A low-pressure chiller has been evacuated to the required vacuum level. Before charging, a technician performs a standing vacuum test and observes that the system pressure rises from 25 inches Hg vacuum to 10 inches Hg vacuum over 10 minutes. What does this pressure rise most likely indicate?

Why the other options are wrong

• A. While temperature changes can affect pressure, a rise from 25 inches Hg vacuum to 10 inches Hg vacuum (a significant loss of vacuum) in a short period is too drastic to be solely attributed to ambient temperature increase, unless the system was previously very cold and rapidly warmed, causing residual moisture to boil. However, a leak is the more common and direct cause of such a rapid and significant loss of vacuum.
• B. If the vacuum pump was not isolated, the pressure would likely drop further if the pump was running, or stay stable if the pump was off and the system was tight. A rise indicates external ingress.
• C. If residual refrigerant were boiling off, the pressure rise would typically stabilize at a higher but still sub-atmospheric pressure corresponding to the refrigerant's saturation temperature, or it would continue to rise if there was a large amount. A continuous rise suggests ingress.