As mentioned, there are several sources of gas entering a vacuum system. In order to quantify the gas load in a vacuum system it must be leak tight and vacuum clean, meaning there are no finger prints, back streamed hydrocarbon pump oil, build up of oversprayed films, or other sources that may outgas at low pressures. Once an operator is assured that their system is clean and leak tight, the hunt for tramp gases can go to the next level. In some cases the contribution to gas load from permeation can be calculated or at least estimated based on known factors like the permeation rate of certain gasses through O-rings like Viton® or other FKM products.
Using a small vacuum science training system as an example there are some basic tests that can be done to quantify system performance. A fundamental test to do on any system is the Rate of Rise test. This test, when evaluated according to some practical vacuum criteria, can determine if a system is clean and leak free, leaking, or dirty, or dirty and leaking.
Figure 1: Vacuum science training system at the KJLC headquarters in Jefferson Hills, PA
The initial objective is to measure the total gas load on the system independent of the pumping stack. This handy performance measure can provide insights into whether a system is clean and leak tight while also considering gas loads emanating from the system due to desorption and permeation. Gas load, or Q, is calculated using the following formula:
Q = (P1 – P2) * V
T
Where:
Q = Gas load
P1 = Base vacuum pressure of the system in Torr, after 4 – 24 hours of pumping
P2 = End of test pressure, in Torr, after 120 seconds (or more)
V = Volume of the vacuum chamber, in liters
T = Time, in seconds
The chamber on the vacuum science training system is nominally 8.25” tall and 12” in diameter. That converts to 15,343 cm3 or 15.3 liters. After pumping for about 24 hours it achieved a pressure of 2.02 x 10-6 Torr. To do the test the system was isolated from its pumping stack and the moment the valve closed a timer is started. In the next 120 seconds the internal pressure rose to 1.78 x 10-4 Torr. Using the formula above that translate to a rate of rise of:
2.24 x 10-5 Torr.liters/second
Note that it takes in the volume of the chamber so the measure is independent of size. For an unbaked vacuum system there are some rules of thumb on how to evaluate this factoid.
If the rate of rise is less than 1 x 10-5 Torr.liters/second, the system is in excellent shape, clean and leak free.
1 – 3 x 10-5 Torr.liters/sec = Good, clean leak free
4 – 6 x 10-5 Torr.liters/sec = Probably dirty, maybe a small leak
7 – 9 x 10-5 Torr.liters/sec = Probably dirty and leaking
1 x 10-4 Torr.liters/sec = Dirty and leaking
The rate of rise for the vacuum science training system is acceptable at less than 3 x 10-5 Torr.liters/sec. With this number we can now estimate the contribution via permeation to the rate of rise due to elastomer O-rings.
Figure 2: Formula to calculate the permeation rate through an O-ring (Ref: 1)
The vacuum science training system utilizes a variety of flanges, temperature gauges, conductance constraints and engineered leaks. Primarily it has two large Viton® O-rings on the top and bottom of the chamber. Most other seals are the metal sealed ConFlat®design. With a chamber diameter of 12”, the length of the pair of O-rings is 96 cm and the height of the O-ring is 5mm, for an exposed area of 48 cm2. The pressure gradient in the system is 760 Torr less 2 x 10-6 Torr, or 759.000008 Torr. The permeation coefficient for Viton® or FKM type O-rings for air at room temperature is about 10-10 cm3.cm/sec.cm2.Torr (see below).
Figure 3: The relation between permeation rate and temperature for various gasses through Viton®, (Ref: 2)
With only a modest amount of hand waving, the contribution to the gas load from the O-rings can be calculated and is about 1.824 x 10-9 Torr.liters/sec. This is decades away from the calculated leak up, or rate of rise, results of 2.24 x 10-5 Torr.liters/sec. If the mission it to achieve a lower base pressure for the system, below 2 x 10-6 Torr, as a practical approach the first effort should be to lower the gas load, but the O-rings are not the first place to look.
References:
1. O-ring permeation coefficient chart for common materials, Marco Rubber & Plastics, available on line, 2018.
2. Comparison of permeation of atmospheric gases through Viton O-ring gaskets for different initial conditions, Makfir Sefa, and Janez Setina, Journal of Vacuum Science & Technology A 35, 041603 (2017); doi: 10.1116/1.4984292
Category: Vacuum Systems
Sub-Category: CF Flanges & Components
Related Topics: O-ring, Assembly, Maintenance
