What's the Difference Between Continuous Emissions Monitoring and Periodic Measurement?

Posted by Earl Parker on Apr 11, 2017 2:00:00 PM

Navigating the complexities of the current and future regulatory environment can pose quite a challenge to many facilities that fall under EPA air emissions standards for particulate matter (i.e. dust). One of the most complex parts of compliance involves monitoring of your pollution control devices. While installing a dust collection system is complicated enough, often times facilities get hung up on the monitoring and reporting side of things rather than on the technical aspects of their collector. environmental_compliance_engineer.jpg

One issue that comes up frequently for both new and existing plants is whether to conduct periodic monitoring or to implement a continuous emissions monitoring (CEM) system. As we will see the issue depends not only on the particular air permit for each plant but also on several practical factors. 

What Is Periodic Monitoring?

This method involves taking readings of PM concentrations at regular intervals. Periodic monitoring can be done either automatically or manually. Automatic monitoring uses devices that capture a reading at a fixed interval and then return a result. Manual systems usually involve the taking of samples to be analyzed later in a lab, where results can take minutes or days to receive. Some samples are taken over a fairly short period of time (few seconds) others may be taken over longer periods of time such as hours or days in order to establish a larger average emissions level. 

EPA Method 5 is one form of stack testing that can be done an outside firm at regularly intervals as required by a facility’s air permit. Other examples of visual periodic testing currently in use for particulate emissions include EPA method 9 and 22 where a trained person conducts a visual inspection test of the stack to determine Triboelectric monitoring display being used for continuous emissions monitoringopacity levels emitted from the stack. 

What is Continuous Emissions Monitoring?

This method uses fixed instruments to continually take readings during normal operation. These systems can take the form of CEMs, which continually measure particulate concentration levels in the gas stream. Examples of this kind of system include opacity meters and triboelectric dust detection systems which provide a relative indication of the amount of particulate either through an opacity level or a pico amp signal. Some other systems also make use of continuous sampling, whereby a sample is continually extracted from the air stream for analysis, such as chemiluminescence analyzer for measuring NOx, or a Beta Gauge for particulate sampling.  Other systems are used as arrestment failure CEMs, or failure alarms that trigger when a required level is no longer met. Examples of this include broken bag detectors (Both opacity based and certain triboelectric based systems) that trigger alarms when a sudden spike above acceptable emissions levels is detected. 

Advantages of Continuous Emissions Monitoring over Periodic Measurements

Periodic readings generally become less effective the more variable the emissions or process conditions. In these cases where intermittent sampling would be unrepresentative, or would be required too frequently to be practicable, a continuous emissions monitoring system makes the most sense. In addition, many applications that utilize dust collection systems for control of particulate matter emissions will find that periodic sampling will not meet the stringent requirements of modern air permits or compliance regulations.  Periodic sampling may prove impractical from an operational perspective as well. Data gathered from continuous emissions monitoring helps improve operations and enable predictive maintenance planning that can lead to additional cost savings. 


Periodic monitoring techniques may seem to provide the required service at a lower cost. However, for many applications the use  of periodic emissions  monitoring cannot meet the requirements of today’s air permits. On the other hand, many facilities are now explicitly required to implement and maintain continuous emissions monitoring strategies as part of their basic air permits and environmental regulations. When deciding on an emissions monitoring strategy decision makers should take care to consider all of their compliance requirements as well as any additional benefits to be had from using CEM vs a periodic monitoring strategy. 

Would you like to learn more about the advantages of continuous emissions monitoring technology such as triboelectric dust detection systems? Contact Auburn today for a free consultation or download our free eBook now!

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Topics: Particulate Monitoring, Environmental Monitoring