\documentclass{chowto}

\title{Drift Chamber Gas System Operation}
\howtotype{expert} % ``expert'', ``user'', ``reference''
%\experiment{Name of experiment} % Optional
\author{H. Fenker}
\category{general} % Subject area of this document

\maintainer{H. Fenker} % Optional
\date{December 18, 2007} % Can use \today as the argument

%To attempt to make pretty-looking pdf files...(not standard howto)

\begin{document}
\providecommand{\degg}{\ensuremath{^{\circ}\ }}

\begin{abstract}
This document provides detailed setup information for the drift
chamber gas mixing system, as well as the correct procedure for
refilling the alcohol supply and changing gas bottles. This
information is intended for use by {\bf gas system experts only.}  For
day-to-day shift worker instructions, refer to the corresponding {\it
user} howto document.

{\bf Please Note:} The Hall-C Drift Chamber Gas System was
significantly changed in early 2007. It is now a {\it pressure-driven
system} whereas it had been flow-controlled. Long-time Hall-C staff
and users will find that the system operates quite differently now.
\end{abstract}

%=====================================================================
%=====================================================================
\section{Overview}

The drift chamber gas is composed of 50\% Argon and 50\% Ethane (by
volume), bubbled through isopropanol maintained at a temperature such
that the gas mixture contains approximately 1\% alcohol vapor. The
mixing system that produces this gas is housed in the Hall-C gas shed
(Bldg. 96c). The bottles supplying the gas to the mixing system are
attached to two two-bottle manifolds outside the gas shed, within the
fenced-in gas bottle yard.

A single mixing system provides the gas for both the SOS and the HMS
detectors. Gas is delivered to the hall at a pressure of about 9 psi
(500~Torr) above atmospheric pressure.  Each detector's flow is
controlled by its own individual needle-valve with flowmeter, located
in the appropriate shield house, near the detector. The job of the
mixing system is to simultaneously maintain the delivery pressure and
mixing ratio by providing whatever total flow rate of gas (between
zero and 5.28 standard liters per minute [SLM]) is demanded by the
detectors.  This is accomplished by controlling two flow valves (argon
and ethane) so that they each flow the same volume of gas while
keeping the output pressure nearly constant. The mixing system flow
diagram is shown in Fig. \ref{fig:gas_mixer_diagram}.
%=====================================================================
%=====================================================================
\section{Gas Interlock System}
The flow of gas from the supply bottles may be automatically shut off
by normally-closed solenoid valves installed in the primary argon and
ethane manifolds. Several conditions such as over-temperature, fan
failure, gas leak, and fire alarm must all be in the non-alarm state
before these valves will open. Alarm conditions are indicated on the
gas system alarm panel on the lower-left side of the center
counting-house console.

When any of the required conditions is not satisfied the sounder on
the panel in the counting room will make an annoying noise and both
solenoid valves will close. The audible alarm may be silenced by a
toggle switch on the panel. Be certain to return it to the ``on''
position as soon as the fault is cleared.

The most confusing, but most common alarm condition is ``Low
Pressure''.  The solenoid valves will not remain open unless there is
already ample pressure on the output side of both valves. This
prevents, for example, the flow of pure ethane to the drift chambers
when the argon bottle is empty. The way to clear this condition is to
make sure there are no other faults and that both argon and ethane
manifolds are properly pressurized and fitted with non-empty bottles;
then press and hold the ``override'' button on the alarm panel for
several minutes. This button forces the solenoid valves to open even
if there are fault conditions present. If all is well, gas will flow
through the valves and clear the ``low pressure'' condition so that
the button may be released.

\begin{figure}[hbt]
\psfig{figure=drift_gas_system-gasmixer2007.eps,width=\textwidth,bbllx=12,bblly=12,bburx=750,bbury=590}
\caption{Diagram of Hall~C Gas Mixing System\label{fig:gas_mixer_diagram}}
\vspace{0.5cm}
\end{figure}
%=====================================================================
%=====================================================================
\section{Operating the Mass Flow Controller.}

The gas flow is controlled by a MKS~647 controller and mass flow
control valves.  Delivery pressure is sensed at the mixer output by a
{\it Baratron} pressure transducer.  The 647 is menu driven from a
display on the front panel using a keypad with numeric and cursor
controls for input. It features a non-volatile memory so that most of
its settings are retained even if the unit is unpowered.  The
pressure-regulation parameters do need to be reset if power is lost,
however, so the controller is powered through an uninterruptible power
supply (UPS).

