1、2009 ASHRAE 471ABSTRACT 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 th
2、e most commonly used CO2sensors in HVAC systems, namely the non-dispersive infrared (NDIR) type. The procedures presented here provide a methodology to test and evaluate NDIR CO2sensors for accuracy, linearity, repeatability, hysteresis, humidity sensi-tivity, temperature sensitivity, and pressure s
3、ensitivity.The test and evaluation procedures presented in this paper are all inclusive in that they range from procuring the CO2sensor to comparing the performance of the sensors. Specifi-cally, a procedure is presented to both procure CO2sensors from the manufacturers and to maintain quality contr
4、ol by controlling the storage and handling of the sensors. Further, it describes the apparatus and instrumentation, along with test conditions, used to test the sensors. Additionally, it outlines a detailed experimental procedure to evaluate the accuracy of the sensors. Finally, a discussion is pres
5、ented on analyzing and comparing the performance of CO2sensors by using the test data. Partial results of the accuracy test and evaluation of the CO2sensors and the results of the linearity, repeatability, hysteresis, humidity sensitivity, temperature sensitivity, and pressure sensitivity evaluation
6、 are included in this paper. The full test results will be presented in a later publication.INTRODUCTIONControlling ventilation air flow rates using CO2-based demand controlled ventilation (DCV) offers the possibility of reducing the energy penalty associated with over-ventilation during periods of
7、low occupancy, while still ensuring adequate levels of outdoor air ventilation (Emmerich and Persily 2001). A report prepared for DOE (Roth et al. 2005) suggests that DCV can reduce both heating and cooling energy by about 10% or about 0.3 quadrillion Btu (316 quadrillion Joules) annually.Carbon-dio
8、xide (CO2) sensors are gaining popularity in building HVAC systems to monitor indoor air CO2concentra-tion and to control outdoor air intake rate. The sensing tech-nology most commonly used for HVAC applications is the optical method of non-dispersive infrared (NDIR). The perfor-mance of these senso
9、rs is crucial not only to ensure energy savings but also to assure indoor air quality. In CO2-based DCV systems, the CO2level of indoor air is monitored and the outdoor air flow rate is adjusted based on the sensor output to maintain acceptable CO2concentration in the occupied space. Sensors which r
10、ead high will call for more outdoor air leading to an energy penalty. Sensors which read low will cause poor indoor air quality. CO2sensors are reported to have technology-specific sensitivities, and unresolved issues including drift, overall accuracy, temperature effect, water vapor, dust buildup,
11、and aging of the light sources, etc. (Dougan and Damiano 2004). Fahlen et al. (1992) evaluated the performance of two CO2sensors, one photo-acoustic type and one IR spectroscopy type, in lab tests and long term field tests. The lab tests included performance and environmental tests. The authors conc
12、lude that the error of measurement is normally well within 50 ppm at a measured level of 1000 ppm. However, the test results show the deviation up to -300 ppm at 2000 ppm. The output of one sensor increased dramatically during envi-An Experimental Evaluation of HVAC-Grade Carbon Dioxide Sensors Part
13、 I: Test and Evaluation ProcedureSom S. Shrestha Gregory M. Maxwell, PhDStudent Member ASHRAE Member ASHRAESom S. Shrestha is a PhD candidate and Gregory M. Maxwell is associate professor in the Department of Mechanical Engineering, Iowa State University, Ames, IA.LO-09-044 2009, American Society of
14、 Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permiss
15、ion.472 ASHRAE Transactionsronmental testing and never recovered back to its normal value.A pilot study that evaluated in-situ accuracy of 44 NDIR CO2sensors located in nine commercial buildings indicated that the accuracy of CO2sensors is frequently less than is needed to measure peak indoor-outdoo
16、r CO2concentration differences with less than 20% error (Fisk et al. 2006). Thus, the authors conclude that there is a need for more accurate CO2sensors and/or better maintenance or calibration. The evalua-tion was performed either by multi-point calibration using CO2calibration gas or by a single-p
17、oint calibration check using a co-located and calibrated reference CO2sensor. The test was not conducted in a controlled environment hence the effect or humidity, temperature and pressure variation on the sensor output was not considered in the study.Further review of the literature reveled that the
18、re is no present standard method of test available by which CO2sensors are evaluated. Therefore, an experimental procedure for testing and evaluating the sensors was developed and is presented here. This procedure provides a detailed description of the methodology to evaluate the performance of wall
