CO Detection and Measurement

Why CO Detection Matters

Carbon monoxide is the leading cause of accidental poisoning deaths in the United States, killing more than 400 people every year and sending over 50,000 to emergency rooms. For HVAC technicians, understanding CO detection is not optional - it is a life-safety skill that protects both the technician and the building occupants.

Carbon monoxide (CO) is a colorless, odorless, tasteless gas produced by incomplete combustion of any carbon-based fuel. Natural gas, propane, oil, wood, and charcoal all produce CO when they burn without sufficient oxygen. Because CO cannot be detected by human senses, instrumentation is the only reliable way to identify its presence.

Physical Properties of Carbon Monoxide

CO has a molecular weight of 28.01, making it slightly lighter than air (molecular weight 28.97). This means CO tends to mix evenly with room air rather than sinking to the floor or rising to the ceiling. This is why CO detectors should be placed at breathing height - not near the ceiling like smoke detectors, and not near the floor.

CO binds to hemoglobin in the blood approximately 200 to 250 times more readily than oxygen. When inhaled, CO displaces oxygen on hemoglobin molecules, forming carboxyhemoglobin (COHb). This is why even low concentrations can be dangerous over extended exposure periods - the CO accumulates in the bloodstream faster than the body can eliminate it.

Types of CO Detection Instruments

There are three primary types of CO detection instruments that NCI technicians must understand:

1. Electrochemical Sensors - The most common type used in professional combustion analyzers and personal CO monitors. These sensors contain an electrolyte solution and electrodes. When CO molecules diffuse through a membrane into the sensor, a chemical reaction produces a tiny electrical current proportional to the CO concentration. Electrochemical sensors provide accurate readings in the 0-2,000 ppm range with resolution as fine as 1 ppm. They typically have a lifespan of 2-3 years before the sensor cell must be replaced.

2. Metal Oxide Semiconductor (MOS) Sensors - Found in many consumer-grade CO alarms. These sensors use a heated tin dioxide film that changes electrical resistance when CO is present. MOS sensors are less precise than electrochemical sensors and are more susceptible to cross-sensitivity from other gases. They are adequate for residential alarm use but not for professional combustion analysis.

3. Infrared (IR/NDIR) Sensors - Non-dispersive infrared sensors measure CO by detecting the absorption of specific infrared wavelengths. IR sensors do not degrade over time like electrochemical sensors, making them extremely stable and long-lasting. However, they are more expensive and are typically found in laboratory-grade instruments and fixed industrial monitoring systems.

Sensor Cross-Sensitivity

Electrochemical CO sensors can be affected by other gases, producing false readings. Hydrogen (H2), hydrogen sulfide (H2S), and sulfur dioxide (SO2) are the most common cross-sensitive gases. Quality combustion analyzers include an H2-compensated CO sensor or apply a mathematical correction factor to account for the hydrogen typically present in flue gas. When purchasing or specifying a combustion analyzer, always verify it includes H2 compensation.

Carbon monoxide is a colorless, odorless gas that binds to hemoglobin 200-250 times more readily than oxygen. Professional HVAC technicians must use electrochemical sensor instruments - not consumer MOS alarms - for accurate CO measurement in combustion testing.