FAQs: Frequently Asked Questions

What Are CEMS?
CEMS is the acronym for Continuous Emissions Monitoring System. CEMS were developed to demonstrate compliance with regulatory requirements and to track emissions released into the air by facilities considered Major Emitters by the Federal Environmental Protection Agency.

CEMS can be broken down into various components:

1.	Continuous Opacity Monitoring Systems (COMS): Opacity monitoring systems measure the amount of particles being released into the air. COMS are designed and regulated by Title 40 of the Code of Federal Regulations, Part 60. Typical COMS contain an optical head, reflector, and remote controller module. Signals from the system are sent to a data recorder.
2.	Continuous Emissions Rate Monitoring System (CERMS) or Volumetric Flow Monitor: Flow monitors measure the rate of exhaust gases being sent through a duct or stack before they are exhausted into the atmosphere.
3.	Gaseous Continuous Emissions Monitoring Systems (CEMS): The two most prevalent sampling strategies for gas CEMS are Full Extractive and Dilution Extractive: A. Full Extractive: A sample is continuously drawn from the gas stream through a heated filter head, which removes particulate, sent through a heated sample line to a condenser. The condenser removes moisture from the sample by lowering the gas’ dew-point. The dry gas is then sent to individual analyzers to be evaluated. B. Dilution Extractive: A sample is continuously drawn from the gas stream, diluted with high grade instrument air, and sent down a sample line to ambient level gas analyzers to be analyzed.

Dilution systems and Full Extractive systems each have their distinct advantages.

Dilution Extractive		Dilution Extractive
Advantages		Disadvantages
•	Clean sample sent to analyzer rack	•	Subject to CO2 interference in sonic orifice
•	Heated sample line not needed due to low gas dew point	•	More labor intensive to perform in-situ probe maintenance (depending on length of sample probe)
•	No sample gas conditioner eliminates component that may fail	•	Requires high quality instrument air to have no trace levels of gases being analyzed.
•	Samples SO2 very well because there is no moisture interference	•	More difficult to troubleshoot
•	Exhaust from left over sample is at ambient concentrations	•	O2 must be measured on separate system
•	Fast response time	•	Subject to temperature swings in process which can affect dilution ratio
•	Uses less calibration gas
Full Extractive		Full Extractive
Advantages		Disadvantages
•	Simple design, easier to troubleshoot	•	Subject to interference in gas conditioning system
•	Sample probe is not very maintenance intensive	•	Probe and umbilical need to be heated above sample gas dew-point
•	Can sample to very low levels with minimal interference	•	Exhaust from left over sample contains full concentration of gases
•	Sample can be sent to other analyzers requiring wet sample, such as HCl and THC analyzers	•	Slower response time
•	Sample can be sent to other analyzers requiring wet sample, such as HCl and THC analyzers	•	Uses more calibration gas

Data collected from the CEMS are sent to a Data Acquisition and Handling System (DAHS) where data is compiled, averaged and stored. The DAHS also controls system calibrations, purges, and system alarms.