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Guide to DDC

Chapter 2

Input Devices and Sensors

Gas Concentration Measurements

With the increased interest in indoor air quality and the need to monitor potentially dangerous gases, gas concentration measurements have become increasing more prevalent in DDC system design. Many devices are currently available for use in HVAC applications.

Types of Gas Concentration Measuring Devices

There are many types of gas measuring devices available for use with DDC systems. Currently, the three most common gases measured in HVAC applications are carbon monoxide, carbon dioxide, and refrigerant gases.

Carbon Monoxide

Carbon monoxide is a poisonous gas that is most commonly generated as the byproduct of the incomplete combustion of carbon based fuels. Carbon monoxide is generated by all fuel burning equipment, including internal combustion engines. Carbon Monoxide detectors are used to operate ventilation equipment to prevent carbon monoxide levels from becoming unsafe. They are also used to warn facility owners and occupants of unsafe levels in garages, loading docks, tunnels, and other areas where vehicles are operated. Solid state sensing technology is most commonly used. Single or multiple sensing point versions are available that can provide contact closures at one or more set levels and/or analog signals that are proportional to carbon monoxide concentration.

Carbon Dioxide

Carbon dioxide is a non-toxic gas produced by the respiration of living organisms, by the complete combustion of carbon, and by photosynthesis in green plants. Carbon dioxide exists in the air in the amount of 320-350 parts per million. Carbon dioxide concentration inside of buildings has been related to general ventilation adequacy and is commonly monitored by DDC control systems as a measure of indoor air quality and ventilation adequacy. It is also measured by DDC systems and used to control outdoor air fans and dampers to keep the concentration below set levels.

The most commonly used sensing technology is Non-Dispersive Infra-Red (NDIR). This is based on the principle that carbon dioxide gas absorbs infrared radiation at the 4.2 m wavelength. Attenuation of an infrared source can be related to the gas concentration in air in the range of 0-5000 parts per million with a general accuracy of plus or minus 150 ppm or 50 ppm over narrower ranges.

Refrigerant Gas

Refrigerant gas detectors have been in widespread use since safety codes for mechanical refrigeration required their use in the operation of emergency ventilation systems to evacuate hazardous concentrations of refrigerant gas in machinery rooms and other applicable enclosed areas.

Detectors broadly sensitive to families of CFC and HCFC gases commonly used, as refrigerants are available. Gas specific detectors are also available to detect individual refrigerant gases including CFC, HFC, HCFC and ammonia specific to the equipment in use. The most commonly used are infrared (IR), photo-acoustic, and solid state sensing technologies. Single or multiple sensing point versions are available that can provide contact closures at one or more set levels and/or analog signals that are proportional to refrigerant concentration.

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