A common question is whether pressure-rise testing is acceptable. It answers whether a chamber leaks; helium mass spectrometry answers how much and where. The gap is five orders of magnitude — a limit of the method, not of the instrument price.
1. Leak rate turns tightness into a number
Q = Δp·V/Δt. A 1 m³ vessel changing 1 Pa in 1 s carries 1 Pa·m³/s. For scale: a 4 L bicycle inner tube losing 1 bar over 30 days is 1.5×10⁻⁴ Pa·m³/s; a refrigerator leaking 10 g of R134a over ten years is 7.1×10⁻⁷. The refrigerator is two orders tighter than the tube and still far from a semiconductor chamber.
2. Pressure rise measures more than leakage
With the valve closed, the rising pressure contains real leakage plus wall outgassing. Water outgassing decays as 1/t, approximately 3×10⁻⁹/t (hours) in mbar·L/(s·cm²). A freshly cleaned chamber can outgas several orders above its true leak for hours, so the reading is outgassing, not leakage, unless the background is measured and subtracted first.
3. Units differ by a factor of ten
1 mbar·L/s = 0.1 Pa·m³/s. Reading 1.5×10⁻⁴ Pa·m³/s as 1.5×10⁻⁴ mbar·L/s relaxes the requirement tenfold. Units in a contract matter more than the instrument model.
4. Temperature is not in the formula
Q = Δp·V/Δt has three quantities and no temperature term; the source material neither discusses temperature effects on pressure rise nor provides a correction (not covered). Any claim of temperature compensation should come with the formula.
5. Why helium testing reaches 10⁻¹³
The mass spectrometer must operate below 10⁻⁴ hPa, maintained by the instrument’s own pumps. The turbopump compresses air far more than light gases, so air is held back while helium passes — it acts as a selective filter rather than a pump. Counterflow covers 1 to 10⁻⁹ Pa·m³/s and dominates production use; main flow is more sensitive but needs 10⁻² hPa on both specimen and instrument. A quartz-window variant uses a 7 µm SiO₂ membrane with a 5×10⁻⁹ detection limit.
6. What each method can reach
Spray helium outside an evacuated specimen: < 5×10⁻¹³ Pa·m³/s at a 1000 hPa differential. Sniffing a pressurised specimen: < 1×10⁻⁸, specimen must withstand overpressure. Accumulation: mainly above 1×10⁻⁵, and slow. For a 10⁻⁹ verdict only vacuum-mode mass spectrometry qualifies; sniffing is floored by the 5 ppm helium naturally present in air.
7. Large chambers test slowly
In molecular flow (< 10⁻³ hPa) speed is set by specimen volume and effective helium pumping speed; above 1 hPa, laminar flow puts the backing pump in charge and everything slows. Longer waits on large chambers are geometry, not operator error — plan the response time into the schedule.
8. Three sources of misreading
5 ppm atmospheric helium; helium dissolved in oil-sealed backing-pump oil diffusing back into the high-vacuum side (open the gas ballast after a large leak); and no purge after a strong signal.
9. How we do it
Helium mass spectrometry against the criterion agreed on the project drawing. The capability we publish is the 1×10⁻⁹ Pa·m³/s range, with every unit tested rather than sampled. The method references GB/T 36176-2018 and GB/T 32218-2015, and the report is dispatched with the chamber. Pressure-rise testing is not used as an acceptance criterion here for one reason: it cannot separate leakage from outgassing, nor account for temperature error. For process judgement and rough localisation it remains entirely adequate; the sheet that gets signed calls for helium.
Leak testing is a verdict, not a step. A verdict that cannot be stated precisely makes the report meaningless. The same verdict applies to your project once a drawing is submitted.
Frequently asked questions
Can pressure-rise testing replace helium leak testing?
For process judgement, yes; as an acceptance criterion, no. The pressure-rise method cannot separate leakage from outgassing and carries no temperature correction. The document you sign has to rest on helium testing.
What magnitude can helium leak testing reach?
External helium spray in vacuum mode reaches the 5×10⁻¹³ Pa·m³/s range. Our factory acceptance testing follows the criterion on the project drawing; the capability we disclose externally is the 1×10⁻⁹ Pa·m³/s range.
Why does a large chamber take so long to test?
In the molecular-flow regime (below 10⁻³ hPa) the response time is governed by chamber volume and effective helium pumping speed (V/S). Above roughly 1 hPa, in the viscous regime, it is governed by the backing pump — the larger the chamber, the slower the response.