%Gas Handling System manual                                 Steve Chappell

\chapter{The Gas \& Vacuum Handling System}
\section{Overview \& System Description}

These notes are designed to give an outline of the procedures to be
followed at the ANU when operating the MEGHA gas \& vacuum handling
system.  Due to the complexities of the system there are of course a
number of different ways of achieving the same result when cycling
MEGHA. Indeed, the inbuilt flexibility that allows for many different
modes of operation means that only certain of the procedures below will
be applicable to any one experiment. {\bfseries\itshape It is therefore
advised that operators of this system use these notes only as a guide
and use their own logic when making command decisions!}

The gas rack is formed from two 19" racks which are bolted together to
form one continuous internal space. This contains the vacuum control
and gas handling systems. The main electrical power enters the rack on
the right hand side via a 30A connector. It then goes to a power
control unit for distribution. This unit contains three contactors and
associated connectors to run the three vacuum pumps plus a 13A
connector for the mains distribution board. The Programmable Logic
Controller (PLC), vacuum gauges and pump controllers are powered from
this board.

Figure \ref{gasrack} shows the schematic diagram of the gas \& vacuum
handling system that appears on the front panel of the control
rack\footnote{Please remember that this diagram {\itshape is} schematic
and does {\itshape not} always show the physical location of pipework
connections. For example, the roughing line is usually connected to the
top of the chamber}. MEGHA consists of three distinct volumes:  The
target chamber A and the volumes made up by the detector modules B \& C.
Each of the modules can be connected in parallel as belonging to either
volume B or volume C using the gas manifold mounted to the rear of the
chamber. Any modules not required may be replaced by a blanking plate
attached to the vessel A.  The two volumes B and C may be operated
independently to give, for example, two different gas pressures in two
sets of modules simultaneously. If only one gas pressure is required
then all modules may be connected in parallel to form one volume B or C.
On the schematic diagram the module volumes are connected across the two
sets of boxes ``B/C Vacuum'' and ``B/C Gas''. The former (B/C Vacuum)
indicates connection to the backing pressure which draws gas from B/C
Gas through the modules. The vacuum and gas pipework in the figure is
indicated by solid lines and the electrical wiring by dashed lines.
Logical control of the pneumatic valves and pumps is provided by
pushbuttons on this panel labelled with the valve identifications VA1 to
VA20. VA1, VA2 and VA3 are the large plate valves connecting the chamber
to the beamline and turbo pump ``units'' TPU1 and TPU2. These units
consist of a turbo pump plus rotary backing pump. RP3 is the backing
rotary pump.
\begin{figure}
\centerline{\psfig{file=./figs/gasflo.ps,height=16cm}}
\vspace{2.0cm}
\caption{MEGHA gas \& vacuum flow logic.}
\label{gasrack}
\end{figure}

The Programmable Logic Controller forms the heart of the system
separating the pushbuttons and the user from the solenoid operated
pneumatic valves. The PLC uses ``Ladder Logic''\footnote{Further
information on Ladder Logic and programming the PLC can be found in the
appendices to this manual.} to process status information before
allowing the valves and pumps to be switched.  If the user attempts to
open or close a valve or switch a pump using the pushbuttons when it is
foolish to do so they will be rewarded with a beep from the gas control
alarm and no operation will be performed. Once a valve has been opened
succesfully {\itshape i.e. confirmed to have ACTUALLY opened by the PLC
monitoring the status of switch sensors at the valve}\footnote{Note
that VA13, VA16, VA17, VA18 and VA19 do not have switch sensors at the
valve. VA2 and VA3 have two switches, one closes on valve open, the
other on valve close. The status of VA1 is given by the control unit
adjacent to the beamline. The remaining valves have a single switch
which makes contact on valve open.} the relevant pushbutton will be
lit. The pump control buttons (TPU1, TPU2 and RP3) are latched with
distinct on (pressed in) and off (out) positions. These buttons do not
connect to the PLC. They simply connect the pumps directly to the power
control unit which turns the pumps on or off.

