Panasonic PT-DZ12000U Operating Instructions Manual page 123

A. Passive and Diffusion
Passive and diffusion sampling of air requires the
substance being sampled to come into contact (or near
contact) with the container, collection media, or detection
assembly.
Examples of this sampling method are; smoke, CO, and
CO monitors where the substance in the air (in this case the
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smoke or CO or CO ) migrate throughout the space where the
detectors are located and when the substance enter into the
active volume of the detector an alarm is generated.
Another example is the Electret for sampling radon and
thoron. A charged disc is placed inside a container which has
an opening through which air can migrate. The radon and
thoron in the air will ionize the air inside this container due to
the radioactive decay of these gases and their progeny. The
ionization of the air then causes the charged disc to lose part
of its charge. When the Electret is collected later the
remaining charge on the disc is determined and the amount
of charge lost is related to the radon and thoron concentration
of the air the Electret was exposed to.
Colorimetric detectors are also used in a passive mode for
the detection of toxic airborne substances. A color change or
color intensity change indicates the detector has reacted to
the toxic substance. The substances detected by this method
are gases such as chlorine and ammonia and for military and
homeland security applications colorimetric detectors for
biological and chemical weapons are used.
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