California's Building Energy Efficiency Standards Title 24 [1] has the following accuracy specifications for CO2 sensors used for demand controlled ventilation (DCV): “ CO2 sensors shall be certified by the manufacturer to be accurate within plus or minus 75 ppm at a 600 and 1000 ppm concentration when measured at sea level and 25°C, factory calibrated, and certified by the manufacturer to require calibration no more frequently than once every 5 years. Upon detection of sensor failure, the system shall provide a signal which resets to supply the minimum quantity of outside air to levels required by Section 120.1(b)2 to the zone serviced by the sensor at all times that the zone is occupied.” Selection of CO2 Sensors for DCVNearly all CO2 sensors marketed for DCV applications contain specifications certifying that they meet the requirements of Title 24. The available data, albeit somewhat dated, indicate that, in practice, CO2 measurement accuracy can be much poorer than indicated by manufacturer’s specifications [7, 8]. Available data on CO2 sensor accuracy [9], independent from manufacturers’ data, do not show that specific sensor design types are consistently superior.
This figure shows results from this project (Task 3) where accuracy of seven CO2 sensors were compared with a co-located reference instrument. Triplicates of seven CO2 sensors were placed in three spaces: general office space, conference room, and elementary school classroom. CO2 concentrations were measured minute by minute for two years. The figure shows percentage of time when absolute value of error was less than 75 ppm, when CO2 concentration in the studied space was at roughly 700 ppm (650 to 750 ppm) as measured by the co-located reference instrument. The seven CO2 sensors by five manufacturers (AirTest, BAPI, Gas Sensing Solutions, Telaire, and Vaisala) included different types: single beam/single wavelength, single beam/dual wavelength, with and without ABC, and different IR sources (incandescent light bulb, MEMS emitter, LED). Results show CO2 sensor accuracy differed by manufacturer and by sensor type. CO2 sensor accuracy also varied somewhat by study spaces.
Many of the CO2 sensors available for DCV applications incorporate an auto-calibration system that periodically resets the sensor’s calibration based on the lowest measured CO2 concentration during a prior time period, assuming that that lowest measured concentration is approximately 400 ppm – a typical outdoor CO2 concentration. This auto-calibration procedure is sometimes referred to as an “automatic baseline adjustment” or “automatic baseline calibration” (ABC). While this auto-calibration scheme is suitable for many spaces, sensors with this type of auto-calibration scheme are not suitable for spaces with indoor CO2 concentrations consistently maintained above the outdoor concentration. Spaces unsuitable for this type of sensor include buildings with continuous occupancy. Also, concentrations of CO2 in some classrooms can stay above 400 ppm overnight throughout a school week, although concentrations may still fall below 400 ppm on weekends.
In selection of CO2 sensors for DCV, the maximum measurable concentration of the sensor is another consideration. Many of the available products can measure CO2 concentrations up to 2000 ppm, or up to 5000 ppm. In general, accuracy will be improved if the maximum measurable concentration of the sensor is not far above the maximum concentration encountered in practice. For example, if the CO2 sensor is to be used to maintain CO2 concentrations below 1000 ppm, a sensor with a maximum measurable concentration of 2000 ppm will usually be preferable to a sensor with a maximum measurable concentration of 5000 ppm. |
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