Fluke CO-220 Fluke Air Quality Measurement - Making the Number Add Up Understa - Page 4

In terms of response time, both technologies perform well. But response time is also affected by the overall design of the test instrument. If the temperature sensor is enclosed in a massive housing, the surrounding material is liable to affect the speed with which air heats or cools the sensor, thus slowing response time. Minimal response time requires minimizing mass, and that means shrinking components. If carried too far, this could affect instrument durability. Stability and drift. The thermistor temperature sensor is very stable and does not drift like some other sensors. However, the temperature sensing function can drift over time due to other components used and the layout of the circuit design. In design and manufacture, care must be taken in the layout of the printed circuit board that connects with the thermistor. Ionic contamination from soldering flux residues can cause performance degradation. This increases as the impedance of the circuit increases. Good analog design is essential to achieving optimal temperature performance. User guidance. Allow for instrument 'settling time' when measuring temperature. The mass of a temperature tester may slow response time in rapidly changing conditions. In simple terms, it takes a while for the instrument to reach ambient temperature. In addition, the instrument's electronic circuitry will perform differently as temperatures vary, so the circuitry too must settle and stabilize. Humidity Sensor technologies. The device traditionally used to measure humidity is the sling psychrometer. This instrument consists of two bulb thermometers, one of which is surrounded by a wet cloth. When the device is slung through the air, moisture evaporates from the wet bulb and cools the thermometer. The dryer the air, the greater the temperature difference between the wet and dry thermometers. A more convenient alternative is a hand-held humidity meter that uses a capacitance sensor technology. A semi-permeable membrane in the sensor becomes more conductive as humidity increases and moisture penetrates the membrane. The meter interprets this change in terms of humidity level. Technology characteristics. The sling psychrometer is highly accurate when used correctly. It is also relatively slow and cumbersome to use. The hand-held meter is much faster and easier to use, but over time the sensor membrane will be affected by airborne contaminants that reduce its ability to absorb moisture. The sensors cannot be cleaned, so if they are accidentally contaminated or simply 'get old,' they must be replaced. Stability and drift. Users should be aware that the performance of humidity meters will decline over time. The rate of change will depend on how and where they are used. User guidance. Testing to determine accuracy and calibrate the instrument requires sophisticated and expensive test equipment beyond the means of the individual user. Instruments can be returned to the manufacturer or sent to an independent testing laboratory to verify their performance. If out of spec, the sensor of an expensive instrument may be replaced. In the case of a less costly instrument, the user will probably choose to replace the entire tool. Airborne particles Sensor technologies. The particle counter uses a pump to pull a sample of air into a space called the view volume, where particles intersect with a laser beam. The particles reflect differing amounts of light based upon their size. Photo detectors "see" these light flashes and convert each one to a millivolt signal. Larger particles reflect more light and create a stronger signal. Signals within a certain millivolt range are counted in one size "bin," particles in another range are put into another bin, and so on. Technology characteristics. Accuracy can be affected when the counter is used where extremely high levels of particles are present. Particles may collect within the intake passage and measurement chamber. If these particles are dislodged during a subsequent test, they can cause a misleading spike in the particle count. Stability and drift. Laser particle counter technology is generally stable over time, but the manufacturer's recommended calibration interval should be observed to maintain optimal performance. User guidance. Operating the air pump for a period of time before taking a reading will help to flush particles out of the instrument. Users can calibrate the instrument for a zero particle count by applying a HEPA (high efficiency particle air) filter over the air intake port. Carbon dioxide Sensor technologies. CO2 sensors use a non-dispersive infrared technology. Incandescent light is projected through a small sample cell called the "bench link". The CO2 present in the test sample will absorb a specific wavelength of the projected light. A filtered infrared detector at the other end of the chamber measures the amount of light at that wavelength that passes  Fluke Corporation Making the numbers add up: Understanding specifications and performance of IAQ test instruments