Ammonia, often present in exhaust gas samples, is a polar molecule gas that interacts with walls of the gas sampling and analysis equipment resulting in delayed instrument response.
A set of experiments quantified various materials and process parameters of a heated sample line system for ammonia (NH3) response using a Fourier Transform infrared spectrometer (FTIR). Response attenuation rates are due to mixing and diffusion during transport as well as NH3 wall storage. Mixing/diffusion effects cause attenuation with a time constant 1-10 seconds. Wall storage attenuation has a time constant 10-200 seconds.
The effects of sample line diameter and length, line temperature, line material, hydrated versus dry gas, and flow rate were examined. All of these factors are statistically significant to variation of at least one of the time constants.
The NH3 storage on the sample system walls was calculated as a function of the experimental test as well. In this case, line length and diameter were not statistically significant, but line temperature, flow rate, and material were. Hotter sample lines, hydrated gas streams, moderate to high flow rates, and small stainless steel lines gave the fastest response and least NH3 storage.