The temperature of the control
system must be maintained within the
operating range of the MKS~647 (15\degg C - 40\degg C), the pressure
transducer (0\degg C - 50\degg C), and the mass-flow meters and valves
(0\degg C - 50\degg C).  Therefore,
the heating and cooling systems in the gas shed must be maintained in
working order. The temperature is indicated by the
red LED display labeled \emph{Controller Temperature},
located in the right-hand rack of the gas shed.

%=====================================================================
%=====================================================================
\subsection{Settings for Normal Operation}

A summary of all of the settings required to make the controller
function properly is given in Table~\ref{tab:mixer_nominals}. The
table also shows which screen contains each parameter. Instructions
for setting parameters are given below. Detailed instructions for
configuring and operating the MKS~647 can be found in the
manufacturer's instruction manual\cite{647C_EN_0504A1}.

\begin{table}[hbt]
\begin{minipage}[h!]{\textwidth}
{\scriptsize
\begin{center}
\begin{tabular}{|l|c|l|}
\hline
Parameter   & Set To   & {\it Controller Screen}/Comments\\ \hline
\multicolumn{2}{|c|}{\bf Manual Valves  } &  Valves are labeled        \\
Valves 1, 2, 4, 8 & OPEN                  &         \\
Valves 3, 5, 6, 7 & CLOSED                &         \\ \hline
\multicolumn{2}{|c|}{\bf Pressure PID Loop Settings   } & {\it Pressure Control} (Fig. \ref{fig:pressure_control})  \\
Pressure    & 500 Torr &                   \\
PID Mode    & AUTO     &                   \\
PID GAIN    & 4.0      &                   \\
PID INTEG   & 10.0     &                   \\
PID LEAD    & 0.000    &                   \\ \hline
\bf Mixture     & 1        & {\it User} (Fig. \ref{fig:user_display})/ Lower-right corner\\ \hline
\multicolumn{2}{|c|}{\bf Gas Composition for Mixture 1} & {\it Gas Composition} (Fig. \ref{fig:gas_composition})\\
Channel 1   & 1.000    &  (Argon)                 \\
Channel 2   & 1.000    &  (Ethane)                 \\
Channel 3   & 0.000    &                   \\
Channel 4   & 0.000    &                   \\ \hline
\multicolumn{2}{|c|}{\bf MFC Valve Size } & {\it Range Selection} (Fig. \ref{fig:range_selection})  \\
\multicolumn{2}{|c|}{\bf  / Gas}             & {\it Gas Selection} (Fig. \ref{fig:gas_selection})  \\
Channel 1   & 2.0 SLM / Ar         & provides 2.78 SLM Argon\\
Channel 2   & 5.0 SLM / C$_2$H$_6$ & provides 2.50 SLM Ethane\\
Channel 3   & \it unused&                  \\
Channel 4   & \it unused&                  \\ \hline
\multicolumn{2}{|c|}{\bf Channels ON/OFF Settings} & {\it Extended Display} (Fig. \ref{fig:extended_display})   \\
Channel 1   & ON       &                   Press ``ON 1'' (\em Argon)  \\
Channel 2   & ON       &                   Press ``ON 2'' (\em Ethane) \\
Channel 3   & OFF      &                   Press ``OFF 3'' (\em not in use) \\
Channel 4   & OFF      &                   Press ``OFF 4'' (\em not in use) \\ \hline
\multicolumn{2}{|c|}{\bf Pressure Transducer} & {\it Pressure Setup}  \\
Controller  & STD      &                   \\ 
Range F.S.  & 1000 Torr&                   \\ \hline
\multicolumn{2}{|c|}{\bf MFC Valve Controls     } & {\it Mode Selection} (Fig. \ref{fig:mode_selection}) \\
Channel 1   & PID      &                                   \\
Channel 2   & SLAVE / 1&                                   \\
Channel 3   & INDEP    &                                   \\
Channel 4   & INDEP    &                                   \\ \hline
\bf Alcohol Temp. & $2^\circ $ C & Electronic Temperature  \\
                  &              & Control Box             \\ \hline
\hline
\end{tabular}
\end{center}
}%end of \scriptsize
\end{minipage}
\caption{Normal Valve and Parameter Settings for the Gas Mixing System.
\label{tab:mixer_nominals}}
\end{table}
%=====================================================================
\subsection{General Operation of the Mass Flow Controller}
If the controller screen is dark, press {\bf ESC} to awaken the
display.  Many screens merely provide a menu of other screens you may
access: simply press the item number you desire. To go up one level in
the menu hierarchy, press {\bf ESC}. The \emph{Menu Tree} for the 647C
controller is shown in Fig. \ref{fig:command_tree}.

