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Chamber, leak and detector
Pressures are in mbar, throughput Q in mbar L/s. The chamber obeys V dP/dt = Q - S P, so the steady pressure is Q/S and the time constant is V/S. Sliders for rates are logarithmic.
Test actions
Model checks
The science
What is modelled
A chamber of volume V is pumped with speed S. Gas enters through a leak (rate Qleak) and by outgassing of the walls. When helium is sprayed at the leak, a fraction of the incoming gas is helium, and the detector, which measures helium throughput, rises with a finite delay set by the pump time constant, the gas mixing at the leak and the detector response.
Equations
- Total pressure:
V dP/dt = Q - S P, solved exactly over each step:P(t+dt) = P e-S dt/V + (Q/S)(1 - e-S dt/V). - Helium partial pressure obeys the same equation with
QHe = Qleak f + Qout × 5.2 ppm, where f is the helium fraction at the leak (low-pass filtered with the mixing time). - Detector reading:
S PHepassed through a first-order lag.
Checks run in the page
- Steady pressure against Q/S.
- Valve closed: the pressure rises linearly, P0 + Q t / V.
- Fully sprayed leak: the reading settles at the helium throughput.
- Clear-down time after the spray stops, against the V/S estimate.
Honest simplifications
A single pumping speed, no molecular-flow conductance between the leak and the pump, no helium permeation or pump back-streaming and a simplified detector. The detector noise is a deterministic sinusoid sum. Not a calibrated leak-testing instrument.
What to try
- Switch on the spray and read how long the detector takes to cross the alarm level.
- Press Large chamber, slow: the response is much slower because V/S is large.
- Lower the true leak below the alarm threshold and see that the helium never trips the alarm.