Fluke 983 Fluke Air Quality Measurement - Making the Number Add Up Understandi - Page 2

Limited and imprecise standards. Outside the industrial workplace, absolute standards for most indoor air conditions and air pollutants do not exist. Guidelines, not specific limits, are the rule. Government has been slow to establish specific standards to control levels of indoor air pollutants. With few exceptions, science has yet to convince lawmakers that indoor air contaminants cause specific health problems that demand legislation. In 1994, the U.S. Occupational Safety and Health Administration (OSHA) filed a notice of proposed rulemaking for indoor air quality in non-industrial workplaces, but withdrew the proposal in 2001. The management of indoor air quality remains primarily a private, not public, concern. Test instrument performance. Another important consideration is the accuracy of test instruments over time and in varying environmental conditions. Instrument standards and performance specifications, together with appropriate testing and calibration, are the keys to making accurate, repeatable and defensible measurements. Accurate performance over time is essential for valid, repeatable results and effective remediation. Yet the technology used to measure some air quality parameters, such as CO2 and CO, is inherently subject to drift and change as days and weeks go by. It's important for the practitioner to understand these characteristics and know what they can expect from their instruments-and what steps to take to achieve high quality results. Calibration and traceability. Without documented proof that a test instrument has been calibrated against a known standard, called traceability, test results may be difficult to defend against a challenge. In a court case, failure to prove valid measurement, made by instruments that were calibrated and employed as specified by the manufacturer, could result in liability for the organization and the air quality professional. Instrument usage and operator error. Measuring the characteristics of an invisible, changeable gas, using instruments that may perform within spec for a limited time, or in a narrow range of conditions, puts the responsibility on the operator to understand each test tool's limitations and to maintain and use those instruments correctly. What is avoidable operator error? Consider what could happen if a technician drove to a job site to assess indoor air quality. In the middle of winter, this tech left his instruments in his unheated garage over night. Carrying the chilled instruments into the job site, he ran a quick temperature check. The results are way off. The temperature tester's electronic circuits are accurate only within a specified temperature range, and the instrument is still very cold. Only by letting the tool warm up to room temperature will this tech get the accurate result he's after. Specifications and the spec sheet Every measurement is based on our belief that the instrument is going to give us the "real" reading. Specifications quantify both the likelihood of getting accurate readings and the risk of seeing inaccurate readings. A specifications document is a clearly written description of an instrument's performance. It should quantify the instrument's capabilities objectively, under well-defined operating conditions. Good specifications will be complete, identifying all factors that affect the instrument's ability to deliver accurate measurements. For air quality instruments, such factors could include humidity, temperature, altitude and air pressure. Specifications should also be clear and objec- tive. You should expect that the information is both accurate and complete. The spec sheet should identify measurement uncertainty specs and modifiers that affect the uncertainty, as well as operating limits (such as temperature) that constrain the environment in which the uncertainty specifications will hold true. Drift and test tool calibration Time and temperature are crucial for determining uncertainty. Electronic components experience small changes (or "drift") over time. The electrochemical nature of some air quality sensors makes their readings even more likely to drift over time. Because of drift, test instrument uncertainties are valid only for a specified period of time, which usually coincides with the recommended calibration cycle. At calibration, the clock starts over again and the uncertainties are valid for another period. Temperature affects the performance of every component in an instrument. Instrument designers make every effort to build circuits that compensate for temperature variation. The ability to operate at various temperatures is captured in a specified operating range and is often accompanied by a temperature coefficient.  Fluke Corporation Making the numbers add up: Understanding specifications and performance of IAQ test instruments