This is the fourth paper in a four-part series reporting on the test and evaluation of typical wall-mounted carbon-dioxide sensors used in building HVAC applications. Fifteen models of NDIR HVAC-grade wall-mounted CO₂ sensors were tested and evaluated to determine the effect of ageing on the sensors’ performance. In all, 45 sensors were evaluated: three from each of the 15 models. Among the 15 models tested, eight models have a single-lamp, single-wavelength configuration, four models have a dual-lamp, single-wavelength configuration, and three models have a single-lamp, dual-wavelength configuration. All single-lamp, single-wavelength sensors and one single-lamp, dual-wavelength sensor incorporate an “automatic baseline adjustment” algorithm in the sensor’s electronics package.

Each sensor was tested under “as received” conditions, and then, over the course of one year, performance tests were conducted at four-month intervals. All tests were conducted at 40% relative humidity, 73oF (22.8oC) temperature, 14.70 psia (101.35 kPa) pressure, and 1100 ppm CO₂ concentration. For each sensor, the readings from the four tests were compared in order to evaluate the effect of continuous operation on the sensor’s performance. The test results showed a wide variation in sensor performance among the various manufacturers. In one year, the maximum deviation in a sensor’s reading was observed to be 420 ppm (38%) while the minimum deviation in a sensor’s reading was observed to be 0 ppm.