The FP-31 is a highly sensitive portable gas detector specific for formaldehyde detection. It uses a photoelectric photometry method which utilizes colorimetric tablets for detection. A tablet in placed into the instrument, and then a room air sample is pumped onto the tablet for either a 15 minute or a 30 minute period. If formaldehyde is present, it will cause the chemically impregnated tablet surface to darken or stain.  The stain darkness is read by an optical sensor in the FP-31, and then the instrument calculates the formaldehyde concentration.