In general, to change a parameter displayed on the controller screen
use the {\bf left/right} arrow keys to move the cursor to the item you
wish to change. Then either use the number keys to enter the value
desired for that item (numeric parameter) or use the {\bf ENTER} or
{\bf up/down} keys to cycle a parameter through its available settings
(configuration parameter). Numeric parameters may be incrementally
modified by using the {\bf up/down} arrow keys. To make certain that a
new parameter becomes active, move the cursor off of the parameter
after you have entered the new value.

The initial menu upon startup is the {\bf Main Menu}
(Fig.~\ref{fig:main_menu}).  For normal operation use the {\bf User
Display} menu (Fig.~\ref{fig:user_display}). It shows the amount of
each gas currently flowing, the total gas flow, and the current
delivery pressure. This display also shows which of several possible
pre-defined gas mixtures is selected. These mixtures are configured 
on the {\bf Gas Composition} screen, Fig. \ref{fig:gas_composition}.
For normal operation, we use
only mixture {\bf \#1}, (number shown on the lower-right of the
display). {\bf Only on this screen can this parameter be changed.}
Mixture {\bf \#2} is usually configured to provide 100\% argon for
purging flammable gas out of the chambers.

The {\bf Extended Display} menu (Fig.~\ref{fig:extended_display})
shows actual flow, flow set point, units, valve full-scale range, gas
calibration factor, whether that channel is enabled, and whether each
channel is operating in master, slave, PID, or independent mode. This
display is most useful to a system expert wishing to verify the system
parameter settings.  Most parameters cannot be modified from this
screen, however.

