Customers send a chamber size and one line — “down to this pressure in a few minutes” — and leave the rest to the supplier. The calculation is not hard, and doing it yourself tells you whether the quoted model makes sense.
Step one: volume, including the piping. Chamber volume is length × width × height, but the volume being evacuated is more than the chamber. Add the piping between chamber and pump, which matters on long small-bore runs. Subtract tooling and fixtures inside the chamber. Add buffer tank, filter and valve volumes.
Take a chamber of 800 × 600 × 500 mm with 30 L of tooling inside: 0.8 × 0.6 × 0.5 = 0.24 m³ = 240 L, minus 30 L of tooling, plus about 10 L of piping, giving roughly 220 L net.
Step two: the formula. A working estimate is S = (V / t) × ln(p₀ / p₁), with S in m³/s, V the net volume in m³, t the allowed time in seconds, p₀ the starting pressure (1013 mbar from atmosphere) and p₁ the target. For 220 L to 10 mbar in 90 seconds, V = 0.22 m³: S = (0.22 / 90) × ln(1013 / 10) = 0.00244 × 4.62 ≈ 0.0113 m³/s, which is about 40.7 m³/h.
Step three: margin, then model. The 40.7 m³/h figure is theoretical. Add 20–30% for leakage and outgassing (seals, hoses, workpiece surfaces), 10–20% for line losses through bends, filters and long runs, and about 10% for vane wear. With 40% total, the requirement is around 57 m³/h, which puts you here:
| Model | Speed 50/60 Hz | Note |
|---|---|---|
| V15/25/40H | 40 / 45 m³/h | short on margin; cycle time stretches |
| V0063 | 63 / 75 m³/h | meets the duty, oil-sealed |
| V0160 | 160 / 190 m³/h | well oversized; about 23 s theoretical, roughly 30 s with losses |
Where the process forbids oil contamination, the largest oil-free option is V25/40 at 40 / 45 m³/h — so either accept a longer cycle, run two pumps in parallel, or shrink the chamber.
Two common mistakes. Treating ultimate pressure as working pressure: ultimate is what a blanked-off pump reaches, while real stable pressure sits higher because of leakage and outgassing. A 10 mbar duty suits a 0.1 mbar pump; a 10 mbar pump will never get there. And ignoring gas type: the formula assumes air at 20 °C. Water vapour cuts effective speed well below nameplate and demands an open gas ballast; light gases such as hydrogen and helium also reduce a vane pump’s speed. For those, raise the margin to 50%.
What to send us. Net volume including piping, target pressure, allowed time, gas type and temperature, dust or corrosive content, and available supply. With all six the sizing holds; miss one and the model is a guess.