An Improved Methodology for Calibrating Hydrazine Monitors

摘要

This paper describes a method for calibrating MDA 7100 hydrazine monitors in the United States Air Forceinventory. The calibration system consists of a KinTek 491 reference gas generation system, ahumidifier/mixer system which combines the dry reference hydrazine gas with humidified diluent or carriergas to generate the required humidified reference for calibrations, and a gas sampling interface.The KinTek reference gas generation system itself is calibrated using an ORNL-constructed coulometrictitration system to verify the hydrazine concentration of the sample atmosphere in the interface module. TheKinTek reference gas is then used to calibrate the hydrazine monitors. Thus, coulometric titration is onlyused to periodically assess the performance of the KinTek reference gas generation system, and is notrequired for hydrazine monitor calibrations. One advantage of using coulometric titration for verifying theconcentration of the reference gas is that it is a primary standard (if used for simple solutions), therebyguaranteeing, in principle, that measurements will be traceable to SI units (I.e., to the mole).The effect of humidity of the reference gas vs. the humidity of the MDA 7100 Chemcassette environmentwas characterized by using the results of concentrations determined by coulometric titration to develop aMDA 7100 reading correction factor graph based on the difference in relative humidity between the KinTekreference gas stream and the MDA 7100’s Chemcassette environment . Using this calibration method,calibration uncertainty has been reduced by 50% compared to the current method used to calibrate hydrazinemonitors in the Air Force inventory and calibration time has also been reduced by more than 20%. Adetection limit of hydrazine using coulometric titration is discussed.We observed a “bias” (I.e., a difference in the slope of our calibration line from the theoretical value) in thecoulometric titration calibration line, even though coulometric theory suggests otherwise. The authorsspeculate on the reasons for this bias in the paper.Finally, the paper describes precautions that need to be taken during the calibration setup. For example,hydrazine is so reactive that relatively lengthy exposures of tubing to the air during manual sampling aresufficient to “upset” the previously conditioned tubing and reduce the observed hydrazine concentrationssubstantially. Using ball-and-socket joints permit very rapid connects/disconnects of tubing, andsubstantially improves accuracy.