A Thermally Shielded Atmospheric Pressure Standard Leak Calibrator

Abstract

A method is described for the calibration of capillary and permeation standard leaks. The technique utilizes a capacitance manometer to detect the small pressure change in a defined volume caused by the influx of gas from a standard leak. Corrections are derived for the small volume changes which occur during manometer operation. All measurements are made at atmospheric pressure. A double-walled aluminum and copper cylinder design is utilized to decrease temperature related pressure changes. A limiting drift rate of approximately 4×10−8 Torr/sec (Q=3×10−9 std cm3/sec) in the manometer indication was traced to manometer diaphragm “microcreep.” A method to reduce this background to ≃6×10−10 std cm3/sec is described. The unique aspect of the calibrator is its ability to calibrate Freon standard leaks under usage conditions, i.e., using Freon gas with atmospheric pressure on the down-stream side of the leak, down to 10−9 std cm3/sec. Error of measurement ranges from +10% to 15%. Calibration results for several leaks are presented.