Detector Electrical Circuitry (continued)

TM 10-6665-297-13&P 1-2 GAS DETECTOR UNITS (continued) 1-2.2 Detector Electrical Circuitry (continued) thus disturbing the electrical balance of the paired resistances. The resulting voltage changes are in proportion to the concentration of gas in the atmosphere surrounding the detector. Automatic compensation for changes in ambient temperature, humidity, and pressure is provided by the reference element in the opposite half of the resistance bridge. As the opposing companion element changes resistance with changes in ambient air conditions--exactly as does the sensing element-- the bridge remains in balance. Therefore only the combustible gas-to-air proportional mix, independent of other atmospheric conditions, will produce an output signal from the detector. For CD800/830 (single-channel) models, a matched pair of 15-ohm resistors installed on the control module terminal strip, Figure 1-2, can be removed and replaced by leadwires to a second detector to provide more intensive monitoring of a critical area. Readings then sum gas conditions at both detectors so that, for example, a 20-percent L. E. L. reading may equal 10 percent plus 10 percent or 14 percent plus 6 percent, etc. Such installations do not indicate at which of the two detectors the more dangerous atmospheric condition prevails, therefore the installation of single-channel systems with two detectors should be limited to areas where corrective or protective measures can be applied in the vicinity of both detectors at once. Conversely, CD802/832 (dual-channel) models designed to monitor two detectors independently may be used to monitor only a single detector. If so, a matched pair of 15-ohm resistors must be installed on the control module terminal strip in place of the leadwires from a second detector. 1-2.3 DETECTOR MODELS. (Figure 1-3) Different environments frequently require different detector housings and air sampling techniques. Five detector models (for which up-to-date Factory Mutual and Canadian Standards Association listing are available from the manufacturer) are available to meet these varying needs: (1) Models 23-4012: remote mounting, explosion-proof, weatherproof detector housing; diffusion sampling (Figure 1-3). Approved for use in hazardous areas as defined by the National Electrical Code (N. E. C.), Class I, Division 1, Groups A, B, C, and D. (2) Model 23-4014: duct-mounting, explosion-proof detector housing; diffusion sampling (Figure 1-3). Approved for use in N. E. C. Class I, Division 1, Groups A, B, C, D hazardous areas. Because the mounting plate may not be a suitable interface between Division 1 and less hazardous areas, this 23-4014 detector is approved for use entirely within a Class I, Division 1 area, or for insertion into a Class I, Division 2 area. 23-9617 7/1/76 1-4