Vapor Stream Dilution by Pulse-Width Modulation
NAVAL RESEARCH LAB WASHINGTON DC
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The ability to prepare accurate calibrated vapor samples is of profound importance in the use and development of analytical instruments for gas phase environmental monitoring. A variety of methods have been developed for the generation of calibrated vapor streams. A pulse-width modulation method for the dilution of calibrated vapor streams is described. The critical assumption of the method is that the flow rates of the vapor and clean carrier gas streams being modulated are identical and constant, and that this condition is maintained even as valves are actuated. Under these conditions, dilution occurs in ratios determined by duty cycle timing. Flow perturbations occurring on valve-actuation can significantly influence whether dilutions achieved match those calculated by duty cycle timing. Methods to eliminate such perturbations by careful placement of flow restrictions in the vent system are described. The effects of organic liquid vapor pressures on the dilution method are also described. Keywords Chemical detectiondetectors Vapor generators Vapor dilution.
- Pumps, Filters, Pipes, Tubing, Fittings and Valves
- Test Facilities, Equipment and Methods
- Fluid Mechanics