Notes:
If you use a fixed-orifice vent-restrictor, you may find that the time required to vent your system
is unacceptably long. You may be able to reduce the vent time if you use an unrestricted vent port
without a vent-restrictor and wait until the pump speed has fallen to 50% of full rotational speed
before you vent the pump.
If your system automatically controls an auxiliary vent-valve, you can use the auxiliary
vent-valve with a suitable vent-restrictor together with a TAV vent-valve (controlled by the EXC
Controller) for dual venting. When you want to stop the EXT pump and vent your system, open
the auxiliary vent-valve when the EXT pump is at full speed and configure the EXC Controller to
open the TAV vent-valve when the EXT pump speed falls to 50% of full rotational speed. If your
auxiliary vent-valve is connected to the purge-port of the EXT pump, select a suitable VRX
vent-restrictor (see Tables 4 and 5) and fit it to the purge-port as described in Section 3.5.1.
Vacuum system volume (l)
Suitable VRX Restrictor
Orifice diameter (mm)
³
25
< 25
< 10
< 5
< 2
VRX70
VRX50
VRX30
VRX20
VRX10
£
0.7
£
0.5
£
0.3
£
0.2
£
0.1
Table 4 - Vent-valve orifice diameter (with atmospheric pressure
at the inlet of the vent-valve)
Vacuum system volume (l)
Suitable VRX Restrictor
Orifice diameter (mm)
³
25
< 25
< 10
< 5
VRX50
VRX30
VRX20
VRX10
£
0.5
£
0.3
£
0.2
£
0.1
Table 5 - Vent-valve orifice diameter (with 2 bar absolute, 2 x 10
5
Pa pressure
at the inlet of the vent-valve)
EXT250M Turbomolecular Pump
17
