Introduction

All vacuum furnaces operate by controlling the gas environment

within the chamber allowing certain transformations of the material

to reach desired parameters. When a leak occurs, integrity of the

material being processed can be compromised. Therefore, a leak

tight system is paramount for consistent, accurate treatment

of any material.

Application Description

Every vacuum furnace eventually develops leaks that might affect

product quality and/or damage internal components. No matter

what the type of furnace all must maintain a leak free environment

to attain purity levels necessary for controlled reactions in the fur-

nace. Those leaks can develop in multiple locations – valves,

feedthroughs, door seals, etc. During process conditions, when

temperatures are elevated, different metals expand at different

rates sometimes opening up leaks that are not present at lower

temperatures.

Large leaks in a vacuum furnace will be very obvious. In these

cases, the furnace will not pump down and/or the treated material

will show clear signs of oxidation. Small leaks however often go

undetected, as the pumping system can easily offset the gas load

of the leak. As a result the vacuum gauge(s) might still show ade-

quate levels misleading the system operators, however, small leaks

can result in major damage and scrap depending on the

application or process.

Alternative Leak Detection Methods

Observe the Pumpdown Cycle

Compare the furnace pump down cycle with a previous cycle made

when the system was in a good working order. Evaluation of the

pressure vs. time curve, like the example shown in Figure 1, can

indicate the presence of a leak. If the vacuum level is slow

to reach the original base pressure, then outgassing is suspected.

Outgassing is simply additional gas load and can come from a vari-

ety of sources. With outgassing the system pressure will decrease,

albeit more slowly than normal. A real leak however will cause the

pumpdown to stall at a higher than desired pressure.

Perform Rate of Rise (RoR) Test

During this test the operator can close the valve between the

vacuum pump and the chamber. This stops the evacuation process.

After a short stabilization time, observe in Figure 2 the rise in pres-

sure (⌬P) over time (⌬T) or, in other words, a vacuum decay.

2 Leak Detection for Furnaces www.agilent.com/chem/leakdetection

VS Series Leak Detectors for Vacuum Furnaces

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Stabilization

Outgassing

Real Leak

valve

Real Leak

Pressure (mTorr)

Time (min)

Figure 1

Rate of Rise Plot

Figure 2

–1

–3

Pressure (Torr)

Real Leak

Outgassing

Clean System

0 5 10 15 20 25 30

Time (min)

Chamber Pumpdown Characteristics