Delivery pressure set-point and pressure \emph{PID-loop} control parameters
may be configured from the {\bf Pressure Control} screen
(Fig.~\ref{fig:pressure_control}).
%
\begin{figure}[tb]
\begin{center}
\framebox{
\begin{minipage}{.65\textwidth}
\footnotesize
\begin{itemize}
\item MAIN MENU (Fig. \ref{fig:main_menu})
   \begin{enumerate}
   \item [1] USER DISPLAY (Fig.\ref{fig:user_display})
   \item [2] EXTENDED DISPLAY (Fig.~\ref{fig:extended_display})
   \item [3] PRESSURE CONTROL (Fig.~\ref{fig:pressure_control})
   \item [4] DIAGNOSTICS
      \begin{enumerate}
      \item [4.1] ERROR LISTING
      \item [4.2] SIGNALS
      \end{enumerate}
   \item [5] INSTRUMENT SETUP
      \begin{enumerate}
      \item [5.1] RANGE SELECTION (Fig.~\ref{fig:range_selection})
      \item [5.2] GAS SELECTION (Fig.~\ref{fig:gas_selection})
      \item [5.3] MODE SELECTION (Fig.~\ref{fig:mode_selection}) 
      \item [5.4] ZERO ADJUST
      \item [5.5] TRIP LIMITS
      \item [5.6] GAS COMPOSITION (Fig.~\ref{fig:gas_composition})
      \end{enumerate}
   \item [6] SYSTEM SETUP
   \item [7] PRESSURE SETUP 
   \item [ ]
   \item [9] INFORMATION
   \item [0] POWER OFF
   \end{enumerate}
\end{itemize}
\end{minipage}
} %%end of \framebox{
\caption{Command Tree for the MKS-647C Control Panel}
\label{fig:command_tree}
\end{center}
\end{figure}
%
\begin{center}
\begin{figure}[hbt]
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-main_menu.eps,width=2.6in,height=1.8in}
\caption{MKS~647 Main Menu\label{fig:main_menu}}
\end{minipage}
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-user_display.eps,width=2.6in,height=1.8in}
\caption{MKS~647 User Display\label{fig:user_display}}
\end{minipage}
\end{figure}
\end{center}
%
\begin{center}
\begin{figure}[hbt]
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-extended_display.eps,width=2.6in,height=1.8in}
\caption{Extended Display Screen\label{fig:extended_display}}
\end{minipage}
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-pressure_control.eps,width=2.6in,height=1.8in}
\caption{Pressure Control Screen\label{fig:pressure_control}}
\end{minipage}
\end{figure}
\end{center}
%=====================================================================
\subsection{Gas Flow Rates}
\label{sec:gas_flow_rates}
The flow rates are adjusted automatically by the controller in order
to maintain a constant delivery pressure at the output. Only the flow
{\em ratios} should be set by the operator. We use a 1:1 ratio, set on
the {\bf Gas Composition} screen (Fig.~\ref{fig:gas_composition}), as
indicated in Table~\ref{tab:mixer_nominals}.

The average total flow should equal the sum of the flows to all of the
detectors in the shield house. (Note that the ball-type flowmeters in
the shield house are calibrated for nitrogen. The approximate
multiplier to convert these readings for 50/50 Argon-Ethane is 0.9 .)

System configuration parameters specifying the full-scale flow
capacity (for nitrogen) of each valve, the types of gases actually
flowing through each valve, and the mode of control for the valves are
set in the screens pictured in Figs.~\ref{fig:range_selection},
\ref{fig:gas_selection}, and \ref{fig:mode_selection}. These figures
show the nominal settings for the Hall-C system.

\begin{center}
\begin{figure}[hbt]
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-gas_composition.eps,width=2.6in,height=1.8in}
\caption{Gas Composition Screen\label{fig:gas_composition}}
\end{minipage}
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-range_selection.eps,width=2.6in,height=1.8in}
\caption{Range Selection Screen\label{fig:range_selection}}
\end{minipage}
\end{figure}
\begin{figure}[hbt]
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-gas_selection.eps,width=2.6in,height=1.8in}
\caption{Gas Selection Screen\label{fig:gas_selection}}
\end{minipage}
\begin{minipage}{2.7in}
\psfig{figure=drift_gas_system-mode_selection.eps,width=2.6in,height=1.8in}
\caption{Mode Selection Screen\label{fig:mode_selection}}
\end{minipage}
\end{figure}
\end{center}

%=====================================================================
\subsection{To set the Delivery Pressure:}

Navigate to the {\bf Pressure Control} menu. The pressure set-point (in
Torr) is indicated at the bottom-center of the screen. This value
should be se to 500.0. Note that the system can not respond instantly
to a change in requested gas pressure: it has no way to release excess
pressure and must wait for the detector systems to consume it
\footnote{An \emph{over-pressure relief valve} releases gas through a
small oil bubbler in the gas shed if the pressure exceeds about
600~Torr.}; it cannot build up pressure any faster than the
flow-control valves can supply it. It may take thirty minutes or so
for the pressure regulation system to stabilize at a new set-point or
stabilize in response to a change in total gas consumption. However,
you should be able to observe a change in the gas flow within a few
seconds (possibly up to a minute) after a set-point change.

%=====================================================================
\subsection{To turn gas flow on or off:}

The gas flow can be turned on or off while in any menu.  In the
Extended Display menu the bottom line displays ``ON'' or ``OFF'', by
channel, to show which mass flow valves are enabled.  ``ON" must be
displayed in the bottom row of the Extended Display menu for gas to be
flowing in a particular channel.

