time no proper zero can be achieved, the instrument will give a short notice signalling its
LED’s after the procedure.
The red and green LED will blink turn-by-turn for a few seconds to indicate that the auto-zero wasn’t able to
find a zero (because of too much noise in the signal). This is mostly the case when the instrument is placed in
a vibrating environment.
When ready zeroing after trying several (max. 4) times, the final result for the zero value will be a moving
average value of all attempts. The instrument will save this zero value into its non-volatile memory and will
keep this value until a next zero-procedure will be performed.
This value (Sensor Input Zero Scale Adjustment) can be accessed (read/written), via parameter 218 in the
parameter table of FlowDDE (note: Depending on firmware of instrument).
The mini CORI-FLOW will accept a proper zero only if the measured signal is within a limited noise band.
Best way to achieve this is to avoid external noise influences. However, when this is not possible, filter settings
of the mini CORI-FLOW can be changed to improve noise immunity.
Important:
Always make sure that there is absolutely no flow when the instrument is performing the (auto-)zero procedure
and there are no vibrations or pulsating inlet pressures.
9.17.050 page 25