{\bfseries Note that no protection is implemented for the
pumps other that that provided by the turbo pump control units}.

The system also incorporates four manual needle valves NV1 to NV4. The
controls for NV1 to NV4 are mounted immediately below the schematic
diagram. NV4 is used to regulate the flow of the N$_2$ ``gas purge''
from the modules. If there is insufficient pressure at the pressure
switch PS2, VA13 will not open. NV2 and NV3 allow adjustment of the
pumping pressure and N$_2$ admit to the module volumes and NV1 a similar
function on the chamber volume. Needle valves NV1 to NV3 may be
bypassed using the valves VA6 to VA8.

Two gas bottles are indicated on the diagram labelled A and B with a set
of status lights under PLC control. Bottle A is the ``Master'' cylinder
and B the ``Slave''. Gas is delivered to the gas flow system through
VA14 to VA19. The PLC automatically selects which bottle to place
``on-line'' by inspecting the status of the pressure switches PS3 to
PS6. If there is insufficient pressure at PS1 VA16 will not open. If the
gas pressure in bottle A is not above the threshold set on PS2, VA18
will not open. If it is not below the threshold set on PS6, VA19 will
not open. Similarly, the pressure provided by bottle B must fall between
the limits set by PS3 and PS4 before VA17 will open. Pressure
transducers PT1 and PT2 measure the gas pressure and feedback to control
valves CV1 and CV2 which adjust the gas flow supply.

Four pressure gauges are included in the system. GP1 and GP2 are of the
Pirani type and measure the target chamber and backing pressures
respectively. GC1 and GC2 are cold cathode sensors monitoring the target
chamber and beamline at high vacuum.

The gas flow controls, pressure readouts and Turbo pump status
indicators are mounted in the left hand side of the control rack.

\section{Important Notes}

{\bfseries 1)} A vacsorb unit is required for roughing.\\
{\bfseries 2)} The small turbo pump (TPU1) is now mounted on the downstream beamline.\\
{\bfseries 3)} The target chamber, downstream beamline and upstream
beamline to the valve VA1 form a single volume, albeit with large
impedances to gas flow through the collimation system and the chamber
exit pipe.\\
{\bfseries 4)} The beamline valve VA1 is controlled by a unit adjacent
to the beamline where it is designated as ``Valve A''. The status of
this valve is given by the Normally Open pair connected to terminals 7
and 8 of the protection unit. This is indicated by the LED behind the
VA1 button on the MEGHA gas rack.\\
{\bfseries 5)} A Normally Open Pair connects to the Valve A and
beamline protection unit terminals 3 and 4. This pair closes and
indicates ``Good'' Target Vacuum for GC1 pressures below the set point
on the MEGHA vacuum gauge controller.\\
{\bfseries 6)} Undesirable pressure fluctuations were found when
attempting to rough the MEGHA system with carbon vane pumps. It is
therefore inadvisable to use them for roughing when modules with mylar
windows are in place.\\
{\bfseries 7)} At the time of writing no provision had been made to
connect gauge GC2 to the beamline. The beamline pressure can be
calculated by reading the appropriate ion pump currents.\\
{\bfseries 8)} The Automatic Bottle change system was also found to be
highly sensitive to slight changes in gas flow. The system should be
set to manual and Bottle B used in preference to Bottle A until further
notice.\\
{\bfseries 9)} There is no mesh over the chamber port sealed by VA3. To
prevent screws flying into the turbo pump when this valve is opened
ensure that none are left loose in the chamber!

\section{Alarms}
If a failure occurs the control system will automatically attempt to
shut ALL valves. The failsafe for the pneumatic valves VA4 to VA20 is
in the shut position. They require compressed air pressure to remain
open so will always close on a compressed air fail. Unfortunately, this
is NOT true of the large area plate valves VA1 to VA3 which require
compressed air to {\itshape change} position and CAN remain in the open
OR closed position on a compressed air fail.

