The CVS-1 system, sometimes termed variable dilution sampling, is designed to
measure the true mass of emissions. . A large positive
displacement pump draws a constant volume flow of gas through the system. The exhaust of the vehicle is mixed with filtered room air and the mixture is then drawn through the pump. Sufficient air is used to dilute the exhaust in order to avoid vapour condensation, which could dissolve some pollutants and reduce measured values. Excessive dilution on the other hand, results in very low concentration with attendant measurement problems.
A pump with capacity of 30-350 cfm provides sufficient dilution for most vehicles.
Before the exhaust-air mixture enters the pump, its temperature is controlled to
within +or – 10F by the heat exchanger. Thus constant density is maintained in the sampling system and pump. A fraction of the diluted exhaust stream is drawn off by a pump P2 and ejected into an initially evacuated plastic bag. Preferably, the bag should be opaque and manufactured of Teflon or Teldar. A single bag is used for the entire test sample in the CVS1 system.
Because of high dilution, ambient traces of HC, CO or NOx can significantly
increase concentrations in the sample bag. A charcoal filter is employed for leveling ambient HC measurement. To correct for ambient contamination a bag of dilution air is taken simultaneously with the filling of the exhaust bag.
HC, CO and NOx measurements are made on a wet basis using FID, NDIR and
chemiluminescent detectors respectively. Instruments must be constructed to accurately measure the relatively low concentrations of diluted exhaust.
Bags should be analyzed as quickly as possible preferably within ten minutes after
the test because reactions such as those between NO, NO2 and HC can occur within the bag quite quickly and change the test results
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