Fig. 2-9 Schematic configuration of a thermal micro-balance A micro-balance is wholly installed in a vacuum vessel. There are a heater, a gas inlet, and a pumping port around a sample piece. An electric current is excited by electromagnetic induction in a coil located near a permanent magnet, when the balance moves. This current can be converted to the weight.

Fig. 2-10 Time development of a weight change of the piece of 500 degrees cent. titanium alloy (18.77129g) due to hydrogen accumulation. The alloy is used for fusion facilities The titanium alloy increase its weight in stepwise behavior (like breaths) as a function of time. The vertical line designate molecule numbers obtained from the gas weight.

A gas quantity released from solid material is usually obtained from a pressure increase in a constant volume of the vessel. We have a high sensitivity technique of measuring the pressure increase, but complex surface conditions of the inner wall cause an error. There is no room for error, if we measure directly a weight of gas absorbed in the solid material using a micro-balance. In term of quality control of vacuum, it becomes important to get an exact amount of gas absorbed in the solid material in many cases such as a fusion device and vacuum processing of food and so on. A micro-balance is used exploiting electromagnetic forces to measure a very small quantity (Fig. 2-9). We need particular ideas in the combination of electronics and a computer system to obtain a stable null point in a high sensitivity condition. A system is composed of a sample piece, a heater which discharges gas from the sample and a gas inlet. We measured the weight increase of the piece of titanium alloy in making a constant gas injection under a constant temperature of 500 degrees cent. The weight increase with time development is shown in Fig. 2-10. The titanium alloy does not absorb hydrogen at one stretch, that is to say, absorbs and adsorbs hydrogen like human breaths in a stepwise increasing accumulation. This phenomenon is interesting in a point of view of surface physics.

Reference M. Hamasaki et al., Fabrication of Electromagnetic Force Balance for Vacuum Application, Shinku, 36 (3), 125 (1993).

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