\subsection{Procedure on Hearing the Alarm}

All valves should have been closed automatically. The
user is prevented from operating the valves at this point.

On hearing the buzzer press the reset button ONCE to inhibit the
annoying buzzer noise unless you like it! Take note of the panel
display. Valves that were open when the failure occured will flash. If
you are satisfied that the problem has been resolved press the reset
button again to clear the alarm condition. If all is well the panel
LED's will stop flashing, the gas supply system will activate and you
can start opening valves again.  If not the alarm will persist.\\
\\
Please Remember to:\\
{\bfseries\itshape 
1) Protect the beamline!\\
2) Ensure biases are off all the detectors!\\
3) Check pumps!\\
4) Always turn off the Vacuum Gauge Controller before disconnecting gauges.\\
5) Leave the Turbo Pump Controller Units switched on at
the units themselves.}

\section{Gas Control Power Up}

When starting the system from scratch,\\
\\
{\bfseries\itshape
Ensure that...\\
1) Mains Power is Connected and ON.\\
2) The Compressed Air Line is attached and ON.\\
3) N$_2$ Supply pipework is connected and ON.\\
4) Pumps TPU1 and TPU2 (both turbo and rotary) are connected with
switches OFF on the gas rack panel but ON at the pumps and controllers
themselves.\\
5) Air supply to VA2 and VA3 is connected and ON. Note VA1 is now
connected to the controller adjacent to the beamline. The connections
on the gas rack should always be blanked off.\\
6) Water switch cables from VA2 and VA3 are connected. No connections
are required to the VA1 sockets.\\
7) Backing Pump RP3 is OFF on the gas rack panel but ON at the pump.\\
8) A Vacsorb unit is available.\\
9) Pressure Guages are present and cabling is attached.\\
10) Gas flow manifold and Module pipework is connected.\\
11) Gas bottles are connected (open hand valves and check pressure reading).\\
12) Manual Override is DEACTIVATED.\\
13) The PLC RUN key is set to STOP.\\}
\\
Note the status of:\\ 
{\bfseries\itshape 
a) Downstream beamline! : Is beamline there?!, Pumps ON?, Pressure?\\
b) Upstream beamline! : Pumps ON?, Pressure?, CLOSE VALVES.\\
}\\
{\itshape Go Ahead press the green power ON button.}\\
All fuse lights on the PLC front panel should illuminate.\\
{\itshape Now start the PLC program by turning the PLC RUN Key to RUN.}\\
ALL control panel LED's should light and the buzzer sound.
{\bfseries\itshape If this does not happen Seek Help BEFORE proceeding:
the incorrect software may be loaded into the PLC or some LED's/the
buzzer may be faulty.}\\
In the unlikely event that everything has worked the control
display should indicate:\\
{\bfseries 1)} which gas bottle has been automatically placed on-line by
opening either VA18 and VA19 or VA17.\\
{\bfseries 2)} that all other valves are closed.\\
{\bfseries 3)} that the compressed air supply is connected.\\
\\
You can be sure that if there is a problem lights will flash and the
alarm will sound!

\section{Gas \& Vacuum Cycling Procedures}

The general procedures described in this section assume a standard setup
where modules are operated with both gas control channels B and C. It
should be clear that for experiments using only channel B, that the
channel C controls and valve operations can be ignored.  Similarly, for
measurements not requiring gas control at all one can follow these
procedures and ignore all reference to operations of the gas cycling
system. Some of the possible variations and corresponding amendments to
the standard procedure are listed below.

\subsection{Variations on the Standard Procedure}
\subsubsection{Gas Channel C NOT in Use}
e.g. All modules connected in parallel as volume B.\\ 
Valves VA4, VA7, VA10, VA15 and NV2 should always remain closed.\\
Gas control valve CV2 should be kept closed.
\subsubsection{Gas Channel B NOT in Use}
e.g. All modules connected in parallel as volume C.\\
Valves VA5, VA8, VA11, VA14 and NV3 should always remain closed.\\
Gas control valve CV1 should be kept closed.
\subsubsection{No Gas Volumes Required}
e.g. For MEGHA strip detector array experiments.\\
Apply the above two amendments ensuring that VA16 also remains closed.\\ Disconnect the gas bottles by closing the regulator hand valves.

