Merck Millipore Mobius Chrom 20 System User Manual page 168

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One of the capabilities of the system is the ability to form gradients based on conductivity. It
allows a gradient to be defined in terms of the conductivity (in μS) of the fluid entering the
column. The software employs a hybrid feed-forward-feedback algorithm to manipulate the
gradient valve to achieve the desired conductivity.
In addition to gradients based on conductivity, users can program gradients the traditional way
by specifying the primary pump (Pump 1) percentage (%pri).
• Gradient Control Fixed Speed Example: If the pump-based Loop Mode is set to Fixed Speed
with a setpoint of 50% for the total pump speed and Gradient Control is enabled with a
Percent Primary setpoint of 60%, Pump 1 would have an individual setpoint of 30% (60% of
50%) and pump 2 would have an individual setpoint of 20% (40% of 50%). The sum of the
two pump speeds equaling 50%.
• Gradient Control Flow Example: If the pump based Loop Mode is set to Flow with a setpoint
of 3.0 Lpm for the total system flow and Gradient Control is enabled with a Percent Primary
setpoint of 60%, Pump 1 would have an individual setpoint of 1.8 Lpm (60% of 3.0 Lpm)
and pump 2 would have an individual setpoint of 1.2 Lpm (40% of 3.0 Lpm). The sum of the
two pump flows equaling 3.0 Lpm.
• Gradient Control Linear Flow Example: If the pump based Loop Mode is set to Linear Velocity
with a setpoint of 300 cm/hr for the total system velocity and Gradient Control is enabled
with a Percent Primary setpoint of 60%, Pump 1 would have an individual setpoint of 180
cm/hr (60% of 300 cm/hr) and pump 2 would have an individual setpoint of 120 cm/hr
(40% of 300 cm/hr). The sum of the two pump velocities equaling 300 cm/hr.
Linear gradients and/or combinations of step and linear gradients can be executed only through
an operation.
The Mixing Lookup Table allows to minimize the conductivity control error when performing
conductivity gradient (step or linear). The table compensates for the nonlinear relationship
between mixing percentage and measured conductivity.
Fill in the lookup table based on experimental trials. When the system is stabilized, in the
Calibrate segment, snap the current conductivity reading and current primary pump speed
percentage. These readings are used as reference points on the table. Choose the number of
points (maximum of 20 points) to include in the lookup table.
Note To use the Mixing Lookup Table, the holdup volume must be different from 0.
User Guide