Reduced Leak Calibration Uncertainties by the Outgassing Quantification Method

Standard leaks are used throughout industry for various applications, such as gas transfer, calibration of mass spectrometers, and calibration of leak detectors. The ability to provide a controlled delivery of gases at relatively low flow rates makes standard leaks a popular choice for calibration standards. This ability also allows for precise quantities of gas to accumulate over time. The accumulation of gas from standard leaks is used as a “transfer standard” for making in-process adjustments to measurement instrumentation. With the advent of tighter quality control constraints, the quantification and control of leak rates is becoming an increasingly important matter, placing increased demands on the understanding of leak behavior and on the accurate measurement of leakage rates.

Because of stringent quality constraints imposed by industry and technological advancements, it is essential to develop a reliable and accurate method of calibrating standard leaks. The purpose of this research was to design, implement, and validate a leak calibration stand prototype that can detect and reduce systematic errors that lead to high calibration uncertainties of standard leaks. The main contributors to high calibration uncertainty were caused by internal leaks and the outgassing from the internal surfaces, fittings, and valves within the vacuum chamber. In order to quantify the rate at which the internal surfaces of the vacuum chamber outgassed, a custom vacuum chamber was constructed inside an oven to test the effects of outgassing at various pumpdown intervals and baking temperatures. Statistical methods were used to obtain a better insight on the selection of materials, pumpdown intervals, and baking temperatures needed to achieve low and repeatable outgassing rates.

This research concluded that the quantification of leakage and outgassing will completely eliminate the once unknown uncertainty component of pressure measurements in vacuum. This discovery led to the realization that an additional isolated volume can be used to “capture” the accumulation of leakage and outgassing and to correct for pressure measurements during standard leak calibrations.