19、-mounted CO2sensors for accuracy, linearity, repeatability, hysteresis, humidity sensitivity, temperature sensitivity, and pressure sensitivity.Further, this paper presents the details of the experimental test apparatus and instrumentation being used for the test. Additionally, steady-state criteria
20、 for recording data from the CO2sensors are also discussed, along with some preliminary test results.HVAC CO2SENSORSFor HVAC applications, two CO2sensor technologies are available: photoacoustic and NDIR. Of these the NDIR is the most commonly used technology for DCV application. As shown in Figure
21、1, the essential components of a NDIR CO2sensor include an IR (infrared) radiation source, detector, opti-cal bandpass filter, and an optical path between the source and the detector which is open to the air sample. The bandpass filter limits the IR intensity that is measured in a specific wave-leng
22、th region. The detector measures this intensity which is proportional to the CO2concentration. The main configura-tions used for HVAC grade CO2sensors are: (1) single-beam single-wavelength, (2) dual-beam single-wavelength, and (3) single-beam dual-wavelength.IR light interacts with most molecules b
23、y exciting molec-ular vibrations and rotations. When the IR frequency matches a natural frequency of the molecule, some of the IR energy is absorbed.While carbon dioxide has several absorption bands, the 4.26 m band is the strongest. At this wavelength, absorption by other common components of air i
24、s negligible. Hence, CO2sensors use the 4.26 m band. Quantitative analysis of a gas sample is based on the Beer-Lambert law (Equation 1), which relates the amount of light absorbed to the samples concen-tration and path length.(1)whereA = decadic absorbanceI0= light intensity reaching detector with
25、no absorbing media in beam pathI = light intensity reaching detector with absorbing media in beam path= molar absorption coefficient (absorption coefficient of pure components of interest at analytical wavelength)c = molar concentration of the sample componentl = beam path lengthFrom Equation 1 it i
26、s evident that the attenuation of an IR beam at 4.26 m is proportional to the number density of CO2molecules in the optical path. For gases, the molecular density is directly proportional to the pressure and inversely propor-tional to the temperature. Thus temperature and pressure corrections must b
27、e applied when using IR absorption to deter-mine CO2concentrations. Operational and environmental conditions affect the performance of all CO2sensors. An unavoidable operational effect is a result of the degradation of the IR light source over time. Since the principle of operation is based on measu
28、red attenuation of the IR beam, a decrease in lamp intensity affects the sensor output. Environmental conditions such as dust, aerosols and chemical vapors may also affect the sensor performance by altering the optical properties of the sensor components due to long-term exposure to these contaminan
29、ts. To minimize the effects of air-born particulates, sensor manu-facturers use a filter media across the opening of the sensors optical cavity where the air sample is analyzed. Various techniques are used by CO2sensor manufactur-ers to compensate for the long-term effects of operational and environ
30、mental conditions. Some sensors automatically reset the baseline value (normally 400 ppm) according to a mini-mum CO2concentration observed over a time period. However, the logic used to reset the baseline and frequency of correction varies with manufacturer, and often it is not well documented. Thi
31、s technique relies on the fact that many Figure 1 Schematic of a NDIR CO2sensor.A log10I0I()cl=ASHRAE Transactions 473buildings experience an unoccupied period during which CO2levels drop to outdoor levels. Other compensation techniques include dual-beam single-wavelength and single-beam, dual-wavel
32、ength designs. The working principles, physical construction, advantages and disadvantages of NDIR CO2sensors are well documented in the literature (Raatschen (1990), Emmerich and Persily (2001), Schell and Int-House (2001), Fahlen et al. (1992).PROCUREMENT AND HANDLING OF THE SENSORSHVAC CO2sensors
33、 are available with various options such as digital display, selectable output signal (voltage or current), output relay and selectable CO2 operating ranges (with 0 to 2000 ppm being the most common). For this research, preference was given to the sensor models that meet Title 24 criteria of the Cal
34、ifornia Energy Commission (CEC 2006) for CO2sensors that can be used for DCV. Among the acceptance criteria are requirements that the CO2sensor(s) have an accuracy of 75 ppm, and a calibration interval of at least five years. Similarly, preference was given to the sensor models with 0 to 10 V output
