Bronkhorst®
Instruction Manual mini CORI-FLOW ML120
9.17.097G
18
What is zero-stability?
Due to mechanical construction of the sensor tubes each
mini CORI-FLOW
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 (temporarily) 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 (process) and fluid conditions.
In ideal situations, where actual process conditions do not change, this error will remain the same.
See below for possible reasons of change of zero-stability.
Model*
DN (mm)
Zero-stability
Nominal flow
ML120
0.25
< 0.01 g/h
100 g/h
* Zero-stability depends on the (mini) CORI-FLOW model
In practice zero-stability turns out to be better than the values in the table, but for calculation we will take worst case
values.
Process conditions
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 will mainly change when one or more of the following items change significantly:
·
Temperature (of fluid or environment)
·
Mounting of the (mini) CORI-FLOW instrument
Less important items:
·
Pressure
·
Density of fluid
·
Vibrations working on instrument via environment
·
Pulsation of supply pressure working on instrument
Zero Procedure
There are two ways to perform zeroing of a (mini) CORI-FLOW instrument:
1. With the micro-switch
2. Through digital communication
Always make sure that there is absolutely no flow when the instrument is performing the (auto-)zero procedure and there
are no mechanical vibrations or pulsating inlet pressures.
If the instrument has problems finding a proper and stable zero, it will repeat the auto-zero procedure up to 4 times. Each
time when no proper zero can be achieved, the instrument will give a short notice, signaling its LEDs after the procedure.
The red and green LED will blink alternating for a few seconds to indicate that the auto-zero was not able to find a zero
point (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 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 the
next zero-procedure is performed. The mini CORI-FLOW will accept a proper zero point 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.
2.10.1 Zeroing using micro switch
The zero-point of each instrument is factory adjusted. If so required the zero point may be re-adjusted
over RS232, fieldbus or by means of using the micro switch.
Procedure for zeroing by micro switch:
1. Set process conditions
Warm-up (for a minimum of 30 minutes), pressurize the system and fill the instrument according to the process
conditions.
2. Stop flow
Make sure no flow is going through the instrument by closing valves near the instrument. The setpoint must be zero.
