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<br />precisely metered amount of hydrogen added to the incoming sample. The water vapor product of <br />this reaction is removed by drawing the sample through water permeable Nafion tubing over which <br />a dry stream of nitrogen passes. The drive to equalize the concentration on either side of the Nafion <br />membrane pulls the water into the nitrogen stream, which is then exhausted from the instrument. The <br />sample then passes through the detector containing a radioactive source emitting thermal electrons. <br />A voltage potential (in this case pulsed) applied across the detector forces the free electrons to <br />collect at the detector anode, providing a steady-state current in the nano-amp range. As an electron- <br />capturing compound such as SF 6 enters the chamber, electrons are absorbed by the compound, fewer <br />pass to the anode, and a decrease in current is observed. An increase in concentration of the trace <br />gas results in a measurable current decrease. An electrometer, electronic filtering and amplification <br />then condition the detector signal for the user. Both the catalytic conversion and the sample air <br />drying process are very temperature sensitive, therefore a means has been provided to monitor and <br />control process temperatures to within plus or minus one degree Celsius. <br /> <br />Specifications <br />Range: 5 to 5000 pptv <br />Response Time: < 400 ms (0-63% ofa step change in SF6 concentration) <br />Lag Time: ~ 1-2 s <br /> <br />Calibration: <br />Multiple-point calibration performed onboard before each flight; inflight checks were <br />accomplished as time permitted. <br /> <br />4.11 Liquid Water Content <br /> <br />A CSIRO-King developed instrument for the measurement ofliquid water content (L WC), <br />manufactured by Particle Measuring Systems as PMS Model KL WC-5, was installed outboard below <br />the right wing on the research aircraft. The sensor element is a heated wire coil maintained at a <br />constant temperature much higher than the ambient air or any liquid water which may impact on it. <br />The coil is the dependent resistance in a bridge circuit and the power used to keep it heated is directly <br />proportional to the mass of water impacting on it. <br /> <br />Specifications <br />Range: 0 to 1, or 0 to 6 g m-3 <br />Response Time: 50 ms <br />Resolution: 3.5 mat 75 knots <br /> <br />Calibration <br />Manufacturer: Performed December 1988. <br />Periodic checks are accomplished in the field to assure that the unit is responding properly to <br />water impacting on the coil. Comparisons are also made with the FSSP and OAP probes. <br /> <br />Operational procedures <br />The instrument was installed and connection made to the data acquisition system before the <br /> <br />15 <br />