[Dsg-mvt] MVT gas mixing system temperature control

[mdefurne]

Dear all,

The gas mixture must be as homogeneous as possible over time. The gain 
of the detector depends quite strongly on the fraction of isobutane. 
Having a poor fraction of isobutane will result in a dramatic increase 
of gain, triggering sparks which damage the detector (locally things 
vaporize). Moreover one would have to permanently play with the HV 
setting to compensate for the gain fluctuation due to the change in the 
gas mixture if the later is not stable enough.

Consequently I highly recommend the most permanent and stable solution.

Kind regards,

Maxime



Le 28/11/2017 à 11:12, George Jacobs a écrit :
> Hi Bob,
>
> There are 2 issues which need to be addressed now that the mass flow 
> controllers are subject to the exterior ambient temperature.
>
> First - The isobutane gas, C4H10, has a very low vapor pressure. The 
> gas cylinder heater blanket will maintain temp in the cylinder. The 
> line from the gas cylinder to the mass flow controllers must be heated 
> to prevent condensation. The mass flow controllers must also be heated 
> to prevent C4H10 condensation. Once the C4H10 gas is mixed with the 
> argon, heating the lines is no longer required.
>
> Second - The mass flow controllers are not rated below 50F. Winter 
> temperatures can go much lower than 50F. Temperatures could get as low 
> or lower than 0F. A temporary solution of heat tape may work at some 
> level, but temperatures will vary greatly over time and the mass flow 
> controllers may not operate correctly.  The resultant gas mixture will 
> vary over time, due to changes in the outside temperature. A better 
> solution is required for this mixing system to operate within 
> specifications and to supply the proper gas mixture to the detectors. 
> At some point, an improper gas mixture may cause damage or reduce the 
> lifetime of this detector.
>
> I recommend working on a more permanent solution. One solution may be 
> to place a heated cover over the mass flow controllers and insulate 
> them from the metal mounting plate they are attached to.
>
> As a temporary solution, heat tape can be used. Most heat tapes are 
> capable of temperatures that could damage whatever it contacts. We do 
> not want to damage the mass flow controllers or anything else, so 
> great care must be taken in it's use.
>
> George
> -- 
> George Jacobs
> Jefferson Lab (TJNAF)
> STE 12
> 12000 Jefferson Ave.
> Newport News, VA 23606
>
> (office) 757-269-7115
>
> (cell)   757-876-0480
>
> (email)jacobsg at jlab.org
>
> (website)https://userweb.jlab.org/~jacobsg
>
>
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