Stack samples must be collected at what condition relative to the flow of gases through the stack?

Stack samples must be collected at the same speed of flow that gases normally pass through because this ensures that the samples are representative of the actual emission conditions during normal operations. Sampling at this speed allows for accurate measurement of pollutant concentrations and helps maintain the integrity of the data collected. If samples were taken at a higher or lower speed than normal, it could lead to unrepresentative concentrations of gases, affecting the validity of the sampling results and potentially leading to erroneous conclusions regarding emissions or compliance with environmental regulations. Consistency with the normal flow rate is crucial for obtaining reliable data that reflects the routine operational status of the source.