FEI Quanta FEG 250, User'S Operation Manual, Page: 23 / 228

System Operation: Vacuum Modes
3-5
Clicking the OK button after selecting an appropriate pole piece radio
button (cone or a detector with an integrated PLA) informs the system
that it is mounted on the lens insert. The system starts pumping to the
LoVac mode. From that point on, this information is automatically
used until the system is vented again, when the PLA is set to
“unknown”. Clicking the
Cancel button leaves the system in its actual
status mode (Pumped or Vented).
Using Gas
ESEM and Low Vacuum modes allow the user to image samples in a
gaseous environment, which can be selected in the drop down list
box:
• the Water vapour from a built-in water reservoir located in the back
part of the microscope console,
Note:
On occasion the water reservoir needs to be filled (see Chapter 8).
• the gaseous environment supplied by the user via the Auxiliary
inlet placed on the back of the console.
Caution!
Maximum overpressure for Auxiliary gas and Nitrogen inlets is
10 kPa (0.1 atm). The
Nitrogen inlet is used only for venting the
chamber with air or the nitrogen preferably.
When using a particular pressure limiting aperture, there are pressure
limits for different gases.
Note:
*) Combustible gases (acetylene for instance) must always be used
with respect to safety issues.
**) The argon use should be minimized to a short time, because the
IGP’s are not optimized for pumping of it at all.
TABLE 3-1 MAXIMAL CHAMBER PRESSURE [PA (TORR)]
UNDER DIFFERENT GASEOUS ENVIRONMENT
Working Gas 500 µm Aperture 1000 µm Aperture
Water - H
2
O 2 700 (20) 750 (5.5)
Nitrogen - N
2
4 000 (30) 750 (5.5)
Air
Carbon Dioxide - CO
2
Nitrogen Dioxide - NO
2
Helium - He 2 000 (15) 500 (4)
70 % He + 30 % H
2
1 500 (12) 400 (3)
Argon - Ar
**)
1 000 (7) 200 (1.5)
C
x
H
y
*)
4 000 (30) 750 (5.5)