1) As I promised a couple of weeks ago, I measured the impedance of the
kind of flow tube that I want to have on the Region One sectors.
I used a Matheson model FM-1000 flow tube with a #202 flow tube, which
has a maximum throughput of 0.7 liters/minute. This is the ordinary
type of flow tube that you have seen many times, not the high precision
type, but the "cheap" version one uses for approximate flow rate
measurements and control. The needle valve was left wide open as it
would be in CLAS, so that the tube primarily monitors flow rate. We
would plan to have six of these in parallel, one for each sector. In
case the flow to the six sectors were not equal, or if the leak rate in
one sector was a bit more than the others, we could balance the flow by
judicious use of the needle valves.
The answer is that the open flow tube has an impedance which is
proportional to the flow rate to a good app[roximation; indeed it acts
like a "resistor." At a flow rate of 0.4 liters/minute (the expected
Region One flow rate) the differential pressure drop across the tube is
1.0 +- .1 inches of water, as measured by a Magnehelic guage. (Thanks
go to Gary Wilkin for setting up the hardware.) This corresponds to
.0024 atm, or 0.036 psi of pressure drop.
My guess is that this is not too great for the gas system to
handle. I'd like to get a quantitative judgement on this from someone
who understands the gas handling system. I picture putting these flow
tubes on the input side of the chamber, with individual exhaust bubblers
on the output side (the bubblers have about 0.05" of oil pressure drop,
less than the flow tubes, if you ignore surface tension effects). For
Region One there is no gas contamination issue since the gas coming out
of Region One will be discarded. CMU will provide the flow tubes and
the bubblers on rack-mount panels. All we need is the space to install
them. I picture rolling (or fixed) racks on the left and right sides of
CLAS. Other Regions could also mount gas flow monitoring devices on
these racks, since ours are only about ten inches high. Can JLab
provide the racks?
2) I've been trying to read up on non-flammable gas mixtures. So far
this has amounted to reading the book by Blum and Rolandi. There are
more major detectors than I thought using non-flammable gas. Of 21
chambers that are featured in this book, 5 use non-flammable mixtures.
To me the most attractive one, on paper at least, is the one used by the
central drift chamber at Mark II. They used Argon 89%, CO2 10%, and CH4
1%. It is known from JADE that if the CH4 concentration rises to 5%
while CO2 is reduced, whiskers will start to grow; but if the methane
is reduced again to 1% the whiskers go away. By hearsay I have heard
that the methane is in the mixture to "plug a hole" in the photon
absorption spectrum of CO2. I guess that the high concentration of Argon
means the gas is still "hot," which is what we probably want. A lot more
research could be done on these kinds of gas mixtures. Should it? How
seriously are we considering the non-flammable gas mixture scenarios?
Regards, Reinhard.