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Frequently Asked Questions

The Rate of Rise, or ROR, test is handy to determine if a vacuum system leaks. While it will not identify where a system is leaking it is a good first pass to determine what level of leak detection may be required to find exactly where a leak exists if there is one.

Using the formula:

Q = ((P2 – P1)*V)/T where the result is in Torr.Liters/Second Where:

Q = Throughput, or Gas Load, or Rate of Rise
P1 = Pressure at the beginning of the test
P2 = Pressure at the end of the test
V = The volume of the chamber, in Liters
T = The time between readings of P1 and P2 , in Seconds

As an example, we performed an ROR test on a small vacuum system we use for our Practical Vacuum Course. This system was designed specifically to enable students to operate, deconstruct and reconstruct a simple vacuum system using good vacuum cleanliness practices while learning concepts such as conductance, pump throughput, effective pumping speed, and the impact of things like finger prints on base pressure.

Figure 1. Training system for the Lesker U Practical Vacuum Technology course

This small system is typically kept under vacuum and handled with the best available vacuum practices, so we don’t expect to find a leak. However, we can determine the health of the system looking at its ROR.

To perform the ROR test we pumped on the system overnight using a small rotary vane pump and an adorably tiny turbo pump. Using this pumping stack our (unbaked) system achieved a base pressure of 7 x 10-6 Torr. The chamber was then valved off from the pumping system and the pressure rise was timed with a stop watch. After 100 seconds the pressure in the system increased to 1.4 x 10-4 Torr. The volume of the chamber is nominally 15 liters.

Using the equation above:

Q = ((1.4 x 10-4 Torr – 7 x 10-6 Torr)*15 liters)/100 seconds
which reduces to, roughly, to a ROR of 2 x 10-5 Torr.Liters/sec

Once any vacuum chamber is isolated from the pumping system, there will always be a pressure increase due to pure gas desorption―typically water and other gases desorbing from the chamber walls. This is illustrated in the following graph. The pure gas desorption curve starts out with an aggressive slope and then ‘flattens’ out as the rate of desorption decreases. If the chamber has a leak and we were able to isolate the pressure increase due to that defect, it would be a straight line of fixed slope from the point at which the pumps were isolated up to atmosphere. The combination of the two curves is an obvious indicator of a leak.

Figure 2. Pure gas desorption curve combined with a leak
[Ref. A User’s Guide to Vacuum Technology, O’Hanlon]

The magnitude of the leak up rate can determine if a vacuum system is leak tight, dirty, leaking, or dirty AND leaking. In general, a ROR can be interpreted according to the following table:

ROR < 1 x 10-5 Torr.Liters/sec = Excellent, leak free and clean
ROR = 1 to 3 x 10-5 Torr.Liters/sec = Leak tight and clean
ROR = 4 – 6 x 10-5 Torr.Liters/sec = May be dirty
ROR = 7 – 9 x 10-5 Torr.Liters/sec = May be leaking or dirty
ROR > 1 x 10-4 Torr.liters/sec = May be leaking AND dirty

In any case, it is important to make sure a vacuum chamber is vacuum clean before chasing leaks. So if the ROR test yields a leak up rate on the order of 4 x 10-5 Torr/Liters/sec or larger the first course of action should be to break vacuum and do an appropriate cleaning of the vacuum chamber. It is also a good idea to check the health of the pumping system. Chamber cleaning and pump system diagnostics will be discussed in future FAQs.

In the case of our small training system, with an ROR of 2 x 10-5 Torr.Liters/sec, we can be assured that the system is leak tight and clean. It better be, considering the number of students going through the program!

Category: Vacuum Systems

Sub-Category: CF Flanges & Components

Related Topics: Assembly, Maintenance, Safety, Pressure, Leaks

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