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<p>Dear all,</p>
<p>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. <br>
</p>
<p>Consequently I highly recommend the most permanent and stable
solution. <br>
</p>
<p>Kind regards,</p>
<p>Maxime<br>
</p>
<p><br>
</p>
<br>
<div class="moz-cite-prefix">Le 28/11/2017 à 11:12, George Jacobs a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:171e7193-e230-6a89-35c0-a4e00b243039@jlab.org">
<meta http-equiv="content-type" content="text/html; charset=utf-8">
Hi Bob,<br>
<br>
There are 2 issues which need to be addressed now that the mass
flow controllers are subject to the exterior ambient temperature.
<br>
<br>
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. <br>
<br>
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.<br>
<br>
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.
<br>
<br>
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.<br>
<br>
George<br>
<pre class="moz-signature" cols="72">--
George Jacobs
Jefferson Lab (TJNAF)
STE 12
12000 Jefferson Ave.
Newport News, VA 23606
(office) 757-269-7115
(cell) 757-876-0480
(email) <a class="moz-txt-link-abbreviated" href="mailto:jacobsg@jlab.org" moz-do-not-send="true">jacobsg@jlab.org</a>
(website) <a class="moz-txt-link-freetext" href="https://userweb.jlab.org/%7Ejacobsg" moz-do-not-send="true">https://userweb.jlab.org/~jacobsg</a></pre>
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