Sensors

The output reliability from an air quality sensor depends on its inherent accuracy, sensitivity, zero drift, and calibration. The inherent accuracy and sensitivity are a function of the the instrument's design and its operating principle. Zero drift can be either an electronic phenomenon or an indication of difficulties with the instrument. In instruments that use an optical path, lenses become dirty. In wet chemical analyzers, the flow rates of reagents can vary, changing both the zero and the span (range) of the instrument. Because of these potential problems, every instrument should have routine field calibration at an interval determined in field practice as reasonable for the sensor.

Environmental engineers calibrate an air quality sensor using either a standard gas mixture or a prepared, diluted gas mixture using permeation tubes. In some cases, they can currently sample the airstream entering the sensor by using a reference wet chemical technique.

The operator of air quality sensors should always have a supply of spare parts and tools to minimize downtime. At a minimum, operator training should include instruction to recognize the symptoms of equipment malfunction and vocabulary to describe the symptoms to the person responsible for instrument repair. Ideally, each operator should receive a short training session from the instrument manufacturer or someone trained in the use and maintenance of the instrument so that the operator can make repairs on site. Since this training is seldom possible in practice, recognition of the symptoms of malfunction becomes increasingly important.

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