Fluke CO-220 Fluke Air Quality Measurement - Making the Number Add Up Understa - Page 3

Instruments are designed to deliver measurements within a specific temperature range, generally around room temperature. Outside that range, results will deviate. It may be possible to calculate an adjustment factor or temperature modifier for use in adjusting results obtained in these conditions. The user should review instrument documentation or consult the manufacturer to determine how changing environmental conditions affect instrument performance, and how to compensate. Other environmental factors, such as storage temperature, humidity, air density, and electromagnetic radiation, can also affect uncertainty. There are a number of steps air quality professionals can take to prevent these performance characteristics from adversely affecting their results. First, they should seek out, evaluate and compare manufacturers' claims regarding instrument accuracy and stability. As mentioned above, not all specs are equal. This review should also include an evaluation of each manufacturer's reputation for quality and performance. After the user has chosen an instrument, it is important to keep that tool properly calibrated in accordance with manufacturer's instructions. Unlike a yardstick, an air quality test instrument's performance will change over time. Calibration and, when necessary, replacement of out-of-date or worn-out sensors will bring the tool back to peak performance. If renting, ask to see up-to-date proof of calibration for the instruments you will use. Performance characteristics of indoor air quality test technologies Each characteristic of indoor air quality is measured using a specific type of sensor. In some instances, instrument engineers can choose from several alternative sensor technologies. In every case, their decisions about which sensor technology to use, and how to employ it, must take into account a number of performance parameters: • Accuracy • Response time • Stability over time, and in varying environmental conditions • Strength, durability and longevity • Portability and ease of use • Ease of adjustment and calibration • And always, cost In most cases, product designers must make tradeoffs to deliver an affordable end product that meets user needs and expectations. A 'perfect' instrument is something few would be willing to pay for. In the following section we review the technologies used to measure the various air quality parameters, the nature and limitations of those technologies and what users need to know to get the best results. Air temperature Sensor technologies. Though household thermometers may employ expanding liquid (alcohol or mercury) or a bimetallic strip attached to a pointer, professional instruments generally use one of two sensor technologies: the thermocouple or the bead thermistor. Most common in the service industry is the thermocouple, which looks like a bead on the end of two wires. The sensor is a combination of two metals that when joined together and presented with a temperature, creates a voltage differential across the connected wires. Thermocouples have little mass and therefore respond quickly to temperature changes. This is significant when measuring air, a gas with relatively low density and limited ability to quickly heat or cool a material. The thermistor uses a different technology. A small resistor in the device receives a voltage or current. Resistance in the device varies as temperature changes, causing output current or voltage to change as well. A third electronic technology is infrared, used in non-contact thermometers. Infrared thermometers do not measure air temperature, but measure the infrared radiation emitted from surfaces. Infrared thermometers provide greatest accuracy at short distances, and provide at best an indirect indication of air temperature. Technology characteristics. Comparing thermistor and thermocouple technologies reveals several performance differences. Because simpler circuitry is needed to convert thermistor signals into temperature readings, instruments using thermistors are likely to cost less than those using thermocouples. Thermocouples can perform from near absolute zero to thousands of degrees. Thermistors operate over a limited temperature range (approximately -30 °F to 180 °F). This range is likely adequate for indoor air quality applications.  Fluke Corporation Making the numbers add up: Understanding specifications and performance of IAQ test instruments