This is a comparative evaluation of errors in testing engines using reference methods and portable analyzers. New Mexico Environment Department (NMED) Air Quality Bureau has published a guidance document, Standard Operating Procedure (SOP), Use of Portable Analyzers in Performance Tests. EPA provides guidance on reference method testing in 40 CFR 60, Appendix A. This document is based primarily on these two sources.
Reference method testing and portable analyzer testing are capable of comparable accuracy. The aggregate errors for each approach, if the guidance documents are followed rigorously and other sources of error are ignored, are approximately 15% to 19% for reference method testing, and 28% for portable analyzer testing (note that this is the worst-case product of errors, based on the sum of the allowable or physically realistic error and drift for each component of the test method, and not a prediction of error).
However, the guidance documents for each approach do not address all potential sources of error. Potential errors in determining volumetric flow for portable analyzer testing by calculation (the use of 'F' factors), in particular, can be very large and are not addressed by the guidance documents.
Primary sources of error in determining volumetric flow by means of 'F' factors for use in conjunction with a portable analyzer that are not addressed by guidance documents (including EPA's Method 19) are as follows.
- Moisture from combustion
- Accuracy of fuel flow measurement
- Fuel gas composition other than assumed
These error sources are generally not addressed, and can have significant impact on calculated volumetric flow. The errors are not with the method, but are failures of guidance documents to address these sources of error.
This paper analyzes the allowable errors in each component of both reference method and portable analyzer testing as described by their respective guidance documents, and addresses potential error sources not addressed by the guidance documents, estimating the potential error from each.