Where to Install CEMS Probes, Flow Sensors, and Opacity Monitors

Title 40, Appendix B to Part 60 — through Performance Specification 1 (Opacity), Performance Specification 2 (CEMS), and Appendix A (Flow) — clearly defines where ports for CEMS probes, flow sensors, and opacity monitors need to be located on a stack or duct.

For any stack monitoring equipment to produce defensible data and pass a Relative Accuracy Test Audit (RATA), the sample location needs to reflect the actual emissions profile of the source. That means prioritizing laminar flow and avoiding areas where stratification or turbulence could produce skewed results. Each device type has its own placement criteria.

Flow Sensor Location Requirements

Flow sensors carry the most stringent siting requirements of the three — and need the longest run of straight stack or duct to meet them.

The standard requires sampling and velocity measurement points to be located at least eight stack or duct diameters downstream and two diameters upstream from any flow disturbance — bends, expansions, contractions, visible flames. Where that isn't physically possible, an alternative location may be selected: at least two diameters downstream and a half diameter upstream from any disturbance.

CEMS Probe Location Requirements

For CEMS probes, the regulations provide siting guidance structured as recommendations rather than hard minimums — though meeting them is the practical standard for passing certification.

For the overall measurement location: it should be at least two equivalent diameters downstream from the nearest control device, point of pollutant generation, or any point where the concentration or emission rate may change, and at least a half equivalent diameter upstream from the exhaust outlet or control device.

For point CEMS, the measurement point should be either no less than 1.0 meter (3.3 ft) from the stack or duct wall, or within (or centrally located over) the centroidal area of the stack or duct cross-section.

For path CEMS, the effective measurement path should meet one of the following: be entirely within the inner area bounded by a line 1.0 meter (3.3 ft) from the stack or duct wall; have at least 70 percent of the path within the inner 50 percent of the stack or duct cross-sectional area; or be centrally located over any part of the centroidal area.

Opacity Monitor Location Requirements

Opacity port siting is more stringent than CEMS probe placement but less demanding than flow sensor requirements.

The selected measurement location must be: (1) at least four duct diameters downstream from all particulate control equipment or flow disturbance, (2) at least two duct diameters upstream from a flow disturbance, (3) free from condensed water vapor, and (4) accessible for maintenance.

Where opacity siting can't meet those criteria, the regulations permit alternative locations if the facility can demonstrate — to the satisfaction of the Administrator or delegated agent — that the alternative location or light beam path produces results equivalent to a compliant location.

What This Looks Like in Practice

The common thread across all three is that you need a meaningful run of straight, undisturbed stack or duct before the monitoring point. What counts as "meaningful" changes depending on the instrument: CEMS probes require two equivalent diameters from the last disturbance, opacity monitors require four, and flow sensors require eight. Each should also have one to two diameters of straight run after the monitoring point before another elbow, device, or exit.

For both CEMS probes and flow sensors, procedures exist for taking traverse point measurements across a cross-section to confirm that a location provides a representative sample — important when geometry doesn't allow ideal siting.

Getting placement right from the start reduces problems during RATA testing and certification. It also gives you a cleaner baseline for ongoing QA/QC, since poor probe siting creates data quality issues that compound over time.

Questions about your CEMS siting or upcoming RATA?

Alliance Technical Group's CEMS and DAS team supports facilities through the full lifecycle of continuous emissions monitoring — from initial siting review through RATA preparation, StackVision integration, and ongoing CEMS maintenance.

Contact us to talk through your specific CEMS setup.