The following files are attached:

1) gasrep03.txt - text file
2) gascst.jpg - estimate of gas costs for running
3) gsyscst.jpg - estimate of gas system cost
4) gassys01.jpg - schematic of proposed gas system
5) flo10.jpg - thermistor flowmeter response with helium
6) flo12.jpg - thermistor flowmeer response with DME

				Robert

E614 GAS SYSTEM                                     APRIL 8, 1998



                     DESIGN CONSIDERATIONS

1) Contamination by diffusion

     - Helium into DME (affects drift times)
     - Helium into CF4/i-C4H10 (affects gain, drift times)
     - DME into helium (affects depolarization near target)
     - air into any of the gas systems
     - contamination level = (diffusion rate)/(fresh gas supply rate)
           ==> want high flow rates

2) E-Field Stability

     - cathode foil deflection
          - foil tension
          - differential pressure across foils
          - flow rates
          - flow o/p impedances
                ==> want low flow rates

3) Density Stability

     - ambient temperature, pressure
     - local heat sources/sinks (effects depend to some extent on flow rate)
          ==> want high flow rates

4) Construction materials
     - suitability for use with DME

5) Cost
     - cost of gas system (see gsyscst.jpg)
     - cost of gasses ==> want low flow rates (see gascst.jpg)
     - interrelationship of cost and all of the above


-  the flow rate per chamber/gap will be constrained by the optimization of 1,2,3 and
5 above.
                                
                                
        ISSUES OUTSTANDING AT LAST COLLABORATION MEETING

1) Improved design for flow into chamber

     - have constructed and tested a "flattened kapton straw" gas injection system
     - input impedance    7 Torr @ 200 cc/min
               ==>  ~ 0.7 Torr @ 20 cc/min
     - can probably use standard swagelok ferrules and fittings to seal to external
tubing

2) Measure o/p impedance of chambers

     - measurements with kapton straw ==> impedance ~ 1 mTorr/(l/min)
          - difficulties ensuring measurement straw remains open
          - at limit (+/- 1 mTorr) of measurement system
           ==> not a great deal of confidence in accuracy of measurement

     - an EXTREMELY pessimistic calculation based on measured input
impedance of straw
          - assume parallel straws of same area as donut (OD - ID) X
circumference
          ==> impedance @ 20 cc/min ~ 5 mTorr
     - or consider that average crossectional area of the gap between planes ~ 8
cm^2
          
     - conclude that backpressure due to chamber o/p impedance will be
insignificant

3) Measure foil deflection as function of differential pressure (or flow rate)

     - not done yet
     - should have opportunity when chamber disassembled for wire repair
     - also need to know effect of deflection on chamber performance
          - Garfield can tell us this?

4) Verify helium diffusion rates for our foils

     - this was measured (TN-6)
     ==> expect ~ 0.015 cc/min/chamber diffusion rate of helium into DME
     - DME flow rate of 20 cc/min/chamber ==> helium contamination ~ 0.1%
     - Garfield (R.Poutissou) indicates expect ~ 1% change in drift time per 1%
change in He
     ==> flow rate and stability not a serious problem for flows ~ 20
cc/min/chamber

     - NOTE:  above assumes the only source of helium is diffusion thru foils.
          - pinholes in foils or any DME-helium seals could easily overwhelm
diffusion
          - all chamber subassemblies should be checked for diffusion as
they are built

5) Density stability
     - have decided that it's sufficient to monitor temperature, pressure and correct
data

                      PROPOSED GAS SYSTEM
          (See gassys01.jpg)

1) Flow rate and stability, and chamber o/p impedance not as important as
first thought

     - flow rates varying from 2 cc/min to 200 cc/min will probably not affect
chamber        performance
     - need to do foil deflection tests to absolutely confirm this
==> a much simpler gas system than first envisaged

2) Differential pressure to push gas to surface is significant

     - differential static pressures of 10 meter columns of gas:
     - dP (DME-He) ~ 1.3 Torr
     - dP (FI-He) ~ 2.4 Torr
     - dP (He-air) ~ -0.76 Torr
==>  - need to pump gas to surface
     - need pressure control of exhaust gas lines at beam level

3) Still need to ensure that flow rates in each chamber within (albeit wide)
acceptable limits

==>  - need flow monitoring and control for each chamber and each He gap
     (can be simply a manual regulating valve and visual or electronic flow
readout)
     - separate input lines for each chamber and He gap

4) Other features

     - exhaust lines can come to common exhaust manifold
       (inside or outside of magnet??)
     - pressure control at common portion of exhaust lines  
     - slave DME and CF4/isobutane pressures to helium pressure
     - pressure relief bubblers on each common exhaust line before pressure
control

5) Recycle gas ??

