Setting the
Pump Size
switches
Note
Select the pressure display unit with the rear panel
Display
Units
switch. For the display to read correctly, the pump size
selection switches must both be set to the correct pump size.
At low pressures, the display may fluctuate considerably. This
is a result of electrical noise in the ion pump.
Set the
Pump Size
switches to the pumping speed value closest to the
pump's actual air speed. For example, if a 270 1/s pump is used, set
the switches to 220 1/s position (left switch down and right switch up
as viewed from the rear panel).
The accuracy of the pressure reading is dependent on line voltage,
temperature condition and history of the pump. For example, as the
pump ages, leakage currents develop and limit minimum pressure that
can be measured. For this reason, pressure measurements should only
be used as a general indication of pressure inside the vacuum system.
The pressure is calculated using the formula:
Ptorr 5 x 10 -
3
x 1/N
where I is the pump current N is the number of pump elements.
a 120 1/s pump has 4 elements
a 220 1/s pump has 8 elements
a 400 1/s pump has 16 elements
a 700 1/s pump has 32 elements
To arrive at other units of measure, multiply Ptorr by the correct
conversion constant shown in Table 3-3.
Table 3-3. Pressure Conversion Constants
To convert From:
To:
torr
pascal
torr
micron
torr
millibar
pascal
torr
pascal
micron
pascal
millibar
micron
torr
micron
pascal
micron
millibar
millibar
torr
millibar
pascal
millibar
micron
Multiply by:
133
1 X 10
+3
1. 333
7.5 x10-
3
7.5
7.5 X 10-2
1 X 10-
3
0.1333
1.333 X 10-
3
0.75
100
750
Operation
3-5
