S ENSORS 0 C 2 MAKING SENSE OUT OF SENSORS CO2 and air quality sensors are not interchangeable, but both can be important for building indoor air quality control. Here is a review of what they can and can't do. BY MIKE SCHELL, AIRTEST TECHNOLOGIES With the growth of the perature and humidity. indoor air quality industry, a The biggest drawback number of new technologies of these sensors is that have burst onto the market- they have no way of dis- place. One of these tech- tinguishing a potentially nologies, often called an air harmful air contaminant quality sensor, volatile or- from a harmless gas (e.g., ganic compound (VOC) perfume or after shave). sensor, or IAQ sensor has Because an air qual- generated great interest as ity sensor will react in dif- a possible approach to con- ferent ways to different trolling air quality in all types gases and compounds or of buildings. mixtures in the air, there Many have wondered if is no way to calibrate these sensors can be used them to a known measure the same way as CO2 sen- of contaminant concen- sors. While both sensor tration or air quality. technologies can be useful Some manufacturers in indoor air quality control, suggest that individual they are not interchange- sensors can be tuned to able. Each has its own par- the building space in ticular applications. which they are operating. In such an application, A CLOSER LOOK each individual sensor in Air quality sensors are a building space would be often called total contami- AirTest's carbon dioxide controllers are designed to regulate fresh air adjusted to provide a low nant sensors, because they delivery to a space to ensure that target cfm-per-person ventilation rates for indoor air quality are maintained. output signal when the measure and react to a building is perceived to broad range of diverse com- have good air quality. The pounds in the air. The sensor reaction is nonspecific sensor would provide a higher signal These sensors utilize an interactive, to any one gas; its output reflects the when more contaminants are in the chemical-based oxidizing element. When this total effect of a wide variety of com- space. element is exposed to various compounds pounds in the air on the sensor. The com- While this approach may work in in the air, the sensor will vary it’s electrical pounds it reacts to can be VOCs, aero- general, it is limited by the inability of resistance and provide an output signal. sols, hydrocarbons or just changing tem- this type of sensor to have any defined S ENSORS Comparison Broadband Air Quality Sensors Carbon Dioxide Sensor Gases Measured Wide range of non-specific gases. Carbon Dioxide Measurement Units Cannot be referenced to any known measure but parts per million (ppm) can be individually “tuned” to a building space. Measurement Range Typically 0-5 or 0-10 units 0-5,000 ppm Resolution 1 unit (non-quantifiable) ±20 ppm Common Interference Temperature & Humidity- Also, low levels of silicon None vapors and other hydrocarbons may cause damage. Calibration Cannot be calibrated to any referenced standard. Use calibrated gas or recently calibrated instru- Will tend to normalize in the direction of conditions it ment as reference. sees most often over the short term. Type of Technology Interactive Sensor chemically reacts with pollut- Non-Interactive Sensor does not come in contact ants and eventually changes or degrades sensitiv- with sampled air. CO2 concentration determined ity over time. by infrared light interference of CO2 gas. Drift Unpredictable. 100 ppm per year or less (linear). Can be reduced to 30 ppm annually with self-calibrating software. Best Application To measure changes in indoor conditions where Ventilation control to ensure that target cfm-per- unusual non-occupant related pollutant sources are person outside air ventilation rates are maintained a concern (e.g. occupied printing plants, VOC clean- at all times in occupied spaces. Energy savings ing materials). can result where occupancy is variable or inter- mittent (e.g., schools, meeting rooms, offices). Correlation to Ventilation Rate Levels will probably be higher when there is less The differnce between inside and outside con- outside aire ventilation and inside sources of sen- centrations can ve directly related to the cfm-per- sor-sensitive pollutants are higher. person of outside air delivered (e.g., a 700 ppm differential = cfm/person). Recognized in the ASHRAE 62- No. The “Air Quality Procedure” in the standard al- The “Ventilation Rate Procedure” in the standard 1989 Standard “Ventilation for Ac- lows for control of ventilation based on reducing con- establishes specific cfm-per-person guidelines for ceptable Indoor Air Quality” centration of specific contaminants. Since the sen- various applications . Appendix D of the standard sor does not sense specific gases in known con- provides the rationale to correlate CO2 concen- centrations, it is impossible to know if acceptable con- tration to specific cfm-per-person ventilation rates. taminant concentrations have been achieved. TABLE 1: A comparison of CO2 sensors and air quality sensors, also called VOC or IAQ sensors measure of air quality, as well as the fact Air quality sensors continually drift and humidity. As a result, it is sometimes that it reacts in different ways to differ- as chemical compounds react with he difficult to determine if the air quality ent contaminants. sensor surface. Once installed, these conditions measured from season to sea- A critical feature of any good sen- types of sensors will tend to zero or sta- son are actually similar, even if the sen- sor, whether it measures temperature, bilize themselves in whatever environ- sor output may be the same. humidity, or various gases, is that it ments they have been exposed to over However, a low reading will still pre- should have very little drift. Drift is the the past few days or hours. sumably indicate better air quality than tendency for a sensor to shift its cali- The sensitivity of these sensors also a higher reading when compared over a bration over time. is dramatically affected by temperature few days. S ENSORS TESTING REVEALS when the presses begin operation. In a tilation based on occupancy. An air qual- LIMITATIONS commercial building, it might indicate ity sensor cannot be used to determine In a recent article written by Sal when the cleaners are using potent de- the ventilation rate of a space. Ignello, manager of Indoor Quality En- tergents or de-waxers. In a new build- A CO2 measurement cannot indi- vironments at Johnson Controls, he indi- ing, it might indicate when furniture or cate if outside air quality is good, al- cated that their testing had found some carpets are off-gassing a significant num- though a high outside CO2 level (over limitations to IAQ sensors. He stated: ber of contaminants. 600 ppm) can indicate the outside air is -Sensitivity of these sensors is not Most air quality sensors provide a 1-5 quite polluted. A CO2 sensor controls the sufficient to detect low levels of VOCs or 1-10 scale of output. It is impossible to ventilation rate in occupied spaces. that are known to affect building occu- determine what a "3' reading means ver- If a space is unoccupied, a CO2 con- pants. sus a "9," because of the non-specific na- troller is typically designed to set air in- One study monitored 240 spaces in ture of the sensor. It can tell you some- take volume at a minimum setting; it can- 15 buildings. Measurements were made thing has changed, but it cannot really in- not sense the buildup of other contami- with a single air quality sensor at the dicate if the change is significant or quan- nants within a space. Carbon dioxide same time as a trained human sensory tifiable from an air quality perspective. (CO2) should not be confused with car- panel made judgment of the space odor Future product development will fo- bon monoxide (CO), a highly toxic by- level. There was no correlation between cus on trying to make sensors more con- product of combustion of furnaces, fire- the sensor's indications and the panel's sistent, as well as referencing them to a places, and automobiles. perception of the area's IAQ. known standard of air quality, Ideally in Carbon dioxide sensors are best -Temperature and humidity often a real application, once an air quality used to identify and control outside air overwhelm the sensor, making it less sensor is tuned to the space, a control ventilation rates in non-industrial build- sensitive to other contaminants. strategy would attempt to keep levels as ings where cfm-per-person ventilation - Sensors can be heavily biased toward low as possible. rates, as mandated by codes and stan- reaction to one compound over another. dards, must be maintained. Testing showed that an IAQ sensor THE DIFFERENCES Measurement and control of CO2 in will have the same response to 5,000 ppm Air quality sensors and CO2 sensors a building can ensure that outside air is be- of methane as 250 ppm of carbon mon- measure very different things. In fact, ing circulated in the right proportion to the oxide (CO), 90 ppm of acetone, or 50 because carbon dioxide is an inert gas, it distribution of people within the building. ppm of ethanol. These tests confirmed is one of the few elements that will not If the space is subject to intermittent or that high and dangerous levels of toxic cause an air quality sensor to react. variable occupancy, costly and unneces- pollutants like CO could be masked or Also, most CO2 sensing technology sary over-ventilation can be avoided.