For a leak test in vacuum systems, an acceptable vacuum level generally depends on the type of application and the sensitivity required. In most industrial settings, including vacuum furnaces for heat treating tool steel, the following guidelines can be considered:

1. Rough Vacuum Testing: In rough vacuum systems (pressures above 1,000 microns or 1 Torr), an acceptable leak rate could be around 50–100 microns per minute of pressure rise after isolating the system. This is often used in preliminary leak tests before achieving high or ultra-high vacuum.
2. High Vacuum Systems: In high vacuum systems (pressures below 10 microns or 0.01 Torr), an acceptable leak rate is typically much lower, often around 5 microns per minute or less. This is crucial for heat treatment applications where minimal oxidation and contamination are required.
3. Ultra-High Vacuum: For applications that require ultra-high vacuum (below 1 micron or 0.001 Torr), the leak rate must be extremely low, often specified as less than 1 micron per hour. Leak rates here are typically measured using helium mass spectrometry rather than simple pressure rise tests.

Practical Example for Vacuum Furnaces

In your case, where you oversee vacuum furnaces for heat treating, maintaining a leak rate of 10 microns per minute or less would generally be acceptable, especially if high vacuum is necessary for preventing oxidation of tool steels. This ensures that any leak is small enough to not significantly impact vacuum quality or process outcomes during typical operating cycles.