\subsection{Evacuation}
\label{evac}
{\bfseries\itshape Ensure biases are off all the detectors!}\\
{\bfseries\itshape Protect the beamline!}\\
{\bfseries\itshape Ensure chamber is sealed!}\\
\\
{\bfseries 1) Starting from:}\\
VA4, VA5, VA6, VA7, VA8, VA9, VA10, VA11, VA20 Open.\\
Remaining valves closed including needle valves!\\
{\itshape Nb: VA17 to VA19 are controlled by the automatic gas feed.
The pipework from VA14 to CV1 and VA15 to CV2 should be at good vacuum
from the previous gas purge.}\\
Connect the rotary pump RP3.\\
{\bfseries\itshape Monitor the backing pressure on GP2 and the chamber
pressure on GP1 then GC1.}\\
\\
{\bfseries 2) At the Atmospheric Pressure of the day:}\\
Make a note of the various gauge readings. You will need to vent the
chamber back up to within a few torr of these values.\\
Close VA6, VA7, VA8, VA10, VA11, VA20. \\
Only the volume A-B-C pressure equalisation valves VA4 and VA5 and the
roughing line valve VA9 should be open.\\
Start RP3, Open Hand Valve and open VA12.\\
Slowly open NV1 to pump the combined volumes A, B and C
{\bfseries\itshape at a rate of 0.1 to a few times 0.1 torr per second}
to prevent significant pressure differences between the volume A and
volumes B and C. Pumping in this manner will cause the evacuation of B
and C to lag behind that in A, keeping the windows pressured in the
right direction (i.e. outward).\\
Open NV1 further as the pressure drops to maintain the rate of
decrease. {\bfseries If the pressure differential between the volumes
becomes greater than a few torr, close NV1, wait for the pressure to
equalize, then proceed.}\\
\\
{\bfseries 3) At $<50$ torr:}\\
Close NV1, VA12 and RP3 Hand valve.\\
Open the needle bypass valve VA6.\\
Open VA12.\\
Slowly Open RP3 using it to control the rate of decrease.\\
\\
{\bfseries 4) At $<1$ torr:}\\
Switch from RP3 to vacsorb. Close VA12 and RP3 Hand valve. Open
Vacsorb. Open VA12.\\
\\
{\bfseries 5) At $<10^{-1}$ torr:}\\
Close the pressure equalisation valves VA4 and VA5.\\
Close VA6, VA9 and VA12.\\
Ensure small turbo unit TPU1 is operational and at speed.\\
Open the small turbo valve VA2.\\
Ensure large turbo unit TPU1 is operational and at speed.\\
Open the large turbo valve VA3.\\
\\
{\bfseries 6) Continue to pump modules using the backing line:}\\
Open RP3 hand valve then VA12.\\
Open VA10 and VA11.\\
GP2 should be $<10^{-1}$ torr before opening VA7 and VA8.\\
\\
{\bfseries 7) At chamber pressure $<10^{-5}$ torr:}\\ 
Ensure that the pressure in the
beamline is Good. Check GC2 and ion pump gauges.\\
Open the beamline VA1.

