Do Your CO Readings Seem Low? Maybe it's the Wrong Sensor

We are often asked about how to properly take CO measurements here at Testo. So we decided to make a video demonstrating how CO measurements can be affected by naturally-occurring NOx.

For more information about Testo's complete line of emission analyzers, please go to www.testo350.com. To learn more about Testo's complete line of combustion analyzers, please go to www.testoUSA.com/combustion.

What is a Pellister?

A Pellister (CxHy Sensor) is a heated sensor that measures total hydrocarbons. Also know as a catalytic or heated bead sensor, it consists of a specially matched pair of precision resistive thermal devices (RTDs) overlaid with two different coatings. The first is covered with a propriety catalyst that creates an exothermic reaction to the presence of targeted hydrocarbons. The second RTD is covered with an inert coating and acts as a reference. Both RTDs are heated to approximately 950 °F to increase the catalytic reaction rate.

In the presence of hydrocarbons the catalytic RTD will have higher heat value than the reference. The RTDs are tied to two legs of a "Wheatstone Bridge" to maximize the output and stability. Testo's unique cross calibration techniques essentially make the HC module blind to interference gases such as carbon monoxide and free hydrogen.

What is NOx?

NOx is a term used to describe nitrogen oxides. NOx is a reactive gaseous compound that is one of the criteria air pollutants regulated by the USEPA Clean Air Act. The two primary oxides of nitrogen involved in air pollution are NO and NO2. Nitric oxide (NO) is colorless and essentially odorless. Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent smell. NOx (NO & NO2) can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections. NOx is also a precursor to the formation of ground level ozone (smog) and a contributor to acid rain.

NOx emissions are produced during the combustion of fuels. The primary man-made sources of atmospheric NOx include; area mobile sources (cars and trucks), off-road sources (construction and agricultural machines, trains, etc.) and stationary sources (power plants, industrial boilers & heaters, etc,).

In a combustion process, three types of NOx can be formed:

• Thermal NOx: From high temperature combustion in the presence of free oxygen.
• Fuel NOx: From the nitrogen bound in the fuel when it is oxidized during combustion.
• Prompt NOx: From the combustion flame and the ambient nitrogen in the ambient air.

To learn more about how testo products measure NOx, check out testo350.com.

What are Electrochemical sensors?

Electrochemical sensors are devices that measure flue gas constituents (O2, CO, NO, NO2, SO2, H2S) through the principle of ion selective potentiometry The sensor contain a electrolytic matrix that is designed for a specific gas to be detected. Two or three electrodes (again gas specific) are placed in this matrix and an electrical field is applied. Flue gas enters the sensor and chemically reacts (oxidation or reduction) on the electrode releasing electrically charged particles (ions). This reaction causes the potential of this electrode to rise or fall with respect to the counter electrode. With a resistor connected across the electrodes, a current is generated which is proportional to the concentration of gas present. The output is converted then displayed as a concentration (typically in ppm, percent, or as a mass unit (i.e. lbs/hr or mmbtu).

Standard electrochemical sensors are affected by various environmental factors including: temperature, pressure, and other combustion gases. However, testo sensors are designed to eliminate these effects. Mounted on each sensor is a circuit board that contains calibration data, linearity data, and other critical information. This technology enables the sensor to be pre-calibrated at testo and installed in the field as a simple plug-in device. No need to have calibration gases on site. Additional technological advancements in sensor and analyzer design include the use of:

• Continuous temperature compensation to provide accurate response regardless of ambient temperature fluctuations.
• Control of sample pressure and automatic flow rate monitoring to eliminate pressure related effects.
• Sensor temperature control by using heated thermoplastic sensor blankets.
• Integrated on-board interference filter media that eliminates the hassle of replacing NOx beads
• Automatic cross compensation for interference gases.

To learn more about testo's complete line of products, check out testo350.com.