Experiences in vacuum testing of the 44″ cyclotron of Osaka University are discribed, which we have carried out since the end of last year.
In order to operate this cyclotron at the pressure of about 10^-3μHg, we had to detact so small leakage that was difficult to locate by usual leak location method and it was undesirable to use volatile liquids such as alcohol which filled up small leakage temporarily.
By the use of the massspectrometer using hydrogen gas we could detect leakage as small as 0.01 μl/min. easily.
Sensitivity proper to its leak detector decreased to about 1/51/10, owing to the fluctuation of the background of hydrogen peak.

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After the ingenious studies of Blears and Leck, we have investigated the differential method of leak detection by using ionization gauges. We attempted a special probing technique and estimated the substitute sensitivity factor φ of ethylether, chloroform and aceton, at 5.4, 3.5, 3.5 respectively. And we also pointed out the complex phenomena in actual leak detection. With differential condensation and ionization gauges, the minimum detectable change in air was 1×10 ^-4μHg.

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Notes on the differential method of Leak detection using two pirani qauges is teseribed.
Carbon deoxide, Buthane, Ether and Ethyl Alcohol as probe gases were trapped by a liquid air trap.
Buthane was proved the best and the minimum detectable leak rate was 2x10^-3μl/sec.

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Details of the design and constructions of lowimpedance high vacuum Valves are described.
The valve lid is rotated II/2 radians with the combined action of a cam and a spring so that the lid may have the position completely parallel to the direction of flow.

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The sensitivity of Fogel type ionization gauge was determined in the pressure range between 5×10^-5mm and 3×10^-4mm. of mercury by comparison with a Pirami gauge, which had been carefully calibrated by a McLead gauge. The vacuum factor K… the ratio of collector current to emission current… was found to amount to 0.61μA/mA. for oxygen and 0.35μA/mA. for hydrogen at 1×10^-4mm. of mercury.
Next, Bayard type was compared with Fogel type in the region extending from 2×10^-7 to 5×10^-5mm. of mercury. The residual vacuum factor due to the electron emission from the collector surface of Fogel type was estimated to be 1.6±0.5×10^-4μA/mA.

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A four stage oil diffusion pump has been const-ructed and its characteristics have been measured. The pump is all made of metal (in let opening diameter 613in.), and has a large boiler area, geometry of its second stage being specially designed after the idea of P. Alexander. The maximum speed abtained is ca. g30l/sec at 10^-4 mmHg, corresponding to 44% Ho-crefficient, (for the throat area of the first stage) and the critical backing pressure is about 4×10^-1mmHg.

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A new method of Trapping oil Vappor back diffusing from the fine side jet of an oil diffusion pump, formarly proposed with some experimental results, was applied to three stage specially designed fractionating pump.
The ultimate vacuum of 7.4×10^-6mm Hg was reduced to 1.4×10^-6mm Hg by using the trap. More effect of the trap is expected by introducing an other oil suitable for the purpose.

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