For gas to flow, two conditions must be met: 
\begin{enumerate}
\item Each channel (1 and 2) must be enabled by pressing ``ON" and then
the desired channel number.
\item The entire system must be enabled by pressing
``ON" and then ``ALL" from the keypad. 
\end{enumerate}
Thus, each valve can be controlled individually using ``ON/OFF~-~\emph{Channel Number}'',
or all flow can be controlled using ``ON/OFF~-~ALL''. If the system
is enabled, the status line at the bottom of every screen will
indicate ``FLOW ON GAS ON''. {\bf Note:} because Valve-2 (ethane) is 
normally slaved to Valve-1 (argon), when Valve-1 is disabled there will
be no flow through Valve-2.

%=====================================================================
%=====================================================================
\section{To Change a Gas Bottle}

The argon and ethane supply bottles should be replaced by new (full)
bottles when the bottle content drops below about 10\% of its
capacity.  For argon, the bottle content is directly indicated by the
bottle pressure: a new bottle usually contains 2000 to
3000~psig. Argon bottles should be changed whenever the bottle
pressure is found to be below about 200~psig. 

Ethane bottles, on the
other hand, contain liquefied ethane. Thus the bottle pressure is just
the vapor pressure of ethane at whatever the current temperature
happens to be. At 70\degg F this is about 544~psig. The pressure gauge
will not tell you how much ethane is left in the bottle until it reads
zero! Instead, we measure the ethane content by observing the weight
of the bottle and comparing it to the weight when the bottle was
full. A standard B-size cylinder contains about 32~pounds of ethane.
The ethane cylinders on the manifold sit on scales which have been 
pre-set to indicate the net weight of ethane in the bottle.
Numbers in the green portion of the dial indicate ethane remaining.
If the indicator points to the red portion of the dial, the bottle
is empty.

Handling and connecting bottles of compressed gas require special
knowledge.  The high pressure gas stored in the cylinders (bottles)
constitutes significant stored energy. Mishandling of a gas bottle can
pose a lethal hazard! Refer to the JLab ESH\&Q Manual\cite{bi:jlabehs}
for safe handling practices. If you do not already know how to safely
manipulate compressed gas hardware, have a knowledgeable person train
you.

After attaching a new gas bottle to the supply manifold, check the
connection for leaks using \emph{Snoop} or a similar leak detector.

%=====================================================================
%=====================================================================
\section{The Alcohol Bubbler}

To reduce the rate of aging of the wire chambers, the operating gas
contains a small quantity of alcohol vapor. The vapor is added by
bubbling the argon/ethane mixture through liquid alcohol. The
temperature of the alcohol controls the alcohol vapor pressure, which
determines the amount of vapor added to the gas. The alcohol content
also affects the electron drift velocity in the wire chambers, so it
must be held approximately constant.

Gas is bubbled through the liquid alcohol inside the glass
dome vessel in the refrigerator. The dome is covered by a
perforated steel cylinder as a precaution against breakage. The
alcohol level is controlled by a float valve inside the metal cold
reservoir, which is also inside the refrigerator. As long as there is
alcohol in the warm reservoir (sitting on top of the refrigerator),
the liquid levels inside the refrigerator will remain constant. A
drain valve (\#7) inside the refrigerator is available for emptying
all liquid from the system.  It is for use by experts only and should
remain closed during normal operation.

{\bf The reservoir should be refilled before it becomes empty to
maintain a head of liquid over the float valve.  This will prevent air
from entering the system.}
%=====================================================================
\subsection{To by-pass the alcohol system}

Open valve {\bf 3}, then promptly close valves {\bf 1 \& 2}, in that order!

\noindent To restore flow through the bubbler, open valves {\bf 1 \& 2} then
close valve {\bf 3}.

