Performance evaluation of carbon-dioxide sensors used in building HVAC applications
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Carbon-dioxide sensors are widely used as part of a demand controlled ventilation (DCV) system for buildings requiring mechanical ventilation, and their performance can significantly impact energy use in these systems. Therefore, a study was undertaken to test and evaluate the most commonly used CO2 sensors in HVAC applications, namely the non-dispersive infrared (NDIR) type.
Fifteen models of NDIR HVAC-grade wall-mounted CO2 sensors were tested and evaluated to determine the accuracy, linearity, repeatability, hysteresis, humidity sensitivity, temperature sensitivity, and pressure sensitivity of each sensor as well as effect of long-term ageing on sensor performance. All tests were conducted in a chamber specifically designed and fabricated for this research. 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.
The accuracy, linearity, repeatability, and hysteresis of the sensors were evaluated at a fixed relative humidity, temperature, and pressure, by varying CO2 concentrations from 400 ppm to 1800 ppm. The test results showed a wide variation in sensor performance among the various manufacturers and in some cases a wide variation among sensors of the same model.
The humidity sensitivity was evaluated by varying the relative humidity from 20% to 60% while holding the CO2 concentration, temperature, and pressure fixed. The temperature sensitivity was evaluated by varying the temperature from 66yF (18.9yC) to 80yF (26.7yC) while holding the gas composition and pressure fixed. The pressure sensitivity was evaluated by varying the pressure from 14.70 psia (101.35 kPa) to 11.80 psia (81.36 kPa) while holding the gas composition and temperature fixed.
The test results showed that while humidity sensitivity of most of the sensors is negligibly small, some sensors are strongly affected by humidity. The test results also showed that the effects of temperature and pressure variation on NDIR CO2 sensors are unavoidable. For the range of temperature and pressure variation in an air-conditioned space, the effect of pressure variation is more significant compared to the effect of temperature variation.
The long-term ageing effect was evaluated at four month intervals for one year. The result showed a wide variation in ageing effect among manufacturers. Some sensor models showed a nominal ageing effect of less than 30 ppm deviation in one year; whereas, all three sensors of one model showed significant ageing effects, up to -376 ppm deviation, in one year at 1100 ppm CO2 concentration.