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Combustion of organic motor vehicle fuels produces carbon dioxide and water (H2O) vapor (and also products of incomplete combustion, e.g. hydrocarbons and carbon monoxide, at lower concentrations). The Constant Volume Sampling (CVS) system, commonly used to condition auto exhaust for sampling and analysis, provides for controlled ambient air dilution of the engine exhaust. Water condensation can be a problem during CVS system sample conditioning, depending upon vehicle fuel composition and fuel economy, and diluent air humidity and exhaust/diluent ratio. This paper describes a “spreadsheet” procedure for detailed, second by second, determination of diluted exhaust dew point and the necessary CVS system flow rates to avoid H2O vapor condensation.