Vol.2 No.3 2009
Research paper : A marked improvement in the reliability of the measurement of trace moisture in gases (H. Abe)−208−Synthesiology - English edition Vol.2 No.3 (2009) the traceability to the SI may be guaranteed by calibrating the standard using a national standard of the same quantity established in another country, and many developing countries often select this method. In contrast, many advanced countries develop a standard called a primary measurement standard that is directly traceable to the SI. The method adopted depends on the technology, cost, and level of the domestic industry. Because there are advanced semiconductor and process industries in Japan, AIST decided to develop its own primary measurement standard.Another very important task is selecting the generation method for the trace moisture generator. Here, AIST selected a unique method, different from those of national metrology institutes of other countries. The reasons were because we had a specific plan for this unique method, we considered it possible to achieve the world’s highest precision if this unique method was successful, and the reliability of the standard could be increased by maintaining multiple national standards using different methods around the world, rather than relying on a single method.3.1 Selection of the generation methodThere are several methods of generating trace moisture, and AIST selected a method called the diffusion tube method. Figure 4 shows a schematic diagram of the trace moisture generator using the diffusion tube method. A diffusion cell is composed of a small water vessel and a diffusion tube made of metal such as stainless steel, and this is placed in the generation chamber under controlled temperature and pressure. The water vessel of the cell contains water, and water vapor with pressure that responds to a given temperature is generated inside the vessel. The vapor passes through the diffusion tube and travels to the generation chamber. Trace moisture is generated by mixing the vapor with flow-controlled dry gas in the chamber. The amount-of-substance fraction of water in the gas is determined by measurement of the mass of moisture that evaporates per unit time (evaporation rate) and the mass of dry gas that flows per unit time (flow rate).National metrology institutes other than AIST commonly employ a method called the frost point method as the core of their low-humidity and trace moisture generation system. Figure 5 shows a schematic diagram of the frost point generator. A gas is passed through a saturation chamber that is saturated with water vapor evaporated from ice at a constant temperature, and humid gas is generated by mixing the vapor and gas. When saturation is complete and the state of equilibrium is achieved, humid gas with a frost point equivalent to the temperature of ice is generated. Trace moisture can be generated using low-temperature ice (-75 ºC or less). The amount-of-substance fraction of water in the generated gas is determined by measuring the ice temperature, the pressure inside the generation chamber, as well as the vapor pressure formula of ice and the enhancement factor of ice Term 3 . Although the principle of this method is simple, it is highly reliable for generating humidity.Table 1 gives a comparison of the advantages and disadvantages of the two methods. One of the advantages of the diffusion tube method is that it does not require the achievement and confirmation of saturation, which are necessary in the frost point method. In the frost point method, complete saturation in the saturation chamber is necessary, but the state of equilibrium is difficult to achieve in the low-temperature range because the amount of vapor Fig. 4 Schematic diagram of diffusion-tube trace-moisture generator.Temperature andpressure controlDry gas (N2)Trace moisturein gasDiffusion cellDiffusion tubeWater vesselTrace moisturein gasIceGasHeatexchangerConstanttemperatureSaturationchamberFig. 5 Schematic diagram of frost point generator.Past performance as a primary standardZero gasNecessaryMeasurement of extremely small mass loss rateVapor pressure equation and enhancement factorNot necessaryAchievement and check of saturationFrost pointDiffusion tubeGeneration methodNot necessaryNot necessaryNot necessaryNecessaryNecessaryNecessaryNoYesTable 1 Comparison of diffusion tube method and frost point generation method.