When sourcing a helium leak detector, the most critical specification is the Minimum Detectable Leak Rate (MDLR), which for high-end vacuum applications should be at least 5x10^-12 Pa·m³/s. Additionally, evaluate the pumping speed of the internal vacuum system; a higher helium pumping speed (e.g., >2.5 L/s) ensures faster response and recovery times. You should also verify the inlet pressure tolerance, as units capable of starting at higher pressures (e.g., 1500 Pa) significantly reduce cycle times in production environments.