the time t required to evacuate a system from an initial pressure P0
to a final presssure P1 is estimated with the following equation:
t = (V/S).Ln(P0/P1)
t: evacuation time [h]
V: system volume [m³]
S: average pump suction capacity [m³/h]
P0: initial pressure at t0
P1: final pressure at t1
The equation is applicable for reasonably tight systems (neglecting a possible leakage and /or degassing),
and when the pump suction capacity S is nearly constant over the entire operating range.
p.V = m.(R/MG).T
V = m.(R/MG).T/p
V: pump suction capacity [m³/h]
m: mass flow (load or leakage) [kg/h]
R = 8,314
MG: Mol.Weight [g/Mol]
T: Temperature [K]
p: Pressure to be maintained [Pa]
estimate leakage rates with "pressure rise vs. time" test
The system with a known total volume V have been evacuated down to to a pressure P1 (in the range 1 - 50 mbar).
The system is then isolated and the time required for the pressure to rise to P2 (to be < 500mbar) will be used to estimate the leakage.
q = 0,1 . V . (dP/dt)
q: leakage rate [kg/h]
V: total system volumeVolumen [m³]
dP: pressure rise [mbar] in the system
dt: time [mn]
N.B.: K=0,1 is used for an application under rough vacuum, for an air leakage (or similar MW), without product degassing.
This test can also be used to check leakage rates on regular basis (comparing the record)