RKI Instruments FP-40 - Manual (Page 16)

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6.4 7 DETECTION PRINCIPLE
The Gas detection TAB test paper is treated with special chemicals and an illuminating agent.
When gas is blown onto the TAB paper face, the paper emits illumination by chemical reaction, and this
causes the paper to change color. The amount of color change is determined by the level of
formaldehyde exposure, and the time of exposure.
For example, when formaldehyde (HCHO) contacts the paper, chemicals impregnated into the
paper combine with HCHO to form compounds, and these compounds change the paper from white to
yellow color.
The instrument creates a light beam that reflects off the test paper at the end of the detection cycle. The
intensity of the light beam is affected by the color or darkness of the detection TAB paper, and this
intensity is measured by a light sensor. The level of light intensity measured is correlated (from an
exposure curve stored in the instrument) to a particular level of Formaldehyde exposure to the tab. This
concentration is then displayed at the end of the detection cycle.
Light (LED)
Photo sensor (Photo diode)
GAS IN
GAS OUT
Chamber
Detection
TAB