Leaks are the bane of vacuum systems. While often masked by issues of vacuum cleanliness, outgassing or artifacts of the process running in the vacuum chamber, leaks can be the major contributor to poor ultimate vacuum pressures. One handy approach to determine if you have a leaking chamber is to plot the rate-of-rise in chamber pressure over time. The rate-of-rise test involves taking a vacuum clean chamber and pumping it for 4 or more hours to its base pressure. This enable to desorption and exhaust of most of the gases that have adsorbed to the chamber interior and its components. When the base pressure has stabilized the chamber is valved off from the pumping system and the rise in pressure is the monitored over time. A good vacuum technologist will have a log book for their vacuum system and have done a rate of rise test on delivery.
The formula for rate of rise in a vacuum system is:
Q = (P2-P1)V/t
Where P1 is the base pressure of the system in Torr, P2 is the pressure in Torr after the high vacuum valve is closed after t seconds and V is the volume of the chamber in liters The units for rate of rise are Torr-liters/sec.
The rate of rise of a chamber with a volume of 6 liters at an increase in pressure of 3 millitorr/minute would be 18 millitorr-liters/minute. Converting that to Torr-liters/sec would be 3 X 10e-4 Torr-liters/sec. For any vacuum system, even ones that are pristine in terms of vacuum cleanliness, there will be a characteristic rate of rise curve. The graphic bellow shows the distinction between the rate of rise curve related solely to desorption of gases from chamber walls and components in the vacuum system. It is labeled ‘pure gas desorption’. There is also a line for ‘pure leakage’. So in a system that has a leak the observer will see a combination of the two curves.
Converting Torr-liters/sec to (atmospheric) standard cc/sec one would be 1 Torr X 1 Atm/760Torr – 1 liter X 1000cc/1 liter / second or about 0.76 atmospheric-cc/sec = 1.0 Torr-liters/sec. Keep in mind that a vacuum clean leak tight vacuum system has a rate of rise due to gas coming off the walls and out of the SS is typically 1 X 10e-5 Torr-liters/sec.
Category: Vacuum Systems