VS Series Leak Detectors for Vacuum Furnaces

Introduction

All vacuum furnaces operate by controlling the gas

environment within the chamber allowing certain transfor-

mations 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 para-

mount 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 furnace. 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 open-

ing 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 adequate levels mis-

leading 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 Pump Down 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 addi-

tional gas load and can come from a variety 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 evacu-

ation process. After a short stabilization time, observe

in Figure 2 the rise in pressure (⌬P) over time (⌬T) or,

in other words, a vacuum decay.

Vacuum decay is the difference in the vacuum levels

at the beginning and end of the measurement divided

by elapsed time and can be measured quite accurately.

In the industry, it is normally expressed in microns per

hour. (For most vacuum applications, a vacuum decay

exceeding 10 microns per hour is usually unacceptable).

Observing the pump down cycle and performing a rate of

rise test are affected by the overall cleanliness of the fur-

nace and might not lead to an immediate detection of the

problems. Moreover, it will be quite difficult and time con-

suming to determine that the problems are caused by a

leak in the vacuum system.

–1

–3

Pressure (Torr)

Real Leak

Outgassing

Clean System

0 5 10 15 20 25 30

Time (min)

Fig 1. Chamber pumpdown characteristics

0 5 10 15 20 25 30 35 40

Stabilization

Outgassing

Real Leak

valve

Real Leak

Pressure (mTorr)

Time (min)

Figure 2. Rate of Rise plot