Fluke CO-220 Fluke Air Quality Measurement - Making the Number Add Up Understa - Page 5

through the chamber. As CO2 levels increase the gas absorbs more light, which reduces the strength of the electric signal emitted by the detector. Technology characteristics. Design affects both the performance and accuracy of a CO2 tester. The length of the test chamber or "bench" is important, because a longer chamber enables the light to pass through a larger air sample and more CO2 molecules before reaching the detector, making greater accuracy possible. A test instrument that pumps air through the test chamber will respond faster than a dispersion unit that does not pump the air. Changes in air temperature, pressure and density all will affect the accuracy of CO2 test results, so CO2 meters must be adjusted before use to compensate for changes in air pressure and temperature. Some instruments are built to compensate automatically for these changing conditions. Stability and drift. The CO2 sensor will degrade and drift over time due to loss of sensitivity and declining bulb performance. Airborne containments will pass through the sensor filter (assuming there is one) and accumulate on the interior walls, emitter and detector of the sensor. This contamination will affect the intensity of the light source, as well as the signal strength of the optical filter/ detector. The more contaminants (smoke, dust, etc.) the sensor sees, the faster the degradation of the signal strength. Another factor affecting longterm stability of the sensor is degradation of the light source. Like any bulb, it will burn out. User guidance. A CO2 sensor should be calibrated about once a year to compensate for the reduced output of the bulb and collection of contaminants. CO2 meters can be tested and user-calibrated using a standard span gas that contains a known percentage of CO2. A rough calibration can be achieved using outdoor air, which should contain 350 to 450 parts per million of CO2. Dry nitrogen, which contains no CO2, may be used as a zeroing gas. Carbon monoxide Sensor technologies. Carbon monoxide testers use an electrochemical gel sensor technology. Technology characteristics. The gel sensor has a limited life span (two years) and its accuracy can be affected by changes in both temperature and ambient humidity levels. Stability and drift. Prolonged exposure to humidity levels below the ideal 50 percent RH level can dry the gel sensor and cause the readings the instrument delivers to drift out of spec. The changeable nature of CO sensor technology means that users must calibrate their CO meters frequently (monthly) to ensure accuracy. Most CO meters can be user-calibrated using a span gas containing a known percentage of CO. They can be zero calibrated (zeroed) in free air. User guidance. Calibrate the instrument when you receive it, to ensure that it is set for your environment. Then, if your environment changes (for instance, if humidity increases during the summer), recalibrate. Simply transporting the instrument from a humid outdoor environment into a dryer air-conditioned space should not cause problems. Environmental changes over a longer period are the issue. Conclusion Accuracy is fundamental for those who measure, monitor and control air quality in workspaces. By choosing instruments carefully based on an in-depth understanding of their specifications and performance characteristics, using them properly, and maintaining and calibrating them as recommended, HVAC and IAQ professionals can ensure themselves and their clients of accurate measurements and effective guidance to improve indoor air quality and comfort. Fluke. Keeping your world up and running.™ Fluke Corporation PO Box 9090, Everett, WA USA 98206 Fluke Europe B.V. PO Box 1186, 5602 BD Eindhoven, The Netherlands For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: http://www.fluke.com ©2006 Fluke Corporation. All rights reserved. Printed in U.S.A. 8/2006 2722838 A-EN-N Rev A  Fluke Corporation Making the numbers add up: Understanding specifications and performance of IAQ test instruments