The following are some estimates of the atmospheric pressure tracking
capabilities of the proposed gas system.  If we know the capacity of the
various gas system elements, and the flow rate capabilities of the
controllers supplying/removing the gas from them, then the rate of pressure
change can be calculated from the relation:

F = C dP/dt		where C is capacity in liter/Torr
				F is flow (liter/min)
				dP/dt is rate of change of pressue (torr/min)

Helium Containment Volume

NOTE: The following assumes the helium containment vessel is a RIGID
structure, i.e. that it does not expand with differential pressure.  If it
is not rigid, the capacitance cannot be calculated until its elasticity is
determined.  An inflatable structure would significantly increase the
capacitance and render the following calculations meaningless. 

Volume ~ 1180 liters ==> C ~ 1180/760 = 1.55 liter/Torr
nom flow in ~ 1 liter/min (manually set, range 0 - 5 liter/min)

max increasing dP/dt = (1 liter/min / 1.55 liter/Torr) = 0.64 Torr/min

decreasing dP/dt 
	The output of the helium vessel is just an open tube to atmosphere.  Thus
the output flow rate depends on the differential pressure between the
helium vessel and atmosphere.  The pressure inside the helium vessel will
track atmospheric pressure with a time constant given by Tau = RC
where: R = o/p resistance of tubing ~ 40 mTorr/(liter/min)
	C = 1.55 liter/torr
==>	Tau ~ 0.04 * 1.55 = 0.06 min = 3.7 seconds


Chamber Volumes

	This is more complicated, but I'll do calculation for one xy pair, and
assume the output flow is equally shared by all 44 chambers.  The chamber
capacities are composed of 2 elements; the rigid body capacitance (i.e. the
flat foil volume), and the "active" capacitance due to the flexing of the
foils.  Since the point of the exercise is to keep the foils flat, we can
do the calculation assuming the foils ARE flat, and calculate how fast we
can change the absolute pressure in "rigid flat foiled" chambers.  If we
can supply/remove gas fast enough to track atmospheric pressure variations
under these conditions, then we can keep the foils flat.   Thus we can
ignore the "flexible foil" capacitance and just use the "rigid  body"
capacitance.

Volume xy chamber ~ 2.28 liters ==> C ~ 2280/760 = 3 cc/torr
nom flow in ~ 40 cc/min (manually set, range 0 - 200 cc/min)
max flow out ~ 200 cc/min (automatic control 0 - 200 cc/min)
	Note: there are 2 of 20 cc/min flow inputs per xy pair

max increasing dP/dt = 40/3 = 13.3 Torr/min

max decreasing dP/dt = (200-40)/3 = 53.3 Torr/min


Atmospheric Pressue Variations in Vancouver (anecdotal)

The fastest pressure change I've monitored is a change of ~ 4% in 2 hours, or 
~ 0.25 Torr/min.  The worst case we have with the proposed gas system is
increasing the pressure in the helium volume where our maximum rate is 0.64
Torr/min at our nominal input flow of 1 liter/min.  Even if the helium
volume lagged atmospheric pressure, the worst that would happen is air
would seep back thru the o/p tubing into the helium volume .  In any case
... we can always increase the nominal helium input flow rate if the margin
of safety is deemed to be too tight.

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