System must be evacuated to less than 1.0 Millibar absolute. After the system has been evacuated the vacuum pump should be isolated from the system. As a guide, with constant ambient conditions, the vacuum should not rise more than 0.13 Millibar in one hour. A greater rate of rise may indicate a leak or the presence of moisture. Absolute vacuums must be measured using accurate measuring equipment selected for the specific application.
With this method it is only possible to determine the total leak rate. After the test object is evacuated with a vacuum pump or a vacuum pump system, a valve is used to isolate the test object from the vacuum pump. The pressure will then rise as a function of time. Pressure rise can exist due to outgassing from the surfaces of the test object. This pressure rise tends to tail off in the direction of a saturation level. If in such a case the time allowed for monitoring the pressure rise is too short, a leak will be indicated which in reality does not exist. If one waits long enough for the pressure to rise, the outgassing process can then be disregarded, so that the leak rate can be determined from the known volume of the test object and the measured pressure rise over a fixed rise time. In practice, where outgassing and leak rate are added together, the detectable leak rate depends on the volume of the test object, the obtained ultimate pressure and the outgassing from the test object. In connection with a very large test object, this method is time consuming if extremely low leak rates are to be determined in the fine and rough vacuum range. In practice, we have found a recorder to be useful in these circumstances.