Air leaks in a vacuum furnace are a prime cause of damage to furnace internals such as shielding or insulation, as well as heating elements and load supports. A rate of rise test is a quick way of checking a furnace's integrity during the normal production schedule using only the instrumentation available on the average control panel.

The procedure is to pump down the furnace using its pumping system. The main vacuum valve is then closed, and the pressure inside the furnace is observed over some timed interval, usually ten or fifteen minutes. That information is then used to calculate the pressure rise over a one hour period.

Ideally, the test is repeated after the furnace soak cycle, just before backfilling for cooling. Leaks that cannot be detected in a cold furnace may appear when  at temperature. SECO/WARWICK typically  provides an optional automatic rate of rise test sequence into the furnace control system, which is performed both before the heating cycle and after the soak, and can be set up to either alert the operator, or lock out the next cycle, or both.

A tight furnace will have a pressure rise of only around fifteen microns per hour or less. If the rate of rise is over fifty microns per hour, the furnace should be scheduled for leak detection when production permits.

Rates over a hundred microns per hour mean that damage is probably being done, and the leaks should be located as quickly as possible.

Rich Roberts is the Project Engineer for the USA Vacuum Team