Question

When using Constant Volume Sampling (CVS) to measure tailpipe emissions, what is the fundamental requirement for the system to ensure accurate pollutant concentration data?

Accepted Answer

The fundamental requirement for accurate Constant Volume Sampling is to maintain a constant total volumetric flow rate of the mixture consisting of vehicle exhaust and dilution air. Because the absolute concentration of pollutants in the raw tailpipe exhaust changes constantly as the vehicle accelerates or decelerates, measuring the concentration directly is prone to error. By adding a large, constant volume of clean ambient air to the exhaust, the system creates a dilute mixture with a stable total flow rate. This process ensures that the mass of a pollutant is directly proportional to its concentration in the sample bag. Because the total volume of the mixture is held constant by a critical flow venturi or a positive displacement pump, the concentration measured in the dilute sample represents the true mass emission rate of the vehicle over the duration of the test cycle regardless of fluctuating exhaust gas flow rates from the tailpipe.

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When using Constant Volume Sampling (CVS) to measure tailpipe emissions, what is the fundamental requirement for the system to ensure accurate pollutant concentration data?

The fundamental requirement for accurate Constant Volume Sampling is to maintain a constant total volumetric flow rate of the mixture consisting of vehicle exhaust and dilution air. Because the absolute concentration of pollutants in the raw tailpipe exhaust changes constantly as the ....