Biomedical Engineering Reference
In-Depth Information
due to the small sample intake, a fast switching of samples, selective selection of
samples in a gas flow (e.g., plug flows), and lock-in type process for sample gen-
eration and detection is possible.
A combination of the pressure stage and the microvaporizer will also ensure that
the vaporized samples will not condense on their way to the ionization chamber,
even if the total system is at room temperature.
8.4
Miniaturized Vacuum Pumps
Another part of the periphery that is not yet in accordance with the above-mentioned
applications is the vacuum system of the PIMMS. As mentioned in Sect.
4
, the gas
input to the system is very low. In continuous operation barely 150 mL/min are fed
into the PIMMS. All available vacuum pumps on the market are oversized in pump
power by several orders of magnitude and are correspondingly large, heavy, and
energy consuming. These currently commercial vacuum pumps (Pfeiffer MVP
006-4 and Pfeiffer HiPace™ 10) are used with a total volume of about 2.5 L and a
weight of 3.6 kg. For a real mobile field device a miniaturized vacuum pump system
is advantageous. Efforts have been made to develop a miniaturized vacuum pump
using microsystems technology. At the moment three pumping principles and their
transfer to the MEMS domain are investigated also at the TUHH.
As in high vacuum generation in macroscopic scale, the micropump is divided
into two parts. As a backing pump for the pressure range from a few 10
2
Pa to atmo-
spheric pressure a micro-sorption pump is under investigation, which relies on sur-
face adsorption effects, as well as a scroll pump, which ranks with its displacement
principle among the classic backing pumps.
The high precision with which structures can be etched using DRIE allows for a
precise alignment of the structures with minimal clearance between scrolls. Wear
problems can be avoided by deposition of polymer films. First experiments have
been done with a pump with 10 mL/min pump capacity.
Sorption effects in the macroscopic scale are usually used for non-continuous
pumping. They contain a highly porous sorption material like activated carbon or zeo-
lites with a huge inner surface. The sorption material is usually cooled down by liquid
nitrogen. They are regenerated by heating the sorption material to temperatures of
several hundred degrees Celsius after a disconnection of pump and vacuum-chamber.
This pumping principle can be transferred to the microsystems. In particular the
low thermal masses allow for fast heating and cooling. Hence with two or more
pumping units, which work parallel in a push-pull cycle, a quasi-continuous pump-
ing can be obtained. Due to the short cycle time freshly outgassed sorption material
is available most of the time and because the very low temperatures used in macro-
scopic pumps are not necessary for the pumping of most gases, a cost-effective
cooling with Peltier-elements is sufficient.
For the lower pressures the principle of the diffusion pump is investigated, that
uses the momentum transfer between colliding molecules [
36
] .
