Finding the Invisible
A pinhole smaller than a bacterium can ruin a vacuum system, admitting enough gas to prevent reaching target pressure. Helium leak detection — spraying helium on suspected leak locations while monitoring with a mass spectrometer — is the gold standard for finding such defects. The technique achieves sensitivity to leak rates as small as 10⁻¹² Pa·m³/s, detecting just a few million helium atoms per second.
How It Works
A magnetic-sector mass spectrometer tuned to mass 4 (helium) monitors the gas composition inside the vacuum system. When helium sprayed on the outside finds a leak path, it enters the system, reaches the detector, and produces a signal proportional to the leak rate. The partial pressure rise equals the leak rate divided by the effective pumping speed: P = Q/S.
Response Time and Volume
The time constant for signal buildup is τ = V/S — the system volume divided by the pumping speed at the detector. For a 10-liter system with a 5 L/s detector pump, response time is 2 seconds. Large industrial vessels may have response times of minutes, requiring patience and systematic spray patterns for accurate leak localization.
Practical Leak Hunting
This simulation models the detection process: you set the leak rate, system parameters, and background noise, then observe the signal-to-noise ratio and response time. Real leak hunting follows a systematic approach — start at the top of the vessel (helium rises), spray methodically, wait for the response time at each location, and reduce helium flow once a region is identified to pinpoint the exact defect location.