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Source: Foxboro Co.

Source: Foxboro Co.

chart recorder, magnetic tape data storage, or an online data transmission system. The output of most air quality sensors is in analog form. This form is suitable for direct input to a strip chart recorder; but in automated systems, the analog signal is commonly converted to a digital signal. For those sensors that have linear output, the signal can go directly to the recorder or transmission system. For sensors with logarithmic output, it may be advantageous to convert this signal to a linear form.

Many early automated air quality monitoring systems in the United States had difficulty with the data transmission step in the system. In some cases, this difficulty was caused by attempts to overextend the lower range of the sensors so that the signal-to-noise ratio was unfavorable. In other cases, matching the sensor signal output to the data transmission system was poor. With developments and improvements in systems, these early difficulties were overcome.

Online systems, i.e., those that provide an instantaneous readout of the dynamic situation monitored by the system, add an additional step to air quality monitoring systems. Data from continuous monitors can be stored on magnetic tape for later processing and statistical reduction. In an online system, this processing is done instantaneously. The added expense of this sophistication must be evaluated in terms of the purposes of the air quality monitoring system. When decisions with substantial community impact must be made within a short time, this real-time capability may be necessary.

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