Thursday, March 21, 2013

Combustion analysis and why CO readings are not always correct

Have you ever compared your CO readings to another analyzer?  You might have noticed that Testo CO reading is lower than expected.  Why such a difference?  The secret lies in the Testo’s addition of the NOx gas filter.

In residential and light commercial combustion analysis there are several important combustion gases that are measured or monitored. Carbon monoxide (CO), however, is arguably the one gas with the most liability associated with it. This is due to safety concerns and simply from the standpoint of good combustion performance. But NOx is also present and can affect the CO readings, it’s the carbon monoxide levels that you just tested that will keep you up at night.

While performing a combustion test, the O2 and CO are right in front of you on the analyzer display, but it’s easy to overlook all the other gases flowing through the flue pipe,  like CO2, nitrogen, NOx, SO2, water vapor, and so on. It’s these other gases that need to be accounted for when testing furnaces, boilers heaters and other combustion sources. Why, you ask are these other gases important? Some of them can change the output of sensors, and it’s important to know this when you have critical decisions or combustion adjustments to make. This way you are making them based on good information.

How Electrochemical Sensors Function
Electrochemical (EC) sensors rely on the chemical reaction between the combustion gases and the chemicals/materials inside the sensor. As the targeted gas, (i.e., CO) comes in contact with the sensor materials, a reaction takes place. Specifically, between the metal electrodes and chemicals which in-turn will create an electrical output. Technically speaking, this process is call a redox reaction (oxidation or reduction) depending upon the sensor. When the flue gas component (i.e., CO) is no longer present, the chemical reaction and electrical output stops.

The technology behind an EC sensor is well established and is designed to detect both low and high levels of a specific gas through different sensor designs and materials. The key to a quality EC sensor is its’ ability to isolate a single measurement parameter while remaining unaffected by the wide range of other components in the flue gases.

How NOx Affects CO Sensor Readings
If a sensor within a combustion analyzer responds to gases other than what it’s intended to measure, an incorrect measurement will be displayed. This will occur when a CO sensor - without a NOx scrubbing filter - is exposed to flue gas. All flue gas contains some concentration of NOx (for simplification, NOx refers to Nitrogen Oxide). The concentration of NOx tends to be higher with elevated temperatures and low O2 concentrations. When NOx enters the CO sensor, additional reactions take place that increase sensor output.  It is impossible to know how much of the output is due to CO and how much is the result of NOx. 

Many government weatherization programs and performance testing guidelines require corrective action when specific levels of CO are detected. This can prove to be time consuming and costly when these corrective actions are based on potentially false CO readings.

CO Sensor Design
The design of a CO sensor should include a way to eliminate the cross interference effect to other flue gases, namely Nitrous Oxide (NO).  If the CO sensor is designed incorrectly, the CO measurement will be wrong. For example, if the  CO sensor is exposed to flue gas containing 100ppm CO and 200ppm NO (remember NO may not be displayed or measured on the combustion analyzer), the CO sensor will react with a positive output for CO and an additional positive output for the NO. As such, the CO sensor will calculate 100ppm of CO and also calculate an additional 50-100ppm of NO, resulting in a displayed value of CO as much as 200ppm.  As a result, technicians will attempt to mechanically correct the burner to lower CO levels. The correction will be wrong and will negatively impact burner/system performance.

Testo’s Sensor Design
For decades, Testo has used CO sensors with integrated NOx filters as a standard design. These filters “scrub” and remove NOx from the gas stream. Ultimately, this prevents NOx from ever reaching the CO sensor. This NOx filter is designed to last longer than the life of the sensor so there is no maintenance required when used under normal operating conditions. In some of Testo’s  analyzers the filters can be replaced in the field. Go to to see more of Testo’s line of combustion analyzers.

Confidence in Testo Sensors
Filtering NOx from the combustion gas stream before it goes through the sensor is fundamental to achieving accurate CO readings. Without a NOx filter the validity of the CO reading will always be in question.

If there is ever a concern in your CO readings, it’s easy to confirm proper function by using calibration gas. Connect and flow NO calibration gas to your analyzer. The CO sensor should show 0ppm. If a filter is not present, expect to see a 25%, to over 100%, value of the NOx gas in your CO reading. Although this procedure is easy to do, most contractors don’t carry calibration gas. So to eliminate the questions, use the proper sensor, and analyzer for accurate combustion testing.

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