1-3.1.8 Alarm and Warning Reset Circuits (continued)
CD802/832 relays K2, K3, and K4 latch and reset in the same way as K1 and K2 of the CD800/830
Circuit modifications for remote resetting, or for automatic resetting of warning and alarm relays as
alarm conditions are cleared may be accomplished, if desired, during or after the installation of a
system. (See Subsection 2-7. ) A common reset button for several modules may be provided, in which
case individual reset switches would not be incorporated in the system. Kits for conversion to common
reset are available for systems with standard individual control module reset connections.
1-3.1.9 Display Meter. The display meter, located on the front panel of the control module, continuously
indicates gas-to-air mixtures at the remote detect- or in terms of percentage of the lower explosive
limit. With fresh air at the detector, the indicator reads zero. But catalytic oxidation of gas on the
detector sensor produces unequal outputs from the two halves of the detector circuit, which causes a
differential amplifier (U3 for CD800/830; U1 for CD802/832) in the control unit to amplify the difference
and produce a proportionate current flow through the indicating meter (Figure 1-12).
The CD802/832 module meter responds either to the channel selected by the setting of the channel
selector control, or, if the control is set to AUTO, to the channel with the strongest signal.
In addition to the mechanical adjustment-to-zero screw on the meter front, two electrical adjustments
prepare the meter for proper indication of gas air mixtures. A ZERO potentiometer (CD800/830: R24;
CD802/832: R9, R13) screw adjustment (Figures 1-4, 1-5, 1-9) serves to balance the input voltage to
amplifier U3 (with detector in fresh-air or nitrogen-inerted condition for setting), to assure that the output
signal fed to the meter is zero. After the indicator has thus been set to zero, a GAIN potentiometer
(CD800/830: R10; CD802/832: R22) screw adjustment (Figures 1-4, 1-5, 1-9) provides the amount of
amplification gain required to drive the indicator needle full-scale in the presence of a 100-percent L. E.
L. gas-air mix at the detector. Because each combustible gas has its own rate of heat energy release
upon oxidation, the GAIN adjustment must be set with respect to the kind or kinds of gas to be
1-3.1.10Relay External Circuits. (Figures 1-13, 1-14) Solenoid operated contacts for controlling external lamps,
buzzers, horns, fans, sprinklers, door closers, etc., are provided by control unit relays K1, K2, and K3
for all models, and an additional relay K4 for CD802/832 dual-channel models. Warning relay K1 and
alarm relay K2 of CD800/830 models, and warring relay K2 of CD- 802/832 models each have two
isolated normally open contacts and two