\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{Name of person maintaining document} % Optional
\date{April 4, 2003} % Can use \today as the argument


\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.
\end{abstract}

\section{Overview}


The drift chamber gas is composed of 50\% Argon and 50\% Ethane (by volume),
bubbled through isopropanol at 1\degg C. This results in gas containing
approximately 1\% alcohol vapor. The mixing system that produces this
gas is housed in the gas shed. 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.

\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 overtemperature, 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
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 us from flowing, for example,
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 for several
seconds. 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 many be released.

\section{Settings for Normal Operation}

Refer to the gas system flow diagram, Fig. \ref{fig:gas_mixer_diagram}
\begin{figure}
\psfig{figure=drift_gas_system-mixer_diagram.eps,width=6in,bbllx=12,bblly=12,bburx=750,bbury=590}
\caption{Diagram of Hall~C Gas Mixing System\label{fig:gas_mixer_diagram}}
\end{figure}


For normal operation, with the alcohol systems in use, the valves
should be set as follows:

{\bf For The HMS:}

Open - 3, 4, 11, 14; Closed - 12, 13, 17, 18, 19, 20.

The nominal flows set in the mass flow controller are:
\begin{itemize}
\item Channel 3 (Argon): 0.3~slpm (standard liters per minute)
\item Channel 4 (Ethane): 0.3~slpm
\end{itemize}


Unless the gas \#3 Mass flow control valve is installed, valve \#6 should
always be closed.

{\bf For the SOS:}

Open - 1, 2, 7, 10; Closed - 8, 9, 15, 16, 19, 21.
\begin{itemize}
\item Channel 1 (Argon): 0.15~slpm
\item Channel 2 (Ethane): 0.15~slpm
\end{itemize}


Unless the gas \#3 Mass flow  control valve is installed, valve \#5 should
always be closed.

\section{Operating the Mass Flow Controller.}

The gas flow is controlled by a MKS 647 controller and mass flow
control valves.  The 647 is menu driven from a CRT in the front panel and
with a keypad with cursor controls. The 647 features a non-volatile memory
so settings are retained even if the unit is unpowered.  The initial menu
upon startup is the Command Menu.  For normal operation use either the User
Display menu (Command menu item \#1) or the Extended Display menu (Command
menu item \#2).  The User Display menu shows actual flow in each channel and
the total flow in all channels.  The Extended Display menu 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, or independent mode.

\subsection{To set flow rates:}

The flow rate set points are adjusted from the Extended Display
menu.  There are two methods to change valve flow rate set point.  If you
want to enter a specific value you must first turn off the flow in that
channel or all of the channels.  Using the cursor keys move the cursor to
the desired channel.  Enter the desired flow rate.

The flow rate set point can be changed with gas flowing using the
cursor keys. In the Extended Display mode move the cursor to the desired
channel using the left/right cursor keys.  The set point can then be
adjusted up or down using the cursor up/down keys.

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

The gas flow can be turned on or off while in any menu.  When any
of the mass flow valves are open the green LED labeled ``GAS ON" on the 647
is lit.  When none of the gas flow valves are open the red ``STAND BY" LED
will be flashing.  In the Extended Display menu the bottom line displays on
or off, by channel, to show which mass flow valves are enabled.  The green
LED must be lit and an ``ON" must be displayed in the bottom row of the
Extended Display menu for gas to be flowing in a particular channel.

{\bf Turning the gas on or off is done in two steps which can be done in
either order.}
Each channel must be enabled by pressing ``ON" and then
that channel number.  The command input must be enabled by pressing ``ON"
and then ``ALL" from the keypad.  This allows a single channel or all of the
enabled channels to be turned on or off at once.  Both steps must be
performed initially, but thereafter only one of the steps need be performed
to cycle the gas flow on or off.

To turn gas off in a single channel press ``OFF" and then the
desired channel.  If you want to close all the valves simultaneously, press
the ``OFF" key and then the ``ALL/0" key.  To turn gas back on you must
reverse whichever sequence you used to stop the gas flow.  For example if
you turned the gas off by pressing ``OFF" and then the channel number, it
must be turned back on by pressing ``on" and then the channel number.  If
you turn off all the channels by pressing ``OFF" , ``ALL" you must turn it
back on by pressing ``ON" , ``ALL."

\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 liquified
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 guage
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.
Thus, when the bottle weight is about 30~pounds less than its full weight,
the bottle should be replaced. Recent (as of April 2003) experience
indicates that full bottles weigh $165\pm 1 lbs.$

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 EH\&S 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.

