CO Detection Technology
The three most common CO detection technologies available today are: 
· Chem-optical, 
· Electro-chemical &
· Semiconductor.
 

CHEM-OPTICAL (GEL CELL) TECHNOLOGY

Chem-optical technology alarms are also known as gel cell or biomimetic technology alarms. These alarms utilize a type of sensor that simulates hemoglobin in the blood.

Biomimetic sensors utilize a material that mimics the response of human hemoglobin to carbon monoxide. In the presence of carbon monoxide gas, the amount of infrared light which will pass through the sensing material declines. Alarms using this kind of sensor use external circuitry to monitor the transmittance of infrared light through the sensor. The rate of change of the transmittance is used to calculate carbon monoxide gas concentrations.Biomimetic sensors demonstrate acceptable immunity to other gases that may be present.Biomimetic sensors are mechanically simple devices.Alarms using these sensors have field demonstrated a dependable sensor life in excess of three years.

One main drawback that remains is that the sensor can non-reversibly accumulate carbon monoxide and other contaminants over time, which can eventually lead to false and/or nusiance alarms. Some chem-optical (gel cell) alarms on the market today contain an expensive relacement battery and/or sensor, which must be replaced periodically.
 

ELECTRO-CHEMICAL

Electro-chemical technology alarms are usually battery powered. This type of sensor typically has a limited lifetime of about 2 - 5 years. Some manufacturers' models will require its battery and/or sensor to be changed periodically by installing an expensive replacement. Other manufacturers' models have sealed housing that requires the entire unit to be discarded once the battery power supply is depleted.

Electrochemical sensors typically use platinum as a catalyst and acid as an electrolyte to break down carbon monoxide gas and release electrons. The electrons induce a small current which creates a change in potential at external measurement points. Alarms utilizing this type of sensor use external circuitry to monitor the changes in potential and use this information to calculate the concentration of CO gas.

Electrochemical sensors are mechanically much more complex than semiconductor sensors but can provide more accurate measurements of CO concentrations. Modern electrochemical sensors demonstrate good immunity to interferent gases. Careful design and processing is necessary to ensure accuracy across humidity extremes. Historically, electrochemical sensors have been prone to leakage due to:

· Corrosion of electrical contacts
· Destruction of sealing surfaces in the body of the sensor
· Expansion of the electrolyte volume
 
The current that flows between the two electrodes is proportional to the amount of CO present. This means that the detector can give an accurate reading (in parts per million of CO), from low levels that may be hazardous over long periods of time, to high concentrations that present an immediate danger.
 

SEMICONDUCTOR TECHNOLOGY

There are a variety of CO alarms that utilize semiconductor or tin dioxide technology available on the market today. Unlike alarms which utilize chem-optical or electro-chemical technology, semiconductor detectors do not require expensive replacement sensors. However, not all semiconductor CO alarms are alike. Some manufacturers utilize semiconductor sensors that are not designed to be used in a CO specific alarm and may be prone to false and/or nuisance alarms.
Semiconductor sensors utilize a controlled quantity of tin dioxide (SnO2) as a sensing element. The sensing material is heated by a small electric heating element and carbon monoxide gas is catalytically broken down at the surface of the sensing element. Electrons are released in this process and are absorbed by the sensing element. This increase in charged particles lowers the resistance of the sensor.In an alarm using semiconductor sensors, electronics are used to measure the sensor resistance and from this to calculate the carbon monoxide concentration. Semiconductor sensors are mechanically simple and are electronic in nature, therefore, they are very long-lived and very reliable. Current designs demonstrate excellent immunity to other gases that may be present. Millions of semiconductor CO alarms manufactured over the past ten years have provided historical data which demonstrates a dependable sensor life of more than ten years.
 

SAFETY PATCH           

Another type of CO detector that you may have seen consists of a small plastic square with a patch of orange crystals on it, which turn black if CO is present. These detectors are designed to be stuck to a wall near a gas fire or boiler, and provide a cheap and easy safety check.The orange-to-black colour change is the result of simple redox chemistry. The orange crystals contain palladium (II) chloride which is reduced by CO to form palladium (0), ie metallic palladium, which is dark grey:

CO + PdIICl2.2H2O ® CO2 + Pd0 + 2HCl + H2O

The crystals also contain copper (II) chloride. This oxidises the palladium metal back to palladium (II) chloride, when the sensor is exposed to CO-free air. The copper (II) chloride is reduced to copper (I) chloride in the process:

Pd0 + 2CuIICl2.2H2O ® PdIICl2.2H2O + CuI2Cl2

The final stage in regeneration of the system is oxidation of the copper (I) chloride back to copper (II) chloride by atmospheric oxygen:

CuI2Cl2 + 2HCl + H2O + 1/2O2 ® 2CuIICI2.2H2O