With the recent acceptance of the Bag Mini-Diluter (BMD) for low-level exhaust emissions measurement, there is an increased interest in exhaust flow measurement technology throughout the industry. Accurate tailpipe exhaust flow measurements can be considered the most “difficult” aspect of Bag Mini-Diluter systems due to the heat, water content, and transient behavior of the samples.
Acceptance by the EPA and ARB of the Bag Mini-Diluter sampling system was based on the design and performance of the Flow Technologies E-Flow system with UGF-5 ultrasonic electronics. A number of noteworthy advancements have been made to this exhaust meter since the Agency's acceptance, and there appears to be a number of new technologies entering the market. At this time, ultrasonic flow measurement is the most prevalent and robust technology, however, other alternatives are worthy of consideration.
The primary focus of this paper is to update the industry as to performance capabilities of current certification systems and to describe the requirements and desired features for future products. We will outline the general guidelines for exhaust flow meter performance in terms of accuracy, repeatability, and response characteristics. The tools needed to verify meter performance are described and data are presented for various exhaust flow meter designs. We will address the impact of the performance criteria on FTP and HwFET test results, and finally, we will recommend the performance improvements needed by industry to enhance the value of today's products.