\section{To by-pass the alcohol system}

\noindent For the HMS:\\
Open valves 12 \& 13, then close valves 11 \& 14, in that order!

\noindent For the SOS:\\
Open valves 8 \& 9, then close valves 7 \& 10, in that order!

\section{The Alcohol Bubblers}

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 dirft velocity in the wire chambers, so it must be held 
approximately constant. 

\subsection{To refill the alcohol bubblers:}

The alcohol bubbler system features a refill system that allows
filling directly from the bottle, minimizing exposure of the alcohol to air
and reducing the possibility of a spill.
{\bf The reservoirs should be refilled
before they become empty to maintain a head of liquid over the float valve
which will prevent air from entering the system.}
In the back of the gas
system rack is a holder for gallon sized alcohol bottles and a cap with dip
tube.  Place a new bottle in the bottle holder and replace the cap with the
cap with dip tube.

\subsection{Step-by-Step Instructions for Refilling the SOS Alcohol Bubbler}
\em{These steps must be individually completed in the order listed!}\\
Refer to Fig.~\ref{fig:gas_mixer_diagram}. 
\begin{enumerate}
\item{If needed, install a full bottle of alcohol in the back of the gas racks as mentioned in the preceeding paragraph.}
\item{{\em Open valves 8,9. Close valves 7,10} to Put the SOS alcohol bubbler in BYPASS.}
\item{{\em Close valve 16} to isolate the warm reservoir gas from the bubbler.}
\item{{\em Open valve 15} to bleed off the warm reservoir gas pressure.}
\item{Certify that the pressure feeding valve 19 is 1 psi or less so that you 
do not cause the alcohol bottle to explode when you pressurize it. {\em Wear
safety glasses!!}}
\item{{\em Open valve 19} to pressurize the alcohol bottle.}
\item{{\em Open valve 21} to flow alcohol into the warm reservoir.}
\item{When the alcohol level in the sight-glass is within 2cm of the top, stop
the flow of alcohol: {\em Close valve 21.}}
\item{{\em Close valve 19.}}
\item{{\em Open valve 16.}}
\item{{\em Close valve 15.}}
\item{{\em Open Valves 7 and 10. Close Valves 8 and 9.}}
\item{Record what you did in both the gas logbook and the electronic logbook.} 
\end{enumerate}

\subsection{Step-by-Step Instructions for Refilling the HMS Alcohol Bubbler}
\em{These steps must be individually completed in the order listed!}\\
Refer to Fig.~\ref{fig:gas_mixer_diagram}. 
\begin{enumerate}
\item{If needed, install a full bottle of alcohol in the back of the gas racks as mentioned in the preceeding paragraph.}
\item{ {\em Open valves 12,13. Close valves 11,14} to Put the HMS alcohol bubbler in BYPASS.}
\item{{\em Close valve 18} to isolate the warm reservoir gas from the bubbler.}
\item{{\em Open valve 17} to bleed off the warm reservoir gas pressure.}
\item{{\em Open valve 19} to pressurize the alcohol bottle.}
\item{{\em Open valve 20} to flow alcohol into the warm reservoir.}
\item{When the alcohol level in the sight-glass is within 2cm of the top, stop
the flow of alcohol: {\em Close valve 20.}}
\item{{\em Close valve 19.}}
\item{{\em Open valve 18.}}
\item{{\em Close valve 17.}}
\item{{\em Open Valves 11 and 14. Close Valves 12 and 13.}}
\item{Record what you did in both the gas logbook and the electronic logbook.} 
\end{enumerate}

\subsection {Alcohol Temperature Control}

To keep the alcohol temperature (and thus the vapor pressure) constant,
the alcohol bubblers are housed in refrigerators which are controlled by
electronic temperature regulators having 1~C\degg  sensitivity. Both 
controllers are located on a shelf in the left rack of the gas mixing
system. Normally, the actual temperature in each refrigerator is
indicated on the front panel of the controller. Both controllers should
be set to maintain a temperature of 1\degg  C.

\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 15 psig.


\section{Related {\it Howtos}}
\begin{itemize}
\item MKS 647 Mass Flow Controller Howto \cite{howto:MKS_controller_TM}
\item Flammable Gas Detector System \cite{howto:flam_gas_detector}
\item Gas System Interlock Panel \cite{howto:gas_interlock_panel}
\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.
% $Log: drift_gas_system.tex,v $
% Revision 1.2  2003/04/04 16:05:29  saw
% (hcf) added notes on bottle changing, ethane bottle pressure.
%
% Revision 1.1  2003/03/03 20:08:14  saw
% Initial Checkin
%
