New Technology Supplement

Sensing a Change

There are a number of features to consider when choosing the perfect carbon monoxide sensor

• By Jack Ogden
• Oct 01, 2006

NEARLY every residential and commercial dwelling includes flame-fueled devices of some sort. These devices may include gas and hot water heaters, gas and oil furnaces, clothes dryers, fireplaces, vehicles, charcoal grills, engine-powered tools and lawn mowers. If any of these is not properly vented or malfunctions, carbon monoxide can be produced.

undefinedCarbon monoxide, or CO, is an odorless, colorless, tasteless and highly toxic gas. If inhaled, it will immediately absorb into the bloodstream, producing a toxic compound that will interfere with the body's ability to absorb and transport oxygen to vital organs.

Carbon monoxide, or CO, is an odorless, colorless, tasteless and highly toxic gas. If inhaled, it will immediately absorb into the bloodstream, producing a toxic compound that will interfere with the body's ability to absorb and transport oxygen to vital organs. The rate at which the body will feel the effects of CO depends on several variables, including the concentration of CO in the air, the length of exposure, current health status and lifestyle activity level.

Decoding the Sensor
Because you cannot see, smell or taste CO, the only way to protect yourself from the deadly gas is with CO detectors. Although the CO detector market is one of the fastest growing in the life safety industry, security professionals sometimes have trouble certain how to choosing the best CO detector.

There are three common types of CO sensing technologies to choose from: metal oxide semiconductor, biomimetic and electrochemical.

MOS sensors were the first CO sensors invented. An internal heating device heats tin oxide (SnO₂) to at least 250 degrees Celsius. When heated, oxygen atoms in the air bond to the SnO₂. If CO mixes with the oxygen atoms, it will release electrons into the semiconductor, increasing the current. This increase in current sets off the alarm in the detector. The drawback is that MOS detectors typically draw more current than other technologies due to the power consumed by the heater.

The second type of technology is biomimetic. Inside a biomimetic sensor, an infrared light passes through a synthetic hemoglobin membrane. When CO is present, the amount of light transmitted decreases and the alarm is activated. One drawback of biomimetic detectors is that as CO and other contaminants accumulate over time, these detectors tend to send a false alarm.

The newest, most refined sensors available today are electrochemical sensors. The sensors provide accurate measurements (in parts per million) of CO concentrations. This is done through sensors that use a platinum/acid combination to promote the reaction between CO and the oxygen in air. The electrons produced by this combination induce a small current between the two electrodes, which is proportional to the amount of CO in the air. External circuitry monitors changes in the current to calculate the concentration of CO gas. Readings are taken from low levels that may be hazardous over long periods of time to high concentrations that present an immediate danger.

Empowering the Detector
Installation locations vary by manufacturer. According to the NFPA, all carbon monoxide detectors "shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms," and each detector "shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit."

Manufacturers' recommendations vary to a certain degree based on research conducted with each specific detector. Make sure to read the provided installation manual for each detector before installing.

Although detectors require specific power sources, system-connected, monitored CO detectors are the most reliable choice. There are three main power sources for CO detectors.

Battery-powered CO detectors require annual battery replacement. When the battery has reached its maximum lifespan, the detector will chirp at regular intervals to alert homeowners. However, some homeowners become annoyed by the chirping. Instead of replacing the battery, they remove it. Therefore, battery-powered CO detectors require discipline (checking the battery) and maintenance (replacing the battery) by the homeowner.

A 120-volt powered detector runs off the main power supply in the dwelling. Like battery-powered detectors, most of these detectors do not have monitoring capabilities. So, if your detectors lose power in a blackout, for example, you may not realize your detectors are not working -- unless the detector has a battery backup feature. Plus, a power outage is a crucial time to protect against CO because people tend to use more CO-generating appliances, such as space heaters and generators, for comfort.

A 12/24-volt device is the most reliable means of powering a CO detector. These system-connected CO detectors can be wired to either a security or fire panel. This type of system offers monitoring by a central station to provide extra protection if the residence is empty, if the residents are sleeping or if the residents are already suffering the effects from CO. If a problem arises with the detector or the CO concentration hits dangerous levels, the owner may not only be notified by the detector and the control panel, but also by the central station. If the owner is unreachable, the central station may send the proper authorities to investigate. Procedures will vary from station to station, but the residents will always be guaranteed protection 24/7.

Following the Rules
Many state and localities now require CO detectors. As CO research continues to educate the world on the dangers of carbon monoxide, states and municipalities are passing legislation that requires CO detectors to be placed in residential and commercial units. As of July, 11 states and dozens of municipalities have legislation in place, and nine more states and several municipalities have legislation pending. Each code may have different CO detector requirements. With legislation on the rise, check with the local authority having jurisdiction for local CO detector requirements.

System-connected CO detectors should be more than just listed to UL-2075 -- it should be specifically designed for system operation. UL-2075 requires CO detectors to have a trouble relay and wiring supervision. CO detectors should be designed around these life-saving features to guarantee that if a problem exists with the sensing circuit or if the detector has reached its maximum lifespan, the trouble relay will send a message to the control panel.