35、, and with an operating range of 0 to 2000 ppm.Carbon-dioxide sensors used for HVAC controls applica-tion are either duct mounted or wall mounted; however, only wall-mounted sensors meet the Title 24 acceptance criteria. Wall mounted sensors package the sensing element and elec-tronics in a single u
36、nit that is mounted to a base plate secured to the wall. Most wall-mounted sensors provide a port where calibration gas can flow across the sensing element for “field calibration”.Sensors are available from numerous suppliers. In some cases sensors are sold directly through the manufacturer while in
37、 other cases manufacturers produce products which are sold under a variety of product names. Due to the competitive nature of the sensor business and the various after markets, it is difficult to know how many “unique” sensor products there are. In this study, fifteen models of sensors with three of
38、 each model were purchased for the tests. The sensors are divided into three groups: A, B, and C, where each group contains one sensor of each model.The sensors were ordered in two separate batches over a period of several weeks to increase the probability that they would come from different manufac
39、turing lots. Manufacturer provided guidelines for installation and operation of the sensors were adhered to.After receiving all sensors, an “as received” test was conducted for each sensor to check its functionality. This check is not a part of the formal testing of the sensors. The “as received” te
40、st consists of connecting the sensor to the proper power supply, waiting for the appropriate warm-up time, and measuring the output signal from the sensor. Handling of the sensors is always noted on log sheets.All sensors used for testing are mounted on one of three fixtures specifically designed fo
41、r this research (referred to hereafter as “trays” and described more fully in Experimental Apparatus and Instrumentation section). Each tray holds one of the three groups (A, B or C) of sensors. Prior to testing, all trays were placed in the lab station and the sensors were powered up for a three we
42、ek period before commencing the first formal test. This time period provided assurance that all sensors acclimate to the conditions (temperature, humidity and CO2concentration) in the laboratory and that sensors which “self-calibrate” over a period of several weeks are given adequate time to complet
43、e the calibration process. Ambient conditions in the laboratory (temperature, relative humidity, and CO2concentration) are continuously recorded to provide a record of the environmental conditions.EXPERIMENTAL APPARATUS AND INSTRUMENTATIONA test chamber (hereafter, chamber) was designed and fabricat
44、ed for this study. Figure 2 is a schematic diagram of the test system used. The sealed chamber is constructed of 8 in. (20.3 cm) square 0.25 in. (6.35 mm) wall steel tubing. An external water jacket is used to maintain the desired tempera-ture inside the chamber. Flanges are welded to each end of th
45、e chamber to enable removable end plates with gaskets to be attached. The front endplate is made of Lexan while the rear endplate is 0.25 in. (6.35 mm) steel. The chamber was sized to accommodate one tray of test sensors at a time. Air cylinders are connected through one end plate and are used to co
46、ntrol the pressure inside the chamber during the pressure sensitivity tests. The chamber vent valve (valve #8) is partially closed to pressurize the test chamber to sea-level pressure while allowing continuous flow of the gas mixture through the test chamber during accuracy, linearity, repeat-abilit
47、y, hysteresis, humidity sensitivity, and temperature sensi-tivity tests.A gas mixture of CO2and N2is supplied to the test cham-ber from a commercially-available gas-mixing system1. The gas-mixing system uses mass-flow controllers calibrated using a primary flow standard traceable to the United State
48、s National Institute of Science and Technology (NIST). The system is capable of producing gas mixtures from 334 to 3333 ppm CO2(1% accuracy) at a flow rate of 3 liters/min. The gas mixing system is annually calibrated as recommended by the manufacturer. The technical specification of the gas mixing
49、system is provided in Table 1. The dry-gas mixture from the gas mixing system is “bubbled” through a deionized water column to add water vapor (humidity). The bubbler is capable of producing relative humidity ranging from dry gas (no humidification) to approx-imately 80% relative humidity.Adding water vapor to a dry gas mixture changes the mole fraction of the gases in the mixture; therefore, the concentra-tion of CO2in a mixture will decrease as water vapor content increases. To achieve a desired CO2concentration under 1.EnvironicsS-4000474 ASHRAE Transactionshumid condi
