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 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 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 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
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