This issue looks at the regulatory requirements for probe placement on a stack or duct for compliance systems.

The EPA spells out very clearly where ports for Continuous Emissions Monitoring System (CEMS) probes, Flow Probes and Continuous Opacity Monitoring (COMS) systems should be located on a stack or duct.  This is called out in Title 40, Appendix B to Part 60 – Performance Specification 1 (Opacity), Performance Spec 2 (CEMS), and Appendix A (Flow).

In order to get good readings from any stack monitoring equipment and pass a Relative Accuracy Test Audit (RATA), the location where the sample is taken needs to be as representative as possible to the total emissions of the affected facility.  Laminar flow is the goal rather than trying to take a reading where stratification or turbulence might exist which could give spurious readings.  Each device has its own placement criteria.

CEMS probe:

Here are what the regs call out to help insure a representative sample:

CEMS: It is suggested that the measurement location be (1) at least two equivalent diameters downstream from the nearest control device, the point of pollutant generation, or other point at which a change in the pollutant concentration or emission rate may occur and (2) at least a half equivalent diameter upstream from the effluent exhaust or control device.

  • Point CEMS: It is suggested that the measurement point be (1) no less than 1.0 meter (3.3 ft) from the stack or duct wall or (2) within or centrally located over the centroidal area of the stack or duct cross section.

Flow probe:
These are the most stringent and require the longest length of straight stack or duct.

  • Path CEMS. It is suggested that the effective measurement path (1) be totally within the inner area bounded by a line 1.0 meter (3.3 ft) from the stack or duct wall, or (2) have at least 70 percent of the path within the inner 50 percent of the stack or duct cross-sectional area, or (3) be centrally located over any part of the centroidal area.

Sampling and/or velocity measurements are performed at a site located at least eight (8) stack or duct diameters downstream and two diameters upstream from any flow disturbance such as a bend, expansion, or contraction in the stack, or from a visible flame. If necessary, an alternative location may be selected, at a position at least two stack or duct diameters downstream and a half diameter upstream from any flow disturbance.


Select a measurement location that is (1) at least 4 duct diameters downstream from all particulate control equipment or flow disturbance, (2) at least 2 duct diameters upstream of a flow disturbance, (3) where condensed water vapor is not present, and (4) accessible in order to permit maintenance.

Here is a sample illustration showing how the equivalent diameters are measured on a stack or duct:

It’s important to note that in all the criteria there are exceptions that allow for stacks and ducts where there simply is not enough straight run to accomplish placement at 2, 4 or 8 diameters from a “last disturbance”. 

The main criteria being that a user must demonstrate that the location offers a representative sample. 

For both CEMS and Flow there are procedures spelled out where test points can be measured across a predetermined number of traverse points to find a representative location.

For opacity “…you may select locations and light beam paths, other than those cited above, if you demonstrate, to the satisfaction of the Administrator or delegated agent, that the average opacity measured at the alternative location or path is equivalent to the opacity as measured at a location meeting the criteria…”.  equivalent to the opacity as measured at a location meeting the criteria…”. 

Essentially a long run of straight stack or duct is needed after any disturbance whether it be an ID fan exit, the beginning of a straight part of a stack after a transition, or after an elbow. 

How far down from that last disturbance is different for each instrument: CEMS probe is two equivalent diameters, opacity is four, and flow is eight.  In all cases there should be 1-2 diameters after the unit of straight stack or duct before an exit, another elbow, or another transition or device. 

Having these as a minimum will help insure passing certification and RATA tests and make for less headaches all around with your compliance equipment.

If you would like a copy of the Siting Criteria, please contact us and we will be happy to send it along (jnowak@monsol.com, 908-500-4010).

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