Metrohm 823 Membrane Pump Unit - Manual (Page 15)

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2 Overview of the instrument
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3 Pump chamber 4 PTFE membrane
5 Eccentric
Principle
The membrane pump is located in the pump head. The membrane made
of PTFE 4 is moved up and down by an eccentric 5. This results in a peri-
odic volume change of the pump chamber 3. On the downstroke, the
medium to be pumped is therefore aspirated through the inlet valve 2. On
the upstroke, the membrane forces the medium out of the pump chamber
through the outlet valve 1.
The following features characterize the membrane pump:
￭ Self-aspirating, due to the ingenious membrane technology.
￭ Dry run safe, i.e., the pump is not damaged if gases are pumped
instead of liquids.
￭ Operational in any position.
￭ Small-sized and thus space-saving.
￭ High chemical resistance:
As chemically resistant materials such as PTFE, PP and FFPM are used in
all parts that come into contact with the media, nearly all neutral and
aggressive media may be pumped.
Note
If you use particularly aggressive media, make sure that the above-men-
tioned materials are resistant to them prior to using the instrument.
In addition, the membrane pump has a very long lifetime. The only
expendable parts are the membrane and the valve plates.
The dosing accuracy of the membrane pump is considerably further
increased by a valve at the pump outlet. This valve has the following three
functions:
￭ Pressurizing function
When media are pumped into systems with fluctuating pressures, into
a vacuum or from an overpressure area, the pressurizing valve ensures
a constant backpressure.
￭ Overflow function
The valve prevents an excessive rise in pressure in the system on the
pressure side.
￭ Anti-injector function
When the pump is used for dosing into piping systems with high flow
rates, the pressurizing valve prevents an injector effect (aspiration of
ambient air as a result of the underpressure).