Vacuum Overview

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The Vacuum Overview control panel is available only under the control panel selection at the bottom right (its size is too large to fit otherwise). The overview display the current status of the vacuum system. The overview displays the following elements:

• pumps (indicated by their vacuum-technology symbols)
• pumping lines and volumes
• gauges
• valves
• pressure read outs
• process information

N2 indicates connections for inlet of nitrogen gas when a certain volume is vented (let up to air).

 
     Pumps

The microscope contains a number of pumps, of different types. The various pumps are indicated by their vacuum technology symbols:

	Ion-getter pump (IGP)
	Oil-diffusion pump (ODP)
	Rotary or pre-vacuum pump (PVP)

Pumps that are active are shown in black on the inside, inactive pumps in gray.

The microscope contains the following pumps:

• IGP1 pumps on the specimen area.
• IGP2 pumps on the gun area.
• IGP4 pumps on the liner tubes at the condenser and selected-area aperture level.
• ODP pumps on the projection chamber.
• PVP pumps on the buffer tank (backing of the ODP) and on the projection chamber before the ODP.

 
     Pumping lines and volumes

Lines and volumes are indicated in colors, with the blue colors indicating active lines and volumes, and a lighter color indicating lower pressure (higher vacuum). Yellow indicates SF₆ (in the gun).

 
     Gauges

Gauges are vacuum elements that are used to measure pressures (in addition to pressure measurements derived from the current of the Ion Getter Pumps). Three types of gauges exist, Pirani's, Pennings and combined Pirani-Pennings. The following gauges are present in the vacuum system:

• Pirani 1  -  Measures pressure in the buffer tank.
• Pirani 2  -  Measures pressure in the backing line (line going from main vacuum system to rotary pump).
• Penning 3  -  Measures pressure in projection chamber.
• Pirani 5  -  Measures pressure in lines behind column. Used to determine if it is safe to switch on IGPs.

 
     Valves

Valves are indicated by a butterfly-like symbol. In closed valves the 'butterfly wings' touch each other, for open valves there is an opening between the 'wings'. Valves with a 'capital T'-like symbol between the 'wings' indicate a manual valve. Of the valves present in the system, the following have special importance:

• V4 and V7 are the column valves, which are under user control (as well as automatic control by the vacuum system).
• The unmarked valve to the right of V8 (the airlock valve) is the valve inside the CompuStage goniometer that is opened and closed by the rotation of the specimen holder during insertion or retraction. It is therefore a manual valve (as indicated by the T on the valve).

 
     Pressure read-out

The gray arrows indicate the values of the pressures read-out (units are as selected in the Vacuum Settings control panel). The arrows roughly point to the vacuum gauge or pump used to read out the pressure.

 
     Process information

The process information lists up to four messages about the status of the vacuum system. Processes that will take some time are listing the expected time needed to finish the process. 

The times displayed should be seen as an indication only as they may depend on settings not currently known to the vacuum system. For example, the start-up of the ODP typically takes some 20 minutes. However, if the ODP is still warm, the start-up time may be considerably less and the displayed value will thus appear to have been wrong. Since the vacuum system cannot measure the temperature of the ODP but only knows if is it warm or not warm, it cannot accurately predict the time needed. 
A similar situation applies to pumping on the airlock with the PVP. Initially the display will typically say 5 minutes (the maximum allowed time for the PVP to empty the backing line). Once the backing line has reached sufficiently low pressure (usually a very short time), the time displayed will change to the actual airlock pumping time (thus seemingly 5 minutes may change to 30 seconds in just a few seconds).

Page last modified on 09/03/01