Vol.2 No.3 2009
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Research paper : A marked improvement in the reliability of the measurement of trace moisture in gases (H. Abe)−215−Synthesiology - English edition Vol.2 No.3 (2009) in the accuracy and stability of the indications, while the product of Company B showed problems in response and accuracy. Because a chilled-mirror hygrometer can directly measure the frost point, it has the highest reliability among instruments measuring humidity. However, it was demonstrated from this result that the measurement in the trace moisture range was not straightforward, even with the chilled-mirror hygrometer.Performance tests were carried out on some conventional aluminum oxide capacitive sensors that are widely used in industry. The products of five companies were tested, and the lower limit of the measurement range specified in the catalogs was a frost point of -100 ºC (14 nmol/mol (ppb)) or less for all the products. The flow rate of gas introduced into the sensor was 0.6 L/min to 1.0 L/min, and dry purging (the removal of residual moisture) was performed for over one month before the test using the trace moisture of 12 nmol/mol (ppb). The long purging period was set to investigate the performance under actual conditions, because conventional aluminum oxide capacitive sensors are often used as an alarm indicating the presence of moisture in the processes that involve drying, and the sensors are exposed to an extremely dry atmosphere for a long time. The test was performed in the range of between approximately 12 nmol/mol (ppb) and 100 nmol/mol (ppb) (a frost point between about -100 ºC and -90 ºC). The product of one of the five companies indicated an error, that the level of moisture was below the measurement range, and measurement was not possible. The results of the products of the other four companies are shown in Fig. 15. The sensors exhibited problems in response, accuracy, and sensitivity of the indications. Similar tests were performed in joint research between BOC Edwards, Ltd. and NPL, and it was reported that some sensors required 8 h or more to respond to the moisture concentration change of 300 nmol/mol (ppb) 850 nmol/mol (ppb)[24]. Because the conventional aluminum oxide capacitive sensor uses the equilibrium of moisture adsorption in the pores of aluminum oxide, it is deduced that the slow response is responsible for the long time required to reach the equilibrium state in the trace moisture range. Therefore, technological development to increase the response is necessary for the measurement using conventional aluminum oxide capacitive sensors in the trace moisture region.Previously, there was no trace moisture standard, and it was difficult to change the moisture concentration quickly in the trace moisture region. Therefore, there was no method of reliably testing the performance of trace moisture analyzers, particularly in terms of response, but recently, this has become technologically possible owing to the development of the trace moisture standard and trace moisture generation technology.6 Future developmentsTrace moisture measurement traceable to the SI was realized by establishing the national standard, the commercial availability of the CRDS trace moisture analyzer, and organizing the calibration service system. The results of the performance tests presented in chapter 5 clearly show that the reliability of the trace moisture measurement has increased markedly in recent years. However, a CRDS trace moisture analyzer is necessary to perform such measurements, which not all people can easily access because it is an expensive instrument. Therefore, from the perspective of the dissemination of highly reliable trace moisture analyzer, there are still issues concerning the commercial availability of a measuring instrument. To solve these issues, we must wait for the development of low-cost high-performance trace moisture analyzers. Manufacturers are advancing Fig. 13 Comparison of standard value and indication of CRDS trace moisture analyzer.Fig. 14 Comparison of standard value and indication of chilled-mirror hygrometer.StandardIndication1501001005000200Time / hxw / (nmol・mol-1)Time / hTime / hxw / (nmol·mol-1)xw / (nmol·mol-1)Company ACompany BStandardIndicationStandardIndication002040600100200010020030040050100150

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