7/13/2007

3He pressure gauges and target thickness:

Before the XEM running in 2004, the target group pressurized the 3He
gas panel to look for leaks and check the pressure gauges. Pressure
readings from this test were:

Gauge      Measured pressure (psi)
-----      -----------------
9702	     199.8      <--- read out in epics
9703	     197.0
9704	     198.5
9705	     104.5

Obviously, 9705 was messed up. This was later addressed.

Note that 9702, the sensor we use to determine the 3He target density
run-by-run, is systematically higher than the other 2 useful measurements.
If we take the "best" estimate of the pressure to be the average of the
three, this yields P_ave=198.43. So 9702 is 1.37 psi. Meek tells me the 
discrepancy is not likely to scale much with pressure, so at the ~120 psi
operating point, the absolute size of the difference will be about the same.

Assuming our nominal reading from 9702 is 1.37 psi too large, means the 
run-by-run target density is 0.51% too large [at our operating pressure,
the density changes about 0.56%/1.5 psi].  so, if we wish to take this into
account in our analysis, we should scale the 3He target densities (thicknesses)
by 1/1.0051.