Leak detection—Ultrasensitive techniques employing the helium leak detector

The development of closures for electron devices often requires that leaks far below the usual range of the helium mass spectrometer-type leak detector be localized and measured. It has been possible, by using new techniques, to extend the sensitivity of detection several orders of magnitude beyond the normal limit of the instrument. Two highly sensitive methods with similar techniques but separate sensitivity ranges are discussed. In seals similar to those of color television picture tube closures, it is noted that the character of small leaks is such that long times are required to reach a constant maximum rate of helium leakage. Methods for predicting this maximum rate from early observations are discussed. Methods for establishing procedures necessary for sensitivity requirements for any particular seal type are given. Time and sensitivity limitations are discussed. Increases in the sensitivity of detection of the order of 40,000 are attained easily. Leaks as small as 1.5 × 10^-10µliters/second have been detected and measured in 22-inch rectangular color television picture tube closure seals. In one year, a leak of this size would only cause an increase in pressure in an ungettered tube of 10^-7mm Hg. This represents an increase in the sensitivity of detection of about one million.