     - needs cost/benefit and technical analysis 
          - e.g.  how do we do it?
          - will it affect chamber performance?
     - subject for ongoing and future discussions
     - approximately « of typical recyling system already present in current design
     - can be "appended" to current proposed design


                A CHEAPER FLOWMETER/CONTROLLER?

     Much of the cost of the current design is due to the requirement of 82
channels of flow measurement and control.  The initial cost estimate was based on
the "standard" rotameter used at TRIUMF.  Less expensive rotameters are
available, but we need to be careful that they are DME proof.  We could likely
reduce the per channel flowmeter cost to ~ $150 - $200.
     It would be nice to have an electronic readout of the flows (not available from
rotameters).  Typical mass flow controllers (MFC's) cost ~ $1400.  There is a
cheaper one available for about $ 400 (1989 price), but it is made of plastic, and
may not be DME proof.
     We have been investigating the possibility of using thermistors in "self
heating mode" to provide flow rate information.  If enough current is passed through
a thermistor, the heat generated lowers its resistance until eventually it stabilizes at
some temperature (resistance).  A flow of gas around the thermistor will then cool it,
thus changing its resistance.  The amount of change is proportional to the mass flow
rate and heat capacity of the gas flowing by it.  Ambient temperature effects can be
nulled out by using another identical thermistor (not in the gas flow) in a bridge
circuit.
     We have constructed and tested such a device.  The thermistor worked well
as a DME flowmeter.  Stable (over several days) signals of ~ 100mV per 10 cc/min
flow were obtained.  Similar tests with argon were also encouraging.  Unfortunately,
tests with helium were disappointing.  The signals were very small (~ 10 mV per 10
cc/min flow) and voltage drifts of greater than 60 mV were observed.
     At this time we aren't sure what causes the problems with helium flow
measurements.  An obvious candidate is the small molar heat capacity of helium,
which leads to small signals, and high susceptability to air leaks into the reference
thermistor cavity.  Other possibilities include the low viscosity of helium (we may
have laminar rather than turbulent flow around the thermistor bead).  The high
thermal conductivity of helium may also cause problems in that it makes the
thermistor more sensitive to ambient temperature changes.
     We are currently constructing the "Mark II" version of the thermistor
flowmeter which we hope will solve some of the problems.
     These thermistors cost ~ $50 (1990 prices) for a matched pair.  With the
addition of a control valve (~ $50??) we may be able to produce a reasonably
inexpensive electronic readout flowmeter/controller system.
     The following graphs show examples of the performance with DME and
helium.  A commercial MFC was used to control and monitor the flow to the
thermistor flowmeter.  The MFC signal and the thermistor signal were continuously
recorded using a datalogger.  (See FLO10.JPG, FLOW12.JPG)


                   FUTURE ACTIVITIES REQUIRED

1) Measure foil deflection as function of differential pressure (flow rate)

2) Determine the relationship between foil deflection and chamber performance

3) Determine effects of helium on CF4/isobutane chamber performance

4) Test the effects of DME on various materials used in chamber and gas system
construction

5) Continue tests of thermistor flowmeters
     - investigate other inexpensive flowmeter/controller possibilities

6) Investigate recycling techniques and possible problems

7) Finalize proposed (non recycling) design, prepare accurate cost estimate and start
ordering parts.


                   FUTURE DECISIONS NECESSARY

1) To recycle or not

Filename: GASCST.JPG

Filename: GSYSCST.JPG

Filename: GASSYS01.JPG

Filename: FLO10.JPG

Filename: FLO12.JPG

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