Metrohm 806 Exchange Unit - Manual (Page 28)

4.1 Filling the dosing cylinder and dosing
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806 Exchange Unit
When ejecting the titrant, the piston rod of the drive moves upwards the
dosing piston in the dosing cylinder. The flat stopcock is in dosing posi-
tion. The switching lever is directed to the right. The liquid in the cylinder
is pushed trough the flat stopcock into the dosing tubing.
After switching the flat stopcock, i.e. rotating the flat stopcock (switching
lever is directed to the left), liquid is aspirated out of the filling tubing as a
result of the dosing piston being pulled downwards by the piston rod of
the drive.
Because of the fact that the exchange units are interchangeable, the cou-
pling of the piston rod exhibits a low mechanical tolerance which has an
effect on the accuracy of the dosing when the dosing piston changes its
direction of movement. This tolerance is mechanically compensated by the
drive during automated procedures.
The piston movements are controlled by the precise electronic fine
mechanics of the drive. Independent of the cylinder volume, it exhibits a
resolution of 20'000 increments across the entire piston stroke.
The rate, at which a solution is to be dosed and the rate, at which the cyl-
inder is to be filled, depend on the cylinder volume of the exchange unit.
The maximum and minimum filling and dosing rates are:
Cylinder volume (mL) 1 5 10 20 50
max. filling- dosing rate
(mL/min) 3 15 30 60 150
min. filling- dosing rate (µL/
min)
10 (depending on device)
NOTE
Enter lower rates for high-viscosity liquids.