\subsubsection{Example Pump Down Curves}
An example evacuation sequence for the MEGHA system is shown in Table
\ref{ptimes} and Fig. \ref{pcurve}. These data were measured for 3
modules connected as volume B. The pressure transducer PT2 (normally
connected to volume C) was connected so as to monitor the chamber
pressure in the range:  atmosphere to zero torr. Note that a confident
user may well be able to evacuate faster than this {\itshape hopefully}
foolproof scheme!
\begin{table}
\begin{center}
\begin{tabular}{lll}\hline
{\bfseries Event No.} & {\bfseries Time (mins)} & {\bfseries Operations}\\
\hline \hline
0 & Start Point & VA5, 6, 8, 9, 11, 17, 20 OPEN. Rest Closed.\\
  &     &  Turbo's at Full speed. RP3 On.\\
1 & 0 & Open RP3 Hand Valve.\\
  &   & Close VA6, VA8, VA11, VA20.\\
  &   & Open VA12. Crack Open NV1.\\
2 & 12 & NV1 Now Fully Open.\\
3 & 40 & PT1 and PT2 equal.\\
4 & 160 & RP3 Hand Valve Closed.\\
  &     & VA6 Open. Crack Open RP3 Hand Valve.\\
5 & 170 & Switch from RP3 to Vacsorb.\\
6 & 173 & Close VA5.\\
7 & 175 & Close Vacsorb Hand Valve.\\
  &     & Close NV1, VA6, VA9, VA12.\\
  &     & Open RP3 Hand Valve.\\
  &     & Open VA11, VA12.\\
8 & 179 & Open VA2 then VA3.\\
9 & 186 & Open NV3 Fully.\\
10 & 189 & Close NV3. Close RP3 Hand Valve.\\
   &     & Open VA8. Crack Open RP3 Hand Valve.\\
11 & 244 & Open VA1 (Valve A to beamline). Valve B in closed position.\\
12 & 499 & Open Valve B to the accelerator.\\
\hline
\end{tabular}
\end{center}
\caption{Event list for Example Evacuation Sequence.}
\label{ptimes}
\end{table}

\begin{figure}
\special{psfile=./figs/pcurve.ps hoffset=-40 voffset=-520 hscale=80 vscale=80}
\vspace{18.0cm}
\caption{Example Pump Down Curves.}
\label{pcurve}
\end{figure}

\subsection{Introducing Gas}
{\bfseries\itshape Ensure biases are off all the detectors!}\\
{\bfseries\itshape Protect the beamline Close VA1!}\\
{\bfseries\itshape Protect the turbo pumps Close VA2 and VA3!}\\
\\
{\bfseries 1) Starting from Evacuated Volumes A,B and C.}\\ Valves as
at end of Evacuation procedure and the above precautions: VA7, VA8,
VA10, VA11 and VA12 are open.  Rest CLOSED including needle valves!
{\itshape Nb: VA17 to VA19 are controlled by the automatic gas feed}.\\
{\bfseries\itshape Monitor the backing pressure on GP2 and the chamber pressure
on GC1.}\\
\\
{\bfseries 2) Check the Gas feed system.}\\
The gas feed valves VA14 and VA15 should be closed.\\
VA18 and VA19 or VA17 should be open. VA16 should be closed.\\
Ensure Gas Controller 1 is operational and CV1 is CLOSED.\\
Ensure Gas Controller 2 is operational and CV2 is CLOSED.\\
The controllers should be set to INT and on the 10V range. Their
potentiometer dials should be wound to zero. The Phase lead and gain
are typically set to 6 secs and 95\%, respectively.\\
Open VA16.\\
\\
{\bfseries 3) Admit gas to volume B:}\\
{\bfseries\itshape Monitor Gas B pressure on PT1.}\\
Close VA8.\\
Open the gas admit valve VA14.\\
Dial in the required set point pressure.\\
Switch Gas Controller 1 to MANUAL. Use the potentiometer to slowly open
CV1 and bleed gas into volume B.\\
{\bfseries\itshape The flow rate should not exceed 0.1 to a few times 0.1 torr per second.}\\
{\bfseries\itshape Watch closely PT1 and GC1 for any indication of a gas
leak across the intermediate window. e.g. a rise in volume A pressure.}\\
\\
{\bfseries 4) At a torr or so above the required Gas B pressure:}\\
Switch CV1 to CLOSE. Wind potentiometer to zero.\\
Check backing pressure GP2.\\
Adjust NV3 for a suitable pump out rate measured on PT1.\\
{\bfseries\itshape The flow rate should not exceed 0.1 torr every few
seconds or so.}\\
At approximately the desired pressure. Turn Gas Controller 1 to AUTO.
The pressure should quickly settle down to cycle about the required set
point value.\\
\\
{\bfseries 5) Admit gas to volume C:}\\
{\bfseries\itshape Monitor Gas C pressure on PT2.}\\
Close VA7.\\
Open the gas admit valve VA15.\\
Dial in the required set point pressure.\\
Switch Gas Controller 2 to MANUAL. Use the potentiometer to slowly open
CV2 and bleed gas into volume C.\\
{\bfseries\itshape The flow rate should not exceed 0.1 to a few times 0.1 torr per second.}\\
{\bfseries\itshape Watch closely PT2 and GC1 for any indication of a gas
leak across the intermediate window. e.g. a rise in volume A pressure.}\\
\\
{\bfseries 6) At a torr or so above the required Gas C pressure:}\\
Switch CV2 to CLOSE. Wind potentiometer to zero.\\
Check backing pressure GP2.\\
Adjust NV2 for a suitable pump out rate measured on PT2.\\
{\bfseries\itshape The flow rate should not exceed 0.1 torr every few
seconds or so.}\\
At approximately the desired pressure. Turn Gas Controller 2 to AUTO.
The pressure should quickly settle down to cycle about the required set
point value.\\
\\
{\bfseries 7) Ready to Run?!}\\
Open the chamber to the small (VA2) then large (VA3) turbo pumps.\\
Ensure that the chamber pressure GC1 is $<10^{-5}$ torr.\\
Ensure that the pressure in the beamline is Good. Check GC2.\\
Open chamber to the beamline (VA1).

