Technical Explanation
Throughput and Leak Rate
In vacuum technology, throughput Q is defined as pressure × volumetric flow rate. Its natural SI unit is Pa·m³/s, which is dimensionally equivalent to power per unit area. Vacuum engineering also uses mbar·L/s or Torr·L/s as leak rate units.
Many process tools specify flows in sccm (standard cubic centimeters per minute). Using NIST conversion coefficients at standard conditions, this tool adopts:
1 sccm = 0.0168875 mbar·L/s = 0.001689 Pa·m³/s
Rate-of-Rise Method
In a pressure rise (rate-of-rise) test, the chamber is isolated from the pump and the pressure increase is measured from p₁ to p₂ over a time interval Δt. With volume V, the total gas load or leak rate is:
q_L = V · (Δp / Δt)
When expressed in mbar and liters, q_L has units of mbar·L/s. This method is widely used to quantify leaks and outgassing in vacuum systems with only a gauge and known volume.
Ultimate Pressure Estimate
With effective pumping speed S_eff and leak/gas load q_L, the approximate ultimate pressure is:
p_ult ≈ q_L / S_eff
If the measured base pressure is significantly higher than this estimate, either the effective pumping speed is lower than assumed or additional gas loads (outgassing, virtual leaks) are present.
References & Disclaimer
Conversion factors follow NIST gas flow unit conversion tables. Pressure-rise and leak-rate relationships follow standard vacuum engineering references (e.g., Leybold, Lesker). For critical applications, verify assumptions about standard conditions, gas species, and measurement accuracy.