The time it takes for helium to reach a leak detector in a vacuum furnace can vary depending on several factors, including the size of the leak, the pressure inside the furnace, the distance between the leak and the detector, and the sensitivity of the detector itself.

In general, helium is often used for leak detection because it is a small, inert gas that can quickly escape from even small leaks and is not normally present in the atmosphere in large quantities. The process typically involves pressurizing the system with helium and then using a detector to sense for the presence of helium outside the system, indicating a leak.

For small leaks, it may take a while for enough helium to accumulate at the detector to register a reading, while larger leaks will result in a more immediate response. The speed of detection can range from seconds to several minutes. It’s important to follow the manufacturer’s guidelines for the specific leak detection equipment being used to get a more accurate estimate.

If you’re looking for a specific calculation of time for helium to reach the leak detector, this would involve complex fluid dynamics calculations that take into account the variables mentioned above.