3.5 GEM Vacuum
Description
The GEM sample tank vacuum system consists of a roughing pump, a turbo pump and a polycold. This vacuum system is controlled from the PLC. Please refer to the PLC section of the manual for information on starting and stopping the sample tank vacuum system.
At the time of writing (December 2002), the GEM vacuum system takes about 7 minutes to go from atmospheric pressure to 1 mbar.
Vacuum Gauges
GEM has 4 vacuum gauges, as follows...
| Gauge | Description | Pressure range | Reading when out of range |
| A1 | Penning in sample tank | 10-3 mbar to 10-6 mbar | 9.93x10-3 mbar |
| A2 | Pirani in sample tank | atmospheric to 10-3 mbar | 8.02x10-4 mbar |
| B1 | Pirani between Rootes pump rotary and turbo | ||
| B2 | Pirani in flight tube | ||
Table. GEM vacuum gauges |
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The digital readouts for these vacuum gauges are to be found on the ground floor level, near to the GEM blockhouse door.
The pressure in the sample tank can also be displayed by the PLC. The PLC will almost certainly say that the vacuum is "bad" - please ignore this.
Logging the Pressure in the Sample Tank
The two sample tank gauge pressures can be monitored by the camac system on the VMS computer. The two relevant sample environment parameters are called TPG3_A1 and TPG3_A2. The pressure readings may be displayed on the dashboard by the use of commands such as cset tpg3_a2/log/disp. If the pressure readings are not displayed on the dashboard then please refer to the pressure logging troubleshooting page.
Collecting Data only when the Pressure is OK
Usually GEM is set up so that data will not be accumulated if the sample tank pressure is above 1.0 millibar. If the pressure is above 1.0 millibar then the instrument will enter a WAITING state until the pressure falls below 1.0 millibar). The advantage of this feature is that you can type BEGIN as soon as you like and then the instrument will not start counting until the pressure is sufficiently low. The upper pressure limit for data acquisition may conveniently be altered by using the command pcheck . Alternatively the user can type a camac command directly, such as cset tpg3_a2/control/hilimit=0/lolimit=-99 (the limits actually refer to the log10 of the pressure, so that the high limit of zero corresponds to a pressure of 1.0 millibar). The feature can be turned off by typing pcheck off , or cset tpg3_a2/nocontrol .
Plotting the Pressure Log
Refer to section 4.4 for information on plotting the pressure log.
Last updated on 10 Dec 2002 by Alex Hannon (a.c.hannon@rl.ac.uk)