\subsection{Venting}
{\bfseries\itshape Ensure biases are off all the detectors!}\\
{\bfseries\itshape Protect the beamline Close VA1!}\\
\\
{\bfseries 1) Starting from Evacuated volume A, Gas filled volumes B \&
C.}\\
Valves as at end of Introducing Gas procedure: VA2, VA3, VA10, VA11,
NV2, NV3, VA12, VA14, VA15, VA16 open.  Rest CLOSED including remaining
needle valves! {\itshape Nb: VA17 to VA19 are controlled by the
automatic gas feed}.\\
\\
{\bfseries 2) Shut off Gas Supply:}\\
Set Gas Controller 1 and 2 to CLOSE CV1 and CV2.\\
Close VA16.\\
\\
{\bfseries 3) Evacuate volumes B and C}\\
Adjust NV2 and NV3 to evacuate gas from B and C.\\
\\
{\bfseries 4) At Volumes B \& C $<10$ torr:}\\
Open VA7 and VA8.\\
Close NV2 and NV3.\\
\\
{\bfseries 5) At Volumes B \& C $<10^{-1}$ torr:}\\
Close VA14 and VA15.\\
Close VA7 and VA8.\\
Close VA10 and VA11.\\
Close VA12.\\
Close the turbo pump valves VA3 and VA2.\\
Open the volume A-B-C pressure equalisation valves VA4 and VA5.\\
Open VA9 and VA13.\\
Slowly open NV4 and NV1. {\bfseries NV4 is not a needle valve in the present system so care must be taken not to open this valve too fast.}
Admit N$_2$ into the combined volume A-B-C.\\
{\bfseries\itshape Ensure no large pressure differential occurs across
the thin windows.} The rate of pressure increase should not exceed a
few 0.1 torr a second, i.e. the reverse of the evacuation procedure.\\
\\
{\bfseries 7) At Volumes A-B-C $1-3$ torr above atmospheric:}\\
Close NV1 and NV4.\\
Close VA13.\\
Open VA20.\\
Slowly Open NV1 then the bypass valve VA6 to allow pressure to decrease
to atmospheric.\\
You may now open the chamber.

\subsection{Changing Targets}

Close VA1 before winding the target mechanism up or down to the
required target.

