Veeco MS40 Operation And Maintenance Manual page 61

Pressing the START pushbutton opens the rough valve. The internal rotary
vane mechanical pump (the roughing pump) or the external roughing pump
(when equipped with the unit) will begin to evacuate the test object. A pirani
gauge attached to the test port monitors the pressure within the test port and
test object.
In systems without an external roughing pump, the internal mechanical
pump serves as both the roughing and fore-line pump. Consequently,
during the roughing state, the fore-line valve is closed. Note that due
to the advanced design of the MS-40 system, the turbo-molecular pump
has an extremely high fore-line tolerance. This allows for much longer
than normal roughing intervals.
The MS-40's computer monitors fore-line pressure using its fore-line pirani
gauge. If fore-line pressure reaches 1,000 millitorr, the MS-40 computer
will stop the roughing cycle by closing the internal roughing valve and
opening the fore-line valve. When fore-line pressure has been brought back
down to 150 millitorr, the roughing cycle is resumed.
Testing begins once the pressure at the test port reaches the pre-set
crossover set point for the appropriate test mode.
The sample gases of the test object disperse throughout the confines of the
Hi-Vac section. As gas molecules enter the ion source chamber of the mass
spectrometer, an electrically heated filament provides a source of electrons
that collide with the gas molecules and produce positively charged ions.
The production of these ions is referred to as ionization. Due to the
differences in mass of the ionized gases, only helium ions have the ability
to traverse the two 90° magnetic sections of the spectrometer housing to
the collector. It is at the collector that the ions are converted into an
electrical current and then processed by the MS-40 computer for display on the
User Panel. The MS-40 has been programmed with the ability to test
using either Mass 3 or Mass 4 helium.
When the unit is equipped with an external mechanical pump, the
unit will "crosses over" into Reverse Flow mode testing only after Gross
Mode testing is complete (assuming a leak has not been detected). In the
vacuum system the internal rough valve is opened and the fore-line valve
remains open. All other valves are closed. During crossover, the
equalization of pressures may cause a temporary increase in fore-line
pressure. This increase is termed a burst. If a burst should cause the
fore-line pressure to increase to over 7,000 millitorr, the test cycle will be
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aborted and ERROR 05 will be posted. If a burst should cause the fore-line
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pressure to increase to over 8,000 millitorr, the filament will be turned off,
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