Introduction to Carbon Monoxide (CO)

Carbon Monoxide (CO) is a colorless, odorless, tasteless gas. CO is found in combustion fumes, such as those produced by vehicles, small gasoline engines, stoves, lanterns, charcoal and wood fires, and gas ranges and heating systems. It is a byproduct when there is insufficient oxygen supply to enable complete oxidation to Carbon Dioxide (CO2).

Carbon Monoxide poisoning occurs after enough inhalation of the gas. Known as the “silent killer,” it is undetectable to the human senses, so people may not know that they are being exposed. CO causes adverse effects in humans by combining with hemoglobin to form carboxyhemoglobin in the blood. This prevents hemoglobin from releasing oxygen in tissues, leading to oxygen deprivation within a human body. However, the effects of Carbon Monoxide exposure can vary greatly from person to person depending on age, overall health, and the concentration and length of exposure.

As Carbon Monoxide is considered a toxic gas, it is particularly dangerous in enclosed areas and requires constant monitoring and detection.

Health Hazards

When present in low concentrations through inhalation, Carbon Monoxide poisoning may produce these symptoms:

  • Headache
  • Fatigue
  • Vertigo
  • Vomiting
  • Chest pain
  • Muscle weakness
  • Flu-like symptoms

When present in high concentrations through inhalation, Carbon Monoxide poisoning may produce these symptoms:

  • Unconsciousness
  • Seizure
  • Death

Exposure Limits

When researching CO gas detection solutions, facility managers should be aware of the following CO exposure limits:

Exposure Limits Agency
50 ppm TWA OSHA PEL (General Industry)
50 ppm TWA OSHA PEL (Construction Industry)
50 ppm TWA OSHA PEL (Maritime)
35 ppm TWA; 200 ppm Ceiling NIOSH REL
1,200 ppm NIOSH IDLH

Technologies for CO Gas Detection

Electrochemical Sensors

Electrochemical sensors are fuel cell-like devices consisting of an anode, cathode, and electrolyte. The components of the cell are selected so that a subject gas, allowed to diffuse into the cell, will cause a chemical reaction and generate a current. The cells are diffusion-limited, meaning that the rate of the gas entering the cell is solely dependent on the gas concentration. The current generated is proportional to the rate of consumption of the subject gas in the cell.

Advantages to using an electrochemical sensor include allowing sensors to be specific to a particular gas in the parts-per-million range, low power requirements, high accuracy, and a lower cost than other gas detection technologies. However, electrochemical sensors are sensitive to temperature, subject to interference from other gases, and have a shorter lifespan the greater the exposure is to the targeted gas.

Sierra Monitor electrochemical sensors provide improved reliability by allowing the gas to diffuse into the sensor through a capillary port, rather than diffusing through membranes. The result is an extremely stable sensor with very low temperature and pressure coefficients, and the capability to monitor gas as a percent by volume (oxygen) and ppm (toxics).

Solid State (Semiconductor) Sensors

Solid state (semiconductor) sensors have a resistance that is affected by oxygen adsorbed on the surface of the sensor. Oxygen atoms capture electrons on the semiconductor surface, thereby increasing its resistance. The sensors can be impregnated with dopants such that the sensor's resistance changes when specific gases displace the adsorbed oxygen.

As solid state sensors are among the most versatile of all sensors, they are simple and robust and can be used in many different applications. Solid state sensors have the ability to detect both low ppm levels of gases, as well as high combustible levels. However, they are not very specific to a particular gas as often required for industrial applications and could potentially provide false alarms. Solid state sensors may be affected by fugitive volatile organic compounds (VOCs), and must be located in areas without these gases present.