\subsection{Replacing Targets Under Vacuum}

{\bfseries\itshape Protect the beamline Close VA1!}\\
{\bfseries\itshape Protect the turbo pumps Close VA2 and VA3!}\\\\
{\bfseries 1) Starting from Evacuated volume A, Gas filled volumes B \&
C.}\\
Valves as at end of Introducing Gas procedure and the above
precautions: VA10, VA11, NV2, NV3, VA12, VA14, VA15, VA16 open.  Rest
CLOSED including remaining needle valves! {\itshape Nb: VA17 to VA19
are controlled by the automatic gas feed}. As a further precaution you may wish to remove the gas from the modules before proceeding.\\
{\bfseries\itshape Monitor the chamber and gas pressures on GC1, PT1 \& PT2.}\\
\\
{\bfseries 1) Venting the target tube assembly:}\\
Ensure that a carbon vane/vacsorb unit is connected to the exchange
tube before evacuating the chamber!\\
Wind target ladder to its lowest setting.\\
Connect N$_2$ supply line to the carbon vane/vacsorb unit.\\
Bleed N$_2$ into the target tube until it reaches atmospheric pressure.\\
Open the target tube \& remove the target ladder.\\
Reload the target ladder \& replace inside the target tube.\\
Close target tube.\\
\\
{\bfseries 2) Returning target ladder to Evacuated volume A:}\\
Evacuate target tube using carbon vane/vacsorb unit.\\
\\
{\bfseries 3) At target tube pressure $<10^{-2}$ torr:}\\
Isolate from the carbon vane/vacsorb unit.\\
Wind target ladder up to operating position.\\
Open chamber to the turbo pumps. Open VA2 and VA3.\\
\\
{\bfseries 4) At chamber pressure GC1 $<10^{-5}$ torr:}\\
Ensure that the pressure in the beamline is Good. Check GC2.\\
Open to the beamline VA1.

\section{The Gas Bottles}
These are located behind the lower left panel of the gas control rack.
For access to the bottles undo the screws on the front and remove this
panel.

If the master gas bottle A empties, VA16 will automatically close. VA18
and VA19 will then shut and the empty LED will light.  Bottle B will go
on-line if it is present and contains gas. Should you wish to change a
gas bottle follow the procedure below.

{\bfseries\itshape Note that it is advisable to close VA16 before
changing the gas bottles. Adjustments to the needle valves NV2 and NV3
and gas control potentiometers may also be needed during automatic or
manual bottle changes.}

\subsection{Changing the Slave bottle B}

Bottle B may be replaced providing VA17 is shut. Check the pressure
reading from the bottle. The pressure switches and indicator lights may
be trying to fool you into believing that the bottle is empty! Close the
hand valve on the bottle regulator.  Replace the bottle. Open hand
valve. Check pressure reading.

\subsection{Changing the Master bottle A}

Bottle A may be replaced providing VA18 and VA19 are shut. Procedure the
same as for bottle B.

{\itshape Nb: The PLC will automatically select which gas bottle to
place on-line after opening the hand valve if running in the AUTO
mode.}

\section{Shutdown}

{\bfseries 1)} Purge Gas from the modules and vent the chamber.\\
{\bfseries 2)} Ensure that all valves are closed, especially VA1 to VA3. {\itshape Nb: VA17 to VA19 are controlled by the automatic gas feed and may remain open}.\\
{\bfseries 3)} Deactivate the pumps TPU1, TPU2 and RP3.\\
{\bfseries 4)} Turn the PLC RUN key to STOP.\\
{\bfseries 5)} Press the red power OFF button.

The remaining valves should shut automatically. If the system is to be
left for some time close the hand valves on the gas bottle regulators,
the compressed air and N$_2$ inlets. Isolate the control rack and pumps
TPU1, TPU2 and RP3 from the mains supply.

\section{Manual Override}
This key allows the automatic gas feed to be set to manual. The level of
protection for the rest of the system remains the same. Particulary
strange requests such as venting the beamline will still not be granted.
You'll just have to rewrite the software to do that!