%=====================================================================
\subsection {Alcohol Temperature Control}

To keep the alcohol temperature (and thus the vapor pressure) constant,
the alcohol bubbler is housed in a refrigerator which is controlled by
an electronic temperature regulator having 1~C\degg  sensitivity. The 
controller is located on a shelf in the right-hand rack of the gas mixing
system. Normally, the actual temperature in the refrigerator is
indicated on the front panel of the controller. The controller should
be set to maintain a temperature of 2\degg  C.

%=====================================================================
\pagebreak[4] 
\subsection{Step-by-Step Instructions for Refilling the Alcohol Bubbler}
{\em These 13 steps must be individually completed in the order listed!}
Refer to Fig.~\ref{fig:gas_mixer_diagram}.\\

\begin{minipage}{\textwidth}
\begin{enumerate}
\footnotesize{
\item{Remove the three screws securing the cover on the alcohol fill-tank 
(the top-most metal tank at the rear of the mixer rack. Carefully remove the lid
without allowing dust or dirt to fall into the tank.}
\item{Fill this tank to about 75\% full from a bottle of 2-propanol.}
\item{Replace the lid and retaining screws.}
\item{Remove the small brass cap which seals the port on the lid.}
\item{Isolate the warm reservoir by {\bf closing valves 4 and 8.}}
\item{Release pressure in the warm reservoir by {\bf slowly opening valve 6 fully.}}
\item{{\bf Open valve 5 fully} to begin the flow of alcohol from the fill-tank to the warm reservoir.}
\item{{\bf Monitor the level} of alcohol in the warm reservoir sight-tube.}
\item{When the level is 2~cm from the top of the sight-tube, {\bf close valve 5.}}
\item{Replace the brass cap on the lid of the fill-tank.}
\item{{\bf Close valve 6, {\it then} slowly open valve 4.}}
\item{{\bf Open valve 8.}}
\item{Record what you did in both the gas logbook and the electronic logbook.}
} %end of scriptsize 
\end{enumerate}
\end{minipage}

%=====================================================================
%=====================================================================
\section{Gas Filters Maintenance}

There are gas filters on the argon and ethane supply lines just inside
the gas shed. These filters should be replaced on a regular schedule.
See Bill Vulcan for details.

%=====================================================================
%=====================================================================
\section{Secure Pressure Regulators}

The gas mixing system is protected from failure or mis-setting of the 
primary pressure regulators (the ones mounted on the manifolds on the
exterior of the gas shed -- near the bottles) by {\it hidden} regulators
mounted just inside the gas shed. It is these regulators which actually
set the maximum supply pressure to the mixing valves. These regulators
should {\em never} be adjusted by other than a gas system expert! The
nominal secondary pressure supplied by both the argon and ethane
secure regulators is 30 psig.

%=====================================================================
%=====================================================================
\section{Ethane Flow Restrictor}

A calibrated flow-restricting orifice is installed at the outlet of
the main ethane high-pressure manifold outside the gas shed. This
orifice passively limits the flow rate of ethane into the gas shed
even if there is a catastrophic failure of the flow and pressure-controlling
devices inside the shed. It is model {\tt IC-DM4-9-SS} manufactured by
{\em O'Keefe Controls Co.}, Trumbull, CT, USA. While it may look like
simply a stainless-steel union fitting, it is in fact a precision part,
and a necessary component of the gas safety system. It limits the flow
of ethane to less than 26.5 SLM, which is a little
less than ten times the maximum capacity of the flow-control system.

%=====================================================================
%=====================================================================

\raggedright  %this fixes formatting of bibliography, too.

\section{Related {\it Howtos}}
\begin{itemize}
\item MKS~647 Mass Flow Controller Manual \cite{647C_EN_0504A1}
\item Base Equipment Shift Checklist Items \cite{howto:base_equip_checklist}
\end{itemize}

\end{document}

% Revision history:
%  1st draft by Howard Fenker 27FEB03 -- taken from existing ops manual.
%  Rev. 1.1 - added notes on bottle changing, ethane bottle pressure.
% $Log: drift_gas_system.tex,v $
% Revision 1.6  2008/01/02 19:48:54  saw
% Move fontenc and pslatex packages to the chowto style.
%
% Revision 1.5  2008/01/02 16:46:03  saw
% Latest update
%
